Sn2 Reaction Essay

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 Comment: The three bonds (one solid line, one wedged line and one dashed line) that are attached to the + carbon atom are turned inside out. If the starting material has Br on the right side of the + carbon atom, the final product will have OH on the left side of the + carbon atom and vice versa.

 Comment: There is only one hump for the energy profile of an SN2 reaction. As we shall see later, there are two humps for the energy profile of an SN1 reaction. One easy way to recall is to remember that SN2 has 1 hump and SN1 has 2 humps.

Example: SN2 Mechanism Involving Electrically Neutral Nucleophile

Mechanism of Nucleophilic Substitution: SN1
Example: SN1 Mechanism Involving a Negatively Charged Nucleophile
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(b) State and explain if a chiral tertiary halogenoalkane would produce an optically active alcohol upon hydrolysis?

Solution
(a) benzyl chloride:

 Comment: There are two types of nucleophilic substitution reactions. Students need to identify the correct type first before describing the mechanism.

A benzylic carbocation is a very stable carbocation, more stable than a tertiary carbocation. This is because the positive charge at the benzylic position can be effectively dispersed throughout the benzene ring via resonance. In general, charge dispersal is more effective through the resonance effect than the inductive effect. Resonance usually beats induction.
The benzene ring is represented by the Kekulé structure to demonstrate the resonance effect.
The resonance effect operates through  bonds and adjacent p-orbitals, and the strength of the resonance does not diminish with distance. This is unlike the inductive effect which operates through  bonds and it diminishes with distance. Hence, the order of reactivity of RX to SN1 can be modified as follows:

Description of
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A tertiary halogenoalkane will undergo SN1 and not SN2 reaction. In an SN1 reaction, the tertiary carbocation intermediate is trigonal planar and the incoming nucleophile can attack the positively charged carbon centre from either side of the plane with equal probability. The end result is a 50:50 racemic mixture which is optically inactive.

Reactivity of Halogenoarenes
Halogenoarenes are generally less reactive towards nucleophilic substitution because of two reasons.
 The C-X bond has a partial double bond character. When the p-orbital of the halogen atom overlaps with the p electron cloud of benzene, its lone pair of electrons is delocalised into the aromatic ring. This results in the partial double bond character of the C-X bond which makes it more resistant to hydrolysis. (Primary reason)
 The benzene ring has its own supply of p electrons. Since a nucleophile carries with it its own electrons, its path of attack will be hindered due to electronic repulsion between the two. (Secondary reason)

 Comment: When asked to explain why halogenoarenes are less reactive towards nucleophilic substitution than halogenoalkanes, choose the primary reason, i.e. partial double bond

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