The major product [X] in the following reaction sequence is:
A. 2-butene
B. 1-butene
C. 2-butyne
D. 1-butyne
The correct answer is: A. 2-butene
The reaction sequence involves the addition of HBr to 1-butyne. The addition of HBr to an alkyne can occur in two ways: 1) 1,2-addition, which produces a 1,2-dibromoalkane; or 2) 1,4-addition, which produces a 1,4-dibromoalkene. The major product of the addition of HBr to an alkyne is determined by the stability of the intermediate carbocation. The more stable the carbocation, the more likely it is to form.
In the case of 1-butyne, the two possible carbocation intermediates are the 1-propyl carbocation and the 2-propyl carbocation. The 1-propyl carbocation is more stable than the 2-propyl carbocation because it is tertiary. Tertiary carbocations are more stable than secondary carbocations, which are more stable than primary carbocations. Therefore, the major product of the addition of HBr to 1-butyne is 2-butene, which is formed by 1,2-addition.
The other options are incorrect because they are not formed by 1,2-addition. Option B, 1-butene, is formed by 1,4-addition. Option C, 2-butyne, is formed by the elimination of HBr from 2-bromobutane. Option D, 1-butyne, is the starting material.