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					Learner Notes


SUBJECT: PHYSICAL SCIENCE

GRADE: 12

CHAPTER / MODULE: ORGANIC CHEMISTRY

UNIT / LESSON TOPIC: ORGANIC MOLECULAR STRUCTURES


By the end of this unit, the Learner should be able to:
    give condensed structural, structural and semi-condensed structural formulae
       alkanes, alkenes, alkynes, alkyl halides, alcohols, carboxylic acids, esters, amines,
       amides, ketones, aldehydes and arene compounds.
    explain the term saturated and unsaturated.
    explain and identify isomers.

    LESSON:
COVALENT BONDING AND INTERMOLECULAR FORCES

COVALENT BONDS
 Covalent bonds occur between two non metals
 Electronegativity difference of 0 non polar covalent
 Electronegativity difference of 0,1 – 1,6 polar covalent
 Electrons are shared in a covalent bond

    Forming a Chlorine molecule
               Each atom has 7 ē in the outer energy level.
               The only way that each can achieve the octet structure, is for a pair of ē’s
                to be shared equally between them:
               Each F atom now shares 1 ē with the other therefore each has the noble
                gas electron structure.



                         Cl Cl
INTERMOLECULAR FORCES
 Weak electrostatic forces (Coulombic forces)
 Holds together molecules (covalently bonded units)
 Between non polar molecules – London forces or instantaneous dipoles
 Between polar covalent molecules – dipole dipole van der Waals forces. Dipole-dipole
   is an attraction between slightly positive and slightly negative sides (poles) of the
   molecule.
 Hydrogen bonding between NOF (Nitrogen, Oxygen and Fluorine) bonded to hydrogen.




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       Inter-molecular forces
Learner Notes


                                      Intramolecular force
                              -
                                      (covalent bond –
                             O        strong)
                     H            H
                          +
                         -
                                      Intermolecular
                         O            force (weak)
                 H            H
                         +
CARBON                                                       STRUCTURES

Important features of Carbon
                                                          12
c There are different allotropes (same element, same phase, different atomic
   arrangement) of C – graphite, coal & diamond.
c Carbon has a valency of 4 (can form 4 bonds), & has 4 valence electrons (outermost
   energy level).
c In the excited state, valence electrons are unpaired i.e. one 2s electron moves to
   higher 2p orbital. i.e. 1s22s1 2p3
c Carbon has the ability to form long chains with other C atoms – called Catenation.
c Graphite is a non-metal that conducts electricity.

Alternative representations of organic molecules
•      Molecular formula:          C6H13OH
•      Condensed formula:
                                      CH3CH2CH2CH2CH2CH2OH

•     Structural formula:



•     Ball + stick & space filled:




HYDROCARBONS
          • Saturated – single bonds only
          • Unsaturated – double and triple bonds

Alkanes
a Single covalent bonds
                                      CH3 - CH2-CH2-CH3
a SATURATED molecule.
                                                   butane
a General formula: CnH2n+2




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Learner Notes



Alkenes
a have one or more C=C double bonds
                                             CH2=CH-CH2-CH3
                                                    but-1-ene
a The general formula: CnH2n.
a UNSATURATED molecule



Alkynes
One or more triple bonds
                                        CHC-CH2-CH3
                                               but-1-yne
General formula: CnH2n-2
UNSATURATED molecule

Naming alkyl groups
                           Alkyl group structure   Alkyl name
                                 CH3 -              Methyl
                                CH3CH2-              ethyl
                             CH3CH2CH2 -             propyl
                           CH3CH2 CH2CH2 -           butyl

FUNCTIONAL GROUPS




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Learner Notes




RULES FOR NAMING ORGANIC COMPOUNDS (IUPAC RULES)
/ Identify the functional group of the molecule – this determines the ending of the name.
/ Find the longest continuous carbon chain and allocate its prefix according to the
  number of carbon atoms in the chain (see table for prefixes).
                                       Name prefixes
                              No of C atoms         Prefix
                                     1               Meth
                                     2               eth -
                                     3              prop -
                                     4               but -
                                     5              pent -
                                     6               hex -
                                     7              hept -
                                     8               oct -
                                     9               non -
                                    10               dec -
/   Number the carbon atoms in the chain. Number them so that the functional group is on
    the carbon of lowest possible number. Double and triple bonds take preference over
    side chains.
/   Name the branched group according to the number of carbon atoms it has and give it a
    number according to the carbon atom it is attached to.
/   If there is more than one branched group of the same kind, use the Greek prefixes di,
    tri, tetra, penta and so on to indicate this.
/   If a halogen atom is attached to the carbon chain, it is treated as an alkyl group, then
    the prefixes: fluoro-, chloro-, bromo -, and iodo – are used.



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Learner Notes



E.g. Write down the IUPAC name of each one of the following compounds:


           H    C  C    H
Ethyne (two carbons and a triple bond)

                H    H        H   H


           Br   C    C        C = C       H


               H     H
4-bromo- but-1-ene (four carbons in main chain, double bond on first carbon, bromine on
fourth carbon)

Homologous Series
     A homologous series is a series of compounds that have the same general formula
     and of which the members in the series differ from each other by the same group of
     atoms and they have the same functional group.

Alcohols
                                       -
   -OH (hydroxyl) NOT hydroxide (OH ) functional group.
   Named using the ending –ol. -OH group gets the lowest possible number:
   Alcohols are oxidized to carboxylic acids when treated with strong oxidizing agents.
   Alcohol molecules have a non-polar hydrocarbon end and a polar –OH section.
   Alcohols are solvents for polar and non-polar solutes.
                                                          Polar end

                 Non -polar
                    end




Carboxylic Acids
 Compounds that have the carboxyl, -COOH functional group
 The ending -oic acid denotes that we are dealing with a carboxylic acid.
 Are relatively weak acids.
                                      O

                              CH3 -C-OH
Esters
o These are compounds that have the CO-O-C functional group.
o Pleasant smelling substances which are responsible for fragrance of fruit & flowers.



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Learner Notes


o Esters arise by the reaction between a carboxylic acid and an alcohol:




o Water is also formed as a result of the reaction.
o All esters consist of two parts;
       an alkyl derived from an alcohol,
       anoate derived from a carboxylic acid (has the double bond O)
o In the example above, ethanoic acid and ethanol react to form the ester, ethyl
  ethanoate.
o NB. Use the oxygen atom as a divider to name the molecule.

Ketones
 Structure:



 A carbonyl group (C=O) is polar.

Aldehydes

 Structure:

                              H                        H

 A carbonyl group (C=O) is polar.


Amines

  Amines are organic compounds that have a nitrogen atom with a lone pair. Amines are
  derived from ammonia, where one or more of the hydrogen atoms are replaced by
  organic chains or cyclic groups. Amino acids are important amines which are the
  building blocks of proteins.

  The functional group of the amines is:

  R-NH2

  Primary amine           Secondary amine             Tertiaryamine

  R       N    H          R       N    R’             R     N     R’

          H                       H                         R”

  Naming of amines consists of the alkyl group ending with amine. For example:




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Learner Notes


  CH3CH2NH2 is called ethylamine or ethanamine

Amides

  An amide is produced when the –OH group of a carboxylic acid is substituted by a –
  NH2 group. The amide gets its name from the acid used to produce it.
       O
       ||
  R    C NH2

  The naming of the amide consists of the carbonyl group followed by the amide.
  CH3CH2 CONH2 is called propanamide.



ISOMERS
     Examine the two structural formulae shown below. Both are structural formulae with
molecular formulae C4H8.

CH2=CH-CH2-CH3
      but-1-ene


CH2-CH=CH2-CH3
      but-2-ene

      These two molecules are said to be ISOMERS.
       Isomers are compounds which have the same molecular formula, but       different
structural formulae.

 ACTIVITIES:


Activity 1

E.g. 1 Consider the following organic compounds:

                                        A HCOOH
                                       B CH3COOH
                                     C HCOOCH2CH3
                                     D CH3CH2COOH
                                    E CH3CH2CH2OH
a. Write down the structural formula for an isomer of E.
b. Write down the IUPAC (systematic) name of B.
c. Write down the name of functional group of C.
d. Write down the formula of the organic acid used in the preparation of C.



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Learner Notes



2      Ethyl propanate has an ester functional group. How is ethyl propanoate made?
Include structural formula diagrams to assist in the explanation. Also name the two organic
reactants.

3

3.1. Briefly explain why these two layers do not mix.
3.2. Why is alcohol able to dissolve in both the cream layer and
the water layer?
3.3 Which layer will be on top?




    REVIEW QUESTIONS:

1       The formula of the organic product formed when ethanol reacts with
        methanoic acid is:

        A       C2H4O2
        B       C3H5O2
        C       C3H6O2
        D       C4H8O2

2       Which one of the following pairs of compounds contains members of the
        same homologous series?

        A       C2H4 and C3H8
        B       C3H6 and C4H6
        C       CH4O and C2H4O2
        D       C2H4O2 and C3H6O2

3       Consider the following structural formula of an organic compound and the
        statements referring to it:




        (i)     The compound belongs to the same homologous series as
                methane.
        (ii)    The compound is a hydrocarbon.
        (iii)   The compound has a higher boiling point than methane.




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Learner Notes


        Which of the above statements are correct?

        A       Only (ii)
        B       (i) and (ii)
        C       (i), (ii) and (iii)
        D       None of (i), (ii) or (iii)


4       The ratio of the number of carbon atoms to the number of hydrogen atoms
        in a particular alkene can be …

        A       4  10
        B       36
        C       26
        D       22

5       The fermentation process of cane sugar can be represented as follows:
              C12H22O11 + Enzymes           CO2 +        P
        The product P is a/an …

        A       Alkane
        B       Alcohol
        C       Ester
        D       Carboxylic acid

This compound is a hydrocarbon. The larger balls represent carbon and the smaller ones
represent hydrogen. Question 6 and 7 refer to this figure.




6. To which homologous series does this compound belong?
A. alcohols
B. alkanes
C. alkenes
D. esters

7. Which of the following is a structural isomer of the compound depicted on the previous



                                                                                   Page 9
Learner Notes


page?
A. 2-methyl butane
B. 2-methyl butene
C. pentane
D. pentene

8. The ability of carbon to form long chains or rings is known as:

A. hydrogenation
B. carbonation
C. cyclonation
D. catenation

9. Propane burns in excess oxygen. Which one of the following gives the products
formed?
A. carbon dioxide
B. carbon dioxide and water
C. carbon and hydrogen
D. carbon dioxide and hydrogen

10. The compound C4H8O2 could be          I. an alcohol
                                          II. a carboxylic acid
                                          III. an ester
   Which of the above is correct?

   A.   I only
   B.   I and II
   C.   I and III
   D.   II and III




11. If bromine is reacted with the above hydrocarbon, what type of a reaction is observed?
A. addition
B. hydrogenation
C. saturation
D. substitution

12. Which two compounds were probably added together to obtain ethanoic acid?



                                                                                   Page 10
Learner Notes


A. ethane and oxygen
B. methane and methanol
C. ethanol and sodium hydroxide
D. ethanol and potassium permanganate

13. Which of the following statements regarding alkynes is incorrect.
A. Their general formula is CnH2n-2
B. Their functional group includes two triple bonded carbons
C. They are more reactive than alkanes
D. Structural isomers are possible, provided the compound contains three or more carbon
atoms.

14. Which one of the following belongs to the same homologous series as C3H8?
A. C2H2
B. C3H6
C. C3H4
D. C4H10

15. From which two substances is ethyl methanoate prepared?
   A. methanoic acid and ethanol
   B. ethanol and water
   C. methanol and ethanoic acid
   D. ethylene methanoate                                        [4]




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