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 Alkanes can also form cyclic structures
                                  CH2             CH2
    CH2          CH2 CH2        CH2 CH2        CH2      CH2
CH2     CH2      CH2 CH2         CH2 CH2       CH2      CH2
Cyclopropane     Cyclobutane    Cyclopentane
                                                     CH2
                                               Cyclohexane


General formula for cycloalkanes: CnH2n

Can be conveniently represented using line segment formulae
 Note:                             H       H
                              H        C        H
                             H C               C H
                                                       NOT
                             H C               C
                                                  H
                              H     C           H
         cyclohexane                                         benzene
                                   H H

Cycloalkane nomenclature can be extended to include substitution

                CH3                        CH3



                                                      CH3
         Methylcyclohexane    1,3-Dimethylcyclohexane
  Only one cycloalkane has a planar structure: cyclopropane
  All others have non-planar structure
                                                  H
                                                      109.5o
    Ideal tetrahedral angle is 109.5o             C
                                        H        H
                                         H
  sp3 hybridised carbons with bond angles very different to
  109.5o will be less stable (higher in energy)
                     H
                                    Bond angle approaching 60o
Cyclopropane      H C     H
                   C H C            Cyclopropane is said to suffer
                        o H         from angle-strain
                  H   60
   All C-H bonds in cyclopropane are eclipsed
Ball-and-stick model of
     cyclopropane
Cyclobutane also suffers from angle-stain, but to a lesser extent

  •Actual structure of cyclobutane is „butterfly-shaped‟

                      H            H
                      C          H C
                  H          C         H
                          H C
                            HH

•This avoids eclipsing of hydrogens on neighbouring carbons
    Cyclopentane has almost zero angle-strain

To relieve torsional strain due to eclipsed C-H bonds,
cyclopentane relaxes into a non-planar structure


              H
H         H            One CH2 group out of the plane of the ring
     H             H
                  H
H
     H   H H
 Cyclohexane

A planar structure would have internal bond
angles of 120o and eclipsed C-H bonds
                                                      120o
 Actual structure relaxes into a chair conformation
 This reduces the bond angle to 109o


                                              o
                                      ~109
                                        HH H
    Geometry about each Carbon             H H
    very close to tetrahedral ideal   HH       H
    •Angle strain ~ zero                  H
                                            HH
                                         H
    All C-H bonds staggered, i.e. torsional strain ~ zero
                                              H
     Newman projection along                         H
     any C-C bond
                                                    H
                                              H
The chair conformation contains two different hydrogen
environments

     HH H                          HH H
        H H                           H H
   HH       H                    HH       H
       H                             H
         HH                            HH
      H                             H
    6 Axial Hydrogens         6 Equatorial Hydrogens
The chair conformation of cyclohexane,
shown in ball-and-stick and space-filling
                models.
At temperatures below 230 K (-43C):
•can observe that two different types of hydrogen
environment are present on cyclohexane
Above this temperature, observe only one hydrogen environment
Reason:       cyclohexane molecules are not static above 230 K
              i.e. exist in different conformations
Undergo ring inversion
  HH H                     H       H                  H
                                                         HH
      H H             H     H              H           H
                                           H      H
HH        H            H               H         H                 HH
    H                                  H                   H
       HH                      H                      HH
   H                       H       H                           H

                     Boat conformation
                  Exists in trace quantities
Note: hydrogens axial in one chair conformation equatorial in
the other
Ball-and-stick model of boat
       cyclohexane
Energy barrier to ring inversion = 43 kJ mol-1
Too low to prevent ring inversion at 25C
Rate of ring inversion at 25C = 2 x 105 s-1
Hence, observe only one time averaged hydrogen
environment at 25C
Both chair conformations equal in energy

Hence, samples of cyclohexane at 25C consist of 50:50
mixtures of the two chair conformations
[plus trace quantity of the boat conformation]
What if one of the cyclohexane hydrogens were replaced by
a methyl group?
                                           H
                                              HH
                                            H
        Cyclohexane                     H
                                   = H           HH
                                              H
                                          HH H
                         CH 3

Methylcyclohexane



The two chair conformations are no longer equivalent

One has the methyl group in an axial position; one in an
equatorial position
              H
                 H CH 3              HH         H
               H                               H
           H                                        H
          H         HH            HH                 CH 3
                 H                         H
             HH H                              HH
                                       H

These interconvert by ring inversion (exist in equilibrium)
[Inversion proceeds through boat conformations which exist
in trace amounts]
Can simplify diagram by omitting the C-H bonds

                 CH3
                                                       CH3

       Methyl axial              Methyl equatorial
In methylcyclohexane, the axial and equatorial conformations
are not equivalent                   View along this bond
[Differ in energy]
                              CH3
    Methyl axial                       Gauche interaction
                                       3.8 kJ mol-1 steric strain
                                       energy
                           CH3
Newman projection      H         CH2   CH2
would look like:
                       H         CH2   CH2
                           H
               CH 3
             H     CH 3
 Compare:                  Gauche conformation of butane
             H         H   3.8 kJ mol-1 steric strain energy
                   H
The steric interactions between the axial methyl and
the 1,3-diaxial hydrogens are evident in the ball and
                    stick structure.
 Two gauche interactions exist in axial methylcyclohexane


         H      CH3      Contribute 2 x 3.8 kJ mol-1
          H              = 7.6 kJ mol-1 steric strain energy
     H             H
         H
                   CH3
                                                 CH3

        7.6 kJ mol-1                No strain energy
        Strain energy
 Difference in strain energy the only difference between the
 conformations
Hence, DG for ring inversion of methylcyclohexane = -7.6 kJ mol-1
  Relationship between free energy (DG) and equilibrium
  constant (Keq) at equilibrium

                        DG = - RTlnKeq
  Hence, at 25C (298K) [and with R = 8.314 J mol-1 K-1]


              - DG                      7600 J mol-1
Keg = exp               = exp
               RT                8.314 J mol-1 K-1 x 298 K

                                   = exp [ 3.07] = 21.5
  Hence, the equilibrium constant (Keq) for

                    CH3
                                                CH3


      At 25C = 21.5
                    [methyl equatorial]       96% equatorial
i.e. Keg = 21.5 =                         
                       [methyl axial]           4% axial
  Sources of alkanes
  •Lower Mol. Mt. (~ < 5 Carbons): natural gas
  •Larger Mol. Wt.: petroluem of crude oil

 Crude oil: complex mixture of hydrocarbons
 Separated into fractions based on boiling point ranges
Boiling point related to molecular weight, i.e to number of carbons

  •< 5 Carbons: gases at room temperature

   •5 Carbons < ~18 Carbons: liquids at room temperature
  •> 18 Carbons: solids at room temperature
•Increasing molecular size results in increasing tendency to
form condensed phases

•Associated with weak intermolecular interactions between
alkane molecules
•London dispersion forces: weak electrostatic attractions
between induced dipoles, i.e. are…

•Van der Waals‟ forces between electrons of one molecule
and nuclei of another

•Extent of attraction increases with increasing molecular size

•Weak interactions compared to hydrogen bonding or ionic
bonding
Solubility of alkanes
•„Like dissolves like‟: alkanes soluble in other alkanes, e.g
petroleum
•[Soluble: single liquid phase results upon mixing]


•Alkanes insoluble in water, i.e are hydrophobic

•Mixtures with water separate into two liquid phases: aqueous
and hydrocarbon
Reactions of alkanes

•Relatively inert; contain only stable C-C and C-H bonds
 •Some important reactions:
1. Combustion, e.g.
 2 C4H10 + 13 O2 → 8 CO2 + 10 H2O
     DH = - 2877 kJ mol-1           i.e. exothermic

2. Steam reforming

 CH4 + H2O → 3H2 + CO
                  N2 ↓        ↓
                       NH3 + CO2 → Urea
3. Reaction with halogens

                     Heat or light       CH3Cl          +   HCl
CH 4    +     Cl2                    Chloromethane
                                     (Methyl chloride)

                           With excess Cl2

                                         CH2Cl2
                                      Dichloromethane


                                       Cl 2
                            Cl2
         CCl4                                CHCl3
  Tetrachloromethane                     Chloroform
 (Carbon tetrachloride)              (Trichloromethane)
  4. Catalytic cracking
  •Fragmentation of alkanes into smaller molecules, e.g:

                               ~ 500oC
 CH3 CH2 CH2 CH2 CH2 CH3                     CH2 CH2

                               Catalyst
                               surface       +    CH3 CH CH2

                                              + others     +   H2

•The products of these reactions are a new type of hydrocarbon
•They are said to be „unsaturated‟ compared to alkanes
•i.e., have fewer Hydrogens per Carbon than alkanes, which
are said to be „saturated‟
Unsaturated hydrocarbons contain Carbon-Carbon multiple bonds

Classes of unsaturated hydrocarbons are defined by the types
of Carbon-Carbon multiple bonds they contain

     Alkenes: contain Carbon-Carbon double bonds

           C C         Carbon-Carbon double bond


      Alkynes: contain Carbon-Carbon triple bonds
           C C          Carbon-Carbon triple bond

  Carbon valency of four maintained in alkenes and alkynes
 Alkenes           Older name: Olefins

Characterised by presence of Carbon-Carbon double bonds

                  R    R
  General                           Where „R‟ = Hydrogen
  structural        C C             or alkyl group
  formula
                  R    R
Two Carbons and all four „R‟ groups are lying on the same plane

Bond angles about each Carbon ~ 120o

                                   120o
               R   R             R    R
                C C 120o          C C
               R   R             R    R
sp3 hybridisation cannot provide the geometry found at
Carbon in Carbon-Carbon double bonds
Alternative hybridisation required



          1s
                   2s      2p         2p         2p


                                 Hybridisation




                                                  pz
          1s
                    sp2    sp2         sp2
         2e-                                     1e-
                    1e-    1e-         1e-
 Three sp2 hybridised orbitals can be arrayed to give trigonal
 geometry

                   120   o
                                    120o


                             120o
The remaining 2pz orbital is orthogonal to the three sp2 orbitals


                                                     2pz orbital
                    2pz orbital



     View along z axis
                                    View along xy plane
s bond formation results from overlap of two sp2 hybridised
orbitals
                          s



 [A s-antibonding orbital is also formed, but this is not
 occupied by electrons]
Overlap of the pz orbitals results in formation of a p bond


                         [A p-antibonding orbital is also
                         formed, but this is not occupied by
                         electrons]
           p
p orbital: has a nodal plane on which lies on the bond axis
p electron density lies above and below the plane containing
the two Carbons and four „R‟ groups

   R            R      View along the
   R
       C   C
                R      Carbon-Carbon bond      R C R

  Note:             constitutes one p molecular orbital
                    i.e. constitutes one p bond when occupied


  Carbon-Carbon double bond:               p
                                        C C
  •One s bond; One p bond                s
    •Both occupied by two electrons
  The bonding in ethene consists of one sp2-sp2
carbon-carbon σ bond, four sp2-s carbon-hydrogen
         σ bonds, and one p-p π bond.
Rotation of one sp2 carbon 90° with respect to another orients
                the p orbitals perpendicular to
        one another so that no overlap (and therefore
                  no pi bond) is possible.
•Rotation about a Carbon-Carbon double bond requires
opening up of the p bond
•Requires large input of energy (~ 268 kJ mol-1)

•Hence, rotation about C=C bonds does not occur at room
temperature
•Consequently, a new form of isomerism becomes possible for
alkenes
•Consider an alkene with one Hydrogen and one alkyl group
„R‟ bonded to each Carbon
•Two structures are possible

      R   R                         R    H
       C C                 or        C C
      H   H                         H   R
•This form of isomerism is known as Cis-Trans isomerism
•[older term: geometrical isomerism]

•The cis isomer is that with like groups on the same side of the
C=C
•The trans isomer is that with like groups on opposite sides
of the C=C


           R   R                     R    H
            C C                       C C
           H   H                     H   R
          Cis isomer               Trans isomer
First two members of the alkene series:
           H    H                H    CH3
            C C                   C C
           H   H                 H   H
            Ethene              Propene
          (Ethylene)          (Propylene)
Note:   H    CH3       H    H
         C C     =      C C       =       CH3 CH CH2
        H   H          H   CH3

Nomenclature:
 •Prefix indicates number of carbons
 •(„eth…‟ = 2C; „prop…‟ = 3C; etc.)
 •Suffix „…ene‟ indicates presence of C=C
Butene   1 2 3 4            Could have C=C between C1 and C2
         C C C C            or between C2 and C3

  1  2  3   4                     1  2  3               4
 CH2 CH CH2 CH3                  CH3 CH CH              CH3
         1-Butene                         2-Butene
 Note:
 1. 1-Butene and 2-butene are structural isomers
 2.
  CH2 CH CH2 CH3        =   CH3 CH2 CH CH2         = 1-Butene

 3. Number indicates starting point of the C=C, i.e. number
 through the C=C
4. Cis-Trans isomerism is possible for 2-butene
•There are two isomeric 2-butenes

          CH3                    CH3           CH3
                        H
                C   C                  C   C
            H                      H           H
                        CH3

          Trans-2-butene          Cis-2-butene
          b.p. 3.7oC              b.p. 0.3oC
          m.p. -139oC             m.p. -106oC
Some other alkenes       1   2  3  4  5
                         CH3 CH CH CH CH3
1   2 3   4
                                      CH3
CH2 C CH2 CH3
    CH3                     4-Methyl-2-pentene
2-Methyl-1-butene
                                2 1
                            H 3
                                CH2CH3
1   2  3  4  5                C
CH2 CH CH CH CH3              C
                            H 4 CH2CH2CH3
                                5  6 7
    1,3-Pentadiene
                              Cis-3-heptene



        Trans-2-decene
Can have cycloalkenes                     CH3
                                                   2
                                              3
                                          4
                                                   1
                        Cyclohexene           5
  Cyclopentene                        3-Methylcyclopentene
       6
   5        1
                           Note:
   4        2                                H H
        3
                                          H C C C H
1,4-Cyclohexadiene                    =
                                            C    C H
                                          H    C
                                              H H
Lycopene molecular structure
Reactivity of alkenes
 •More reactive than alkanes
 •Due to the presence of C=C
         Energy-level diagram for ethene CH2=CH2


             s*CH x 4                      s*CC
Energy

                                p*CC               H           H
                                                       C   C
                                pCC                H           H


             sCH x 4                       sCC
p electrons in alkenes are available to become involved in bond
formation processes
Essential processes in the synthesis of new molecules:
formation of new covalent bonds
Covalent bonds: pairs of electrons shared between nuclei (atoms)
In the synthesis of organic molecules, a major strategy for
forming new covalent bonds is:
donation of an electron pair by one molecular species…
…to form a covalent bond with another, electron deficient
molecular species
Electron pair donating species are known as nucleophiles
 Electron pair accepting species are known as electrophiles
Reaction of a nucleophile with an electrophile results in the
formation of a new covalent bond
 Alkene hydrogenation
•Addition of hydrogen (H2) across a C=C
General reaction
         R   R             H2             H   H
          C C                            R C C R
         R   R          Catalyst          R   R

•Alkene p bond is lost, and two new C-H s bonds formed
•Alkene converted to alkane
•No reaction in absence of catalyst
•Typical catalysts: Palladium (Pd), Platinum (Pt), Nickel (Ni),
Rhodium (Rh) or other metals
•Catalysts usually supported on materials such as charcoal
•E.g. Pd/C “Palladium on Carbon”
  Examples
                            H2 (g)
CH2 CH CH2 CH2 CH2 CH3                CH3 CH2 CH2 CH2 CH2 CH3
      1-Hexene               Pt/C             Hexane

                          2 H2 (g)
CH2 CH CH CH CH2 CH3                  CH3 CH2 CH2 CH2 CH2 CH3
                             Pt/C              Hexane
     1,3-Hexadiene

            CH 3          H2 (g)            CH 3
     CH 2   C CH 2 CH 3              CH 3   C CH 2 CH 3
                           Pt/C             H
      2-Methyl-1-butene
                                        2-Methylbutane
•Reaction occurs at the catalyst surface
•H2 molecules adsorbed onto catalyst surface
•Both Hydrogens added to same face of C=C

                                               H
             CH3          H2 (g)                   CH3

             CH3           Pt/C                    CH3
                                               H
1,2-Dimethylcyclohexene
                                   Cis-1,2-dimethylcyclohexane

•Both Hydrogens added to the same face of the cyclohexene
C=C
•[Cis/Trans naming system can be extended to cyclic systems]
  Addition of HX to alkenes
 General reaction

          R   R      HX           H   X
           C C                   R C C R        X = Cl, Br, I
          R   R                   R   R

  •C=C p bond lost; new C-H and C-X s bonds formed
e.g:
                    HCl            Cl              Cl
  CH 2 CH CH 3                CH 3 CH CH 3      H2C CH 2      CH 3
                              2-Chloropropane   1-Chloropropane
       Propene
                               (only product)     (not formed)

  •To explain this, need to consider the reaction mechanism
Reaction mechanism:
•detailed sequence of bond breaking and bond formation in
going from reactants to products
•Addition of HX to alkenes: reaction involves two steps
           1st Step: Addition of proton (H+)
           2nd Step: Addition of halide (X-)
1st Step
                       H                    H
             C C                         C C

      •Alkene p electrons           •Remaining Carbon short 1
      attack proton                 electron
      •New C-H s bond results       •Carbon positively charged
•Addition of H+ to the alkene p bond forms a new C-H s
bond and a carbocation intermediate
•[or carbonium ion]

2nd Step
           X                                  X      H
                        H
                     C C                        C C

               Halide ion attacks        New C-X s bond
               electron deficient        results
               carbon
•Reaction involves two steps with an intermediate species
•Each step proceeds through a transition state

                            Transition
                              State Transition
                                1      State
   Energy
                                        2




                Reactants      Intermediate
                                                 Products




                               Reaction Coordinate
Reaction of HCl with CH3-CH=CH2

1st Step: addition of H+ to form a carbocation intermediate
Two possible modes of addition

                   H
     CH3 CH CH2                           CH3 CH CH3
or
      H
      CH3 CH CH2                    CH3 CH2 CH2


I.e. two possible carbocation intermediates
Classification of carbocations


    R C H               R C H            R C R
      H                   R                R
  Primary (1o)        Secondary (2o)    Tertiary (3o)
  Carbocation         Carbocation       Carbocation


        CH3 CH CH3               CH3 CH2 CH2
          2o Carbocation           1o Carbocation
The relative order of stability for carbocations is:

  Most stable 3o >           2o       >       1o Least stable
•This is because carbocations can draw electron density along
s bonds; known as an inductive effect
•This effect is significant for alkyl substituents, but weak for
Hydrogens


   R> C       H           R> C            H         R > C <R
                                  >




                                                        >
      H                           R                     R
 Least stabilised
                                                  Most stabilised
Addition of HCl to CH3-CH=CH2 proceeds so as to give the
more stable of the two possible carbocation intermediates, i.e:

             H
                        CH3 CH CH3          CH3 CH2 CH2
CH3 CH CH2
                                              Not formed
 Addition of chloride then
 gives 2-chloropropane            Cl-
 exclusively

                             Cl
                       CH3 CH CH3
 Additions of HX to alkenes which follow this pattern are said to
 obey Markovnikov’s rule
 “Reaction proceeds via the more stable possible carbocation
 intermediate”
Reaction energy diagram for formation of the
 isopropyl and propyl cations from propene
  Other examples

         CH3          HBr                CH3                        CH3
  CH3    C CH2                    CH3    C CH3         not   CH3    C CH2 Br
  2-Methylpropene                        Br                         H
                               2-Bromo-2-methyl-              1-Bromo-2-methyl-
                                    propane                        propane

                CH3      HCl                   CH3                      H
                                                                            CH3
                                               Cl      not

                                                                            Cl
1-Methylcyclohexene               1-Chloro-1-methyl-                    H
                                     cyclohexane              1-Chloro-2-methyl-
                            HCl                   Cl             cyclohexane
 CH 3 CH CH CH 3                      CH 3 CH 2   CH CH 3
     2-Butene                           2-Chlorobutane             Cl
  (Symmetrical alkene)                                       CH3 CH CH2 CH3
                                                              Same structure
 Addition of water to alkenes
•Follows same pattern as addition of HX
•Acid catalysis required

                                H   catalyst         OH
CH 3 CH CH 2      + H2O                        CH 3 CH CH 3
    Propene                                    2-Hydroxypropane
                                                  (2-Propanol)
 Mechanism:
 1. Protonation of C=C so as to give the more stable
 carbocation intermediate

                      H
     CH3 CH CH2                     CH3 CH CH3
  2. Attack on the carbocation by water acting as a nucleophile
          H
            O
          H
                                        H       H
                                            O
          CH 3 CH CH 3               CH 3 CH CH 3

3. Loss of proton to give the product and regenerate the catalyst


          H         H                               + H
                O                        OH
       CH 3 CH CH 3                CH 3 CH CH 3
 •Acid catalysed addition of water often difficult to control
 •A Mercury (II) mediated version often used - oxymercuration
                        i) (CH3CO2)2Hg, H2O
                                                            OH
               CH3
                                                            CH3
                              ii) NaBH4
 1-Methylcyclopentene   (Sodium borohydride)        1-Hydroxy-1-methyl-
                                                       cyclopentane
•Gives exclusively Markovnikov addition

Hydroboration
                        i) "BH3" (Borane)       H OH
               CH3                                      CH3
                        ii) H2O2, NaOH                  H
 1-Methylcyclopentene
                                               1-Hydroxy-2-methyl-
                                                  cyclopentane
 •Gives exclusively anti-Markovnikov addition
Mechanisms of these reactions beyond the scope of this module
Alkene hydroxylation

     R    R          KMnO4        HO  OH
       C C                        R C C R
     R    R       or OsO4          R   R

•Alkene p bond lost; two new C-OH s bonds formed

Alkene epoxidation

      R    R         RCO3H           R O R
        C C                            C C
      R    R     (Peroxy acids)      R    R
                                    Epoxides

•Alkene p bond lost; two new C-O s bonds are formed to
the same Oxygen
   Examples                                OsO4                OH OH
                        CH 3 CH CH 2                    CH 3   CH CH 2
                          Propene                       Propane-1,2-diol

                              CH3CO3H (Peroxyacetic acid)

                           O
                  CH3    CH CH2
                 1,2-Epoxypropane

            H                             H                     H
                    CH3CO3H                     OsO4                 OH
             O
           H                              H                          OH
                                Cyclopentene                     H
1,2-Epoxycyclopentane
                                                     Cis-1,2-cyclopentanediol
Ozonolysis of alkenes
•Ozone (O3): strong oxidising agent
•Adds to C=C with loss of both the p and s bonds
•Products formed are known as ozonides
         R    R         O3         O O
                                R       R
           C C                    C   C
         R                      R   O   R
              R
                                  Ozonide
•Ozonides usually not isolated, but further reacted with
reducing agents
        O O             Zn     R              R
     R       R
       C   C                     C O     + O C
     R   O   R                 R              R
•Formation of two molecules each containing C=O
(Carbonyl) groups
Overall process:
      R    R         i) O3        R            R
        C C                         C O   + O C
      R    R        ii) Zn        R            R

Examples
                         i) O3
  CH 3 CH 2 CH CH 2                   CH 3 CH 2 CH O       +   O CH 2
                         ii) Zn
      1-Butene                                  Aldehydes


         CH3   CH3           i) O3            CH3
            C C                           2     C      O
        CH3      CH3         ii) Zn           CH3

     2,3-Dimethyl-2-butene
                                               Ketone

				
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