Question
Chapter 5
Structure and Preparation of Alkenes
(CnH2n):
Elimination Reactions
The molecular formula of β-Carotene is C40H56.
On catalytic hydrogenation, β-carotene is
converted to a saturated hydrocarbon of
molecular formula C40H78. How many rings
does β-carotene contain?
A) none
B) one
C) two
D) three
Alkene Nomenclature
H 2C
5.1
Alkene Nomenclature
Alkene Nomenclature
H 2C
CHCHCH 2Br
CH3
4) If a substituent is present, identify its position
by number. The double bond takes
precedence over alkyl groups and halogens
when the chain is numbered.
The compound shown above is
4-bromo-3-methyl-1-butene.
(or 4-bromo-3-methylbut-1-ene)
CHCH 2CH3
1-Butene or
But-1-ene
1) Find the longest continuous chain that
includes the double bond.
2) Replace the -ane ending of the unbranched
alkane having the same number of carbons
with -ene.
ene.
3) Number the chain in the direction that gives
the lowest number to the doubly bonded
carbon.
Alkene Nomenclature
H 2C
CHCHCH 2OH
CH3
4) If a substituent is present, identify its position
by number. Alcohol groups take precedence
over the double bond when the chain is
numbered.
The compound shown above is
2-methyl-3-buten-1-ol.
(or 2-methyl-but-3-en-1-ol)
Cycloalkene Nomenclature
Common Alkenyl Groups
3
H 2C
methylene
2
4
vinyl
H 2C
CH
allyl
H 2C
CHCH 2
1
5
CH3
6-Ethyl-1-methylcyclohexene
6
CH2CH3
1) Replace the -ane ending of the cycloalkane
having the same number of carbons with -ene.
ene.
2) Number through the double bond in the
direction that gives the lower number to the
first-appearing substituent.
Question
Name the alkene below according to the
IUPAC system.
5.2
Structure and Bonding in Alkenes
A) 2-bromo-5-heptane
B) 6-bromo-2-heptene
C) 2-bromo-5-heptene
D) 5-bromo-2-heptene
Bonding in Ethylene
Bonding in Ethylene
σ
σ
σ
σ
σ
Framework of σ bonds
Each carbon is sp2 hybridized
Each carbon has a half-filled p orbital.
Bonding in Ethylene
5.3
Isomerism in Alkenes
Side-by-side overlap of halffilled p orbitals gives a π bond.
Isomers
Isomers
Isomers
Isomers
Constitutional
ConstitutionalIsomers
Isomers
different connectivity
H
CH2CH3
C
1-Butene
H 3C
H
H 3C
H
C
H
Constitutional
ConstitutionalIsomers
Isomers
same connectivity;
different arrangement
of atoms in space
C
H
H
C
H 3C
H
H 3C
C
C
H
cis-2-Butene
cis-2-Butene
H
CH2CH3
C
H
H
H 3C
H 3C
C
H
2-Methylpropene
CH3
C
trans-2-Butene
trans-2-Butene
H
C
1-Butene
H
H 3C
C
H
C
CH3
Stereoisomers
Stereoisomers
consider the isomeric alkenes of
molecular formula C4H8
2-Methylpropene
CH3
C
Stereoisomers
Stereoisomers
C
Constitutional isomers
H
cis-2-Butene
cis-2-Butene
H
H 3C
CH2CH3
C
H
C
C
H
C
H 3C
H
1-Butene
H
Stereoisomers
2-Methylpropene
H
H 3C
C
Constitutional isomers
H
H 3C
CH3
C
C
CH3
trans-2-Butene
trans-2-Butene
H
C
C
C
H
H
Question
Which alkene can have cis/trans
stereoisomers?
stereoisomers?
cis (identical or
analogous substituents
on same side) or Z
trans (identical or
analogous substituents
on opposite sides) or E
A)
B)
C)
D)
Question
Of the compounds with the formula C3H5Cl,
which one has cis/trans isomers?
A) 1-chloropropene
B) 2-chloropropene
C) 3-chloropropene
D) all of them
CH3
trans-2-Butene
trans-2-Butene
cis-2-Butene
cis-2-Butene
Stereochemical Notation
H
H 3C
5.4
Naming Steroisomeric Alkenes
by the E-Z Notational System
Stereochemical Notation
The E-Z Notational System
CH2(CH2)6CO2H
CH3(CH2)6CH2
C
C
H
Oleic acid
E : higher ranked substituents on opposite sides
Z : higher ranked substituents on same side
higher
H
cis and trans are useful when substituents are
identical or analogous (oleic acid has a cis
double bond)
cis and trans are ambiguous when analogies
are not obvious
C
lower
The E-Z Notational System
The E-Z Notational System
E : higher ranked substituents on opposite sides
Z : higher ranked substituents on same side
lower
C
C
E : higher ranked substituents on opposite sides
Z : higher ranked substituents on same side
higher
C
C
higher
lower
lower
C
higher
Entgegen (E)
The E-Z Notational System
higher
C
lower
higher
C
lower
Zusammen (Z)
The Cahn-Ingold-Prelog (CIP) System
Question: How are substituents ranked?
Answer:
higher
They are ranked in order of
decreasing atomic number.
lower
C
C
lower
higher
E
higher
C
higher
C
lower
lower
Z
The system that we use was devised by
R. S. Cahn
Sir Christopher Ingold
Vladimir Prelog
Their rules for ranking groups are most
important in relation to stereochemistry and
most be mastered.
Table 5.1 CIP Rules
(1) Higher atomic number outranks lower
atomic number
Br > F
Cl > H
higher Br
Cl
C
lower
F
higher
Table 5.1 CIP Rules
(1) Higher atomic number outranks lower
atomic number
Br > F
Cl > H
higher Br
C
C
H
lower
lower
F
Cl
higher
H
lower
C
(Z )-1-Bromo-2-chloro-1-fluoroethene
Table 5.1 CIP Rules
Table 5.1 CIP Rules
(2) When two atoms are identical, compare the
atoms attached to them on the basis of their
atomic numbers. Precedence is established
at the first point of difference.
(3) Work outward from the point of attachment,
comparing all the atoms attached to a
particular atom before proceeding further
along the chain.
—CH2CH3 outranks —CH3
—CH(C
H(CH3)2 outranks —CH2CH2OH
—C(C,H,H)
—C(C,C,H)
—C(H,H,H)
Table 5.1 CIP Rules
(4) Evaluate substituents one by one.
Don't add atomic numbers within groups.
—C(C,H,H)
Table 5.1 CIP Rules
(5) An atom that is multiply-bonded to another
atom is considered to be replicated as a
substituent on that atom.
—CH2OH outranks —C(CH3)3
—CH=O
H=O outranks —CH2OH
—C(O,H,H)
—C(C,C,C)
—C(O,O,H)
—C(O,H,H)
See the table on the inside back cover of the text.
Question
Which one of the following groups has the
highest rank when precedence is assigned
according to the CahnCahn-IngoldIngold-Prelog system?
A) -CH2Cl
B) -CH=O
C) -C(CH3)3
D) CH2NH2
Question
Assign the configuration of the alkenes below
as Z or E, respectively.
A) E,E
B) E,Z
C) Z,Z
D) Z,E