Question

(a) Write out each of the three reactions below: 1-butanol, 2-butanol and tert-butanol with a mixture of NaBr and NaCl. Show the major product you expected for each one (bromo- or chloro?).

(b) Which reactions do you predict SN1? SN2? Label them below.

(c) For each, briefly explain the basis for your prediction of whether bromo or chloro substitution is the major product.

Answer 1

a) The major products are shown in each case are given below :

b) 1st reaction is going through via S_{​​​​​​N} ² mechanism.

2nd reaction is going through via S_{​​​​​​N} ¹ mechanism.

3rd reaction I going through via S_{​​​​​​N}^{​​​​​1} mechanism.

c) In the 1st case bromo compound is major i.e. 1- bromobutane. The nucleophilicity order is Cl^(-) < Br^{​​​​​(-)} . As Br has larger size and greater polarisibility power and low electronegativity , makes it weaker base than Cl ion. That's why Bromo compound is more favourable . And the TS in this case is more dispersed which makes it more ease for product formation.

In the 2nd case , 2-chlorobutane is major product . Because it goes through by a stable 2° carbocation and produces chloro product.

In the 3rd case, tert- bromobutane is major product. Because it goes through the most stable 3° carbocation.

For S_{​​​​​​N} ¹ mechanism in both 2nd and 3rd case, the chlorine ion is a favourite one to make the product .

As Br ion has a large size and greater polarisibility it detaches itself to make the carbocation. As we find the chloro product is major in both the cases .