Question

Which of the following statement is correct about the mechanism of hemiacetal and acetal formation? A. In the hemiacetal formation mechanism, the oxygen on the alcohol uses its lone pair to "attack the carbonyl carbon before the protonation of the carbonyl oxygen B. The carbocation formation is not favored during the hemiacetal formation mechanism, this is why hemiacetal are generally unstable and very reactive C. In the acetal formation mechanism, hydrolysis of water to the hemiacetal carbon center occurs as the first step to start the reaction D. In the acetal formation mechanism, the carbocation is always going to be a tertiary carbocation because this is the most stable carbocation E. Both the hemiacetal and acetal formation reaction has to be done in acidic conditions because protonation of the oxygen is the first step in both mechanisms

Answer 1

B is the correct Statement. Here is the explanation and mechanism

A Statement A is not Correct because in the hemiacetal formation mechanism the oxygen on the alcohol does not attack the carbonyl carbon. In the first step, Protonation occurs on the carbonyl Carbon.

B Statement B is Correct.

C Statement C is not Correct because Hydrolysis of water to the hemiacetal carbon is not the first step. In the first step protonation of water takes place.

D Statement D is not correct. In the acetal formation mechanism, it is not necessary that carbocation is always going to be a tertiary carbocation.

E Statement is incorrect because Hemiacetals can be formed in the basic conditions also. Yes Acetals can be formed in only acidic conditions.

0:~ R- o-4 CS0H R-i-1 - R-e-H 0- C-H - o-1 Aldehyde R-6-H -0+ -ht CH3-6-1 - CH3 Н. — Сид Hemiacetal 1 ht nach o- (h И , -ht R-én H Rach R-C-4 Сизон t DO-0-0 - a О-си CH3 och Acetal, Scanned with CamScanner

Answer 2

Option ''E'' is the correct option among the following because protonation of oxygen is the first step in both the mechanism so acidic medium is required. In base hemiacetal will not react to form an acetal. And if we are using acetals as a protecting group, they are stable in base. So you can't form acetals in base.

Option A is incorrect because the protonation of carbonyl oxygen takes place in the first step.

Option B is also incorrect beacuse the formation of carbocation takes place (during the first step i.e. protonation of carbonyl oxygen and the positive charge generate on oxygen is in resonance with carbonyl carbon. So the formal positive charge generate on carbonyl carbon also.) and carbocation formed makes the carbonyl carbon more electrophilic, which means a nucleophile can attack it better.  

Option C is incorrect hydrolysis of water does not take place.

Option D is also incorrect in the acetal formation, carbocation is not formed.