130227SCH4U March 01, 2013

Reaction of Halogenoalkanes

CH3 - Cl

Nucleophilic Substitution

What is a nucleophile?

reactants with a non-bonding electron pair

NaOH + CH3Cl

But what's the mechanism?

NaOH + (CH3)3 C Cl

Substitution Reaction

SN2

130227SCH4U March 01, 2013

The formation of the transition state in the SN2 reaction involves the nucleophile attacking and partially bonding to the carbon atom holding the leaving group. The easier it is for this attack to occur, the easier (and faster) the reaction will be. The idea of steric hindrance is an important consideration here. In the SN2 reaction the nucleophile must approach close enough to the carbon atom being attacked for bonding to occur. If large, bulky alkyl groups crowd this approach, the reaction will be hindered. A change in reactivity due to the spatial arrangement of groups of atoms is known as steric hindrance. A bulky substrate, such as a tertiary halogenoalkane, will hinder the formation of the transition state and so slow the reaction.

130227SCH4U March 01, 2013

Summary: primary substrates are more likely to undergo SN2 substitution.

SN1

The tertiary substrate (2-bromo-2-methylpropane) offers considerable steric hindrance to the nucleophile, OH−. Both reactions involve the same leaving group, bromine. Reaction for 2-bromo-2-methylpropane would be predicted to be slow. Reaction, however, is found to be quite rapid, and many, many times faster than the corresponding reaction between bromomethane and hydroxide ion! Why?

130227SCH4U March 01, 2013

This two-step process involves the formation of the intermediate carbocation by the slow, rate-limiting heterolytic fission of the halogenoalkane. The carbocation then rapidly reacts with the nucleophile.

Summary:Tertiary substrate is more likely to undergo SN1 substitution.