Basics of Organic Chemistry
 Alkenes and Alkynes
 Nomenclature
 Isomerism I
 Constitutional isomers
 Stereoisomers
 Cis-Trans isomers
Chapter 13 Supplement
1
Nomenclature: Alkenes
# of Cs
1
2
3
4
5
6
7
8
9
10
MF
Name
C 2H 4
C 3H 6
C 4H 8
C5H10
C6H12
C7H14
C8H16
C9H18
C10H20
ethene (ethylene)
propene
butene
pentene
hexene
Plant
heptene
ripening
octene
hormone
nonene
decene
General formula: CnH2n
Chapter 13
2
Nomenclature: Alkynes
# of Cs
1
2
3
4
5
6
7
8
9
10
MF
Name
C 2H 2
C 3H 4
C 4H 6
C 5H 8
C6H10
C7H12
C8H14
C9H16
C10H18
ethyne (acetylene)
propyne
butyne
pentyne
hexyne
heptyne
Important
octyne
organic
nonyne
synthesis
decyne
starting
material
General formula: CnH2n-2
Chapter 13
3
Nomenclature: IUPAC Rules (very similar to alkane’s)
The main chain should contain the double (or triple)
bond. It is numbered to give the double bond the lowest
number. In addition, if the molecule has more than 1
double bond, use prefix di, tri or tetra-ene (or yne).
e.g.
2,5-dimethyl-2-heptene
2,8-dimethyl-3,6-decadiene
Chapter 13
4
IUPAC Rules (very similar to alkane’s)
Cycloalkenes are numbered to give the double bond 1
and 2 and then the substituents the lowest number.
e.g.
3-methylcyclopentene
1-sec-butyl-3-methyl-1,2,4,5-hexadiene
Chapter 13
5
In-Class Practice
Name the following compounds.
2-methylhepta-1,3,5-triene
6
In-Class Practice
What is wrong with the following names? Give a better
name and structure for each compound below.
2,2,4-trimethylpentene
2,4-dimethyl-1-pentene
4,7-diethyl-5-(2-methylpropyl)decene
4,7-diethyl-5-(2-methylpropyl)-3-decene
7
In-Class Practice
What is wrong with the following names? Give a better
name and structure for each compound below.
4-chloro-3,6-dimethyl-4-heptene
4-chloro-2,5-dimethyl-3-heptene
2-propyl-4-ethyl-4-methyl-7-heptyne
4-ethyl-4-methyl-6-propyl-1-heptyne
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Isomers
 Isomers are different molecules with the same molecular
formula
 Many types of isomers exist
 Example
Consider two compounds with molecular formula C2H6O
These compounds cannot be distinguished based on molecular
formula; however they have different structures
The two compounds differ in the connectivity of their atoms
Chapter 11 & 13 Supplement
9
In-Class Practice
1. Find the molecular formulas for the following compounds.
a)
b)
c)
C10H22
2. Find the molecular formulas for the following compounds.
a)
O
b)
c)
OH
C4H10O
OH
10
Isomerism: Constitutional Isomers and Stereoisomers
Chapter 11 & 13 Supplement
11
Constitutional Isomers
 They are different compounds that have the same
molecular formula but different connectivity of atoms
 They often differ in physical properties (e.g. boiling
point, melting point, density) and chemical properties
Chapter 11 & 13 Supplement
12
More examples
Chapter 11 & 13 Supplement
13
More examples?
 The number of constitutional isomers possible for a
given molecular formula increases rapidly with the
number of carbons
14
Multiple Choice Questions
1. How many constitutional isomers are possible for the
formula C6H14?
a) two
b) three
c) four
d) five
e) six
2. Which of the following pairs are NOT constitutional
isomers?
Cl
a)
b)
Cl
Cl
and
c)
d)
and
and
Cl
and
Cl
Cl
Cl
Cl
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Multiple Choice Questions
Which compound is not an isomer of the others?
a) I
b) II
c) III
d) IV
e) all of the above are isomers
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Stereoisomers
 They are isomers with the same molecular
formula and same connectivity of atoms but
different arrangement of atoms in space
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Cis-trans Stereoisomers (Diastereomers)
 If two identical groups occur on the same side of the
double bond the compound is cis
 If they are on opposite sides the compound is trans
How about Cl2C=CH2?
Chapter 11 & 13 Supplement
18
Cis - Trans Relationship in Alkenes
HH H
H
H
H
H
HH
isomerizes
H
H
H
H
isomerizes
Chapter 13
H
HH
19
Cis - Trans Isomerization (Producing cis-Retinal)
-carotene
FROM
DIET
CH2 OH
retinol
(Vitamin A)
enzymes
20
11-cis-retinal
O
H
Vision and Cis - Trans Isomerization
11-trans-retinal
11-cis-retinal
light
N
N
OPSIN
OPSIN
rhodopsin
metarhodopsin-II
retinal
isomerism
+ light
H
H
O
O
OPSIN
OPSIN
21
Determining Cis - Trans Relationship
C When all substituents are different, it
A
B
D
is difficult or ambiguous to determine
the cis or trans designation.
e.g.
H3C
F
OH
CH3CH2
NH2
(CH3)2CH
CH2CH3
CH3
22
The (E) - (Z) System for Alkene Diastereomers
A
B
C Step 1: Determine the priority of A, B, C, D.
Step 2: If two higher priority groups are on
the same side, it is “Z” (cis).
D
Otherwise it is “E” (trans).
The Cahn-Ingold-Prelog system of determining priority.
1. Priority is first assigned
according to atomic
number. Small is low.
4
2
F
NH2
H3C
1
OH
3
An (E) compound
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The (E) - (Z) System for Alkene Diastereomers
The Cahn-Ingold-Prelog system of determining priority.
2. Priority is assigned at the first point of difference.
2
CH3CH2
(CH3)2CH
1
2
CH2CH3
CH3
3
(E)-3-isopropyl-4-methyl-3-hexene
Or 3-isopropyl-4-methyl-(E)-3-hexene
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Examples
Cl
e.g.
Br
I
9-bromo-6-chloro-3,8-diethyl-5-iodo-9-methyl-(E)-5undecene
e.g.
I
5-ethyl-2-iodo-3-isopropyl-4-methyl-(Z)-2-octene
25
In-Class Practice
Name the following compounds
4-ethyl-5-methyl-(E)-3-heptene
Same group on double bond, no (E) or (Z)
2-methyl-2-pentene
2-(E), 4-(E)-6-methylocta-2,4-diene
2-(E),6-(E)-5,6,8-trimethyl-2,6-decadiene
26
In-Class Practice
Determine if the C=C bonds in
the following compounds are (E) or (Z)
Cl
HO
(E)
O
OH
No E or Z.
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