Transcript Chem 331, Chapter 5: Structure and Preparation of Alkenes

Alkenes
1
Alkanes: CnH2n+2
Alkenes: CnH2n
H
saturated hydrocarbons
unsaturated hydrocarbons
H
CH3
C C
H
ethene
(ethylene)
CH3
CH CH2
C CH2
propene
(propylene)
H
H
CH3
2-methylpropene
(isobutylene)
Cl
C C
a-pinene
(turpentine)
H
H
chloroethene
(vinyl chloride)
H
H
O
OH
oleic acid (a monounsaturated fatty acid)
2
Nomenclature
• alkenes: parent chain contains C=C
C=C gets lowest numbers
position of C=C indicated by lower of the two numbers
CH3 CH2 CH CH2
1-butene
4-methyl-1-butene
3-bromocyclohexene
Br
3
Nomenclature
• polyenes: alkadiene, alkatriene, etc.
1,3-butadiene
5,5-dimethyl-1,3-cyclopentadiene
bicyclo[2.2.1]-2,5-heptadiene
4
Nomenclature
• C=C and OH: alkenol
higher priority group (OH) gets last suffix
and lowest number
OH
2-propen-1-ol
5-methyl-4-hexen-2-ol
OH
OH
2-cyclohexenol
5
Nomenclature
• as side groups:
H2C
H2C
CH
CH CH2
Examples
vinyl chloride
ethenyl (vinyl)
Cl
2-propenyl (allyl)
H2 C
allyl alcohol
C
1-methylethenyl (isopropenyl)
OH
H3 C
CH2
isopropenyl bromide
methylene
CH2
1-vinylcyclohexene
methylenecyclopentane
Br
6
Nomenclature
Question. Give IUPAC names for the following compounds. To check
your answers, click on the arrow.
OH
OH
7
Nomenclature
Geometric isomers: cis and trans
 bond
2p
H
H
H
C C
H
H
H
H
CH3
CH3
H
H
cis-2-butene
H
C C
H
H
no free rotation
H
C C
C C
H
H
overlap
p orbitals
(sp2C + 1sH)
(sp2C + sp2C)
trigonal planar
sp2
CH3
C C
H
geometric isomers
CH3
trans-2-butene
8
Nomenclature
Geometric isomers: cis and trans
cis
OH
trans
cis
OH
cis,
trans
9
Nomenclature
E-Z notation
The cis and trans designations apply only when there are two groups.
cis, trans??
10
Nomenclature
E-Z notation
1.
2.
Determine the higher priority group on each end of the alkene.
If the higher priority groups are:
on opposite sides: E (entgegen = opposite)
on the same side: Z (zusammen = together)
H
CH3 > H
Cl >
CH2CH3
Cl
H
C C
H3C
CH2CH3
C C
CH2CH3
(E)-3-chloro-2-pentene
H3C
Cl
(Z)-3-chloro-2-pentene
11
Nomenclature
E-Z notation
Priority Rules:
1. look at the first atom attached to the alkene carbon
2. higher atomic number = higher priority
3. if same atomic number, higher atomic mass = higher priority
(e.g., D > H, 13C > 12C, etc.)
4. if atoms are identical, examine next atoms along chain and
apply rules 2 and 3.
CH3
CH3 <
CH2
CH3 <
CH
CH3
(H,H,H)
(C,H,H)
(C,C,H)
CH3
<
C CH3 <
CH2 Cl
CH3
(C,C,C)
(Cl,H,H)
Look for higher atomic number at first point of difference.
12
Nomenclature
E-Z notation
Priority Rules:
5. multiple bonds: treat each  bond as if it were another single
bond to that type of atom
H
C
H
C
=
H
C
C
CH3
C
C H >
C
H H
H
(C,C,H; C,H,H)
C
C H =
C
(C,C,H; H,H,H)
C
C
CH3
C
C H >
C
C C
(C,C,C; C,C,H)
O
CH3
CH3
CH3
(C,C,C; H,H,H)
O C
CH3 =
C
O
CH3
13
Nomenclature
Question. Designate each double bond as E or Z, where applicable.
Click on the arrow to check your answers.
H
OH
H
Br
OH
geraniol
14