Alkenes: Structure and Preparation via Elimination Reactions

21 Pages·2013·0.12 MB·English

Naming double bond-containing substituents
IUPAC: alkenyl
1-(2-butenyl)-2-methyl-1-cyclohexene
Naming double bond-containing substituents
IUPAC: alkenyl
1-(2-butenyl)-2-methyl-1-cyclohexene
Some common double bond-containing substituents:
vinyl (ethenyl)
1-methyl-4-vinyl-1-cyclohexene
Naming double bond-containing substituents
IUPAC: alkenyl
1-(2-butenyl)-2-methyl-1-cyclohexene
Some common double bond-containing substituents:
vinyl (ethenyl)
1-methyl-4-vinyl-1-cyclohexene
allyl (2-propenyl)
1-allyl-4-vinyl-1-cyclohexene
Naming double bond-containing substituents
IUPAC: alkenyl
1-(2-butenyl)-2-methyl-1-cyclohexene
Some common double bond-containing substituents:
vinyl (ethenyl)
1-methyl-4-vinyl-1-cyclohexene
allyl (2-propenyl)
1-allyl-4-vinyl-1-cyclohexene
phenyl
4,4-dimethyl-1-phenyl-1-cycloheptene
Naming double bond-containing substituents
IUPAC: alkenyl
1-(2-butenyl)-2-methyl-1-cyclohexene
Some common double bond-containing substituents:
vinyl (ethenyl)
1-methyl-4-vinyl-1-cyclohexene
allyl (2-propenyl)
1-allyl-4-vinyl-1-cyclohexene
phenyl
4,4-dimethyl-1-phenyl-1-cycloheptene
benzyl (phenylmethyl)
(R)-1-benzyl-4-methyl-4-phenyl-1-cycloheptene
When the parent chain contains multiple C=C, use a multiplicative prefix before “ene”:
Cl
(S)-6-chloro-4-methyl-2,4-heptadiene
Alenes are classified according to their degree of substitution of the C=C functional group
H
monosubstituted
H
H
H
disubstituted
H
H
terminally disubstituted
H
trisubstituted
H
tetrasubstituted
How strong is a C=C ? Is it twice the strength of a C–C?
B
F
F
H1s
H1s F
Csp2
Csp2
H1s F H1s
F
∆H , kcal/mol
DBE
. .
H C CH CH + CH 83 1 σ
3 3 3 3
146 1 σ + 1 π
H C CH :CH + :CH
2 2 2 2

ˆ ∆H (1 σ + 1 π) - ∆H (1 σ ) = ∆H (1 π)
BDE BDE BDE
146 - 83 = 63
cis-alkenes and trans-alkenes
Consider all the isomers of the alkene C H :
4 8
are these conformational isomers?
• No! They are isomers that differ by the relative positions of the substituent methyl
groups in space.
• They cannot be converted into each other without breaking a bond.
• Therefore, they are diastereoisomers.
• Why does rotation about the C=C break a bond?
H H H
H
C C C
CH CH CH
H3C C 3 C 3 H C 3
H H C CH
3 3
π π π
bond no bond bond reforms
These diastereoisomers are named just like disubstituted cycloalkanes:
E
• Identical groups on the same side of the C=C (that is, with a 0 dihedral angle)
are
cis-alkenes
E
• Identical groups on opposite sides of the C=C (that is, with a 180 dihedral
angle) are
trans-alkenes