Organic Chemistry

Section 6.5 Addition of Hydrogen Halides and Addition of Halogens 243

3-D Molecule:

Vinylic cation

Tutorial:

Addition of HCl to an alkyne

The electrophile adds to the spcarbon

of a terminal alkyne that is bonded to

the hydrogen.

*One equivalent means the same number of moles as the other reactant.

cation, whereas the intermediate formed when a proton adds to an alkene is an alkyl

cation. A vinylic cationhas a positive charge on a vinylic carbon. A vinylic cation is

less stable than a similarly substituted alkyl cation. In other words, a primary vinylic

cation is less stable than a primary alkyl cation, and a secondary vinylic cation is less

stable than a secondary alkyl cation.

A vinylic cation is less stable because the positive charge is on an spcarbon, which

we will see is more electronegative than the carbon of an alkyl cation

(Section 6.9). Therefore, a vinylic cation is less able to bear a positive charge. In addi-

tion, hyperconjugation is less effective in stabilizing the charge on a vinylic cation

than on an alkyl cation (Section 4.2).

6.5 Addition of Hydrogen Halides

and Addition of Halogens

We have just seen that an alkyne is a nucleophile and that in the first step of the reac-

tion of an alkyne with a hydrogen halide, the electrophilic adds to the alkyne. If the

alkyne is a terminalalkyne, the will add to the spcarbon bonded to the hydrogen,

because the secondary vinylic cation that results is more stable than the primary

vinylic cation that would be formed if the added to the other spcarbon. (Recall that

alkyl groups stabilize positively charged carbon atoms; Section 4.2).

Although the addition of a hydrogen halide to an alkyne can generally be

stopped after the addition of one equivalent* of hydrogen halide, a second addition

reaction will take place if excess hydrogen halide is present. The product of the sec-

ond addition reaction is a geminal dihalide, a molecule with two halogens on the

same carbon. “Geminal”comes from geminus, which is Latin for “twin.”

CH 2

HBr

CH 3 CH 2 CCH 3 CH 2 CCH 3

Br

Br

2,2-dibromobutane

a geminal dihalide

2-bromo-1-butene

Br

electrophile adds here

CH 3 CH 2 CCHCH 3 CH 2 CCH CH 3 CH 2 CCH

2-bromo -1-butene

a halo-substituted alkene

1-butyne

CH 3 CH 2 C CH 2

a secondary vinylic cation

HBr +

+

CH 3 CH 2 CH CH

a primary vinylic cation

+

Br−

HHBr

H+

H+

H+

sp^2

>

R

RC+

R

a tertiary

carbocation

>

R

R RCH C R

+

C+

H

a secondary

carbocation

≈≈>

H

RC+

H

a secondary

vinylic cation

H

HC+

H

a primary

carbocation

a primary

vinylic cation

the methyl

cation

RCH C H

+

relative stabilities of carbocations

most stable least stable

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