Organic chemistry Part 10: Reactivity and reaction mechanisms

10.4.1 Introduction to halogenoalkane reactivity and nucleophilic substitution of haloalkanes with cyanide ion, RX + CN^– ==>

Part 10.4 HALOGENOALKANES

Introduction to the mechanisms of halogenoalkanes (haloalkanes, alkyl halides).

Nucleophilic substitution 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.

• 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, C^δ+-X^δ- due to the difference in electronegativity between carbon and the halogen. The  C^δ+ 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]

mechanism 36 - nucleophilic substitution of a halogenoalkane by cyanide ion (S_N1 'unimolecular' via carbocation)

• S_N1 unimolecular, a two step ionic mechanism via carbocation formation [mechanism 36 above]

  • In step (1) the polar C^δ+-Br^δ- bond of the halogenoalkane splits heterolytically to form a carbocation and a free bromide ion, and this is a reversible reaction and the rate determining step.

  • 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.

mechanism 8 - nucleophilic substitution of a halogenoalkane by cyanide ion (S_N2 'bimolecular')

• S_N2 'bimolecular', a one step mechanism [mechanisms 8 above]

  • The C^δ+-Br^δ- 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.

• FURTHER COMMENTS

  • The mechanisms are the same for chloroalkanes, but slower, due to the stronger C-Cl bond enthalpy.

  • In industry chloroalkanes are more likely to be used because they are cheaper.

  • Many of the comments for hydrolysis apply to this reaction.

  • See Organic mechanisms Appendix 1 for a detailed discussion on

  • The structure, relative stability and theory of carbocations (separate page)