Alkanes
Hydrocarbons
Aliphatic
Aromatic
Hydrocarbons
Aliphatic
Alkanes
Aromatic
Alkenes
Alkynes
Hydrocarbons

Aliphatic
Alkanes
Alkanes are
hydrocarbons in which
all of the bonds are
single bonds.
H
H
H
C
C
H
H
H
Hydrocarbons

Aliphatic
Alkenes are
hydrocarbons that
contain a carboncarbon double bond.
H
H
Alkenes
C
H
C
H
Hydrocarbons

Aliphatic
Alkynes are
hydrocarbons that
contain a carboncarbon triple bond.
Alkynes
HC
CH
Hydrocarbons

The most
common aromatic
hydrocarbons are
those that contain a
benzene ring.
Aromatic
H
H
H
H
H
H
CnH2n+2
Introduction to Alkanes:
Methane, Ethane, and Propane
The Simplest Alkanes
Methane
(CH4) CH4
Ethane
(C2H6) CH3CH3
Propane (C3H8) CH3CH2CH3
bp -160°C
bp -89°C
bp -42°C
Structure of Methane
tetrahedral
bond
angles = 109.5°
bond distances = 110 pm
but structure seems inconsistent with
electron configuration of carbon
Structure of Ethane
C2H6
CH3CH3
tetrahedral
geometry at each carbon
C—H
bond distance = 110 pm
C—C
bond distance = 153 pm
n-Butane
CH3CH2CH2CH3
Isobutane
(CH3)3CH
bp -0.4°C
bp -10.2°C
C5H12
CH3CH2CH2CH2CH3
(CH3)2CHCH2CH3
n-Pentane
Isopentane
(CH3)4C
Neopentane
Number of Constitutionally
Isomeric Alkanes







CH4 1
C2H6 1
C3H8 1
C4H10 2
C5H12 3
C6H14 5
C7H16 9
C8H18
18
C9H20
35
C10H22
75
C15H32
4,347
C20H42
366,319
C40H82 62,491,178,805,831
Nomenclature of Alkanes



Rules for naming compounds are given by the
International Union for Pure and Applied Chemistry
(IUPAC).
To name alkanes:
 Find the longest chain and use it as the name of the
compound.
 Number the carbon atoms starting with the end
closest to the substituent.
 Name and give the location of each substituent.
When two or more substituents are present list them in
alphabetical order.
Nomenclature of Alkanes
1. Determine the number of carbons in the parent hydrocarbon
8
7
6
5
4
3
2
8
1
CH3CH2CH2CH2CHCH2CH2CH3
7
6
5
4
4
CH3CH2CH2CH2CHCH2CH3
CH3
2
2
1
CH3CH2CH2CHCH2CH2CH3
CH2CH2CH3
3
3
CH2CH2CH2CH3
1
5
6
7
8
2. Number the chain so that the substituent gets the lowest
possible number
1
2
3
4
5
CH3CHCH2CH2CH3
CH3
2-methylpentane
1
2
3
4
5
6
7
8
CH3CH2CH2CHCH2CH2CH2CH3
CHCH3
CH3
4-isopropyloctane
CH3
CH3CHCH2CH2CH3
common name: isohexane
systematic name: 2-methylpentane
3. Number the substituents to yield the lowest possible number
in the number of the compound
CH3CH2CHCH2CHCH2CH2CH3
CH3
CH2CH3
5-ehtyl-3-methyloctane
not
4-ethyl-6-methyloctane
because 3<4
(substituents are listed in alphabetical order)
4. Assign the lowest possible numbers to all of the substituents
CH3 CH3
CH3CH2CHCH2CHCH3
CH3
CH3
2,4-dimethylhexane
CH3CH2CH2C
CCH2CH3
CH3 CH3
3,3,4,4-tetramethylheptane
CH2CH3
CH3
CH3CH2CHCH2CH2CHCHCH2CH2CH3
CH2CH3
CH2CH3
3,3,6-triethyl-7-methyldecane
5. When both directions lead to the same lowest number for one
of the substituents, the direction is chosen that gives the lowest
possible number to one of the remaining substituents
CH3
CH3
CH3CHCH2CHCH3
CH3
CH3
2,2,4-trimethylpentane
not
2,4,4-trimethylpentane
because 2<4
CH2CH3
CH3CH2CHCHCH2CHCH2CH3
CH3
6-ethyl-3,4-dimethyloctane
not
3-ethyl-5,6-dimethyloctane
because 4<5
6. If the same number is obtained in both directions, the first
group receives the lowest number
CH2CH3
Cl
CH3CH2CHCH3
Br
2-bromo-3-chlorobutane
not
3-bromo-2-chlorobutane
CH3CH2CHCH2CHCH2CH3
CH3
3-ethyl-5-methylheptane
not
5-ethyl-3-methylheptane
7. In the case of two hydrocarbon chains with the same number of
carbons, choose the one with the most substituents
3
4
5
6
CH3CH2CHCH2CH2CH3
2 CHCH3
1 CH3
3-ethyl-2-methylhexane
(two substituents)
1
2
3
4
5
6
CH3CH2CHCH2CH2CH3
CHCH3
CH3
3-isopropylhexane
(one substituent)
8. Certain common nomenclatures are used in the IUPAC system
CH3CH2CH2CH2CHCH2CH2CH3
CHCH3
CH3
4-isopropyloctane
or
4-(1-methylethyl)octane
CH3CH2CH2CH2CHCH2CH2CH2CH2CH3
CH2CHCH3
CH3
5-isobutyldecane
or
5-(2-methylpropyl)decane
Nomenclature of Alkanes
Types of Carbons




primary (1o) - bonded to only 1 other
carbon
secondary (2o) - bonded to 2 other
carbons
tertiary (3o) - bonded to 3 other carbons
quaternary (4o) - bonded to 4 other
carbons
CH3
CH3
CH3
C CH2 CH CH3
CH3
Conformations are different spatial
arrangements of a molecule that are
generated by rotation about single bonds.
Ethane
Eclipsed conformation
Ethane
Staggered conformation
Projection formulas of the staggered
conformation of ethane
H
H
H
H
H
H
H
H
H
H
H
Newman
H
Sawhorse
Anti relationships
H
H
H
H
180°
H
H
H
H
H
H
H
H
Two bonds are anti when the angle between them is 180°.
Gauche relationships
H
H
H
60°
H
H
H
H
H
H
H
H
H
Two bonds are gauche when the angle between them is 60°.
An important point:
The terms anti and gauche apply
only to bonds (or groups) on adjacent
carbons, and only to staggered
conformations.
12 kJ/mol
0°
60°
120°
180°
240°
300°
360°
Conformational Analysis of
Butane: C2-C3 Rotation
The most stable conformation of unbranched
alkanes has anti relationships between carbons
Hexane
Reactions with Alkanes

The C-C and C-H bonds are very strong. Therefore,
alkanes are very unreactive.

At room temperature alkanes do not react with acids,
bases, or strong oxidizing agents.
Alkanes do combust in air (making them good fuels):
2C2H6(g) + 7O2(g)  4CO2(g) + 6H2O(l)
H = -2855 kJ

Alkanes are very unreactive compounds because they have
only strong s bonds and atoms with no partial charges
However, alkanes do react with Cl2 and Br2
Reaction of Alkane with Cl2 or Br2
Petroleum is a complex mixture of alkanes and
cycloalkanes that can be separated by distillation