organic reaction mechanisms

Doc 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

Part 10.4 Halogenoalkanes

Reaction with cyanide ion

Part 10.4 HALOGENOALKANES - introduction to the mechanisms of halogenoalkanes (haloalkanes, alkyl halides). Nucleophilic substitution by by the nucleophile, the cyanide ion to form nitriles. These revision notes include full diagrams and explanation of the nucleophilic substitution reaction mechanisms of halogenoalkanes (haloalkanes) and the 'molecular' equation and reaction conditions and other con-current reaction pathways and products are also explained when halogenoalkanes react with potassium cyanide to give nitriles.

10.4 HALOGENOALKANES (old names 'haloalkanes' or 'alkyl halides')

10.4.1 Introduction to halogenoalkane reactivity

  • Halogenoalkanes owe their reactivity, especially compared to the unreactive alkanes, to two principal reasons.

  • R3C-X = halogenoalkane/haloalkane/alkyl halide/halogenated alkane etc. X = halogen e.g. Cl, Br or I

  • The carbon-halogen bond is polar, Cδ+-Xδ- due to the difference in electronegativity between carbon and the halogen. The  Cδ+ carbon is then susceptible to nucleophilic attack by electron pair donor neutral molecules (e.g. the nucleophiles :NH3, H2O:) or ions (e.g. :OH-, -:CN).

  • The carbon-halogen bond is usually the weakest bond in the molecule and significantly weaker than the carbon-carbon or carbon-hydrogen bonds.

    • Average bond enthalpies/kJmol-1: C-C 348, C-H 412, both relatively high requiring high activation energies for reaction.

    • Average bond enthalpies/kJmol-1: C-Cl 338, C-Br 276, C-I 238, generally lower resulting in lower activation energies.

      • Even the lowering of the bond enthalpy by 10kJ from C-C to C-Cl, combined with the polarity of the C-Cl bond, makes all the difference when comparing alkane and halogenoalkane reactivity.

  • IMPORTANT NOTE on structure classification

    • In the mechanism diagrams you will see part of the molecular structure shown as R3C

    • PLEASE do not assume this means a tertiary (tert) halogenoalkane (haloalkane).

    • R3C- is used repeatedly to minimise the number of graphic images needed.

    • In general a halogenoalkane (haloalkane) has the structure R3C-X where R = H, alkyl or aryl.

    • A primary halogenoalkane (haloalkane) can be shown as RCH2-X where R = H, alkyl or aryl.

    • A secondary (sec) halogenoalkane (haloalkane) can shown as R2CH-X where R = alkyl or aryl.

    • A tertiary (tert) halogenoalkane (haloalkane) can be shown as R3C-X where R = alkyl or aryl.


10.4.3 The nucleophilic substitution of halogenoalkane by cyanide ion

Organic synthesis of nitriles from halogenoalkanes (haloalkanes, alkyl halides) by reaction with potassium cyanide

  • What is the reaction mechanism for the substitution of a halogen atom in a haloalkane/halogenoalkane with the cyanide ion?

  • e.g. for the reaction

  • R3C-X + CN- ==> R3C-CN + X-   [see mechanisms 36 and 8 below]

  • This is usually achieved by refluxing the halogenoalkane with ethanolic potassium cyanide to form the nitrile.

  • organic reaction mechanisms

    mechanism 36 - nucleophilic substitution of a halogenoalkane by cyanide ion (SN1 'unimolecular' via carbocation)

    • SN1 unimolecular, a two step ionic mechanism via carbocation formation [mechanism 36 above]

      • In step (1) the polar Cδ+-Brδ- bond of the halogenoalkane splits heterolytically to form a carbocation and a free bromide ion, and this is a reversible reaction.

      • In step (2) the negative cyanide ion rapidly adds to the positive carbocation to form the C-N bond and give the nitrile product. The cyanide ion donates a pair of electrons to form the new C-C bond, i.e. the cyanide ion is the nucleophile.

    organic reaction mechanisms

    mechanism 8 - nucleophilic substitution of a halogenoalkane by cyanide ion (SN2 'bimolecular')

    • SN2 'bimolecular', a one step mechanism [mechanisms 8 above]

      • The Cδ+-Brδ- bond is polar, so the electron rich negative nucleophile, the cyanide ion, attacks the slightly positive carbon. The nucleophile (CN-) acts is acting as an electron pair donor (Lewis base) to bond with the 'positive' carbon.

      • Simultaneously the bromine atom is ejected, taking with it the C-Br bond pair of electrons, so forming the nitrile and a bromide ion.



    keywords phrases: reaction conditions formula intermediates organic chemistry reaction mechanisms nucleophilic substitution R3C-X + CN- ==> R3C-CN + X-

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