WHAT ARE ALKANES?
General formula CnH2n+2
 are organic compounds that consist
only of the elements carbon (C) and
hydrogen (H)
 atoms are linked together exclusively
by single bonds
 all carbons are sp3 (tetrahedral) and
all bond angles are 109.5o

NOMENCLATURE



Greek numerical prefix denoting the number
of carbons and the suffix "-ane".
It forms Homologous series - a series of
compounds in which each member differs
from the next member by a constant amount,
members are called homologs)
The first four members of the series (in
terms of number of carbon atoms) are named
as follows:



methane, CH4
ethane, C2H6
propane, C3H8
SOURCES OF ALKANES
1. Petroleum – the principal source of alkanes;
are the end products of anaerobics decay of
plants and animals for several years.
2. fossil fuel, coal – secondary sources of
alkanes
3. Natural gas – contains more volatile alkanes(
low molecular weight); contains 3% higher
alkanes.
Petroleum Constituents
Uses
Fraction
Distillation Temp.
Carbon Number
Heating
Natural Gas
Below 20 degrees
C1-C4
Heating
Solvents for many
organic materials of
low polarity
Petroleum Ether
Ligroin (light
naphtha)
60-100
40-205
C5-C6
C6-C7
Internal combustion
engine
Natural Gasoline
40-205
C5-C10 &
cycloalkanes
Heating
Tractor and jet
Kerosine
175-325
C12-C18 &
aromatics
Heating
Diesel oil
Gas oil( furnace oil) Above 275
C12 & higher
Petroleum wax,
petroleum
jelly(Vaseline)
Lubricating oil
Non-volatile liquid
Long chain
attached to cyclic
structure
Roof and roads
Asphalt and
petroleum *coke
Nonvolatile solids
Polycyclic structure
*coke – paraffin base crude oil; complex HC having a high C:H ratio; fuel in the manufacture of C electrode for the
electrochemical industries.
Crude oil contains hundreds of different hydrocarbons mixed together. To
obtain useful products, the process of fractional distillation is used. The
following diagram shows a schematic of a fractional distillation column.
Longer hydrocarbon chain lengths have progressively higher boiling points,
so they can all be separated by distillation. Crude oil is heated and the
different chains are separated by boiling temperatures.

Refinery and tank storage facilities, like this one in Texas,
are needed to change the hydrocarbons of crude oil to
many different petroleum products.
PHYSICAL PROPERTIES
A. Melting point and boiling point
 Melting (blue) and boiling (pink) points
of the first 14 n-alkanes in °C.
For simple straight-chain alkanes,
boiling
and melting points generally
increase with increasing chain length. (
20-30 degrees rises in boiling point for each
carbon that is added to the chain)
B. Forces of attraction
 Van der waals repulsion
C. Conductivity
 Alkanes do not conduct electricity, nor are
they substantially polarized by an electric
field
D. Molecular geometry
 molecular structure of the alkanes directly
affects their physical and chemical
characteristics
E. Bond lengths and bond angles
 An alkane molecule has only C – H and C –
C single bonds
F. Conformation


free rotation about the C-C single bonds
two conformations, also known as
rotamers
 Staggered
 Eclipsed
CHEMICAL PROPERTIES
In general, alkanes show a relatively low
reactivity, because their C bonds are
relatively stable and cannot be easily
broken. Unlike most other organic
compounds, they possess no functional
groups.
 react only very poorly with ionic or other
polar substances





Nonpolar to slightly polar
Solubility; soluble in nonpolar solvents like
benzene, ether, chloroform and insoluble in
water and other highly polar solvents.
Density; increasing Carbon chain increasing
density but tends to level off at about 0.8,
thus all alkanes are less dense than water.
acid dissociation constant (pKa) values of all
alkanes are above 60
Classes of Carbon and Hydrogen atoms
 1o
Carbon - primary carbon is attached
to only one other C atoms
 2o Carbon - secondary carbon is
attached to two other C atoms
 3o Carbon - tertiary carbon is attached
to three other C atoms
CH3CH2CH2CH(CH3)CH2CH3
1o
2o
2o
3o
1o
2o
1o
Preparation of Alkanes

smaller alkanes can be obtained in pure
form by fractional distillation of
petroleum and natural gas.
1. Hydrogenation of alkenes
CnH2n
H2, Pt/Pd/Ni
CnH2n+2
Example:
CH3
CH
CH2
H2, Pt
CH3
CH2
CH3
2. Reduction of Alkyl Halides
a.
Using Grignard Reagent
Stronger acid
H2O
RMgX
ether
+ Mg
RX
alkyl halides
Grignard Reagent
alkyl magnesium halides
C
Cl
+
Mg
ether
CH3
C
MgCl H2O
CH2
CH3
CH
CH3
Br
sec-butyl bromide
CH3
CH2
CH
C
H
+
Mg(OH)Cl
CH3
Mg
CH3
Mg(OH)X
CH3
CH3
CH3
+
weaker acid
CH3
CH3
CH3
RH
CH3
OH2
MgBr
sec-butyl magnesium bromide
CH3
CH2
CH
H
n-butane
CH3
b. Reduction by metal and acid
RX
+
Zn
+
+
RH
H
+
+
CH3
CH Cl
+
Mg
Zn, H
CH3
CH2CH3
Zn
+
+
+
Zn
+
X-
+
Cl
Cl
CH3CH2
CHCH3
Zn, H
CH3CH2CH2CH3
Br
sec-butyl bromide
n-butane
3. Coupling of RX with organometallic
compounds
RX
Li
RLi
CuX
1o, 2o, 3o alkyl lithium
CH3CH2
CHCH3
Li
R2CuLi
+
Lithium dialkylcopper
CuI
Cl
(CH3CH2
CH
R-R'
R'X
alkyl halides(1o)
)2CuLi
CH3
CH3CH2CH(CH3)(CH2)4CH3
CH3CH3CH2CH2CH2Br
sec-butyl chloride
CH3CH2
Cl
ethyl chloride
Li
n-Pentyl bromide
CH3CH2Li
ethyllithium
CuI
(CH3CH2)2CuLi
+ CH3(CH2)5CH2Br
CH3(CH2)7CH3
Lithium diethylcopper n-Heptyl bromide
n-nonane
4. Wurtz Reactions ( used to produce
symmetrical alkanes)
Na is used.
CH 3 CH 3
CH 3
2CH 3 C
CH 3
Cl
Na
CH 3
C
C
CH 3
CH 3 CH 3
Symmetrical alkane
5.Thermal Decarboxylation of a Carboxylate
Salt
CH3-CH2-COOH + NaOH ----> CH3-CH3 + CO2
Reactions of Alkanes
1. Halogenation(free radical substitution)
H
CH 3CH 2CH 3
CH 3
240 - 400 240 - 400
CH 3
Cl 2
C
CH 3
+
CH 3CH 2CH 2Cl
Cl
H
C
Cl
Cl 2
CH 2CH 3
CH 3
H
C
CH 2CH 3
LIGHT
CH 3
ClCH 2C
CH 3
CH 3
H
+
+
CH 3
C
CH 2CH 2Cl
CH 3
+
CH 3
CH3C
H
C
C
Cl
CH 3
H
+
H
CH2CH3
CH2Cl
CH 3
CH 2CH 3
HALOGENATION




In this reaction, a halogen atom abstracts a
hydrogen atom from an alkane.
Reaction occurs slowly in the dark but rapidly in
sunlight.
The rate at which a hydrogen atom is replaced
by a halogen depends upon its position in the
molecule.
The positions are dependent on the position of
the carbon to which hydrogen is attached.
If the hydrogen atom s attached to a
primary carbon (1˚), the H-atom is said
to be a primary H-atom. If it is
attached to a secondary carbon (2˚),
the H-atom is a secondary H-atom, and
so on and so forth.
 The secondary H-atom is more rapidly
replaced by a halogen compared with
primary H-atom.

A. Chlorination



A chlorine atom abstracts a hydrogen
atom from an alkane.
It is known that the chlorination of an
alkane, promoted by sunlight or artificial
ultraviolet light takes place by free
radical chain reaction.
There are 3 fundamental stages in this
reaction: initiation, propagation and
termination.
EXAMPLE

Free radical chlorination of an Alkane
(when all R's are H, CR3H = methane)
B. Bromination
 The mechanism for bromination is
similar.
 When the alkane is methane, traces of
ethane are found in the final mixture
of products. This provides evidence for
a mechanism involving a methyl radical.
 It would be formed from combining
two methyl radicals: H3C. + .CH3 ==>
H3C-CH3
Reactivity: Cl2 > Br2
Ease of abstraction of H : 3o> 2o > 1o >CH3-X
2. Combustion or complete oxidation
 A reaction wherein alkanes burn in air
or oxygen which then forms the
products CO2 and H2O.

The general formula for combustion is:
flame
CnH2n + 2 + excess O2
n CO2 + (n+1) H2O + heat


An insufficient supply of oxygen leads
to the production of soot,
formaldehyde, or other products.
The heat of combustion of alkanes
increases with chain length, simply
because there is more C and H to burn
along longer chain.
3. PYROLYSIS OR CRACKING


When alkane hydrocarbons are heated to a high
temperature (450-900oC, with/without
superheated steam) they are thermally
decomposed or 'cracked' to form mainly alkanes
of lower C number, alkenes of equal or smaller C
number and hydrogen.
When the temperature is high enough, the
kinetic energy of the particles is sufficient to
cause bond fission on collision, and this initiates
a free radical chain reaction.
EXAMPLE

Free radical thermal cracking of Alkanes

C22H44 => C12H20 + C10H24
C17H36 => C9H20 + C8H16
4. NITRATION
alkanes may be converted into nitro
derivatives by heating the hydrocarbon in
the vapor state with vapors of nitric acid
at a temperature of about 420˚.
 A hydrogen atom is replaced by a nitro
(NO2) group.

WHAT ARE CYCLOALKANES?

general chemical formula ; CnH2n where n =
number of C atoms
CLASSIFICATIONS OF CYCLOALKANES
Small
Ex: cyclopropane and cyclobutane
 Bigger
 Normal
Ex: cyclopentane, cyclohexane,
cycloheptane

Physical Properties
1. Hybridization of carbon is sp3, with 109.5o angle.
Cycloalkanes will experienced angle bending strain in
the formation of a ring due to compression of the
tetrahedral bond angle.
2. Nonpolar molecules.
3. Forces of attraction is van der waals
4. Soluble in nonpolar solvents like CCl4, CHCl3, benzene,
ether.
5.They are more reactive than straight chain alkane
6. They have higher boiling points, melting points and
densities
Shapes/conformation
chair conformation
boat conformation
twisted boat conformation
Note: These three are free of angle-bending
strain.
The most stable conformation is the chair
conformation because it is free from
torsional strain, angle-bending strain and
steric strain.
Boat conformation will experienced torsional
strain, there is also van der waals strain due
to crowding of the flagpole hydrogen which
lies only 1.83A apart.
COMPARISON IN TERMS OF REACTIVITY
 As stability of cycloalkanes decreases the
addition reaction increases.

Undergo cycloaddition
Strain in cyclopropane and cyclobutane
1. Angular strain- deviation from tetrahedral
geometry or angle of 109.5 degrees.

Compression of bond angles

Poor, overlap of angle(109.5o)
2. Torsional strain - deviation from staggered
conformation.
 Exist anytime C-H bonds are eclipsed
 Due to eclipsing of bonds on neighboring
atoms
 All H’s eclipsed
3. Steric Strain
 the presence of van der waals repulsion
 Electronic repulsion that occurs when two
atoms or groups are forced together
 Due to repulsive interactions when two
atoms approach each other to closely
 Van der Waals repulsion
For cycloalkanes, every two “missing”
hydrogens are referred to as one “degree of
unsaturation”.
EXERCISES
Natural Sources of Cycloalkanes
Petroleum and coal
Preparation of cycloalkanes
1. Hydrogenation
hydrogenation
+
3H 2
25 atm
Ni, Pt, Pd
OH
OH
+
3H2
Ni, 150 ~ 200oC
15atm
2. Conversion of some open-chain compounds
into a compound that contains a ring, a
process of cyclization.
CH2Cl
H2C
CH2ZnCl
Zn, NaI
CH2Cl
H2C
CH2Cl
H2 C
H2 C
3. Cycloaddition – reaction in which molecule
are added together to form a ring.
CH3
H3C
C
H3C
H2C
N
diazomethane
+
C
:CH2
CH3
N or H2C
C
ketene
O
heat
:CH2
+
carbene
N2 or CO
4. Conversion of other cyclic compounds
+
H
H2, Ni
OH
5. Dieckmann condensation
6. Rearrangement reaction
7. Wurtz reaction
Reactions of cycloalkanes
1. OXIDATION
+ O2
3CO2 + 4H2O + heat
2.HYDROGENATION
H2, Ni
CH3CH2CH2CH3
3. CYCLOADDITION
H2SO4
+
H , OH
-
Br, FeBr 3
CH 3CH 2CH 2CH 2OH
H2C
Br
CH 2
CH 2
Br
4. Catalytic reforming
cycloalkanes catalytic
aromatic HC
reforming
5. Halogenation
Thank you