Predict the order of reactivity of the following compounds in SN1 and SN2 reactions:
(i) The four isomeric bromobutanes
(ii) C6H5CH2Br, C6H5CH(C6H5)Br, C6H5CH(CH3)Br, C6H5C(CH3)(C3H5)Br.
(i) In SN1 reactions, the order of reactivity depends upon the stability of the intermediate carbocations, therefore, (CH3)3 CBr which gives a 3º carbocation, i.e. (CH3)3C+ is the most reactive. CH3CH2CH(Cl)CH3 gives a 2º carbocation, i.e. CH3CH2CHCH3 and hence is less reactive that (CH3)3CBr. Out of the remaining two 1º alkyl halides, the carbocation (CH3)2CHCH2+ is more stable than the carbocation CH3CH2CH2CH+2 due to greater +I-effect of (CH3)2 CHCH2– group w.r.t. CH3CH2CH2CH2- group and hence the alkyl halide, (CH3)2CHCH2Br is more reactive than CH3CH2CH2CH2Br. Thus, the overall increasing reactivity of the four isomeric bromobutanes towards SN1 reaction follows the order :
CH3CH2CH2CH2Br< (CH3)2CHCH2Br <CH3CH2CH(Br)CH3 < (CH3)3 CBr
The reactivity in SN2 reactions, however, follows the reverse order, i.e., ch3ch2ch2ch2Br > (CH3)2CHCH2Br > CH2CH2CH(Br)CH3 > (CH3)3CBr
since the steric hindrance around the electrohilic carbon (i.e. a-carbon) increases in that order.
(ii) Since the reactivity in SN1 reactions increases as the stability of the intermediate carbocation increases, therefore, (C6H5)C(CH3)(C6H5) Br which gives 3º carbocation, i.e., (c6H5)2C(CH3) is the most reactive. Of the two 2º bromides, the carbocation intermediate derived from C6H5CHCH3 obtained from C6H5CH(CH3)Br i.e. C6H5CHC6H5 is more stable as compared to the carbocation C6H5CHCH3 obtained from C6H5CH(CH3)Br which gives the 1º carbocation, i.e., (C6H5)2C(CH3) is the most reactive. Of the two 2º bromides, the carbocation intermediate derived from C6H5CH(CH3)Br i.e. C6H5CHC6H5 is more stable as compared to the carbocation C6H5CHCH3 obtained from C6H5CH(CH3) Br because it is stabilized by two phenyl groups due to resonance. The fourth alkyl bromide, i.e., C6H5CH2Br which gives the 1º carbocation, i.e. C6H5CH+2 is, however, the least reactive. Thus, the overall reactivity of these alkyl bromides towards SN1 reactions follows the order:
C6H5C(CH3)(C6H5)Br > C6H5CH(C6H5)Br > C6H5CH(CH3)Br > C6H5CH2Br
In SN2 reactions, it is the steric hindrance which determines the reactivity. Since a phenyl group is much bulkier than a methyl group, therefore, C6H5C(CH3)(C6H5) Br is the least reactive followed by C6H5cH(C6H5) Brand C6H5CH(CH3) Br while C6H5CH2Br is the most reactive. Thus, the overall reactivity of these alkyl bromides towards SN2 reactions follows the order :
C6H5CH2Br > C6H5CH(CH3)Br > C6H5CH(C6H5)Br > C6H5C(CH3)(C6H5)Br .