Chapter 2
Alkanes
Hydrocarbons


Compounds that contains only Carbons and
Hydrogens
Two types
◦ Saturated hydrocarbons
 Carbon-Carbon single bond
◦ Unsaturated hydrocarbons
 One or more Carbon-Carbon double bonds or
triple bonds or benzene rings
2.1 Alkanes
Saturated hydrocarbon
 Also referred as aliphatic hydrocarbonds
 General formula: CnH2n+2

2.2 How do we write structural
formulas of Alkanes
Line-angle formula
• A line represents a carbon-carbon bond.
• A vertex and a line terminus represent a carbon atom.
• Hydrogen atoms are not shown in line-angle formulas.
Table 2.1 The first 10 alkanes with
unbranched chains
2.3 What are Constitutional
Isomers?
Constitutional isomers: Compounds that have the same
molecular formula but different structural formulas (a different
connectivity of their atoms).
• For the molecular formulas CH4, C2H6, and C3H8, only one
structural formula is possible. There are no constitutional
isomers for these molecular formulas.
• For the molecular formula C4H10, two constitutional isomers
are possible.
2.3Constitutional Isomers
Problem: Do the structural formulas in each set
represent the same compound or constitutional
isomers?
2.3 Constitutional Isomers

Problem: Do the line-angle formulas in each set
represent the same compound or constitutional
isomers?
2.3 Constitutional Isomerism
Problem: Draw line-angle for the three
constitutional isomers of molecular formula
C5H12.
2.4 How Do We Name Alkanes?
The name of an alkane with a branched chain of
carbon atom consists of:
• a parent name: the longest chain of carbon atoms.
• substituent names: the groups bonded to the
parent chain.
IUPAC Names
The IUPAC name of an alkane
with an unbranched chain of
carbon atoms consists of two
parts:
(1) A prefix shows the number
of carbon atoms in the chain.
(2) The suffix -ane: shows
that the compound is a
saturated hydrocarbon.
Alkyl Groups
Alkyl group: A substituent group derived from an
alkane by removal of a hydrogen atom (Table 2.3).
• Commonly represented by the symbol R-.
• Named by dropping the -ane from the name of the
parent alkane and adding the suffix -yl.
Alkyl groups
Table 2.3
Rules for naming alkanes
1.
2.
3.
4.
Determine the longest chain
of carbon-carbon bonding
without disconnecting the
chain (parent) then give its
name that ends with –ane
Determine the number of
substituents and identify
them. (alkyl ends with –yl)
Determine the position(s) of
the carbons that contains
those subsituents.
If more than one of the same
substituent occurred then
indicate them with Latin
prefixes (di-, tri-, tetra-, etc…)
@ C2 and C4
4. 2,4-Dimethyl
3.
Rules for naming alkanes
1.
6.
If more than two
different substituents,
list them in alphabetical
order
Indicate all position of
carbons that has a
substituent even if
they’re the same
subsituents
Rules for naming alkanes
7.
Name the alkanes in this
order
1. Postition-substituent
then parent
2. Give the substituent
lowest position as
possible
3. Use hyphen to separate
each substituents
4. Use comma to separate
position of carbons that
has the same
substituents
Examples

Write the molecular formula and IUPAC name for these alkanes
Examples

Draw a line-angle formula for these alkanes
◦ 2,3-dimethylbutane
◦ 4-isobutyl-2,5-dimethyloctane
Common Names
Common names; an older system
• The number of carbon atoms determines the name.
• The first three alkanes are methane, ethane, and propane.
• All alkanes with the molecular formula C4H10 are named butanes, all
those with the molecular formula C5H12 are named pentanes, etc.
• For alkanes beyond propane, iso shows that one end of an otherwise
unbranched chain terminates in (CH3)2CH-
For more complex alkanes, use the IUPAC system.
2.5 Sources of Alkanes
Natural gas
• 90 to 95 percent methane.
• 5 to 10 percent ethane, and
• A mixture of other relatively low-boiling
alkanes, chiefly propane, butane, and 2methylpropane
.
Petroleum
• A thick, viscous liquid mixture of thousands of
compounds, most of them hydrocarbons
formed from the decomposition of marine
plants and animals.
Petroleum
Figure 2.3 Fractional
distillation of
petroleum.
2.6 Cycloalkanes
Cyclic hydrocarbon: A hydrocarbon that contains carbon atoms
joined to form a ring.
Cycloalkane: A cyclic hydrocarbon in which all carbons of the ring
are saturated (have only carbon-carbon single bonds).
Cycloalkanes with ring sizes of from 3 to over 30 carbon atoms are
found in nature.
• Five-membered (cyclopentane) and six-membered (cyclohexane)
rings are especially abundant in nature.
2.6 Cycloalkanes
Nomenclature
• To name a cycloalkane, prefix the name of the
corresponding open-chain alkane with cyclo-, and
name each substituent on the ring.
• If there is only one substituent on the ring, there is no
need to give it a location number.
• If there are two substituents, number the ring
beginning with the substituent of lower alphabetical
order.
Examples

Give an IUPAC name for this molecule

Draw its structure using line-angle formula
ethylcyclohexane
2.7 Conformations of Alkanes
Conformation: Any three-dimensional arrangement
of atoms in a molecule that results by rotation about a
single bond.
• Figure 2.5 Three conformations for a butane molecule.
2.7 Cyclopentane
Figure 2.5 The most stable conformation of a cyclopentane ring is an
envelope conformation.
2.7 Cyclohexane
The most stable conformation of a cyclohexane ring is
the chair conformation.
◦ All bond angles are approximately 109.5°.
Cyclohexane
In a chair conformation,
• six C-H bonds are equatorial.
• six C-H bonds are axial.
Figure 11.6 Chair conformation of cyclohexane
Cyclohexane
The more stable conformation of a substituted
cyclohexane ring has substituent group(s) equatorial
rather than axial.
Figure 11.7 Methylcyclohexane
2.8 Cis/Trans Isomers
Cis: on the same side of the ring.
Trans: on the opposite side of the ring,
• In drawing cis-trans isomers of disubstituted
cyclopentanes, we can view a cyclopentane ring edgeon.
2.8 Cis-Trans Isomers
• Alternatively, we can view the cyclopentane ring
from above. Substituents are shown by solid
wedges (above) or dashed wedges (below).
2.8 Cis-Trans Isomerism
• To determine cis-trans isomers in disubstituted cyclohexanes,
we can view a cyclohexane ring either as a planar hexagon or
viewed from above.
• Because cis-trans isomers differ in the orientation of their atoms
in space, they are stereoisomers.
• Cis-trans isomers are one type of stereoisomer.
Examples

Following is a chair conformation of cyclohexane
with carbon atoms numbered 1 through 6.
◦ Draw methyl groups that are equatorial on carbon 1,2 and 4
◦ Draw methyl group that is axial on C2, C3 and equatorial on
C6
Examples

Does the following cycloalkanes show cis-trans
isomerism? Why? For each that does, draw both
isomers
◦ 1,3-dimethylcyclopentane
◦ Ethylcyclopentane
◦ 1,3-dimethylcyclobutane
2.9 Physical Properties
The most important physical property of alkanes
and cycloalkanes is their almost complete lack of
polarity.
• The electronegativity difference between
carbon and hydrogen is 2.5 - 2.1 = 0.4 on the
Pauling scale.
• Given this small difference, we classify a C-H
bond as nonpolar covalent.
• Alkanes are nonpolar compounds and the only
interaction between their molecules are the
very weak London dispersion forces.
Melting Points
Melting and boiling points:
• Boiling points of alkanes are lower than those of
almost any other type of compound of the same
molecular weight.
• In general, both boiling and melting points of
alkanes increase with increasing molecular weight.
• More branching, the lower the boiling point
Physical Properties
Physical Properties
•
Alkanes that are constitutional isomers are
different compounds and have different physical
and chemical properties.
Physical Properties
Solubility: a case of “like dissolves like”.
◦ Alkanes are not soluble in water because they
are unable to form hydrogen bonds with water.
◦ Liquid alkanes are soluble in each other.
◦ Alkanes are also soluble in other nonpolar
organic compounds, such as toluene and diethyl
ether.
Density
◦ The average density of the liquid alkanes listed
in Table 11.4 is about 0.7 g/mL; that of highermolecular-weight alkanes is about 0.8 g/mL.
◦ All liquid and solid alkanes are less dense than
water (1.0 g/mL) and, because they are both
less dense and insoluble, they float on water.
Reactions
Oxidation (combustion)
• Oxidation of hydrocarbons, including alkanes and
cycloalkanes, is the basis for their use as energy
sources for heat [natural gas, liquefied petroleum
gas (LPG), and fuel oil] and power (gasoline, diesel
fuel, and aviation fuel).
2.10Reactions of Alkanes
Reaction with halogens (halogenation)
• Halogenation of an alkane is a substitution
reaction.
• Hydrogen from alkane carbon is replaced with a
halogen
Reactions of Alkanes
IUPAC names of haloalkanes

Halogen atoms act as subsituent
◦ Fluoro◦ Chloro◦ Bromo◦ IodoApply the same rules as for alkyl substituents
Examples

Predict all possible products

Reaction of propane with bromine gives two
products, each with molecular formula C3H7Br.
Draw structural formulas for these two compounds
and give each an IUPAC name
2.10 The Chlorofluorocarbons
Chlorofluorocarbons (CFCs)
• Manufactured under the trade name Freon.
• CFCs are nontoxic, nonflammable, odorless, and
noncorrosive.
• Among the CFCs most widely used were CCl3F
(Freon-11) and CCl2F2 (Freon-12).
CFCs were used as;
• Heat-transfer agents in refrigeration systems.
• Industrial cleaning solvents to prepare surfaces for
coatings and to remove cutting oils from millings.
• Propellants for aerosol sprays.
CFC Replacements
Chlorofluorocarbons (CFCs) cause destruction of the Earth’s
stratospheric ozone layer.
The most prominent replacements are the hydrofluorocarbons (HFCs
and the hydrochlorofluorocarbons (HCFCs).
◦ These compounds are chemically more reactive than CFCs and are
destroyed before they reach the stratosphere.