Question

2. Fill in the following missing organic structures. Consider stereochemistry (R/S and/or E/Z) and possible carbocation rearrangements. OH НСІ (S. 1) H,SO,/A (E1)

Answer 1

Ans:

Reaction A- The first reaction given is the S_N1 type reaction following the formation of a carbocation. HCl adds to the OH group giving -OH₂⁺ ion which leaves giving the formation of the secondary carbocation. This secondary carbocation rearranges to more stable tertiary carbocation by 1,2-hydride shift. The addition of Cl^- ion in secondary carbocation gives the racemic mixture of R- and S-3-chloro-2-methyl pentane, whereas the addition of chloride ion in tertiary carbocation gives only one product as there is no formation of chiral center. The mechanism of the reaction is given below:

en e Y - i. Secondary Carbocation (S)-3-chloro-2- methylpentane (R) 3-chloro-2- methylpentane 1,2-hydride shift “ - i Tertiary Carbocation More Stable 2-chloro-2-methylpentane Major

Reaction B- This reaction is E1 elimination. The addition of acid is the same as in the case of the above reaction and the carbocations are formed in the same way as above. In the case of the secondary carbocation, there are two types of $\alpha$ - hydrogens H_a and H_b and the conjugate base of the acid HSO₄^- remove these hydrogens giving two products X and Y. Whereas in the case of tertiary carbocation there are two types of $\alpha$ -hydrogens H_c and H_d. The removal of H_d gives X product and the removal of H_c gives product Z.

XHSO4 Ye H2SO4/A (E1) ~ Y Ho 2-methylpent-2-ene (E)-4-methylpent-2-ene Secondary Carbocation 1,2-hydride shift HSO4 H Hd 2-methylpent-1-ene Tertiary Carbocation More Stable