Question

Consider the following statements in reference to SN1, SN2, E1, and E2 reactions of haloalkanes. To which mechanism(s), if any, does each statement apply?
1. Involves a carbocation intermediate
2. Is first order in haloalkane and first order in base.
3. nvolves inversion of configuration at the site of substitution
4. Involves retention of configuration at the site of substitution
5. Substitution at a stereocenter gives predominantly a racemic product
6. Is first-order in alkyl halide and zero-order in base
7. Is first-order in alkyl halide and first order in base
8. Is greatly accelerated in protic solvents of increasing polarity.
9. Rearrangements are common
10. Order of reactivity is 3°> 2° > 1° > methyl
11. Order of reactivity of haloalkanes is methyl > 1° > 2° > 3°.

Answer 1

Answer:

1. Involves a carbocation intermediate- SN1, E1

2. Is first order in haloalkane and first order in base- E2

3. involves inversion of configuration at the site of substitution- SN2

4. Involves retention of configuration at the site of substitution- SN1

5. Substitution at a stereocenter gives predominantly a racemic product- SN1

6. Is first-order in alkyl halide and zero-order in base- E1

7. Is first-order in alkyl halide and first order in base- E2

8. Is greatly accelerated in protic solvents of increasing polarity- SN1,E1

9. Rearrangements are common- SN1, E1

10. Order of reactivity is 3°> 2° > 1° > methyl- SN1

11. Order of reactivity of haloalkanes is methyl > 1° > 2° > 3°- SN2

Explanation:

SN1 is mechanism of nucleophilic substitution in which a nucleophile reacts with an alkyl halide by first order mechanism. The reaction is first order in the alkyl halide and zero order in the nucleophile. The rate determining step is the formation of a carbocation. The ease of occurrence of SN1 reaction depends on the ease of formation of a stable carbocation. The order of carbocation stability is; 3°> 2° > 1° > methyl. SN1 mechanism leads to retention of configuration at the site of substitution and a racemization when substitution occurs at a chiral carbon atom.

SN2 is a bimolecular nucleophilic substitution mechanism which is first order in both nucleophile and alkyl halide. It is favoured by aprotic solvents which do not solvate the nucleophile significantly. The order of alkyl halide reactivity by SN2 mechanism is; methyl > 1° > 2° > 3°.

E1 mechanism refers to elimination reaction in which the rate determining step is the formation of a carbocation. It is common for tertiary alkyl halides. It mostly occurs with weak bases. The reaction is first order in the alkyl halide but zero order in the base.

E2 is an elimination mechanism which is first order in both alkyl halide and base. It is favoured by strong and bulky bases. Primary alkyl halides more commonly react by E2 mechanism.