compounds that have the same molecular
formula but different structural formulas
– For the molecular formulas CH4, C2H6, and C3H8,
only one structural formula is possible; there are
no constitutional isomers for these molecular
– For the molecular formula C4H10, two constitutional
isomers are possible.
Question: do the structural formulas in each set
represent the same compound or constitutional
Solution for (a):
(a) They represent the same compound.
Solution for (b):
(b) They represent constitutional isomers.
Problem: draw structural formulas for the five
constitutional isomers of molecular formula C 6H14
Number of Possible Isomers
for Selected Alkanes
• The IUPAC name of an alkane with an
unbranched chain of carbon atoms consists of
(1) a prefix: the number of carbon atoms in the
(2) the suffix -ane: shows that the compound is a
• 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
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.
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
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.
4. If there is one substituent, number the parent
chain from the end that gives the substituent
the lower number.
5. If the same substituent occurs more than
– Number the parent chain from the end that gives
the lower number to the substituent encountered
– 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.
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
7. Do not include the prefixes di-, tri-, tetra-, and
alphabetize the names of substituents first,
and then insert these prefixes
• 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
– 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
Conformation of alkanes
• any three-dimensional arrangement of atoms
in a molecule that results by rotation about a
– following are three conformations for a butane
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
– 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 and boiling points of Alkanes:
– Boiling points of alkanes are lower than those of
almost any other type of compound of the same
– In general, both boiling and melting points of
alkanes increase with increasing molecular weight.
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.
– 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.
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.
– A thick, viscous liquid mixture of thousands of
compounds, most of them hydrocarbons formed
from the decomposition of marine plants and