Chapter 7, Problem 7.34P

Interpretation Introduction

(a)

Interpretation:

The mechanism for the addition of a proton from trifluoromethanesulfonic acid to the alkyne to yield two different carbocations is to be drawn.

Concept introduction:

Carbon-carbon double and triple bonds are electron-rich. They can undergo heterolytic bond breaking. The pi bond can break, and the electron pair can be used to form a bond with an electrophile such as a proton. The electrophile bonds to one of the carbon atoms. The other carbon atom gets a 1+ charge and is called a carbocation. The reaction mechanism is drawn by showing the movement of electron pairs with curved arrows. The curved arrow starts from an electron-rich atom or center of an electron-rich region (a pi bond) and ends on an electron-poor atom, an electrophile, to show the formation of a new sigma bond. If a pi bond is formed, the arrow must end in the region between the corresponding atoms.

Interpretation Introduction

(b)

Interpretation:

It is to be determined which of the two carbocations formed on the addition of a proton to the given alkyne is more stable.

Concept introduction:

Carbocations are positively charged carbon atoms that are electron poor. The carbon is two electrons short of an octet. The charge and the incomplete octet make the carbocation an unstable species. The stability of a carbocation depends on the number of alkyl groups bonded to it and increases in the order $\text{methyl < primary < secondary < tertiary}$.