The structure of the substrate impacts on a nucleophilic substitution reaction:

Question

The structure of the substrate impacts on a nucleophilic substitution reaction: some substrates are better SN2 reactants, others are more suitable as SN1 reactants. a. Which type(s) of halide will undergo an SN1 reaction? b. Which type(s) of halide will undergo an SN2 reaction? c. Predict the order of reactivity (1 = least, 4 = most) of primary, secondary, tertiary, and aryl halides toward nucleophilic displacement by an SN2 reaction mechanism. d. Predict the order of reactivity (1 = least, 4 = most) of primary, secondary, tertiary, and aryl halides toward nucleophilic displacement by an SN1 reaction mechanism. e. Draw the skeletal structure of and categorize the following substrates which you will use in this experiment as a primary, secondary, tertiary, or aryl halide. 1-chlorobutane 1-bromobutane 2-chlorobutane 2-bromobutane 2-chloro-2-methylpropane 2-bromo-2-methylpropane bromobenzene

Explanation / Answer

Ans 1.

a) The tertiary halides undergo SN1 reaction as they can form stable tertiary carbocations in the first step .

b) The primary or 1o halides undergo SN2 reaction as they have less steric hinderance than secondary and tertiary alkyl halides.

c) Since primary halides are most reactive and aryl halides towards SN2 reactions due to steric reasons, the order of reactivity is

1 - aryl halides

2- tertiary

3- secondary

4- primary

d) Aryl halides are least reactive in all nucleophilic substitution reactions as the electron pairs on the halogen atom are in the conjugation with the pi electrons of the ring .

So the order for SN1 reaction mechanism goes as:

1- aryl halide

2 - primary

3- secondary

4 - tertiary

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