1

Chapter 11 Unsaturated Hydrocarbons

11.1

Alkenes and Alkynes

2

Saturated hydrocarbons• have the maximum number of hydrogen atoms

attached to each carbon atom.• are alkanes and cycloalkanes with single C-C

bonds.

CH3—CH2—CH3

Saturated Hydrocarbons

3

Unsaturated hydrocarbons• have fewer hydrogen

atoms attached to the carbon chain than alkanes.

• are alkenes with double bonds.

• are alkynes with triple bonds.

Unsaturated Hydrocarbons

4

Bond Angles in Alkenes and Alkynes

According to VSEPR theory: • the three groups bonded to

carbon atoms in a double bond are at 120° angles.

• alkenes are flat because the atoms in a double bond all lie in the same plane.

• the two groups bonded to each carbon in a triple bond are at 180° angles.

5

Naming Alkenes

The names of alkenes • use the corresponding alkane name.• change the ending to –ene.

Alkene IUPAC Common

H2C=CH2 ethene ethylene

H2C=CH─CH3 propene propylene

cyclohexene

6

Ethene (Ethylene)

Ethene or ethylene• is an alkene with the formula

C2H4.• has two carbon atoms connected

by a double bond. • has two H atoms bonded to each

C atom.• is flat with all the C and H atoms

in the same plane.• is used to accelerate the ripening

of fruits.

7

Naming Alkynes

The names of alkynes • use the corresponding alkane name.• change the ending to –yne.

Alkyne IUPAC Common

HC≡CH ethyne acetylene

HC≡C─CH3 propyne

8

Naming Alkenes and Alkynes

When the carbon chain of an alkene or alkyne has four or more C atoms, number the chain to give the lowest number to the first carbon in the double or triple bond.

CH2=CH─CH2─CH3 1-butene

1 2 3 4

CH3─CH=CH─CH3 2-butene

1 2 3 4

CH3─CH2─CC─CH3 2-pentyne

5 4 3 2 1

9

Learning Check

Write the IUPAC name for each of the following:

1. CH2=CH─CH2─CH3

2. CH3─CH=CH─CH3

CH3

|3. CH3─CH=C─CH3

4. CH3─CC─CH3

10

Solution

Write the IUPAC name for each of the following:

1. CH2=CH─CH2─CH3 1-butene

2. CH3─CH=CH─CH3 2-butene

CH3

|3. CH3─CH=C─CH3 2-methyl-2-butene

4. CH3─CC─CH3 2-butyne

11

Learning Check

Write the IUPAC name for each of the following:

A. CH3─CH2─C≡C─CH3

CH3

B. CH3─CH2─C=CH─CH3

12

Solution

Write the IUPAC name for each of the following:

A. CH3─CH2─C≡C─CH3 2-pentyne

CH3

B. CH3─CH2─C=CH─CH3 3-methyl-2-pentene