Question

Question 11.29, 11.30

What change would you expect on the rate of the S_N2 reaction of 1-iodo-2-methylbutane with cyanide ion if the nucleophile concentration is halved and the alkyl halide concentration is unchanged?

What change would you expect on the rate of the reaction of ethanol with 2-iodo-2-methylbutane if the nucleophile concentration is halved by adding diethyl ether as an inert solvent and the alkyl halide concentration is unchanged?

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Question 11.33

Draw the product you expect from the reaction of (S)-3-iodohexane with ^-CN. Be sure to show stereochemistry

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Question 11.36

Draw the structure of the major organic product you would expect from the reaction of 1-bromopropane with Mg, then H₂O.

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Answer 1

11.29. Ans :- Halved

Explanation :-

In S_N2 reaction, rate is depends upon the concentration of substrate alkyl halide (i.e.1-iodo-2-methylbutane) and concentration of nucleophile (i.e. cyanide ion). i.e.

Rate = K [Substrate]¹[Nucleophile]¹ ............(1)

Reaction is first order with respect to substrate and nucleophile. here K = rate constant

if the nucleophile concentration is halved and the alkyl halide concentration is unchanged, then equation (1) becomes

Rate = K [Substrate]¹ [Nucleophile/2]¹

Rate = K/2 K [Substrate]¹[Nucleophile]¹ ...........(2)

On comparing equation (1) and (2) rate becomes halved as compare to the initial rate.

Hence Rate becomes halved is the correct answer.

11.30. Ans :-unchanged

Explanation :-

Given reaction follow S_N¹ mechanism because given alkyl halide (i.e. 2-iodo-2-methylbutane ) is a tertiary alkyl halide. In S_N¹ reaction, rate is depends upon the concentration of substrate alkyl halide (i.e.2-iodo-2-methylbutane ) only and do not depends upon the concentration of nucleophile (i.e. cyanide ion). i.e.

Rate = K [Substrate]¹............(1)

Reaction is first order with respect to substrate.

if the nucleophile concentration is halved and the alkyl halide concentration is unchanged, then equation (1) remains same i.e.

Rate = K [Substrate]¹...........(2)

On comparing equation (1) and (2) rate remains same

Hence Rate unchanged

11.33. Ans :- (R)-2-Ethylpentanenitrile

Explanation :-

Given alkyl halide i.e. (S)-3-iodohexane is a secondary alkyl halide and nucleophile i.e. CN^- is a strong nucleophile therefore this reaction follow S_N2 mechanism. In S_N2 only backside attack of nucleophile on substrate is possible, therefore 100 % inversion of configuration occurs and the products are (R)-2-Ethylpentanenitrile.

11.36. Sol:-

CH₃CH₂CH₂Br + Mg --------> CH₃CH₂CH₂MgBr (Propylmagnesiumbromide)

CH₃CH₂CH₂MgBr + H₂O ----------> CH₃CH₂CH₃ (Propane) + Mg(OH)Br