AQA A Level chemistry - AS Unit 2: Section 3.2.8 Haloalkanes

3.2.8 Haloalkanes - Nucleophilic substitution

Students should:

understand that haloalkanes contain polar bonds
understand that haloalkanes are susceptible to nucleophilic attack, limited to OH^-, CN^-, and NH₃
understand the mechanism of nucleophilic substitution in primary haloalkanes
understand that the carbon-halogen bond enthalpy influences the rate of hydrolysis
appreciate the usefulness of these reactions in organic synthesis

Nucleophilic substitution

These are reactions in which the bond between the carbon atom and the halogen atom breaks by heterolytic fission. This means that when the bond breaks, one atom gets a lone pair of electron, the other gets none. The result is that ions are formed

R-Cl R⁺ + Cl^-

The most common reaction of the halogenoalkanes is nucleophilic substitution (SN1 and SN2 mechanisms) The type of mechanism depends on the nature of the halogenoalkane - primary halogenoalkanes react via SN2 and tertiary halogenoalkanes via SN1.

The reason for the different mechanisms lies with the stability of the intermediate tertiary carbocation which forms in the case of the SN1 mechanism, whereas a primary carbocation would not be stable encouraging SN2. Another factor encouraging the SN1 mechanism in tertiary halogenoalkanes is the steric hindrance on approaching nucleophile experiences, preventing easy access to the partially positive carbon atom

SN1 mechanism

First, due to the electron withdrawing effect of the halogen, the carbon-halogen bond breaks heterolytically. This forms a carbonium ion intermediate, which is stabilised byu the presence of three +I (positively inductive) methyl groups.

unimolecular nucleophilic substitution, SN1, of hydroxide ions in bromomethylpropane

SN2 mechanism

Rather than completely breaking the bond, the polar bond between the halogen and carbon produces a partial +ve charge on the carbon. This is enough to attract a nuleophile to form a high energy transition state, which effectively has 5 bonds, one to the nucleophile, one with the halogen and 3 others.

This is the rate determining step, hence the second order reaction. The halide ion then breaks off heterolytically forming CH₃Nu + Cl^-.

bimolecular nucleophilic substitution - SN2

Some good nucleophiles are ROH, CN^-, OH^-, and RNH₂.

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Rates of nucleophilic substitution

This depends on:

the identity of the halogen (F, Cl, Br or I)
the nature of the halogenoalkane (1º, 2º or 3º)

The type of halogen determines the bond strength between the carbon and the halogen. F-C is the strongest and consequently fluoroalkanes are the least reactive (slowest rate). The strength of the C-X bonds is a function of the bond length. Longer bonds = weaker bonds.

Primary (1º) halogenoalkanes tend to react via the SN2 mechanism which is slower. Consequently the order of reactivity is 3º > 2º > 1º.

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