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					     21        Introduction to
              Organic Chemistry
      21.1    What is Organic Chemistry?
      21.2    The Unique Nature of Carbon
      21.3    Classification of Organic Compounds
      21.4    Factors Affecting the Physical Properties of
              Organic Compounds


 1
1            New Way Chemistry for Hong Kong A-Level Book 3A
     21.1
        What is Organic
          Chemistry?

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21.1 What is Organic Chemistry (SB p.2)


       Organic Chemistry

        •   Chemistry of the compounds present
            in living organisms.
        •    They all contain carbon.
        •    Organic Chemistry is the Chemistry
             of Carbon.



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21.1 What is Organic Chemistry (SB p.4)

       Natural Sources of Organic Compounds
          Living                                     Carbohydrates /
          things                                     Proteins / Fats /
                                                     Vitamins /
                                                     Antibiotics



                                                              A variety of
                                                            organic products
                                                             obtained from
                                                              living things
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21.1 What is Organic Chemistry (SB p.4)

       Natural Sources of Organic Compounds

                   Fractional distillation /               Alkanes /
                                                           Alkenes /
       Crude   oil destructive distillation
                                                           Alkynes /
       or coal
                                                           Aromatic
                                                           hydrocarbons

                                                             Check Point 21-1
                                                     A variety of
                                                   useful products
                                                    derived from
                                                  crude oil and coal
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21.1 What is Organic Chemistry (SB p.3)

       Development of Organic Chemistry
       as a Science
       In the past …,

                               Chemistry

               Organic                               Inorganic
              compounds                             compounds
           obtained from                          obtained from
          living organisms                      non-living sources
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21.1 What is Organic Chemistry (SB p.3)

       Development of Organic Chemistry
       as a Science
       In 1828, Wohler (a German chemist)




                 (Inorganic                           (Organic
                compound)                            compound)
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21.1 What is Organic Chemistry (SB p.3)

       Development of Organic Chemistry
       as a Science
       Redefining … ...

       Organic chemistry is the study of carbon
       compounds (except CO, CO2, carbonates,
       hydrogencarbonates, carbides and cyanides)
       obtained from natural sources or
       synthesized in the laboratories.


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     21.2
         The Unique
       Nature of Carbon


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21.2 The Unique Nature of Carbon (SB p.5)


        Ability to form four strong covalent bonds

         • Electronic configuration of carbon
           (ground state) : 1s22s22p2




                     Carbon (ground state)



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21.2 The Unique Nature of Carbon (SB p.5)


        Ability to form four strong covalent bonds

        • Each carbon atom has four unpaired
          electrons when excited

        • Tend to form four strong covalent bonds




                     Carbon (excited state)

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21.2 The Unique Nature of Carbon (SB p.5)


        Ability to Catenate

         • Carbon atoms link together to form
           chains of varying length, branched
           chains and rings of different sizes
         • Catenation:
             Ability of atoms in forming stable
              bonds with itself, hence joining up
              into chains or rings

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21.2 The Unique Nature of Carbon (SB p.5)


        Ability to Catenate

        C – C > Si – Si > Ge – Ge > Sn – Sn

        Bond strength  as bond length 




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21.2 The Unique Nature of Carbon (SB p.5)


        Ability to Catenate

        C–C>N–N>O–O

        Bond strength 
        as the number of lone pairs 




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21.2 The Unique Nature of Carbon (SB p.5)


        Ability to Catenate

        CnH2n+2        n = 1,2,3,…(no limit for n)
        SinH2n+2       n = 1 to 6 only  silanes
        GenH2n+2 n = 1 to 3 only  germanes
        SnnH2n+2 Only SnH4 (stannane) exists




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21.2 The Unique Nature of Carbon (SB p.5)

        Ability to Form Multiple Bonds
                            sp3
                                                 4 bonds
                         sp2
                                         1 bond, 3 bonds
                     sp
                                    2 bonds, 2 bonds




        Carbon (excited state)
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21.2 The Unique Nature of Carbon (SB p.6)

           Single bond          Double bond                   Triple bond




                              * X = halogens



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21.2 The Unique Nature of Carbon (SB p.6)




                Example 21-2                        Check Point 21-2




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       21.3
          Classification of
              Organic
            Compounds
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21.3 Classification of Organic Compounds (SB p.7)


        Functional Groups

        •   Organic compounds are classified by
            the the presence of characteristic
            functional groups.




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21.3 Classification of Organic Compounds (SB p.7)



        Functional Groups

        A functional group is defined as an
        atom or a group of atoms that
        effectively determines the chemical
        properties of an organic compound.




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21.3 Classification of Organic Compounds (SB p.7)


        Functional Groups




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21.3 Classification of Organic Compounds (SB p.7)


        Functional Groups

        •   Propane does not react with sodium

        •   Ethanol and propan-1-ol react with
            sodium to give hydrogen gas




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21.3 Classification of Organic Compounds (SB p.7)

        Functional Groups


                                       and


   • have similar chemical properties
         they contain the same functional group –OH
         they are classified into the same
          homologous series — alcohols
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21.3 Classification of Organic Compounds (SB p.12)

        Homologous Series

        A homologous series is a series of
        compounds that have the same functional
        group, and each member differs from
        the next member by a – CH2 – unit in
        their formulae.

           CH4               C2H6                 C3H8                C4H10

                   CH2                    CH2                   CH2
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21.3 Classification of Organic Compounds (SB p.12)
   Number        IUPAC         Molecular           Condensed         Structural
  of carbon       name          formula             structural        formula
   atom(s)                                           formula
        1       Methane            CH4                   CH4



        2        Ethane            C2H6               CH3CH3



        3       Propane            C3H8            CH3CH2CH3



        4        Butane           C4H10         CH3CH2CH2CH3

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            The first four members of straight-chain alkanes
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21.3 Classification of Organic Compounds (SB p.12)
   Number        IUPAC        Molecular            Condensed         Structural
  of carbon       name         formula              structural        formula
   atom(s)                                           formula
        1       Methanol         CH3OH                CH3OH



        2       Ethanol         C2H5OH              CH3CH2OH



        3      Propan-1-        C3H7OH           CH3CH2CH2OH
                   ol


        4       Butan-1-        C4H9OH          CH3CH2CH2CH2OH
                   ol
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            The first four members of straight-chain alcohols
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21.3 Classification of Organic Compounds (SB p.13)

        Homologous Series
    • Members in the same series can be
      represented by a general formula.

        e.g. alkanes: CnH2n+2

              alkenes: CnH2n

              alkynes: CnH2n-2


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21.3 Classification of Organic Compounds (SB p.13)

        Homologous Series
    • Members in the same series can be
      represented by a general formula.
        e.g. alkanols: CnH2n+1OH

              alkanals: CnH2n+1CHO

              alkanoic acids: CnH2n+1COOH



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21.3 Classification of Organic Compounds (SB p.13)

        Homologous Series

         Functional group
                                                               Chemical
          of an organic
                                                              properties
            compound



                Members of a homologous series
                have similar chemical properties


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21.3 Classification of Organic Compounds (SB p.13)

        Homologous Series
        • The physical properties change gradually
          along the homologous series

        • e.g. the longer the carbon chain in the
               molecule ( or the greater the
               molecular mass)
           the greater the attractive force
            between molecules
           the higher the melting point,
            boiling point and density
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21.3 Classification of Organic Compounds (SB p.13)
      Some physical properties of the first 20 members of
                     straight-chain alkanes
                     State (at
 Number
                       room                                         Density of
     of  Molecular                      Melting          Boiling
                   temperature                                    solid / liquid at
 carbon formula                        point (°C) point (°C)
                        and                                        20°C (g cm–3)
 atom(s)
                    pressure)
     1     CH4          Gas                –183            –161           –
     2     C 2H 6       Gas                –172              –89          –
     3     C 3H 8       Gas                –188              –42          –
     4    C4H10         Gas                –135                 0         –
     5    C5H12       Liquid               –130                36      0.626
     6    C6H14       Liquid                –95                69      0.657
     7    C7H16       Liquid                –91                98      0.684
     8    C8H18       Liquid                –57              126       0.703
     9    C9H20       Liquid                –54              151       0.718
  32 10   C10H22      Liquid                –30              174
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21.3 Classification of Organic Compounds (SB p.13)
      Some physical properties of the first 20 members of
                     straight-chain alkanes
                      State (at
  Number
                        room                                       Density of
      of  Molecular                      Melting         Boiling
                    temperature                                  solid / liquid at
  carbon formula                        point (°C) point (°C)
                         and                                      20°C (g cm–3)
  atom(s)
                     pressure)
      11   C11H24       Liquid              –26              196      0.740
      12   C12H26       Liquid              –10              216      0.749
      13   C13H28       Liquid                –7             233      0.753
      14   C14H30       Liquid                –3             260      0.761
      15   C15H32       Liquid                10             271      0.769
      16   C16H34       Liquid                18             287      0.773
      17   C17H36       Liquid                22             302      0.778
      18   C18H38       Solid                 28             316      0.777
      19   C19H40       Solid                 32             330      0.777
   33 20   C20H42        Way Chemistry for Hong Kong A-Level 344
                     NewSolid                 37             3A       0.785
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21.3 Classification of Organic Compounds (SB p.13)




           Example 21-3A                         Example 21-3B




           Example 21-3C                         Check Point 21-3




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         21.4
        Factors Affecting the
        Physical Properties of
         Organic Compounds
       Refer to notes on ‘Bonding and Structure’
       pp.77-92 – intermolecular forces
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.17)

        Main Factors Affecting the
        Physical Properties of Organic
        Compounds
          1. Structure of the functional group
              1.1 Dipole moment of the molecule
              1.2 Formation of hydrogen bonding
          2. Length of carbon chains (London
             dispersion forces)

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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.17)

        Structure of Functional Group

         • Molecules having a polar functional
           group have a higher b.p. than others
           with a non-polar functional group of
           similar molecular masses
            Stronger intermolecular attraction
             among molecules



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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.17)

      Structure of Functional Group
                         Molecule                  Relative          Boiling point
                                                  molecular               (oC)
                                                    mass
      Molecules    CH3CH2CH2OH                         60                97.2
      with polar
                   CH3CH2CH2NH2                        59                48.6
      functional
        groups     CH3CH2Cl                           64.5               12.5
                   CH3CH2COOH                          60                141
     Molecules     CH3CH2CH2CH3                        58                -0.5
      with non-
                   CH3CH2CH=CH2                        56                -6.2
        polar
     functional    CH3CH2CCH                          54                 8.1
  38
       groups
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.18)

        Dipole Moment of Molecule
        • Tetrachloromethane has 4 polar bonds
          in the molecule
        • M.p. and b.p. are very low
           the molecule is non-polar
                 the molecule is tetrahedrally
                  symmetrical
                 the dipole moments of the
                  C  Cl bond cancel each other
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.18)

        Examples of Polar Molecules
        with Net Dipole Moment




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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.18)

        Examples of Non-polar Molecules
        with No Net Dipole Moment




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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.18)

   Solubility of Organic Molecules
        • Depends on the polarity of organic
          molecules and the solvent
        • Non-polar or weakly polar compounds
          dissolve readily in non-polar or weakly
          polar solvents
        • Highly polar compounds dissolve readily
          in highly polar solvents
        • “Like dissolves like”

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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.18)

   Solubility of Organic Molecules




               Hexane in                                Hexane in water
          tetrachloromethane
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.19)
        Why does Hexane Dissolve Readily in
        Tetrachloromethane?




    Intermolecular forces among                      Intermolecular forces
    hexane molecules and those                      between hexane and
     among tetrachloromethane                         tetrachloromethane
             molecules                                     molecules
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.19)

    Why is Hexane Insoluble in Water?




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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.19)

        Formation of Hydrogen Bonding
        • Molecules having OH or  NH2 groups
          are able to form hydrogen bonds
        • Hydrogen bonds affect the physical
          properties of alcohols and amines with
          low molecular masses




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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.20)

        Why does Propan-1-ol have a
        Higher Boiling Point?




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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.20)


        Formation of Hydrogen Bonding
        • Also affect the solubility of a molecule
        • Molecules with OH groups are able
          to form hydrogen bonds with
          surrounding water molecules
           Soluble in water




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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.20)




                                                  Example 21-4A



                                                   Example 21-4B




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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.21)

        Length of Carbon Chains

        • Molecules with higher molecular masses
          have higher m.p., b.p. and density
            Higher molecular masses
                 Large molecular sizes
                 Stronger London dispersion
                  forces among molecules


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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.21)

        Length of Carbon Chains

        • Molecules with branched chains
            b.p. and density lower than its
             straight-chain isomer
         Straight-chain isomers have greater
          surface area in contact with each other
            Greater attractive force among the
             molecules

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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.21)

        Length of Carbon Chains
        • Molecules with branched chains
           m.p. higher than its straight-chain
            isomer
         Branched-chain isomers are more
          spherical
           Packed more efficiently in solid state
           Extra energy is needed to break
            down the efficient packing
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.21)




                                                   Example 21-4C




                            Check Point 21-4



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21.3 Classification of Organic Compounds (SB p.8)
                       General           Functional                   Example
        Family
                       formula             group               Formula    IUPAC name
        Alkane            RH                  (Nil)            CH3CH3        Ethane
                     RCH = CH2
                     RCH = CHR
        Alkene                            Carbon-            CH2 = CH2      Ethene
                     R2C = CHR
                                           carbon
                     R2C = CR2          double bond
                                         –CC–
                      RC  CH             Carbon-
        Alkyne                                                 HC  CH      Ethyne
                      RC  CR           carbon triple
                                            bond


    Aromatic
                          ArH                                              Benzene
  hydrocarbon
                                        Phenyl group
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21.3 Classification of Organic Compounds (SB p.8)

                General           Functional                     Example
    Family
                formula             group                Formula    IUPAC name

                                      X
 Haloalkane        RX                                     CH3Cl      Chloromethane
                                   halo group

                                     OH
    Alcohol       ROH                                    CH3OH         Methanol
                                hydroxyl group

                                    O               CH3  O 
        Ether   RO  R                                         Methoxymethane
                                   oxy group          CH3



  Aldehyde                                                             Methanal

                                carbonyl group
        R = CnH2n+1 –
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21.3 Classification of Organic Compounds (SB p.8)

                  General          Functional                Example
   Family
                  formula            group              Formula   IUPAC name


   Ketone                                                          Propanone

                                 carbonyl group


  Carboxylic
                                                                  Ethanoic acid
    acid
                                 carboxyl group
                   RNH2
    Amine          R2NH                                 CH3NH2    Methylamine
                    R 3N          amino group
                                   CN
    Nitrile        RCN                                  CH3CN    Ethanenitrile
                                   nitrile group
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21.3 Classification of Organic Compounds (SB p.8)
                General           Functional                    Example
   Family
                formula             group                  Formula   IUPAC name

                                                                           Methyl
    Ester
                                                                           ethanoate
                                  ester group


                                                                           Ethanoyl
 Acyl halide
                                                                           chloride
                                  acyl halide
                                    group




    Amide                                                                 Ethanamide

                                 amide group
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21.3 Classification of Organic Compounds (SB p.9)



                General           Functional                    Example
   Family
                formula             group                  Formula   IUPAC name


    Acid                                                                    Ethanoic
  anhydride                                                                 anhydride
                                acid anhydride
                                    group
                                                                     R = CnH2n+1 –




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                    The END




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21.1 What is Organic Chemistry (SB p.4)




        (a) How was organic chemistry defined before 1800s?

         (a) The knowledge of organic and inorganic
                                                                       Answer
             compounds was raised during the 1780s.
             Scientists defined organic chemistry as the
             study of compounds that could be obtained
             from living organisms. They believed that the
             synthesis of organic compounds took place in
             living organisms only.




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21.1 What is Organic Chemistry (SB p.4)
                                                               Back


        (b) How is organic chemistry defined nowadays?

         (b) Nowadays, scientists have discovered that
                                                                        Answer
             many organic compounds can be synthesized
             from inorganic substances. The updated
             definition of organic chemistry is the study of
             carbon compounds, except for carbon
             monoxide, carbon dioxide, carbonates,
             hydrogencarbonates, carbides and cyanides.
             These compounds have been traditionally
             classified under inorganic chemistry.

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21.2 The Unique Nature of Carbon (SB p.5)




                     Why is carbon able to catenate?
                                                                        Answer
        The ability to catenate of carbon is chiefly due to
        the high strength of the CC single bond (bond
        enthalpy of C  C single bond is 356 kJ mol-1).


                                                             Back


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21.2 The Unique Nature of Carbon (SB p.6)



 Would you expect silicon, which is just below carbon in the
 Periodic Table, to catenate to form diverse molecular
 structures? Explain your answer.
                                                                          Answer
        Silicon, unlike carbon, does not catenate to form diverse
        molecular structures. Carbon is able to catenate because
        carbon atoms have a relatively small atomic size. This
        enables a carbon atom to form strong covalent bonds
        with other carbon atoms. However, due to the greater
        atomic size of silicon, its ability to catenate is much lower
        than that of carbon.
                                                                 Back

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21.2 The Unique Nature of Carbon (SB p.7)
                                                              Back


        Would you expect sulphur, which has an
        electronegativity value very close to carbon, to
        catenate? Why?
                                                                       Answer
        The electronic configuration of sulphur is 1s22s22p63s23p4. It
        has only two unpaired electrons. Its atomic size is larger than
        that of carbon. So it has a much lower tendency to catenate
        than carbon.




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21.3 Classification of Organic Compounds (SB p.14)



 Identify the functional group(s) in the following compounds:
 (a)




                                                                        Answer
        (a) Carbon-carbon double bond (                         ) and
            chloro group (Cl)




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21.3 Classification of Organic Compounds (SB p.14)



 Identify the functional group(s) in the following compounds:
 (b)




                                                                     Answer
    (b) Carbonyl group (                   )




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 Identify the functional group(s) in the following compounds:
 (c)




                                                                     Answer
    (c) Amino group (                  ) and carboxyl group (             )




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 To which homologous series does each of the following
 compounds belong?
 (a)
                                                             (a) Ester




                                                                         Answer
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 To which homologous series does each of the following
 compounds belong?
 (b)
                                                             (b) Amide




                                                                     Answer
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 To which homologous series does each of the following
 compounds belong?
 (c)
                                                          (c) Acid anhydride




                                                                     Answer
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 State whether each of the following pairs of compounds
 belongs to the same homologous series. Explain your answer.
 (a)




        (a) No, the first one is a carboxylic acid and the
                                                                         Answer
            second one is an ester.



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 State whether each of the following pairs of compounds
 belongs to the same homologous series. Explain your answer.
 (b)




        (b) Yes, both of them are alcohols.
                                                                        Answer


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 State whether each of the following pairs of compounds
 belongs to the same homologous series. Explain your answer.
 (c)




        (c) No, the first one is an amide and
                                                                        Answer
            the second one is an amine.



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21.3 Classification of Organic Compounds (SB p.16)




    (a) Name the homologous series of organic compounds
        that contain oxygen atoms in their functional groups.

        (a)   Alcohol, ether, aldehyde, ketone,                       Answer
              carboxylic acid, ester, acyl halide,
              amide and acid anhydride




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    (b) Identify and name the functional groups in glucose
        which has the following structure.

                                                     (b)   OH (hydroxyl group)
                                                           and  O  (oxy group)




                                                                     Answer
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    (c) Identify and name the functional groups in the
        following compounds:
                                          (c)   Br (bromo),

                                                                (aldehyde),

                                                                (acyl chloride),

                                                          (carbon-carbon
                                                double bond) groups



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21.4 Factors Affecting the Physical Properties of Organic Compounds
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                  Why is oil immiscible with water?
                                                                     Answer
        Oil molecules do not have free OH groups,
        so they cannot form hydrogen bonds with
        water molecules.


                                                           Back


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(SB p.20)




        The relative molecular mass of glucose is 180.0, but it
                      is soluble in water. Why?
                                                                        Answer
        Glucose molecules have OH groups, so
        they are able to form hydrogen bonds with
        water molecules. Therefore, glucose is
        soluble in water despite it has a high
        molecular mass.                                       Back


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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.20)



 Despite the fact that butan-1-ol and ethoxyethane have the
 same relative molecular mass, they have very different
 boiling points. The boiling points of butan-1-ol and
 ethoxyethane are 117oC and 35oC respectively. Explain the
 difference.
                                                                     Answer




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(SB p.20)                                           Back

  There is an OH group in butan-1-ol. Thus, butan-1-ol molecules are
  able to form hydrogen bonds with one another and the energy required to
  separate butan-1-ol molecules would be much greater. Whereas for
  ethoxyethane, the attraction among the molecules is weak van der Waals’
  forces only. The amount of energy required to break the forces would not
  be great. Therefore, the boiling point of ethoxyethane is lower than that of
  butan-1-ol.




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(SB p.21)                                           Back


    Explain why propan-1-ol is soluble in water but
    1-chloropropane is insoluble in water.




                                                                         Answer
        The  OH group of propan-1-ol molecules enables it to form
        hydrogen bonds with water molecules. Thus it is soluble in water.
        Although 1-chloropropane is a polar molecule, it does not form
        hydrogen bonds with water molecules. So it is insoluble in water.
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.21)




        Which molecule would have a higher boiling point,
           1-bromobutane or 2-bromobutane? Why?
                                                                     Answer
    1-bromobutane would have a higher boiling point.
    1-bromobutane is a straight-chain molecule while
    2-bromobutane is a branched-chain molecule. Straight-chain
    molecules have a greater surface area in contact with each other,
    so greater intermolecular forces exist among the molecules.
    Higher energy is required to break down the intermolecular
    forces among the molecules of 1-bromobutane.
                                                           Back
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.22)



    1-Chlorobutane and 2-chloro-2-methylpropane have the
    same molecular mass, yet their melting points differ. The
    melting point of 1-chlorobutane is –123oC while that of 2-
    chloro-2-methylpropane is –27.1oC. Explain the difference.




                                                                     Answer

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                                                                Back


        Melting point is a measure of how efficient the molecules are packed
        together in the solid state instead of just comparing the van der Waals’
        forces among molecules. Hence melting point is a function of the
        efficient packing of molecules but not the contact surface area.
        1-Chlorobutane is a straight-chain molecule while
        2-chloro-2-methylpropane is a branched-chain molecule. As
        2-chloro-2-methylpropane is more spherical and symmetrical, its
        molecules are packed more efficiently in the solid state.
        1-Chlorobutane is linear in shape and flattened, its packing in the
        solid state is not so efficient. Hence, it has a lower melting point than
        2-chloro-2-methylpropane.


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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.24)




    (a) What are the major factors that affect the physical
        properties of organic compounds?
                                                                         Answer
        (a)   The physical properties of organic compounds
              are mainly affected by the structure of the
              functional groups, dipole moment of the
              molecule, the formation of hydrogen bonding
              between molecules, and the length of carbon
              chains of the molecule.




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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.24)



   (b) The melting point and boiling point of pentane are
       –130oC and 36.3oC respectively while the melting point
       and boiling point of 2,2-dimethylpropane are –15.9oC
       and 9.5oC respectively. Account for the difference in
       melting point and boiling point between the two
       isomers.




        Answer
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        (b)   Pentane is a straight-chain molecule, while 2,2-dimethylpropane is
              a branched-chain molecule. Straight-chain molecules have a
              greater surface area in contact with each other than branched-
              chain molecules. Straight-chain molecules are held together by
              stronger intermolecular forces. Therefore, pentane has a higher
              boiling point than 2,2-dimethylpropane. Molecules of 2,2-
              dimethylpropane are more spherical in shape and are packed
              more efficiently in the solid state. Molecules of pentane are linear
              in shape and flattened, so their packing in the solid state is not
              efficient. Since extra energy is required to break down the efficient
              packing of 2,2-dimethylpropane, 2,2-dimethylpropane has a higher
              melting point than pentane.
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.24)



   (c) Which molecule, hexane or cyclohexane, would have a
       higher melting point? Explain your answer.




                                                                     Answer
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.24)




        (c)   Cyclohexane has a higher melting point than hexane. Molecules
              of cyclohexane are more spherical in shape and are packed more
              eff iciently in the solid state. Molecules of hexane are linear in
              shape and flattened, so their packing in the solid state is not
              efficient. Since extra energy is required to break down the efficient
              packing of cyclohexane, cyclohexane has a higher melting point
              than hexane.




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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.24)



   (d) Arrange the following molecules in increasing order of
       boiling points. Explain your answer.




                                                                     Answer
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(SB p.24)

                                                          Back

    (d)   The boiling points increase in the order:
          butane < propanal < propan-1-ol
          Molecules of butane are non-polar. Their molecules are held
          together by weak instantaneous dipole-induced dipole interactions.
          A relatively small amount of energy is required to separate the
          molecules in the process of boiling. Both propanal and propan-1-ol
          are polar molecules. Molecules of propanal are held together by
          relatively weak dipole-dipole interactions, while molecules of
          propan-1-ol are held together by intermolecular hydrogen bonds.
          Since the intermolecular forces present in molecules of propan-1-
          ol are stronger than those present in molecules of propanal, a
          larger amount of energy is required to separate the propan-1-ol
          molecules in the process of boiling.
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