Organic Chemistry

PROBLEM 6

What orbitals are used to form the carbon–carbon bond between the highlighted carbons?

6.4 How Alkynes React

With a cloud of electrons completely surrounding the bond, an alkyne is an electron-

rich molecule. In other words, it is a nucleophile and consequently it will react with

electrophiles. For example, if a reagent such as HCl is added to an alkyne, the rela-

tively weak bond will break because the electrons are attracted to the electrophilicp p

s

a. CH 3 CH CHCH 3

b. CH 3 CH CHCH 3

c. CH 3 CH C CH 2

d. CH 3 C CCH 3

e. CH 3 C CCH 3

f. CH 2 CHCH CH 2

g. CH 3 CH CHCH 2 CH 3

h. CH 3 C CCH 2 CH 3

i. CH 2 CHC CH

s

Section 6.4 How Alkynes React 241

3-D Molecule:

Ethyne

6.3 The Structure of Alkynes

The structure of ethyne was discussed in Section 1.9. Each carbon is sphybridized, so

each has two sporbitals and two porbitals. One sporbital overlaps the sorbital of a

hydrogen, and the other overlaps an sporbital of the other carbon. Because the spor-

bitals are oriented as far from each other as possible to minimize electron repulsion,

ethyne is a linear molecule with bond angles of 180°(Section 1.9).

The two remaining porbitals on each carbon are oriented at right angles to one

another and to the sporbitals (Figure 6.1). Each of the two porbitals on one carbon

overlaps the parallel porbital on the other carbon to form two bonds. One pair of

overlapping porbitals results in a cloud of electrons above and below the bond, and

the other pair results in a cloud of electrons in front of and behind the bond. The

electrostatic potential map of ethyne shows that the end result can be thought of as a

cylinder of electrons wrapped around the bond.s

s

s

p

HCCH

180 °

180 ° bond formed by

sp–s overlap

bond formed by

sp–sp overlap

HHCC

A triple bond is composed of a bond

and two Pbonds.

S

C

C

H

H

C

C

H

H

a. b. >Figure 6.1

(a) Each of the two bonds of a

triple bond is formed by side-to-side

overlap of a porbital of one carbon

with a parallel porbital of the

adjacent carbon.

(b) A triple bond consists of a

bond formed by sp–spoverlap

(yellow) and two bonds formed

by p–poverlap (blue and purple).

p

s

p

We have seen that a carbon–carbon triple bond is shorter and stronger than a

carbon–carbon double bond, which in turn, is shorter and stronger than a carbon–carbon

single bond. We have also seen that a carbon–carbon bond is weaker than a

carbon–carbon bond (Section 1.14). The relatively weak bonds allow alkynes to

react easily. Like alkenes, alkynes are stabilized by electron-donating alkyl groups. In-

ternal alkynes, therefore, are more stable than terminal alkynes. We now have seen that

alkyl groups stabilize alkenes, alkynes, carbocations,andalkyl radicals.

s p

p

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