IIT-JEE 2009 · Paper 2 · Chemistry Q3

ChemistrySingle correct+3 / -1Question 3 of 57

In the following carbocation, H/CH $_3$ that is most likely to migrate to the positively charged carbon is

_3

at C-4

H at C-4

_3

at C-2

H at C-2

Answer and worked solution

Official answer: D

Best new cation. A 1,2-shift moves the positive charge to the carbon the group leaves.

H or CH₃ from C-4 leaves a tertiary alkyl cation.
H from C-2 leaves a cation on the carbon bearing OH, which the oxygen lone pair stabilises as an oxocarbenium ion ( $\ce{C=O+H}$ , all octets), far better than any alkyl cation. (C-2 has no methyl.)

(D) H at C-2

IIT-JEE 2009 · Paper 2, Chemistry question 3