Organic Chemistry

244 CHAPTER 6 Reactions of Alkynes • Introduction to Multistep Synthesis

When the second equivalent of hydrogen halide adds to the double bond, the elec-

trophile adds to the carbon bonded to the greater number of hydrogens—as

predicted by the rule that governs electrophilic addition reactions (Section 4.4). The

carbocation that results is more stable than the carbocation that would have been

formed if had added to the other carbon because bromine can share the positive

charge with carbon by sharing one of its lone pairs (Section 1.19).

The addition of a hydrogen halide to an alkyne can be stopped after the addition

of one equivalent of HBr or HCl because an alkyne is more reactive than the halo-

substituted alkene that is the product of the first addition reaction. The halogen sub-

stituent withdraws electrons inductively (through the bond), thereby decreasing

the nucleophilic character of the double bond.

In describing the mechanism for addition of a hydrogen halide, we have shown that

the intermediate is a vinylic cation. This mechanism may not be completely correct. A

secondary vinylic cation is about as stable as a primary carbocation, and generally pri-

mary carbocations are too unstable to be formed. Some chemists, therefore, think that

a pi-complexrather than a vinylic cation is formed as an intermediate.

Support for the intermediate’s being a pi-complex comes from the observation that

many (but not all) alkyne addition reactions are stereoselective. For example, the addi-

tion of HCl to 2-butyne forms only (Z)-2-chloro-2-butene, which means that only anti

addition of H and Cl occurs. Clearly, the nature of the intermediate in alkyne addition

reactions is not completely understood.

Addition of a hydrogen halide to an internalalkyne forms two geminal dihalides

because the initial addition of the proton can occur with equal ease to either of the

spcarbons.

If, however, the same group is attached to each of the spcarbons of the internal alkyne,

only one geminal dihalide is obtained.

2,2-dichloropentane 3,3-dichloropentane

2-pentyne excess

CH 3 CH 2 C CCH 3 + HCl CH 3 CH 2 CH 2 CCH 3 +

Cl

Cl

CH 3 CH 2 CCH 2 CH 3

Cl

Cl

CH 3 C CCH 3

2-butyne (Z)-2-chloro-2-butene

HCl

H 3 C

CH 3

CC

Cl

H

H

Cl

HC CH

a pi-complex

δ+

δ−

s

CH 3 CH 2 C+ CH 3

carbocation formed by adding the electrophile to the sp^2

carbon bonded to the greater number of hydrogens

bromine shares a lone pair

with carbon

Br

CH 3

+

CH 3 CH 2 C

Br

H+ sp^2

(H+) sp^2

3-D Molecule:

Pi-complex

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