Organic Nomenclature
H
HCH
H
methane
H
HC
H
ethane
propane
butane
H
C
H
H
HC
H
H
HC
H
H
CH
H
H
C
H
H
CH
H
H
C
H
H
CH
H
Organic chemistry originated as the chemistry of living systems.
In this context organic meant “natural”. But as chemists studied
the molecular structure of living things, it quickly became obvious
that all organic compounds contained carbon as their main
constituent. Thus organic chemistry today is defined as the
chemistry of carbon compounds. In the compounds carbon
atoms are covalently bonded to other atoms in chains. Since
carbon has a valence of four, each carbon atoms forms four
bonds; because of this, the number of possible organic
compounds is limitless, making it the perfect atom for life to be
based upon.
Today chemists have learned to imitate nature and synthesize organic molecules in the laboratory.
Such synthetic organic molecules have become the basis of many industries (such as plastics, textiles, and medicine)
and therefore organic chemistry employs more people than all other fields of chemistry combined.
In this chapter we will study the vast variety of
organic compounds found in nature and those made by man.
We will base our study on the system that is called Organic
Nomenclature. There are actually two systems of Organic
Nomenclature now in use; the older system consists of
common names for organic compounds, the newer system,
called the IUPAC system is the one which we shall study in
this chapter.
H
HC
H
H
H
H
H
HC
H
C
C
C
H
H
H
H
H
H
H
C
C
C
C
H
H
H
H
H
CH
H
pentane
H
CH
H
hexane
The IUPAC names shown on this page with their structural formulas are the basis for all the names of
the organic compounds. The molecules shown here are all members of a group of compounds called alkanes. On
the next page we will begin listing the categories of organic chemistry in outline form.
heptane
nonane
octane
decane
Hydrocarbons
Saturated Hydrocarbons
1. Alkanes
Hydrocarbons are the simplest category of organic compounds. They contain only hydrogen and
carbon. Saturated hydrocarbons are those which contain the maximum amount of hydrogen possible.
Alkanes are hydrocarbons made up of only single covalent carbon to carbon bonds. The molecular formulas
and names of the first ten alkanes are as follows.
CH4
C2H6
C3H8
C4H10
C5H12
methane
ethane
propane
butane
pentane
C6H14
C7H16
C8H18
C9H20
C10H22
hexane
heptane
octane
nonane
decane
The names of all alkanes end in “ane”. Their molecular formulas have the general formula C nH2n +2.
2. Branched Alkanes
Alkanes do not always have their carbons linked together in a straight line. Alkanes may be branched.
The names of the branches are based on the names of the straight chained alkanes with ending “ane”
changed to “yl”. The following is a list of the names of possible branches.
CH3
C2H5
C3H7
C4H9
C5H11
methyl
ethyl
propyl
butyl
pentyl
C6H13
C7H15
C8H17
C9H19
C10H21
hexyl
heptyl
octyl
nonyl
decyl
To name a branched alkane by the IUPAC system we name the longest chain of carbons using the standard alkane
names. The names of the branches are listed in alphabetical order immediately preceding the name of the longest
chain. Each branch is given a number by its position on the main chain. The main chain is always numbered in the
direction to give the smallest possible numbers for the branches. The resulting name is one word.
Sample Problem 1
H
HCH
H
HCH
H
HC
H
H
HCH
H
HCH
HCH
H
H
H
C
C
C
C
CH
H
H
H
H
H
Results in the name
H
HC
H
C
H
HCH
H
C
H
C
H
H
C
H
H
CH
H
If there is more than one of the same kind of branch a prefix is used to indicate how many there are. But
even in this case, a number is still required for each of the branches present. The prefixes are as follows:
di for two
tri for three
Sample Problem 2
H
HCH
HCH
HCH
H
H
H
HC
H
C
C
C
H
H
H
Sample Problem 3
Sample Problem 4
H
HCH
H
C
HCH
H
H
C
CH
H
H
tetra for four
Problems
For each problem: a) Circle the longest chain of carbons. b) Draw a box around
each branch. c) Write the IUPAC name next to the structural formula.
For 7-10 Draw the structure for each IUPAC name.
7) 2,2 – dimethyl - 4- propyloctane
8) 2,3,3,5 - tetramethylhexane
9) 3-ethyl-4,4-dipropylheptane
10) 3-ethyl - 2,2 - dimethylpentane
3. Halogenated Alkanes
Halogen atoms may be attached to a carbon chain much like methyl group. The rules for numbering and
listing halogen atoms are the same as for carbon branches. The following names are used:
-F flouro
-Br bromo
- Cl chloro
-I iodo
Sample Problem 5
H
HC
H
Br
C
Br
C
H
HCH
H
H
CH
H
Unsaturated Hydrocarbons
4. Alkenes
Unsaturated hydrocarbons are those which contain less than the maximum number of hydrogens
possible. Alkenes contain a double bond between carbon and therefore have 2 less hydrogens than their
alkane counterparts. Their molecular formulas have the general formula CnH2n.
Alkene names are the same as those for the alkanes except that the “ane” ending is changed to
“ene”. The IUPAC system requires that a number be placed immediately in front of the name for the longest
chain indicating where the double bond begins. The main chain is numbered from the end to give the
double bond the smallest possible number, regardless of where the branches are placed.
Sample Problem 6
H
HC
H
HCH
C
HCH
H
H
H
H
C
C
CH
H
When there is more than one double bond, the standard prefixes (di, tri, or tetra) are placed
immediately in front of the “ene” ending. The letter “a” is inserted in front of this prefix to make the name
pronounceable. As would be expected, there must be a number for each of the double bonds.
Sample Problem 7
H
HC
H
H
C
H
C
H
C
C
HCH
H
H
CH
H
In the sample problem above, numbering from either direction will give the same numbers for the double
bonds. Therefore our second consideration is to give the branch the smallest number possible. But as
demonstrated in the next sample problem, the double bonds have priority in receiving the smallest number.
Sample Problem 8
H
HC
H
H
H
H
H
H
H
C
C
C
C
C
C
CH
H
HCH
H
5. Alkynes
Alkynes are unsaturated hydrocarbons containing a triple bond between carbons. The name of all
alkynes end in “yne”. Their general formula is CnH2n-2.
Sample Problem 9
HC
CH
Sample Problem 10
HC
C
C
CH
Problems: Write the IUPAC name for each of the following.