About The Authors
Chapter 4
These Powerpoint Lecture Slides were created and prepared by Professor
William Tam and his wife Dr. Phillis Chang.
Nomenclature &
Conformations of
Alkanes & Cycloalkanes
Professor William Tam received his B.Sc. at the University of Hong Kong in
1990 and his Ph.D. at the University of Toronto (Canada) in 1995. He was an
NSERC postdoctoral fellow at the Imperial College (UK) and at Harvard
University (USA). He joined the Department of Chemistry at the University of
Guelph (Ontario, Canada) in 1998 and is currently a Full Professor and
Associate Chair in the department. Professor Tam has received several awards
in research and teaching, and according to Essential Science Indicators, he is
currently ranked as the Top 1% most cited Chemists worldwide. He has
published four books and over 80 scientific papers in top international journals
such as J. Am. Chem. Soc., Angew. Chem., Org. Lett., and J. Org. Chem.
Dr. Phillis Chang received her B.Sc. at New York University (USA) in 1994, her
M.Sc. and Ph.D. in 1997 and 2001 at the University of Guelph (Canada). She
lives in Guelph with her husband, William, and their son, Matthew.
Created by
Professor William Tam & Dr. Phillis Chang
Ch. 4 - 1
1. Introduction to Alkanes &
Cycloalkanes
Ch. 4 - 2
5
e.g.
6
Alkanes and cycloalkanes are
hydrocarbons in which all the carboncarbon (C–C) bonds are single bonds
3
4
1
2
hexane (C6H14)
Alkanes: CnH2n+2
Hydrocarbons that contain
C═C: Alkenes
Hydrocarbons that contain
C≡C: Alkynes
Cycloalkanes: CnH2n
e.g.
cyclohexane (C6H12)
Ch. 4 - 3
1A. Sources of Alkanes: Petroleum
Petroleum is the primary source of
alkanes. It is a complex mixture of
mostly alkanes and aromatic
hydrocarbons with small amounts of
oxygen-, nitrogen-, and sulfurcontaining compounds
Ch. 4 - 5
Ch. 4 - 4
Petroleum refining
● Distillation is the first step in
refining petroleum. Its components
are separated based on different
volatility
● More than 500 different compounds
are contained in petroleum
distillates boiling below 200oC
Ch. 4 - 6
Petroleum refining (Cont’d)
● The demand of gasoline is much
greater than that supplied by the
gasoline fraction of petroleum
● The fractions taken contain a
mixture of alkanes of similar boiling
points
● Mixture of alkanes can be used as
fuels, solvents, and lubricants
Gasoline
● Converting hydrocarbons from other
fractions of petroleum into gasoline
by “catalytic cracking”
catalysts
mixture of alkanes
(C12 and higher)
o
~ 500 C
highly branched
hydrocarbons
(C5 - C10)
Ch. 4 - 7
Gasoline (Cont’d)
CH3
CH3
CH3 C CH2 C CH3
CH3
H
2,2,4-Trimethylpentane (isooctane)
(C12H18)
● Isooctane burns very smoothly
(without knocking) in internal
combustion engines and is used as
one of the standards by which the
octane rating of gasoline is
established
Ch. 4 - 9
Typical Fractions Obtained by
Distillation of Petroleum
Boiling Range of
Fraction (oC)
# of Carbon
Atoms per
Molecule
Ch. 4 - 8
Gasoline (Cont’d)
isooctane
heptane
100
0
"octane
rating"
● e.g. a gasoline of a mixture:
87% isooctane and 13% heptane
Rated as 87-octane gasoline
Ch. 4 - 10
Typical Fractions Obtained by
Distillation of Petroleum
(Cont’d)
Use
Boiling Range of
Fraction (oC)
# of Carbon
Atoms per
Molecule
Use
Below 20
C1 – C4
Natural gas, bottled
gas, petrochemicals
250 – 400
C12 and higher
20 – 60
C5 – C6
Petroleum ether,
solvents
Gas oil, fuel oil, and
diesel oil
Nonvolatile liquids
C20 and higher
60 – 100
C6 – C7
Ligroin, solvents
40 – 200
C5 – C10
Gasoline (straightrun gasoline)
Refined mineral oil,
lubricating oil, and
grease
Nonvolatile solids
C20 and higher
175 – 325
C12 – C18
Kerosene and jet
fuel
Paraffin wax,
asphalt, and tar
Ch. 4 - 11
Ch. 4 - 12
2. Shapes of Alkanes
All carbon atoms in alkanes and
cycloalkanes are sp3 hybridized, and
they all have a tetrahedral geometry
“Straight-chain” (unbranched) alkanes
Butane
Pentane
CH3CH2CH2CH3
CH3CH2CH2CH2CH3
Even “straight-chain” alkanes are not
straight. They have a zigzag geometry
Ch. 4 - 13
Branched-chain alkanes
Isobutane
Ch. 4 - 14
Neopentane
CH3
CH3 CH CH3
CH3 C CH3
CH3
Butane and isobutane have the same
molecular formula (C4H10) but different
bond connectivities. Such compounds
are called constitutional isomers
CH3
Butane
Isobutane
Ch. 4 - 15
C4 and higher alkanes exist as
constitutional isomers. The number of
constitutional isomers increases rapidly
with the carbon number
Molecular # of Possible Molecular
Formula Const. Isomers Formula
C4H10
2
C9H20
# of Possible
Const. Isomers
35
C5H12
3
C10H22
75
C6H14
5
C20H42
366,319
C7H16
9
C40H82
62,481,801,147,341
C8H18
18
Ch. 4 - 17
Ch. 4 - 16
Constitutional isomers usually have
different physical properties
Hexane Isomers (C6H14)
Formula
M.P.
(oC)
-95
B.P.
(oC)
68.7
Density
(g/mL)
0.6594
Refractive
Index
1.3748
-153.7
60.3
0.6532
1.3714
-118
63.3
0.6643
1.3765
-128.8
58
0.6616
1.3750
-98
49.7
0.6492
1.3688
Ch. 4 - 18
3.
IUPAC Nomenclature of Alkanes,
Alkyl Halides, & Alcohols
One of the most commonly used
nomenclature systems that we use
today is based on the system and rules
developed by the International Union
of Pure and Applied Chemistry (IUPAC)
Although the IUPAC naming system is
now widely accepted among chemists,
common names (trivial names) of
some compounds are still widely used
by chemists and in commerce. Thus,
learning some of the common names
of frequently used chemicals and
compounds is still important
Fundamental Principle: Each different
compound shall have a unique name
Ch. 4 - 19
The ending for all the names of
alkanes is –ane
Ch. 4 - 20
Unbranched alkanes
Name
The names of most alkanes stem from
Greek and Latin
one
meth-
two
eth-
three
prop-
four
but-
five
pent-
Structure
Name
Structure
Methane CH4
Hexane
CH3(CH2)4CH3
Ethane
CH3CH3
Heptane CH3(CH2)5CH3
Propane
CH3CH2CH3
Octane
CH3(CH2)6CH3
Butane
CH3CH2CH2CH3
Nonane
CH3(CH2)7CH3
Pentane
CH3(CH2)3CH3
Decane
CH3(CH2)8CH3
Ch. 4 - 21
3A. Nomenclature of Unbranched
Alkyl Groups
Alkyl group
● Removal of one hydrogen atom
from an alkane
Ch. 4 - 22
Alkyl group (Cont’d)
● For an unbranched alkane, the
hydrogen atom that is removed is a
terminal hydrogen atom
CH3 H
CH3CH2 H
CH3CH2CH2 H
Methane
Ethane
Propane
CH3
Ch. 4 - 23
Methyl
(Me)
CH3CH2
Ethyl
(Et)
CH3CH2CH2
Propyl
(Pr)
Ch. 4 - 24
3B. Nomenclature of BranchedBranched-Chain
Alkanes
Rule
1. Use the longest continuous carbon
chain as parent name
7
6
5
4
3
6
CH3CH2CH2CH2CHCH3
5
4
3
2
NOT
1 CH3
2. Use the lowest number of the
substituent
3. Use the number obtained by
Rule 2 to designate the location of
the substituent
1
CH3CH2CH2CH2CHCH3
2CH2
Rule (Cont’d)
CH2
7
6
5
4
3
2
3
4
5
CH3CH2CH2CH2CHCH3
2CH2
CH3
(2-Ethylhexane)
(3-Methylheptane)
1
CH3CH2CH2CH2CHCH3
6 CH2
NOT
1 CH3
7 CH3
(3-Methylheptane)
(5-Methylheptane)
Ch. 4 - 25
Rule (Cont’d)
Ch. 4 - 26
Rule (Cont’d)
4. For two or more substituents, use
the lowest possible individual
numbers of the parent chain
The substitutents should be listed
alphabetically. In deciding
alphabetical order, disregard
multiplying prefix, such as “di”,
“tri” etc.
1
2
4
3
5
6
7
8
(6-Ethyl-2-methyloctane)
NOT
7
8
5
6
NOT
3
4
2
1
2
1
4
3
6
5
8
7
(3-Ethyl-7-methyloctane) (2-Methyl-6-ethyloctane)
Ch. 4 - 27
Rule (Cont’d)
Ch. 4 - 28
5. When two substituents are present
on the same carbon, use that
number twice
Rule (Cont’d)
6. For identical substituents, use
prefixes di-, tri-, tetra- and so on
6
1
2
3
4
5
6
7
5
4
3
2
1
(2,4-Dimethylhexane)
8
(4-Ethyl-4-methyloctane)
NOT
1
Ch. 4 - 29
7
6
5
4
3
2
1
(2,4,5-Trimethylheptane)
NOT
2
3
4
5
6
(3,5-Dimethylhexane)
1
2
3
4
5
6
7
(3,4,6-Trimethylheptane)
Ch. 4 - 30
Rule (Cont’d)
7. When two chains of equal length
compete for selection as parent
chain, choose the chain with the
greater number of substituents
7
6
5
4
1
2
3
4
2
3
Rule (Cont’d)
8. When branching first occurs at an
equal distance from either end of
the longest chain, choose the
name that gives the lower number
at the first point of difference
1
6
5
NOT
6
7
(2,3,5-Trimethyl4-propylheptane)
(only three substituents)
5
4
3
2
1
1
(2,3,5-Trimethylhexane)
NOT
2
3
4
5
(2,4,5-Trimethylhexane)
Ch. 4 - 31
Example 1
6
Ch. 4 - 32
Example 1 (Cont’d)
● Use the lowest numbering for
substituents
4
3
2
4
1
5
6
3
7
6
7
3
2
4
7
2
5
1
1
6
5
3
Ch. 4 - 33
3
2
1
7
● Substituents: two methyl groups
dimethyl
4
6
or
5
6
instead of
● Find the longest chain as parent
4
5
Example 1 (Cont’d)
2
7
1
Ch. 4 - 34
Example 2
● Complete name
4
3
5
2
6
7
1
(3,4-Dimethylheptane)
Ch. 4 - 35
Ch. 4 - 36
Example 2 (Cont’d)
● Find the longest chain as parent
6-carbon chain
8-carbon chain
Example 2 (Cont’d)
● Find the longest chain as parent
9-carbon chain
(correct!)
8-carbon chain
⇒ Nonane as parent
Ch. 4 - 37
Example 2 (Cont’d)
Ch. 4 - 38
● Use the lowest numbering for
substituents
7
8
3
9
5
2
1
3
6
Example 2 (Cont’d)
● Substituents
3,7-dimethyl
4-ethyl
2
1
5
instead of
4
8
9
7
4
9
5
7 6
2
1
3
6
4
(3,6,7)
(3,4,7)
Ch. 4 - 39
8
Example 2 (Cont’d)
● Substituents in alphabetical order
Ethyl before dimethyl
(recall Rule 4 – disregard “di”)
● Complete name
7
8
Ch. 4 - 40
3C. Nomenclature of Branched Alkyl
Groups
For alkanes with more than two carbon
atoms, more than one derived alkyl
group is possible
Three-carbon groups
9
5
2
1
3
6
4
(4-Ethyl-3,7-dimethylnonane)
Propyl
Ch. 4 - 41
Isopropyl
(or 1-methylethyl)
Ch. 4 - 42
Four-carbon groups
Butyl
A neopentyl group
Isobutyl
neopentyl
(2,2,-dimethylpropyl)
sec-butyl
(1-methylpropyl)
tert-butyl
(or 1,1-dimethylethyl)
Ch. 4 - 43
Example 1
Ch. 4 - 44
Example 1 (Cont’d)
● Find the longest chain as parent
(a)
(b)
6-carbon
chain
7-carbon
chain
(c)
Ch. 4 - 45
Example 1 (Cont’d)
(d)
8-carbon
chain
9-carbon
chain
Example 1 (Cont’d)
● Find the longest chain as parent
● Use the lowest numbering for
substituents
(d)
⇒ Nonane as parent
1
2
3 4
5 6
7 8
9
or
9
5,6
1
2
3 4
5 6
7 8
9
or
9
8
7 6
5 4
3 2
Ch. 4 - 46
8
7 6
5 4
3 2
1
4,5
(lower numbering)
1
⇒ Use 4,5
Ch. 4 - 47
Ch. 4 - 48
Example 1 (Cont’d)
Example 1 (Cont’d)
● Substituents
Isopropyl
tert-butyl
● Alphabetical order of substituents
tert-butyl before isopropyl
● Complete name
9
8
7 6
5 4
3 2
1
9
⇒ 4-isopropyl and 5-tert-butyl
8
7 6
5 4
3 2
1
5-tert-Butyl-4-isopropylnonane
Ch. 4 - 49
Example 2
Ch. 4 - 50
Example 2 (Cont’d)
● Find the longest chain as parent
(a)
(b)
8-carbon
chain
⇒ Octane as parent
(c)
10-carbon
chain
Ch. 4 - 51
Example 2 (Cont’d)
9-carbon
chain
Ch. 4 - 52
Example 2 (Cont’d)
● Use the lowest numbering for
substituents
1
2
3 4
5 6
7 8
9
10
5,6
1
2
3 4
5 6
7 8
9
10
or
or
10 9
8 7
6 5
4 3
2
1
⇒ Determined using
the next Rules
Ch. 4 - 53
10 9
5,6
8 7
6 5
4 3
2
1
Ch. 4 - 54
Example 2 (Cont’d)
Example 2 (Cont’d)
● Since sec-butyl takes precedence
over neopentyl
5-sec-butyl and 6-neopentyl
● Substituents
sec-butyl
Neopentyl
● Complete name
But is it
● 5-sec-butyl and 6-neopentyl or
● 5-neopentyl and 6-sec-butyl ?
10 9
4 3
2
1
CH3
CH3 CH CH2 CH3
Rules
● Halogens are treated as
substituents (as prefix)
F: fluoro
Br: bromo
Cl: chloro
I: iodo
● Similar rules as alkyl substituents
2o hydrogen atoms
Ch. 4 - 57
3
Ch. 4 - 58
3F. Nomenclature of Alcohols
Examples
4
Ch. 4 - 56
3E. Nomenclature of Alkyl Halides
1o hydrogen atoms
3o hydrogen atoms
6 5
5-sec-Butyl-6-neopentyldecane
Ch. 4 - 55
3D. Classification of Hydrogen Atoms
8 7
2
1
Cl
Br
2-Bromo-1-chlorobutane
Cl
1
2
Cl
3 4
5
IUPAC substitutive nomenclature:
a name may have as many as four
features
● Locants, prefixes, parent compound,
and suffixes
6
CH3
1,4-Dichloro-3-methylhexane
Ch. 4 - 59
6
5 4
3 2
1
OH
4-Methyl-1-hexanol
Ch. 4 - 60
Rules
● Select the longest continuous carbon
chain to which the hydroxyl is directly
attached. Change the name of the
alkane corresponding to this chain by
dropping the final –e and adding the
suffix –ol
● Number the longest continuous carbon
chain so as to give the carbon atom
bearing the hydroxyl group the lower
number. Indicate the position of the
hydroxyl group by using this number as
a locant
Ch. 4 - 61
Example 4
Examples
OH
2
3
OH
4
1
2-Propanol
(isopropyl alcohol)
5
1
3 2
OH
OH
1,2,3-Butanetriol
4 3
2
1
OH
4-Methyl-1-pentanol
(or 4-Methylpentan-1-ol)
(NOT 2-Methyl-5-pentanol)
Ch. 4 - 62
Example 4 (Cont’d)
● Find the longest chain as parent
8
7
6
5
OH
4
3
2
1
or
OH
1
2
Example 4 (Cont’d)
● Use the lowest numbering for the
carbon bearing the OH group
7-carbon chain
containing the
OH group
⇒ Heptane as parent
Ch. 4 - 63
2
3
OH
4
5
6
7
or
2,3
(lower numbering)
7
6
5
OH
4
3
2
1
5,6
Ch. 4 - 64
Example 4 (Cont’d)
● Parent and suffix
2-Heptanol
1
1
7
6
OH
Longest chain but
does not contain
the OH group
5
4
3
● Substituents
Propyl
2
3
4
5
6
7
OH
● Complete name
3-Propyl-2-heptanol
⇒ Use 2,3
Ch. 4 - 65
Ch. 4 - 66
4. How to Name Cycloalkanes
Substituted cycloalkanes
4A. Monocyclic Compounds
Cycloalkanes with only one ring
● Attach the prefix cycloH2C CH2 =
C
H2
Cyclopropane
H2C
H2C
CH2
Isopropylcyclopropane
=
CH2
C
H2
Cyclopentane
tert-Butylcyclopentane
Ch. 4 - 67
Methylcyclopropane
Example 1
Ch. 4 - 68
Example 2
Br
3
4
1-Ethyl-3-methylcyclopentane
2
1
5
5
6
NOT
NOT
NOT
4
3
5
5
1
2
1
4
1-Ethyl-4-methylcyclopentane
2
5
3-Ethyl-1-methylcyclopentane
Example 3
4
5
6
2
1
OH
4-Ethyl-3-methyl
cyclohexanol
1
4
3
2
1-Bromo-3-ethyl-4-methyl
cyclohexane
Ch. 4 - 70
3
4-Bromo-2-ethyl-1-methyl
cyclohexane
(lowest numbers of substituents
are 1,2,4 not 1,3,4)
Ch. 4 - 69
1
3
2
Br
6
3
4
Cycloalkylalkanes
● When a single ring system is
attached to a single chain with a
greater number of carbon atoms
1-Cyclobutylpentane
NOT
1
6
2
5
3
4
OH
1-Ethyl-2-methyl
cyclohexan-4-ol
● When more than one ring system
is attached to a single chain
(the carbon bearing the OH should have the lowest
numbering, even though 1,2,4 is lower than 1,3,4)
Ch. 4 - 71
1,3-Dicyclohexylpropane
Ch. 4 - 72
4B. Bicyclic Compounds
Example (Cont’d)
Between the two bridgeheads
● Two-carbon bridge on the left
● Two-carbon bridge on the right
● One-carbon bridge in the middle
Complete name
● Bicyclo[2.2.1]heptane
Bicycloalkanes
● Alkanes containing two fused or
bridged rings
Total # of carbons = 7
● Bicycloheptane
Bridgehead
Ch. 4 - 73
Other examples
9
8
7
5. Nomenclature of Alkenes &
Cycloalkenes
2
1
3
4
6
5
7-Methylbicyclo[4.3.0]nonane
8
7
4
5
3
1
6
Rule
1. Select the longest chain that
contains C=C as the parent name
and change the name ending of
the alkane of identical length from
–ane to
–ene
2
1-Isopropylbicyclo[2.2.2]octane
Ch. 4 - 74
Ch. 4 - 75
Rule
2. Number the chain so as to include
both carbon atoms of C=C, and
begin numbering at the end of the
chain nearer C=C. Assign the
location of C=C by using the
number of the first atom of C=C
as the prefix. The locant for the
alkene suffix may precede the
parent name or be placed
immediately before the suffix
Ch. 4 - 77
Ch. 4 - 76
● Examples
1
2
3
4
CH2 CHCH2CH3
1-Butene
(not 3-Butene)
1
2
3
4
5
6
CH3CH CHCH2CH2CH3
2-Hexene
(not 4-Hexene)
Ch. 4 - 78
Rule
3. Indicate the locations of the
substituent groups by the numbers
of the carbon atoms to which they
are attached
● Examples
● Examples (Cont’d)
4
2
1
2,5-Dimethyl-2-hexene
4
3
6
5
3
3
NOT
2
1
2
4
5
1
6
2-Methyl-2-butene
(not 3-Methyl-2-butene)
2,5-Dimethyl-4-hexene
Ch. 4 - 79
Rule
4. Number substituted cycloalkenes
in the way that gives the carbon
atoms of C=C the 1 and 2
positions and that also gives the
substituent groups the lower
numbers at the first point of
difference
Ch. 4 - 80
● Example
1
6
5
2
4
3
3,5-Dimethylcyclohexene
2
3
NOT
4
1
5
6
4,6-Dimethylcyclohexene
Ch. 4 - 81
Rule
5. Name compounds containing a
C=C and an alcohol group as
alkenols (or cycloalkenols) and
give the alcohol carbon the lower
number
OH
● Examples
6
1
5
4
Ch. 4 - 82
● Examples (Cont’d)
OH
5
4
3
2
1
4-Methyl-3-penten-2-ol
(or 4-Methylpent-3-en-2-ol)
2
3
2-Methyl-2-cyclohexen-1-ol
(or 2-Methylcyclohex-2-en-1-ol)
Ch. 4 - 83
Ch. 4 - 84
Rule
6. Vinyl group & allyl group
Rule
7. Cis vs. Trans
● Cis: two identical or substantial
groups on the same side of C=C
● Trans: two identical or
substantial groups on the
opposite side of C=C
Vinyl group
Allyl group
ethenyl
prop-2-en-1-yl
OH
6
1
5
Ethenylcyclopropane
(or Vinylcyclopropane)
4
2
Cl
3
3-(Prop-2-en-1-yl)
cyclohexan-1-ol
(or 3-Allylcyclohexanol)
Cl
Cl
Cl
cis-1,2-Dichloroethene
trans-1,2-Dichloroethene
Ch. 4 - 85
Example
Ch. 4 - 86
Example (Cont’d)
(b)
(a)
6
7
5
6
4
2
3
1
5
4
3
(c)
2
5
4
2
3
1
1
3
4
5
Ch. 4 - 87
Example (Cont’d)
● Complete name
3
4
6
7
Ch. 4 - 88
6. Nomenclature of Alkynes
Alkynes are named in much the same
way as alkenes, but ending name with
–yne instead of –ene
Examples
2
1
1
(d)
6
7
2
5
6
7
4-tert-Butyl-2-methyl-1-heptene
6
7
4
3
5
2-Heptyne
Ch. 4 - 89
2
2
1
1
3
Br
4
4-Bromo-1-butyne
Ch. 4 - 90
Examples (Cont’d)
OH group has priority over C≡C
4
2
3
4
5
6
I
Br
7 8
9
1
3
2
1
NOT
OH
2
3
1
OH
4
3-Butyn-1-ol
10
OH
9-Bromo-7-iodo-6-isopropyl-8-methyl-3-decyne
1
2
5
6
OH
7
4
3
8
8
7
6
4
3
2
5
1
NOT
2-Methyl-5-octyn-2-ol
Ch. 4 - 91
Ch. 4 - 92
7. Physical Properties of
Alkanes & Cycloalkanes
C6H14 Isomer
Boiling Point (oC)
68.7
Boiling points & melting points
63.3
60.3
58
49.7
Ch. 4 - 93
8.
Physical Constants of Cycloalkanes
# of C
Atoms
Name
bp (oC) mp (oC) Density
Ch. 4 - 94
Refractive
Index
3
Cyclopropane
-33
-126.6
-
-
4
Cyclobutane
13
-90
-
1.4260
5
Cyclopentane
49
-94
0.751
1.4064
6
Cyclohexane
81
6.5
0.779
1.4266
7
Cycloheptane
118.5
-12
0.811
1.4449
8
Cyclooctane
149
13.5
0.834
Sigma Bonds & Bond Rotation
Two groups bonded by a single bond
can undergo rotation about that bond
with respect to each other
●
●
Ch. 4 - 95
●
Conformations – temporary molecular
shapes result from a rotation about a single
bond
Conformer – each possible structure of
conformation
Conformational analysis – analysis of
energy changes occur as a molecule
undergoes rotations about single bonds
Ch. 4 - 96
8B. How to Do a Conformational Analysis
8A. Newman Projections
Me
H
H Sawhorse formula
OH
Cl
Et
Look from this
direction
Me
H
Cl
Et
front carbon
H
combine Me
OH
back carbon
H
H
Look from this
direction
φ1 = 60o
a
Cl
Et
OH
Newman Projection
H
H
φ2 = 180o
H
b
H
Hc
H
staggered
confirmation
of ethane
Ch. 4 - 97
60o
CH3
180o
CH3
CH3
0o CH3
CH3
Look from this
direction
CH3
anti
gauche
Ch. 4 - 98
φ = 0o
eclipsed
HH
H
H
H
H
eclipsed
confirmation
of ethane
Ch. 4 - 99
Ch. 4 - 100
9. Conformational Analysis of
Butane
H
H
H
H
Me
H
H
Sawhorse formula
Ch. 4 - 101
Me
Me
H
H
Me
New Projection
formula
Ch. 4 - 102
CH3
H
H
H
CH3
anti conformer
(I)
(lowest energy)
H
CH3
CH3
CH3
H
H
H
H
H
H
eclipsed conformer
(II)
gauche conformer
(III)
= CH3 on front carbon
H
CH3
H
rotates 60o clockwise
H
H
H
CH3
eclipsed conformer
(VI)
CH3
H
CH3
H
H
H
gauche conformer
(V)
CH3
H3C
CH3
H
H
H
H
H
eclipsed conformer
(IV)
(highest energy)
Ch. 4 - 103
10. The Relative Stabilities of
Cycloalkanes: Ring Strain
Ch. 4 - 104
10
10A.
A.
H
●
Angle strain – result of deviation from
ideal bond angles caused by inherent
structural constraints
Torsional strain – result of dispersion
forces that cannot be relieved due to
restricted conformational mobility
H
θ
Cycloalkanes do not have the same
relative stability due to ring strain
Ring strain comprises:
●
Cyclopropane
H
H
H
sp3 hybridized carbon
(normal tetrahedral
bond angle is 109.5o)
H
Internal bond angle (θ) ~60o (~49.5o
deviated from the ideal tetrahedral
angle)
Ch. 4 - 105
Ch. 4 - 106
10
10B.
B.
H
H
Cyclobutane
H
θ
H
H
H
H
H
Ch. 4 - 107
Internal bond angle (θ) ~88o (~21o
deviated from the normal 109.5o
tetrahedral angle)
Ch. 4 - 108
10
10C.
C.
Cyclobutane ring is not planar but is
slightly folded.
Cyclopentane
H
H
H
HH
If cyclobutane ring were planar, the
angle strain would be somewhat less
(the internal angles would be 90o
instead of 88o), but torsional strain
would be considerably larger because
all eight C–H bonds would be eclipsed
HH
H
H
H
If cyclopentane were planar, θ ~108o, very close
to the normal tetrahedral angle of 109.5o
However, planarity would introduce considerable
torsional strain (i.e. 10 C–H bonds eclipsed)
Therefore cyclopentane has a slightly bent
conformation
Ch. 4 - 109
11. Conformations of Cyclohexane:
The Chair & the Boat
6
3D
3
H
5
4
H
1
H
5
3
H
H
4
The boat conformer of cyclohexane is
less stable (higher energy) than the
chair form due to
● Eclipsed conformation
● 1,4-flagpole interactions
(boat form)
(less stable)
H
2
6
H
5
1
(chair form)
(more stable)
H
3
6
2
1
2
4
5
Ch. 4 - 110
H
H
6
HH
H
H
4
4
H
3
H
H
2
1
H H
1
HH
H
(eclipsed)
Ch. 4 - 111
Ch. 4 - 112
Energy diagram
(twist boat)
The twist boat conformation has a
lower energy than the pure boat
conformation, but is not as stable as
the chair conformation
Ch. 4 - 113
Ch. 4 - 114
12. Substituted Cyclohexanes: Axial
& Equatorial Hydrogen Atoms
H
H
H
G
H
G
H
G
H
H
Ch. 4 - 115
1
CH3
CH3
cis-1,2-Dimethyl
cyclopropane
Cl
Cl
H
H
G
% of Equatorial
% of Axial
F
60
40
CH3
95
5
iPr
97
3
tBu
> 99.99
< 0.01
Trans-1,4-Disubstituted Cyclohexanes
CH3
H
H
trans-Diaxial
H
H
cis-1,2-Dichloro
cyclobutane
Cl
trans-1,2-Dichloro
cyclobutane
H
ring
H3C
CH3
flip
CH3
H
Ch. 4 - 118
13
13A.
A.Cis
Cis--Trans Isomerism & Conformation
Structures of Cyclohexanes
CH3
CH3
H
trans-1,2-Dimethyl
cyclopropane
Cl
(axial) G
At 25oC
13. Disubstituted Cycloalkanes
Cis-Trans Isomerism
H
Ch. 4 - 116
(equatorial)
The larger the G group, the more
severe the 1,3-diaxial interaction and
shifting the equilibrium from the axialG chair form to the equatorial-G chair
form
Ch. 4 - 117
H
(axial G)
G
(less stable)
G
G
H
H
(equatorial G)
(more stable)
The chair conformation with axial G is
less stable due to 1,3-diaxial
interaction
1,3-diaxial interaction
3
H
H
H
H
H
H
Axial hydrogen atoms in chair form
H
Substituted cyclohexane
● Two different chair forms
Equatorial hydrogen atoms in chair
H
form
H
H
trans-Diequatorial
H
Ch. 4 - 119
Ch. 4 - 120
Upper bond
H
trans-Dimethyl
CH3
H3C
Cis-1,4-Disubstituted Cyclohexanes
cyclohexane
CH3 chair-chair
CH3
ring
H
H
flip
H
H3C
Lower bond
H
Upper-lower bonds means the groups
are trans
H
Equatorial-axial
Axial-equatorial
Ch. 4 - 121
Cis-1-tert-Butyl-4-methylcyclohexane
H3C
H3C
Trans-1,3-Disubstituted Cyclohexanes
(ax)
CH3
ring
CH3
Ch. 4 - 122
H3C
CH3
H3C
CH3
CH3
flip
ring
(eq)
H3C
H
H
(more stable
because large
group is
equatorial)
(less stable
because large
group is
axial)
H
H
flip
CH3
(eq)
CH3
(ax)
trans-1,3-Dimethylcyclohexane
Ch. 4 - 123
Trans-1-tert-Butyl-3-methylcyclohexane
Ch. 4 - 124
Cis-1,3-Disubstituted Cyclohexanes
H3C
CH3
H3C
H3C
H3C
H
ring
CH3
CH3
flip
H
CH3
CH3
CH3
(more stable
because large
group is
equatorial)
CH3
(less stable
because large
group is
axial)
Ch. 4 - 125
(more stable)
ring
flip
H
H
CH3
CH3
(less stable)
Ch. 4 - 126
Trans-1,2-Disubstituted Cyclohexanes
(eq)
CH3
CH3
(eq)
diequatorial
(much more stable)
Cis-1,2-Disubstituted Cyclohexane
(ax)
CH3
ring
(ax)
CH3
(ax)
CH3
ring
(ax)
CH3
flip
diaxial
(much less stable)
CH3
(eq)
CH3
(eq)
flip
(equatorial-axial)
(axial-equatorial)
cis-1,2-Dimethylcyclohexane
trans-1,2-Dimethylcyclohexane
(equal energy and equally
populated conformations)
Ch. 4 - 127
Ch. 4 - 128
14. Bicyclic & Polycyclic Alkanes
Decalin
(Bicyclo[4.4.0]decane)
H
H
H
cis-Decalin
Adamantane
Cubane
Prismane
H
trans-Decalin
H
H
H
C60 (Buckminsterfullerene)
Ch. 4 - 129
H
16. Synthesis of Alkanes and
Cycloalkanes
Ch. 4 - 130
Examples
16
16A.
A.Hydrogenation
Hydrogenation of Alkenes & Alkynes
H2
Pt, Pd or Ni
solvent
heat and pressure
Ni
+ H2
H H
C C
EtOH
o
H H
25 C, 50 atm.
+
H2
Pd
H
EtOH
H
o
25 C, 1 atm.
2H2
Pt, Pd or Ni
solvent
heat and pressure
H H
C C
H H
Ch. 4 - 131
H
Pd
EtOAc
+ 2 H2
o
65 C, 1 atm.
H
H
H
Ch. 4 - 132
17. How to Gain Structural Information
from Molecular Formulas & Index
of Hydrogen Deficiency
● Saturated acyclic alkanes: CnH2n+2
Index of hydrogen deficiency (IHD)
●
●
Index of hydrogen deficiency (Cont’d)
The difference in the number of pairs of
hydrogen atoms between the
compound under study and an acyclic
alkane having the same number of
carbons
● Each double bond on ring:
2 hydrogens less
● Each double bond on ring provides
one unit of hydrogen deficiency
Also known as “degree of unsaturation”
or “double-bond equivalence” (DBE)
Ch. 4 - 133
e.g.
Hexane: C6H14
Examples
C6H12
and
1-Hexene
Ch. 4 - 134
IHD = 2
IHD = 3
IHD = 2
IHD = 4
Cycloheane
C6H14
C6H12
–
Index of hydrogen
=
deficiency (IHD)
H2
= one pair of H2
=1
Ch. 4 - 135
16
16A.
A.Compounds
Compounds Containing Halogen,
Oxygen, or Nitrogen
For compounds containing
● Halogen – count halogen atoms as
though they were hydrogen atoms
● Oxygen – ignore oxygen atoms
and calculate IHD from the
remainder of the formula
● Nitrogen – subtract one hydrogen
for each nitrogen atom and ignore
nitrogen atoms
Ch. 4 - 137
Ch. 4 - 136
Example 1:
IHD of C4H6Cl2
● Count Cl as H
C4H10
C4H6Cl2 ⇒ C4H8
– C4H8
● A C4 acyclic alkane:
H2
C4H2(4)+2 = C4H10
IHD of C4H6Cl2 = one pair of H2 = 1
● Possible structures
Cl
Cl
Cl or
Cl
or
... etc.
Cl
Cl
Ch. 4 - 138
Example 2:
IHD of C5H8O
● Ignore oxygen
C5H12
C5H8O ⇒ C5H8
– C5H8
● A C5 acyclic alkane:
H4
C5H2(5)+2 = C5H12
IHD of C4H6Cl2 = two pair of H2 = 2
● Possible structures
Example 3:
IHD of C5H7N
● Subtract 1 H for each N
C5H12
C5H7N ⇒ C5H6
– C5H6
● A C5 acyclic alkane:
H6
C5H2(5)+2 = C5H12
IHD of C4H6Cl2 = three pair of H2 = 3
● Possible structures
O
OH
or
OH or
... etc.
Ch. 4 - 139
END OF CHAPTER 4 Ch. 4 - 141
N
CH3
or
C N
... etc.
Ch. 4 - 140