1. Chapter 11:
Alkanes
K. Dunlap
Chem 104

2. Hydrocarbon: a compound composed of only
carbon and hydrogen.

3. Alkanes: hydrocarbons containing only
carbon-carbon single bonds.
– The first two alkanes are methane and ethane.

4. line-angle formula or skeletal formula:
– A line represents a carbon-carbon bond and a line
terminus represent a carbon atom.
– Hydrogen atoms are not shown in line-angle
formulas.

5. The first 10 alkanes with unbranched chains

6. Constitutional isomers:
compounds that have the same molecular
formula but different structural formulas
(different connectivity).
– 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.

7. Constitutional isomers
Question: do the structural formulas in each set
represent the same compound or constitutional
isomers?

8. Solution for (a):
(a) They represent the same compound.
Solution for (b):
(b) They represent constitutional isomers.

9. Constitutional isomers
Problem: draw structural formulas for the five
constitutional isomers of molecular formula C 6H14
Solution:

10. Number of Possible Isomers
for Selected Alkanes

11. Naming Alkanes
• The IUPAC name of an alkane with an
unbranched chain of carbon atoms consists of
two parts:
(1) a prefix: the number of carbon atoms in the
chain.
(2) the suffix -ane: shows that the compound is a
-ane
saturated hydrocarbon.

12. Naming 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

13. Alkyl groups:
a substituent group derived from an alkane by
removal of a hydrogen atom.
– commonly represented by the symbol R-.
– named by dropping the -ane from the name of the
parent alkane and adding the suffix - yl.
yl

14. Naming Alkanes
1. The name for an alkane with an unbranched
chain of carbon atoms consists of a prefix
showing the number of carbon atoms and the
ending -ane.
2. For branched-chain alkanes, the longest
chain of carbon atoms is the parent chain and
its name is the root name.
3. Name and number each substituent on the
parent chain; use a hyphen to connect the
number to the name.

15. Naming Alkanes…
4. If there is one substituent, number the parent
chain from the end that gives the substituent
the lower number.

16. Naming Alkanes…
5. If the same substituent occurs more than
once:
– Number the parent chain from the end that gives
the lower number to the substituent encountered
first.
– Indicate the number of times the substituent occurs
by a prefix di-, tri-, tetra-, penta-, hexa-, and so on.
– Use a comma to separate position numbers.

17. Naming Alkanes…
6. If there are two or more different substituents:
– list them in alphabetical order.
– number the chain from the end that gives the lower
number to the substituent encountered first.
– If there are different substituents in equivalent
positions on opposite ends of the parent chain,
give the substituent of lower alphabetical order the
lower number.

18. Naming Alkanes…
7. Do not include the prefixes di-, tri-, tetra-, and
so on;
alphabetize the names of substituents first,
and then insert these prefixes

19. Naming 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.
– Cycloalkanes of ring sizes ranging from 3 to over
30 carbon atoms are found in nature.
– Five-membered (cyclopentane) and six-membered
(cyclohexane) rings are especially abundant in
nature.
Cyclopentan e
Cycloh exane

20. • 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.
1
Isopropylcyclopentane
4
1-tert- Butyl-4-methylcyclohexane

21. Hydrocarbon
Rings

22. Name these alkanes

23. Conformation of alkanes
• any three-dimensional arrangement of atoms
in a molecule that results by rotation about a
single bond.
– following are three conformations for a butane
molecule.

24. Physical properties of Alkane
• 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.

25. Melting and boiling points of Alkanes:
– 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.

26. Physical properties Cont…
• Solubility: a case of “like dissolves like”.
– Alkanes are not soluble in water; they are unable
to form hydrogen bonds with water.
– 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 insoluble
in water, they float on water.

27. Where do hydrocarbons come from?
1) 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 2-methylpropane.
2) Petroleum
– A thick, viscous liquid mixture of thousands of
compounds, most of them hydrocarbons formed
from the decomposition of marine plants and
animals.

28. Petroleum Distillation

29. Some Common Hydrocarbons:
-fuels
-paraffin
-petrolatum
-plastics
-natural plant waxes

30. More Naming…….