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organic reaction mechanismsDoc Brown's GCE Chemistry  Revising Advanced Level Organic Chemistry

A Level Revision Notes PART 10 Summary of organic reaction mechanisms

A mechanistic introduction to organic chemistry and explanations of different types of organic reactions

10.2 Some reaction mechanisms of ALKANES

The reaction mechanisms of alkanes

Part 10.2 ALKANES - introduction to the reaction mechanisms of alkanes. Free radical chlorination/bromination to give halogenoalkanes (haloalkanes, alkyl halides). Free radical thermal cracking to give shorter alkanes and alkenes Ionic catalytic cracking to give shorter alkanes and alkenes Revision notes include full diagrams and explanation of the mechanisms of alkanes and the 'molecular' equation and reaction conditions and other con-current reaction pathways and products are also explained.


Part 10.2 ALKANES

10.2.1 Introduction to alkane reactivity

  • Alkanes are not very reactive molecules. Most reactions require some energy input to initiate a reaction e.g. high temperature and catalyst for cracking, uv light for chlorination or a spark to ignite them (initiating free radical reactions).

  • A combination of two main reasons account for this lack of reactivity compared to most other homologous groups of organic molecules.

    1. Bond Strength:

      • The single covalent C-C (bond enthalpy 348 kJ mol-1) and C-H (bond enthalpy 412 kJ mol-1) bonds are very strong so bond fission does not readily happen. The carbon atom radius is small, giving a short and strong bond with other small atoms. Therefore the reactions will tend to have high activation energies resulting in slow/no reaction.

    2. Nature of bonding:

      • Carbon and hydrogen have similar electronegativities, so there is no polar bond giving a slightly positive carbon (Cδ+) which can be attacked by electron pair donating nucleophiles. [e.g. see halogenoalkanes (δ+C-Clδ-) or aldehydes/ketones (δ+C=Oδ-)]

      • All the C-C and C-H bonds are single covalent and no region of particularly high electron density susceptible to attack by electron pair accepting electrophiles. [e.g. like a double C=C bond see alkenes which are highly reactive despite the fact the C=C double bond has]

 


 

10.2.2 The free radical substitution chlorination/bromination mechanism of alkanes

The organic synthesis of chloroalkanes and bromoalkanes by reacting chlorine//bromine with alkanes

  • Hydrocarbon molecules such as saturated alkanes are reacted with chlorine or bromine to manufacture valuable chloroalkanes or bromoalkanes e.g.

    • ethane + bromine ==> bromoethane + hydrogen bromide

    • alkanes structure and naming (c) doc b + Br2 ==> (c) doc b + HBr

    • propane + chlorine ==> 1-chlorobutane/2-chlorobutane + hydrogen chloride

    • doc b oil notes + Cl2 ==> (c) doc b + HCl

    • doc b oil notes + Cl2 ==> (c) doc b + HCl

    • Note that there are two possible monosubstitution products possible via this free radical substitution mechanism described and explained below.

    • This is an important organic synthesis process to manufacture organochlorine and organobromine compounds.

  • What is the reaction mechanism of chlorine reacting with alkanes like methane and ethane etc.

  • Under the influence of high temperature (heat) or uv light, alkanes will react with chlorine or bromine via a free radical substitution reaction mechanism.

  • The basic reaction is: R3C-H + Cl2 ==heat/uv==> R3C-Cl + HCl [mechanism 6]

    • R = alkyl e.g. CH3, CH3CH2 etc. or aryl e.g. C6H5, CH3C6H4 etc.

  • The reaction is initiated by higher temperatures e.g. 250-400oC or uv light at room temperature.

  • If other hydrogen atoms are available on the original hydrocarbon then polysubstituted chloroalkanes will be formed

    • e.g. methane => chloromethane => dichloromethane => trichloromethane => tetrachloromethane

      • CH4 ==> CH3Cl ==> CH2Cl2 ==> CHCl3 ==> CCl4

    • propane can form initially 1-chloropropane or 2-chloropropane

      • and then 1,1- or 1,2- or 1,3- or 2,2-dichloropropanes etc. etc.!

      • i.e. CH3CH2CH3 ==> CH3CH2CH2Cl or CH3CHClCH3 

      • => CH3CH2CHCl2 or CH3CHClCH2Cl or ClCH2CH2CH2Cl or CH3CCl2CH3  

      • etc. and ultimately CCl3CCl2CCl3

    • You can write a similar set of equations for ethane, butane and pentane etc...

      • ... and the mechanisms can similarly constructed

organic reaction mechanisms

mechanism 6 - free radical chlorination of an alkane, when all R's are H, CR3H = methane

  • Step (1) is the initiation step when the chlorine molecule is split into two chlorine atoms/radicals by homolytic bond fission by the impact-absorption of the ultraviolet photon. Its quantum of energy, E=hv, must be great enough to break the Cl-Cl bond.

    • Homolytic bond fission means the original pair of (Cl-Cl) bonding electrons is split between the two radicals formed.

    • Step (1) illustrates how to use half-arrows to show a homolytic bond fission step

      • Not all exam boards demand half-arrows, so the 'style' is deliberately varied in the diagram.

    • The red dots represent the unpaired electron on the free radical and the half-arrows show the individual electron 'shifts'.

    • The breaking of the Cl-Cl bond in the chlorine molecules begins the reaction because it is the weakest of the bonds of any reactant molecule involvedBond enthalpies/kJmol-1: Cl-Cl = 242, C-C = 348, C-H = 412, and even the new bond formed, C-Cl, is 338.

    • Free radicals are highly reactive species with an unpaired electron and tend to form a new bond as soon as is possible by e.g. in this case by ...

      • abstracting another atom from another molecule e.g. step (2) H abstracted, and step (3) chlorine abstracted or by pairing up with another radical e.g. steps (4) to (6).

  • Steps (2) and (3) are chain propagation steps, because as well as producing one of the reaction products, a new free radical is also produced to continue the reaction, which is why such reactions are sometimes referred to as 'chain reactions'.

    • Step (2) Illustrates how to use half-arrows in a chain propagation step where an attacking radical abstracts an atom from a stable and complete molecule and another radical is formed in the process.

  • Steps (4) to (6) are three possible chain termination steps which remove the highly reactive free radicals as two unpaired electrons form a new bond, in this case single C-C covalent bonds.

    • Step (5) illustrates how to use half-arrows to indicate a termination step where the unpaired electrons of the two radicals pair up to form a new bond, in this case a C-Cl bond.

  • FURTHER COMMENTS

    • The mechanism for bromination is similar.

    • When the alkane is methane, traces of ethane are found in the final mixture of products.

      • This provides evidence for a mechanism involving a methyl radical.

      • It would be formed from combining two methyl radicals: H3C. + .CH3 ==> H3C-CH3 

 


10.2.3 The free radical thermal cracking of alkanes On separate page now

10.2.4 An ionic mechanism for catalytic cracking On separate page now


keywords phrases: reaction conditions formula intermediates organic chemistry reaction mechanisms free radicals initiation step propagation steps termination steps R3C-H + Cl2 ==heat/uv==> R3C-Cl + HCl [mechanism 6] R = alkyl e.g. CH3, CH3CH2 etc. or aryl e.g. C6H5, CH3C6H4 etc.CH4 ==> CH3Cl ==> CH2Cl2 ==> CHCl3 ==> CCl4 CH3CH2CH3 ==> CH3CH2CH2Cl or CH3CHClCH3 => CH3CH2CHCl2 or CH3CHClCH2Cl or ClCH2CH2CH2Cl or CH3CCl2CH3


APPENDIX -  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)

 


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