Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

13.2

NAMING SIMPLE HYDROCARBONS Straight or continuous chain alkanes are

named by indicating the number of carbon atoms

in the chain with the appropriate

root name

as given in Table 13.1 and adding

ane

to

indicate that the compound is an alkane. Fo

r example, the straight chain alkane C

H 7

(^16)
contains seven carbon atoms in the chain, so its root name from Table 13.1 is
hept

. It is an

alkane, so its name ends in

ane

. Thus, C

H 7

is heptane. 16

All of the carbon atoms in a branched chain alkane are not in the same chain, so they

are named by identifying the

longest continuous chain

with the appropriate root name and

then identifying the type and positions of any side chains that are attached to the longest continuous chain. Side chains can be any type of group, but we restrict our discussion to alkyl groups.

Alkyl groups

are formed by removing one hydrogen atom from an alkane.

The resulting alkyl group binds to a longer chain at the position of the removed hydrogen. Alkyl groups are parts of compounds; they are not compounds themselves. The name of the alkyl group is obtained by adding -yl to the root name. The two simplest alkyl groups are:

methyl: -CH

(^3)
ethyl: -C
H 2
(^5)
The dash before the formula indicates where th
ey bond to a longer chain. Propyl, butyl,
pentyl, etc., groups also exist, but there are more than one of each of these alkyl groups. For example, there are two different propyl groups that differ by the position of the removed hydrogen, which can come from a terminal carbon atom or from the middle carbon atom. We restrict our discussion of naming organic compounds to compounds that contain methyl and/or ethyl groups to avoid this complexity.
The carbon atoms in the longest continuous chain are numbered starting at the end
closest to the side chain, and the position of
the alkyl group is indicated by the number of
the carbon atom to which it is attached. The name of the alkyl group and the name of the longest chain are combined into one word. Fo
r example, 2-methylhexane has a six-carbon
chain with a CH
group attached to the second carbon from the end. To name alkanes with 3
more than one side chain, simply place the names of the alkyl groups separated by hyphens in front of the root name. For example, 2-methyl-3-ethylheptane contains a seven carbon continuous chain with a CH
group on the second carbon and a C 3
H 2
group 5
attached to the third carbon. Two identical
groups are indicated with the prefix
di and their
positions separated with a comma. For example, 2,2-dimethylheptane and 3,4-diethyloctane. The procedure for naming branch
ed alkanes is summari
zed in the margin.
Table 13.1
Roots of names of simple organic compounds
and the names of the corresponding alkanes Number of C atoms
Root of name
Formula of alkane
Name of alkane
1 meth CH
methane 4
2 eth C
H 2
ethane 6
3 prop C
H 3
propane 8
4 but C
H 4
butane 10
5 pent C
H 5
pentane 12
6 hex C
H 6
hexane 14
7 hept C
H 7
heptane 16
8 oct C
H 8
octane 18
9 non C
H 9
nonane 20
10 dec C
H 10
decane 22
To name branched chain alkanes
,

1. Identify the longest cont

inuous chain, select the

appropriate root from Table 13.1 and add the ending -ane

.

2. Number the atoms in the

longest continuous chain,

starting at the end of the chain

closest to the branch.

. 3. The name of the alkane is the number of the position of

the side chain followed by a hyphen, then the name of the alkyl group followed by the name obtained in step one.

To name simple alkenes and alkynes, 1. Identify the longest continuous chain

that contains the

multiple bond

and determine the root of the name from

Table 13.1.

2. Start at the end of

the chain closest to the multiple bond

and number the atoms in the l

ongest continuous chain.

3. Use the number of the first carbon in the multiple bond

as a prefix followed by a hyphen followed by the root with an -

ene

or –yne ending for alkene and alkyne,

respectively. For example, 1-butyne has four carbon atoms and a triple bond between the first and second.

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State

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