Chapter 3
Structure and Stereochemistry of Alkanes
Nomenclature (3-3)
Ú Since the names of all organic compounds
are based on alkanes, a brief summary of
the rules of nomenclature is necessary.
Table 3-2 gives you the names of the first
20 common straight chain alkanes (also see
slide 79).
Ú Systematic Names or IUPAC names:
IUPAC: International Union of Pure and
Applied Chemistry
105
Ú Rule 1
Find the longest continuous chain of C
atoms and use the name of this chain as the
base name of the compound.
106
substituent
(alkyl group)
Name the alkyl (+ position) + the main chain
3-methylhexane or 4-methylhexane
107
Ú If there is a choice between 2 or more
chains of equal length, use the one with
the greater # of substituents, it simplifies
the name.
108
Ú Rule 2
number the longest chain beginning with
the end of the chain nearest a substituent.
You always want the positions of the
substituents to be as low as possible.
3
1
7
2
6
5
4 5
6
4
7
positions of
substituents:
3, 4, 5, 6 = 18
3
2
1
positions of
substituents:
2, 3, 4, 5 = 14
109
Rule 3
Name the substituents attached to the
longest chain as alkyl groups. Give the
location of each alkyl group using the
carbon # to which it is attached.
The name and the position must be
separated by a hyphen.
Use alphabetical order to name the
substituents, not the number of their
positions.
110
Rule 4 (multiple groups)
When 2 or more of the same substituents are
present, use prefixes di (2), tri (3), tetra (4), … to
avoid naming them more than once.
(prefix such as di-, tri- tetra- etc are not used to
determine the alphabetical order)
7
5
4
6
3
2
1
3-ethyl-2,4,5-trimethylheptane
111
Ú Other common alkyl groups:
3 carbons
CH3CH2CH2
propyl
4 carbons
CH3CH2CH2CH2
butyl
CH3
CH3CH2CH
sec-butyl
CH3
CH3CH
isopropyl
CH3
CH3CHCH2
isobutyl
CH3
CH3 C
CH3
tert-butyl
(t-butyl)
112
Ú Classification carbon atoms
H
H
R
R C
R C
R C
H
primary (1o )
carbon
R
secondary (2o )
carbon
R
tertiary (3o )
carbon
113
Ú Functional groups
When functional groups are present, the same
rules apply but the longest chain must include the
functional group (the carbon from chain) and the
functional group must be numbered.
the carbon atom bearing the functional
group is included in the longest carbon
chain
OH
O
114
Ú Reactions of alkanes (3-6)
Alkanes are fairly unreactive. However 3
common reactions are listed below.
– Combustion
CnH2n + 2 + O2
∆
n CO2 + (n + 1) H2O
115
– Cracking makes smaller alkanes from large ones.
Not specific, ie cannot predict which shorter alkanes
will be produced.
H2, ∆
C12H26
catalyst
+
C5H12
C7H16
116
– Halogenation
a more useful reaction. However it has the
tendency to give more than one product.
This reaction will be discussed further in
chapter 4.
117
Structure and conformation of Alkanes (3.7)
– In alkanes, all “C” atoms are hybridized sp3.
– With more than one “C” atom, different
arrangements are possible due to rotation.
– These arrangements formed by rotation along a
single bond are called:
Conformations
– And a specific conformation is caller a:
Conformer
118
Ú Newman Projections are normally
used to represent conformations along
C-C single bonds as seen below for
ethane
Figure 3-5
119
Ú There are many possible conformers of ethane.
While they all exist, one is very stable (lower
energy) while one is higher in energy (less
favoured)
Figure 3-7
120
Ú ө: dihedral angle (angle between C-H bond
of the front carbon and C-H bond of the
back carbon
Ú ө = Oo in eclipsed conformation (molecule in
a higher energy state)
Ú ө = 60o in staggered conformation (molecule
in a lower energy state)
Ú Because the energy is less for staggered
conformation, they are favoured
conformations.
121
Ú Increasing the carbon chain by one carbon
(propane) make the staggered conformation even
more favoured, since in the eclipsed conformer, a
stronger interaction (steric factors) exist between
the H and CH3.
Figure 3-9
122
Ú With more carbons on the chain, (ex:
butane) even more possibilities exist.
Let’s consider the C2-C3 bond:
Figure 3-10
123
Ú The energy associated with the rotation of
one conformer to another one is called:
torsional energy.
Figure 3-11
124
§ The anti-conformation is the lowest energy
conformation, followed by the gauche conformations
and the eclipsed ones, and the highest energy
conformation is always the fully eclipsed conformation.
§ Exercise:
Looking at C2-C3 bond of hexane draw the Newman
projections for the anti and gauche conformations
125
Ú Cycloalkanes (3-10)
Whether a cycloalkane exists or not
normally depends on the ring strain.
Typically, the ring strain is too large
on small rings for them to exist. We
will focus on 5 and 6 carbon in the
ring.
126
Cis-Trans Isomerism (3-11)
Ú Similar to alkenes, cycloalkanes can have cis
and trans geometric isomers
Example: 1,2-dimethyl cyclopentane
CH3 H
H H
CH3
H
trans
H3C
CH3
cis
Ú cis: both substituents on the same side of the
ring
trans: substituents on opposite sides of the
ring
127
Conformations of Cyclohexanes (3-13)
2 extreme conformations are possible because the ring
is not flat (respect bond angle)
Chair Conformation:
Figure 3-19
128
Ú Boat Conformation
Figure 3-20
129
Ú Chair conformation is much more
stable and usually preferred. This is
due to less steric interactions in its
conformation compared to the boat
conformer.
Figure 3-21
130
Ú To draw chair conformation:
– Draw two parallel lines slanted upward:
– Join the two parallel lines with a “V”
131
– In any cyclohexane ring there are 6 axial
positions. These are occupied by hydrogens or
by other singly bonded atoms. The axial bonds
are always vertical (up or down) following the
general direction the “V” points towards. The
other 6 positions are equatorial.
Figure 3-22
132
Ú There are 2 possible forms of the chair
conformation. They are caused by “ring
flip”. When flipping the chair, notice that
the axial position and equatorial position
change from one structure to the other.
133
Ú In going from one chair to the other, the
molecule will go through a series of 3
other conformations:
Figure 3-21
134
Conformation of mono-substituted
cyclohexanes (3-14)
Ú In mono-substituted cyclohexanes, the
favored conformation is normally the one
with the substituent at the equatorial
position. This is due to the steric factors
found in the axial position.
Figure 3-23
135
Ú These steric unfavourable interactions
when a group is axial are called: 1,3diaxial interactions.
Figure 3-26
136
Ú In other words, it is better to have the
substituent in an anti conformation
rather than in the gauche conformation.
Figure 3-24
Figure 3-25
137
Conformations of disubstituted
cyclohexanes (3-15)
Ú With 2 substituents present, the molecule
can be either cis or trans:
138
Ú Problem: Are the following cycloalkanes
cis or trans?
139
Ú With 2 substituents, it is recommended to write
both possible conformers via “ring flip”.
Ú The conformer with the least amount of steric
interactions is normally favored.
140
Ú If 2 substituents of unequal sizes are
present, the larger substituent usually
prefer to occupy the equatorial position.
141
§ Question:
For the molecule of cis-1-Bromo-2chlorocyclohexane, draw the two possible chair
conformations and indicate the most stable.
142
Ú Question:
For the following molecule, draw the most
stable chair conformation.
143
Study Problems
Ú 3-33
Which of the following structures represent
the same compound?
144
Ú 3-34
Draw the following compounds.
3-ethyloctane
cis-1-ethyl-4-methylcyclohexane
isobutylcyclopentane
145
3-44
Draw the two chair conformations of each of the
compounds below. Circle the most stable.
cis-1-ethyl-3-methyl
cyclohexane
trans-1-ethyl-4-methyl
cyclohexane
146
Ú 3-47
Draw Newman projection along the C3C4 bond to show the most stable
conformation of 3-ethyl-2,4,4-trimethyl
heptane.
147