Unsaturated Hydrocarbons

Chemical Formulas and Unsaturation

C C C C C

C C C C C

H H H H H

H H H H H

H C C C C C H

H H H H H

H H H H H

H C C C C

H H H H

H H

C

H

H

H

H

H

Cn

CnH2n

CnH2n+2

CnH2n+2

Chemical Formulas and Unsaturation

C C C C C

Cn

C C C C C

Cn

C C C C C

Cn

CnH2n

H C C C C C H

H H H

H H H H H

C

C

C C

C

CnH2n

C

C

C C

C

HH

H

H

H

H H

H

H

H

Chemical Formulas and Unsaturation

Hydrocarbons

SaturatedHydrocarbons

UnsaturatedHydrocarbons

AlkanesCnH2n+2

CycloalkanesCnH2n

(one ring)

AlkenesCnH2n

(one C=C)

AlkynesCnH2n-2

(one C C)

BenzeneC6H6

CnH2n-6

Review of Carbon Bonding Patterns

Classes of “Unsaturated” Hydrocarbons

120º120º 180º

Lewis Representation of Ethene

C C

H

H

H

HC

H

H C

H

H

N o t e : B o t h o f t h e s e representations picture the two components of the double bond as being identical. This is not the case.

Covalent Bonding in Ethene

The two carbons and all four hydrogens all lie within the same plane. The bond angles are all 120º.

Note: Though the two components of the double bond are pictured the same, one of the bonds is actually weaker than the other.

Supplement on Hybridization

in Ethene

Hybrid Orbitals - Bonding in Methane

Recall that the four orbitals associated with the bonding in an alkane or cycloalkane are the result of “sp3 hybridizaton.”

Hybrid Orbitals - Alkenes

The four orbitals associated with the

bonding in alkenes or cycloalkenes are the result

of “sp2 hybridizaton.”

Covalent Bonding in Ethene

C2H4, ethene, is formed from two sp2 hybridized carbon atoms and four hydrogen atoms:

Lateral overlap of the unhybridized p orbitals, above and below the triangular planes, results in a π bond of the double bond.

Nomenclature of Alkenes

1. Select the longest carbon chain containing the double bond as the parent chain. Replace the “-ane” ending with “-ene.

2. Number the parent chain beginning at end closest to the double bond beginning with the first carbon in the chain.

3. Locate and name the substituents in alphabetical order.

4. Compounds with two double bonds are called dienes.

5. Cyclic compounds are named as cycloalkenes. Carbons in the double bond are always numbered first. Then the ring is numbered in the direction so as to give the lowest numbers to substituents.

Isomerism in Alkenes

H2C CH CH2CH2 CH3

H3C CH CH CH2 CH3

H3C CH2 CH CH CH3

Positional Isomerism

1-pentene

2-pentene

????also

2-pentene

Geometric Isomerism (a form of stereoisomerism)

Alkenes-Nomenclature

Alkenes-Nomenclature

2-hexene 2-methyl-3-hexene

2,3-dimethyl-1-pentene

Alkenes-Nomenclature Practice

2,3-dimethyl-2-hexene

Alkenes-Nomenclature

4-methylcyclohexene

Alkenes-Nomenclature Practice

2,5-dimethyl-2-heptene 3,3-dimethylcyclopentene

Exercises on Alkene Nomenclature

H3C CH2 CH2 C CH2 CH3

CH2

H3C CH CH CH CH CH

CH2 CH3

CH3

CH3

CH3

CH3Cl

CH2CH3

1a)

b)

c)

Geometric Isomerism (a form of stereoisomerism)

Isomerism in Alkenes

Exists for the following cases:

C C

X

Y

A

B

A≠B and E≠D

C C

X

Y

A

B

C C

Y

X

A

B

A result of lack of free rotation around the double bond

X

E

D

E

D

D

E

Isomerism in Alkenes

Isomerism in Alkenes Cis/trans isomers have different physical properties

Isomerism in Alkenes

Cis/trans isomers are not possible for some alkenes.

cis-4-methyl-2-hexene

Alkenes-Nomenclature Practice

trans-5,6-dimethyl-3-heptene

2a) cis-1-chloro-1-butene

b) trans-3-hexene

c) trans-4,4-dimethyl-2-pentene

d) cis-1,3-dichlorocyclohexane

C C

Cl CH2 CH3

H H

C C

CH2

CH2 CH3

H3C H

H

C C

H3C

C CH3

CH3

H3C

H

H

Cl

Cl

Alkynes

Supplement on Hybridization

in Ethyne

Alkynes Structure of the Triple Bond

CH C H

N o t e : B o t h o f t h e s e representations picture the three components of the t r i p l e b o n d a s b e i n g identical. This is not the case.

C C HH

7 sigma bonds

Ethane, CH3CH3The sp3 hybrid orbitals of

two carbons overlap

The remaining sp3 hybrid orbitals overlap with hydrogen s orbitals

5 sigma bonds

Ethene, CH2CH2The sp2 hybrid orbitals of

two carbons overlap

The remaining sp2 hybrid orbitals overlap with hydrogen s orbitals

Ethene, CH2CH2

The remaining unhybridized p

orbitals overlap to form a ∏ bond.

5 sigma bonds and 1 ∏ bond

3 sigma bonds

Ethyne, CHCHThe sp hybrid orbitals of

two carbons overlap

The remaining sp hybrid orbitals overlap with hydrogen s orbitals

Ethyne, CHCH

The remaining unhybridized p

orbitals overlap to form 2 ∏ bonds.

3 sigma bonds and 2 ∏ bonds

Bonding in HCCHH C C H

H

C

C

H

Summary of Hydrocarbon Bonding

Alkynes Structure of the Triple Bond

CH C H

N o t e : B o t h o f t h e s e representations picture the three components of the t r i p l e b o n d a s b e i n g identical. This is not the case.

Nomenclature of Alkynes

1. Select the longest carbon chain containing the triple bond as the parent chain. Replace the “-ane” ending with “-yne.

2. Number the parent chain beginning at end closest to the triple bond beginning with the first carbon in the chain.

3. Locate and name the substituents in alphabetical order.

Alkynes-Nomenclature Practice

4,4-dimethyl-1-hexyne 2,7-dimethyl-4-octyne

Alkynes-Nomenclature Practice

H3C

Cl

Cl

C C CH

CH3

CH3Excess Br2 in the dark

HC C CH2 CH2 C

CH3

CH3

CH3Excess HBr in the dark

Properties of Alkenes and Alkynes

Nonpolar and insoluble in water as are alkanes.

Nonpolar, therefore Boiling points depend on size and shape.

Flammable.

Alkenes may display cis-trans isomerism.

Chemically reactive at the multiple bond.