42
Unsaturated Hydrocarbons
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.
