Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

Alkynes

are hydrocarbons that contain at least one triple bond. The simplest alkyne is

C^2

H^2

(see margin), which is commonly known as acetylene. Carbon atoms involved in

triple bonds have only two electron regi

ons and are sp hybridized and have 180

o bond

angles. The presence of

electrons makes them Lewis basic as well, and their chemistry is π

similar to that described above for alkenes. Alkynes are also unsaturated. Example 13.1

Write condensed structures

corresponding to the skeletal st

ructures shown below.

Note that the numbers of the

carbon positions are included for discussion purposes only.

Position numbers are not usually included and those given here do not represent the normal method of numbering.

1

2

3

4

5

6

1

2

3

O

1

2

3 4

5

O

9

6

8

7

2 1

8

4 5

3

6

7

O

(a)

(b)

(c)

(d)

a) The positions labeled 1 and 6 each have

only one bond drawn to them, but carbon

atoms must have four bonds, so each must

contain three C-H bonds. As a result,

positions 1 and 6 are CH

groups. Two bonds are drawn to positions 2 and 3, so they 3

must each be CH

groups. Three bonds are drawn to positions 4 and 5, so they are CH 2

groups. The condensed structure of (a) is shown in the margin. b) The positions labeled 1 and 3 each have only one bond drawn to them, so the other three bonds to each must be C-H

bonds. Thus, positions 1 and 3 are CH

groups. There 3

are already four bonds drawn to position 2,

so it is a carbon atom with hydrogen atoms

attached. The condensed structure of

(b) is shown in the margin.

c) Positions 1, 4, 7, 8, and 9 are all CH

groups. Position 2 is a CH 3

group. Positions 3, 5 2

and 6 each show three bonds, so they are

CH groups. The condensed structure of C is

shown in the margin. d) Position 1 has four bonds and is a ca

rbon atom with no hydrogen atoms attached.

Positions 2 and 8 are CH

groups. Positions 3 and 4 each show three bonds and are CH 2

groups. Positions 6 and 7 must be CH

groups, because there is only one bond shown to 3

each carbon. Position 5 already has four bonds shown, so there are no C-H bonds required. The condensed struct

ure of (d) is shown in the margin. It contains two CH

(^3)
groups and an oxygen atom attached to a six-
carbon ring. Cyclic structures like the six-
carbon ring are very common in organic chemistry.
H^3
C
H^2 C
CH^2
HC
CH
CH
3
(a)
H^3
C
C
CH
3
CH
2
(b)
H^3
C
H^2 C
CH CH
CH 3
O
CH
3
CH
CH
3
H^3
C
(c)
H^2
C C
CH^2
CH C
HC
CH
3
CH
3
O
(d)
HCCH
180
o
C^2
H^2
, Acetylene
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