Doc Brown's Advanced A Level Organic Chemistry Revision Notes

1.6 Chlorination and bromination of alkanes, reaction mechanisms and uses of products

The reactions of alkanes with halogens are important processes in the chemical industry for the production of a variety of useful products. The reaction between alkanes and halogens (chlorine and bromine) is described by reaction conditions, equations and the detailed mechanisms of halogenation, namely chlorination and bromination.

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A basic introduction to the chemistry of alkanes

The reactivity of alkanes - or lack of it!

Alkanes are not very reactive molecules. Most reactions of alkanes require some 'energetic' input to initiate a reaction e.g. high temperature and catalyst for cracking, uv light for chlorination or a spark to ignite them in combustion. In many cases this involves initiating free radical reactions.

A combination of two reasons account for the lack of reactivity of alkanes compared to most other homologous groups of organic molecules.

  1. Bond Strength: Strong sigma (σ) bonds in alkane molecules

    • The single covalent C-C bonds (bond enthalpy 348 kJ mol-1) and the C-H bonds (bond enthalpy 412 kJ mol-1) in alkanes are very strong so bond fission does not readily happen.

    • The carbon and hydrogen atomic radii are small, giving a short and strong bonds.

    • 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 (C = 2.5, H = 2.1, Δelec = 0.4), so there is effectively no polar bond (non-polar bond) giving a slightly positive carbon (Cδ+) which can be attacked by electron pair donating nucleophiles. [e.g. halogenoalkanes (δ+C-Clδ-) or aldehydes/ketones (δ+C=Oδ-)]

    • All the C-C and C-H bonds in alkanes are single covalent (saturated hydrocarbons) with no region of particularly high electron density like a pi (π) bond electron cloud susceptible to attack by electron pair accepting electrophiles. [e.g. like in the double C=C bond in unsaturated alkenes]

  3. Free radicals are highly reactive species with an unpaired electron e.g. methyl radical CH3, ethyl radical CH3CH2. (the ▪ dot represents the unpaired electron). These free radicals initiate and propagate a rapid chain reaction e.g. in combustion! Similarly, heat or uv light can generate free radicals by splitting halogen molecules such as chlorine and bromine into atoms that can then propagate a chain reaction to form substituted products known as halogenoalkanes (haloalkanes). Although Cl▪ and Br▪ are individual atoms, they are highly reactive free radicals because they have an unpaired electron and seek another electron on another atom to 'pair up' and form a stable bond with this other atom.


The reaction of alkanes with chlorine

As pointed out already, alkanes are not very reactive unless burned and they will react with reactive chemicals like chlorine and bromine when heated or subjected to uv light to form chlorinated alkane hydrocarbons.

  • Despite the reactivity of chlorine you still need something extra to initiate the reaction like uv light at room temperature or higher temperature (250oC - 400oC) in this case.
  • A substitution reaction occurs and a chloroľalkane is formed e.g. a hydrogen is swapped for a chlorine and the hydrogen combines with a 2nd chlorine atom e.g.
    • (i) methane + chlorine ===> chloromethane + hydrogen chloride
      • CH4 + Cl2 ===> CH3Cl + HCl
      • alkanes structure and naming (c) doc b + Cl2 ===> (c) doc b + HCl
    • 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.! (just shown schematically)

        • i.e. CH3CH2CH3  +  Cl2  ===>   {CH3CH2CH2Cl or CH3CHClCH3 }  +  HCl

        • ===> CH3CH2CHCl2 or CH3CHClCH2Cl or ClCH2CH2CH2Cl or CH3CCl2CH3  

        • etc. and ultimately CCl3CCl2CCl3 !!!

        • Because it is quite difficult to control this reaction when chlorine (or bromine later) is in excess, its not easy to control the yield of the desired halogenoalkane.

        • With substitution occurring in different places on the same carbon atom, or different carbon atoms, the variety of possible products  places quite a limitation on this reaction as a means of chemical synthesis. Fractional distillation can be used to separate the products.

    • The basic schematic mechanism for the free radical chain reaction between an alkane and chlorine is given in the mechanism 6 diagram below.
    • organic reaction mechanisms
    • R = H or alkyl in various combinations, when all the R groups 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 - hence each radical has an unpaired electron.

      • 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 involved

      • Bond enthalpies/kJmol-1: Cl-Cl = 242, C-C = 348, C-H = 412, and even the new bond formed, C-Cl, is 338.

      • As already stated, 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 in the propagation steps or by pairing up with another radical e.g. termination steps (4) to (6). The terms propagation step and termination step are explained below.

    • 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 free radical '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.

    • 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 

      • Similarly if the alkane was ethane, traces of butane are formed, similarly, for other alkanes you will traces of an alkane with twice as many carbon atoms as the original reactant alkane.

    • Diagram mechanism 48: I've sketched out the mechanism for chlorine reacting with methane to give chloromethane (the monosubstituted product).
    • Chlorine atoms produced in the initiation step (via uv or heat).
    • In the two propagation steps (a) a chlorine atom abstracts hydrogen atom from methane to form HCl. (b) The resulting methyl radical abstracts a chlorine atom from a chlorine molecule to form the product. BUT, you are left with a chlorine atom/radical that continues the chain reaction via (a).
    • Termination occurs when two radicals meet and combine. This has a very low probability because the free radical concentrations are very small in the first place. This allows propagation steps (a) and (b) to occur several hundred times before termination occurs.
    • Diagram mechanism 52: I've sketched out the propagation steps for the polysubstitution of methane.
    • All the carbon based free radicals involved in the chlorination of methane are pyramidal in shape. The pyramid shape arises from the mutual repulsion of four lots of electrons (best appreciated in the dot & cross diagrams), three bonding pairs and one lone electron. The bond angles would be around 109o (as in methane).
    • (ii) ethane + chlorine ==> chloroethane + hydrogen chloride
      • CH3CH3  +  Cl2  ===>  CH3CH2Cl  +  HCl
      •   +  Cl2 ===> +   HCl
      • Diagram mechanism 49: The mechanism for the monosubstitution of ethane with chlorine.

(iii) butane  +  chlorine  ===>  {1-chlorobutane  or  2-chlorobutane}  +  hydrogen chloride

alkanes structure and naming (c) doc b +  Cl2 {(c) doc b  or  (c) doc b} +  HCl

alkanes structure and naming (c) doc b + Cl2 (c) doc b or  (c) doc b  +  HCl

Diagram mechanism 50: Sketch of the mechanism for the monosubstitution of butane with chlorine. Note the possibility of different abstractions at different positions on the carbon chain giving a mixture of two structural isomer products - the structural isomers 1-chlorobutane and 2-chlorobutane.

(iv) cyclohexane  +  chlorine  ===>  chlorocyclohexane  + hydrogen chloride

alkanes structure and naming (c) doc b + Cl2 (c) doc b  +  HCl


Uses of chloroalkanes

Chloromethane and chloroethane are gases at room temperature, but bigger chloroľalkane molecules are liquids and useful solvents in the laboratory or industry. However, they are still quite volatile and chlorohydrocarbon vapours can be harmful if breathed in. Halogenoalkanes are used as refrigerants.

The reaction of alkanes with bromine

(i) propane  +  bromine   ===>   {1-bromopropane or 2-bromopropane}  +  hydrogen bromide

word equation followed by displayed formula equation, structural formula equation and skeletal formula equation

  + Br2    or (c) doc b   + HBr

alkanes structure and naming (c) doc b  + Br2  {(c) doc b  or (c) doc b}   + HBr

alkanes structure and naming (c) doc b  + Br2  (c) doc b  or (c) doc b   + HBr

Diagram mechanism 51: The mechanism for the monosubstitution of propane with bromine. Again, note the possibility of two structural positional isomers, 1-bromopropane and 2-bromopropane. The mechanism is identical to that for chlorination of alkanes.

(ii) cyclohexane  +  bromine  ===>  bromocyclohexane C6H11Br

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

Further substitution can take place to give three isomeric products of C6H10Bre.g.

+ Br2 (c) doc b (c) doc b (c) doc b  +  HBr

overall ...

cyclohexane  +  bromine  ===>  1,2-   &   1,3-   &  1,4-dibromocyclohexane  + hydrogen bromide

alkanes structure and naming (c) doc b + 2Br2 (c) doc b (c) doc b (c) doc b  +  2HBr

Another example of a multiple substitution of an alkane by a halogen

Note: I don't think iodine is reactive enough to produce iodoalkanes by this method.

Bromine compounds are used as flame retardant additives in polymer compositions and fire extinguishers.


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