CH221 CLASS 6
CHAPTER 3: ORGANIC COMPOUNDS: ALKANES AND CYCLOALKANES,
CONTINUED
Synopsis. This class considers some of the chemical characteristics of alkanes
and cycloalkanes, including cis/trans isomerism in disubstituted cycloalkanes.
Occurrence
Alkanes, along with cycloalkanes and aromatic hydrocarbons, are the major
constituents of petroleum, which is the basis of the organic chemical industry of
most nations. Some branched alkanes are also secreted by animals as
semiochemicals or pheromones (signaling chemicals), an example being 13,23dimethylpentatridecane (CH3(CH2)11CH(CH3)(CH2)9CH(CH3)(CH2)11CH3), the sex
pheromone of the tsetse fly.
Properties of Alkanes
An older name for the alkanes is the paraffins, a word derived from the Latin
parum affinis, meaning “little affinity”. This gives us a clue to the chemical
behavior of alkanes: they are generally unreactive and are inert toward most
common laboratory reagents, under normal laboratory conditions. However, they
do undergo three types of reactions, which are mainly of commercial importance,
as described below.
Combustion
The lower alkanes (methane – butane) are readily combustible, and because of
their high enthalpies of combustion, they are valuable fuels.
CH4 + O2
CO2 + 2H2O;
H = - 890 kJ mol-1
Branched alkanes, such as isooctane (2,2,4-trimethylpentane) are valuable fuels
for the internal combustion engine.
Cracking and Reforming
Cracking is a form of pyrolysis (thermal decomposition in the absence of air).
Higher alkanes of the kerosene and gas oil petroleum fractions (~C 12 – C20) are
cracked in petroleum refineries to give the commercially very valuable alkenes
ethene and propene, as well as lower alkanes:
~ 600oC
C12H26
C3H6 + 4C2H4 + CH4
catalyst
Cracking can be done without a catalyst, but requires higher temperatures (e.g. ~
900oC).
Reforming refers to industrially performed rearrangements of alkanes, especially
straight chain alkanes of 6 – 8 carbon chain length, to give commercially
important products.
Cyclohexane
~500oC
CH3(CH2)4CH3
Hexane
+ H2
catalyst
~ 500oC
catalyst
+ H2
Benzene
CH3
~ 500oC
CH3(CH2)6CH3
Octane
CH3
catalyst
C
CH3
CH2
CH
CH3
CH3
2,2,4-trimethylpentane
Halogenation
Alkanes react with halogens, in the presence of light, via free radical chain
reactions (see textbook, section 5.3):
h
CH4 + Cl2
CH3Cl, CH2Cl2, CHCl3 and CCl4 + HCl
Alkyl groups in other molecules behave in a similar manner, although for
bromination, N-bromosuccinimide (NBS) (in the presence of a peroxide initiator)
is a better reagent than bromine. The order of reactivity is –CH (3o) > CH2 (2o) >
CH3 (1o) > alkene or aromatic CH, as illustrated below:
Br
CH2CH3
CHCH3
NBS,
peroxide
mainly
Melting Points and Boiling Points
Straight chain alkanes show regular increases in both melting point and boiling
point as molecular weight increases, as a result of increasing strength of London
dispersion forces between molecules:
Increased branching lowers an alkane’s boiling point, because branching
prevents efficient packing, thus lowering the overall London dispersion forces.
Compare octane, of b.p. 125.7 oC, with 2,2,4-trimethylpentane (isooctane), of b.p.
99.3oC.
Cycloalkanes
Cycloalkanes or alicyclic hydrocarbons possess rings of carbon atoms, from
cyclopropane (C3) to very large (macrocyclic) rings. The rings can also be fused
or bridged:
Cyclohexane
Decalin
(Bicyclo [4.4.0] decane)
Spiro [5.5] undecane
Bicyclo [2.2.2] octane
a bridged ring
fused rings
Note the nomenclature in the above examples and note also the convenient, but
inaccurate, way of drawing the rings as planar conformations: it will be seen later
that these rings are actually puckered in their most favorable conformations
(Textbook, chapter 4).
Occurrence
Alicyclic rings are of widespread occurrence in nature: for example as
cycloalkanes in petroleum and as rings of various sizes in a host of natural
products, such as carotenoids, prostaglandins, steroids and terpenoids. Some
examples are given below.
CH3
CH3
CH3
CH3
H
H
COOH
Chrysanthemic acid, a natural
insecticide found in
chrysanthemums
-pinene, a terpenoid
HO
Cholesterol, a steroid
C8H17
Chemical and Physical Behavior
Chemical reactivity of cycloalkanes, except cyclopropane and cyclobutane, is
similar to alkanes. Boiling points (but not melting points) behave in a similar
manner to alkanes, with increasing molecular weight.
Naming Alicyclic Compounds
1. Nomenclature for alicyclic hydrocarbons is the same as for the
corresponding open chain hydrocarbons, but using the prefix cyclo.
E.g.
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
or
or
cyclopropane
cyclopentane
CH2
CH2
CH
CH2
CH
or
CH2
cyclohexene
2. Substituents are named and their positions are indicated by numbers, with
seniority being the same as for open chain systems.
E.g.
CH3
CH3
CH3
C
CH2
CH2
or
CH2 CH2
1, 1-dimethylcyclopentane
CH3
CH3
CH3
CH
CH2
CH2
CH2
CH
or
OH
OH
CH2
3-methylcyclohexanol
Note that if a hydrocarbon substituent has more carbon atoms than the ring, the
name is based upon that of a cycloalkyl-substituted alkane:
CH2CH2CH2CH3
1-cyclopropylbutane, NOT 1-butylcyclopropane
3. Multiple bonds occupy positions 1 and 2, unless there is a more senior
substituent present.
E.g.
O
1
1
2
2
3
1,3-cyclohexadiene
2-cyclohexenone
4. Polycyclic compounds contain two or more rings that share two or more
carbon atoms.
E.g.
CH3
CH3
CH3
7
CH3
4
5
3
1
6
7,7-dimethylbicyclo[2.2.1]heptane
(7,7-dimethylnorbornane)
total: 7
ring carbons
2
ring junctions (1, 4)
no. of carbons between
bridgehead = 2 ( ), 2 ( ), 1 ( )
Industrial Aspects
Cycloalkanes, especially cyclopentanes and cyclohexanes, are commercially
very useful:
CH3
reforming
and dehydrogenation
-3H2,
catalyst
heat
catalysts
methylcyclopentane
3H2, Ni
cyclohexane
benzene
O2
catalyst
OOH
O
oxidation
polyamides
(nylons)
Cis-Trans (Geometric) Isomerism in Cycloalkanes
A major difference between cycloalkanes and (open chain) alkanes is that the
structures of the former are less flexible (more rigid) than those of the latter.
Because of the cyclic arrangement of carbon atoms, the energy barriers to
rotation around C-C bonds in small and medium cycloalkanes (C3 –C7) is much
higher than in the analogous open chain alkanes. Consider ethane:
H
H
~ 12 kJ mol-1
H
H
H
H
rotation
"staggered" conformer
H
H
HH
H
H
"eclipsed" conformer
Such a small difference in energy between the two rotational isomers
(conformers) means that, under normal conditions, the two are rapidly
interconverting. Alicyclic rings of small or medium size do not have this flexibility:
cyclopropane is rigid and rotational restriction falls off with increase in ring size,
so although C4 – C7 rings are still severely restricted, larger (macrocyclic) rings
have almost the flexibility of open chain alkanes. Conformational isomerism of
alicyclic ring systems is discussed further in the textbook, chapter 4.
Because of their cyclic structures, cycloalkanes have two sides: a “top” and a
“bottom”, making a special kind of isomerism (called cis-trans or geometric
isomerism) possible in disubstituted (and higher) cycloalkanes, as shown for
1,2-dichlorocyclopropane, below.
Cl
H
CH2
Cl
H
Cis-1,2-dichlorocyclopropane
Cl
H
CH2
H
Cl
Trans-1,2-dichlorocyclopropane
Cis comes from the Latin for "same side" and trans comes from the Latin for "across"
Interconversion of these two isomers by simple bond rotation is not possible
because of the rigidity of the ring (i.e. they are NOT conformers). The same
argument applies to multiply substituted medium rings: although the rings are
less rigid than cyclopropane, it is still not possible to interconvert cis and trans
isomers by simple rotation around C-C bonds. Further examples are given below.
OH
Br
CH2CH3
OH
Cis-1,2-cyclohexanediol
Trans-1-bromo-3-ethylcyclopentane
Note that cis-trans isomerism exists for certain alkenes (see textbook, chapter 6)
and other unsaturated compounds, such as oximes and azo compounds. In all
these cases, interconversion of cis and trans isomers is prevented by the rigid
nature of the double bond system. Cis-trans isomers have the same connectivity,
but differ in the spatial arrangement of the atoms about the rigid center. Like
conformational isomerism,dealt with in the next two classes, and optical
isomerism, considered later (see textbook, chapter 9) cis-trans isomerism is a
form of stereoisomerism. Some examples are given below.
Cl
Cl
C
Cl
C
H
C
H
Cl
trans
CH3
OH
CH3
N
..
C
H
..
N
H
cis
Ph
N
..
cis
Acetaldehyde oxime
OH
(an N-hydroxy imine)
trans
Ph
N
..
1,2-Dichloroethene
C
H
cis
C
H
Ph
N
..
..
N
Azobenzene
Ph
trans
Class Questions
1. Write down structures and names of the three isomers of C5H12.
2. Predict the major product of the reaction below.
CH3
COOH
NBS,
a peroxide
(NBS = N-bromosuccinimide)
3. Name the following.
(a)
Br
(b)
C2H5
Cl
(c)
CH3
CH3
C2H5
CH3
4. Indicate which of the following exhibit cis-trans isomerism.
(a)
(b)
(c)
CH3
CH3
CH3
2
CH3
H
2
2
H
H = deuterium
1.
C5H12
CH3
CH3CH2CH2CH2CH3
Pentane
CH3
CH3
CH3CHCH2CH3
C
CH3
CH3
2,2-Dimethylpropane
2-Methylbutane
CH2Br
2.
COOH
3. (a) 1-Chloro-3-ethylcyclobutane (b) 4-Bromo-2-ethyl-1-methylcyclohexane
(c) 1,1-Dimethylcyclopropane
4. (a) No (b) Yes (c) Yes