RULES FOR NAMING ORGANIC COMPOUNDS


[Rules Section 1]
1. INTRODUCTION TO ORGANIC NOMENCLATURE

The IUPAC nomenclature system is a set of logical rules devised and used by organic chemists to circumvent problems caused by arbitrary nomenclature. Knowing these rules and given a structural formula, one should be able to write a unique name for every distinct compound. Likewise, given a IUPAC name, one should be able to write a structural formula. In general, an IUPAC name will have three essential features:

  A root or skeleton indicating a major chain or ring of carbon atoms found in the molecular structure.
  A suffix designating 'principal' functional groups that may be present in the compound.
  a prefix designating 'minor' functional or substituent groups, other than hydrogen, that complete the molecular structure.

A useful web reference on this topic is:
http://www.cem.msu.edu/~reusch/VirtualText/nomen1.htm


[Rules Section 2]
2. THE HYDROCARBON SKELETON

Choosing the skeletal name:
(i) First look for a principal group or multiple bond and count the number of carbon atoms in the longest continuous chain on which it is attached, and including any carbon atoms in the group.
(ii) If (i) does not apply, look for a benzene ring or cycloalkane/ene ring structure and, in the latter case, count the number of carbon atoms forming the ring.
(iii) If (i) and (ii) do not apply then simply count the number of carbon atoms in the longest continuous straight chain.

Name the skeleton as follows:
C                     meth...
C-C                   eth....
C-C-C                 prop...
C-C-C-C               but....
C-C-C-C-C             pent...
C-C-C-C-C-C           hex....
C-C-C-C-C-C-C         hept...
C-C-C-C-C-C-C-C       oct....
C-C-C-C-C-C-C-C-C     non....
C-C-C-C-C-C-C-C-C-C   dec....

If all C-C bonds are single add: ...ane (for saturated side-chains use: ...yl)
if any C-C bond is double add: ...ene 
if any C-C bond is triple add: ...yne
if a closed ring structure add: cyclo... but if it is cyclohexatriene you must change the name to benzene


[Rules Section 3]
3. ONE PRINCIPAL FUNCTIONAL GROUP

The most common principal groups are:

alcohols		-OH		...ol
aldehydes	-CHO		...al
ketones		>CO		...one
carboxylic acids	-COOH		...oic acid
acid halides	-COCl		...oyl chloride
amides		-CONH2		...amide
nitriles		-CN		...nitrile
sulphonic acids	-SO3H		...sulphonic acid
sulphonamide	-SO2NH2	...sulphonamide

When a compound contains a principal group its name is added as a 'suffix' directly after the skeletal name:
eg. CH3-CH2-OH  ethanol
Note: the final 'e' in the skeletal name is dropped where followed by a vowel.


[Rules Section 4]
4. ONE MINOR FUNCTIONAL GROUP

The most common minor groups are:

Halogen derivatives	-F	fluoro... -Cl chloro... -Br bromo... -I iodo...
Nitration products	-NO2	nitro...
Ethers			-OCH3	methoxy...
Alkyl side chains		-CH3	methyl... -CH2CH3 ethyl...
Aryl side chains		-C6H5	phenyl...

When a compound contains a minor group or side-chain its name is added as a 'prefix' directly before the skeletal name:
CH3-CH2-Cl		chloroethane
CH3-CH2-O-CH3		methoxyethane
CH3-CH(CH3)-CH3	methylpropane


[Rules Section 5]
5. POSITIONAL ISOMERS

If more than one position of the functional group is possible on the skeleton, then the carbon atoms must be numbered, starting from the end nearest to the group. This number, called a locant is attached using a hyphen or hyphens immediately before the name of the  group:

CH3-CH2-CH2Br		1-bromopropane
CH3-CHBr-CH3		2-bromopropane
CH3-CH2-CH2OH	propan-1-ol
CH3-CHOH-CH3		propan-2-ol

The same method applies to denote the position of a multiple bond in the skeleton, where alternative positions are possible:
CH3-CH2-CH=CH2	but-1-ene
CH3-CH=CH-CH3	but-2-ene


[Rules Section 6]
6. TWO OR MORE IDENTICAL GROUPS

The prefix:  di... for two groups (tri... for three groups, tetra... for four groups) is attached to the group name and each group is given its own locant, keeping the numbers as small as possible. The locants are separated using commas and attached by a hyphen to the name:

CH2Br2			dibromomethane
CH2Br-CH2Br		1,2-dibromoethane
CH3-CHBr2		1,1-dibromoethane
CH2OH-CH2OH		ethane-1,2-diol
	(Note: the 'e' is not dropped here as in ethanol)
CH2=CH-CH=CH2	buta-1,3-diene
	(Note: the 'ne' is dropped (or 'a' added) as it sounds better)
COOH-CH2-COOH	propanedioic acid
	(Note: locants are superfluous here)


[Rules Section 7]

7. TWO OR MORE DIFFERENT MINOR GROUPS

The prefixes for minor groups and side-chains are connected in alphabetical order and the carbon atoms in the skeleton chain are numbered from the nearest end to the groups to ensure that the smallest possible numbers are used as locants:

CH3-CHCl-CHBr-CH2-CH3	3-bromo-2-chloropentane

The prefixes for multiplicity of groups (di, tri, tetra) are ignored when placing groups in alphabetical order:

CH3-CHCl-CBr2-CH2-CH3		3,3-dibromo-2-chloropentane


[Rules Section 8]
8. ONE MINOR AND ONE PRINCIPAL GROUP

A principal group or multiple bond always has priority for the lowest locant,i.e. the carbon atoms in the skeleton chain are numbered from the nearest end to the principal group or multiple bond:

CH3-CHBr-CHOH-CH3	3-bromobutan-2-ol
CH3-CHBr-CH=CH2	3-bromobut-1-ene
CH3-CHBr-CH2-COOH	3-bromobutanoic acid


[Rules Section 9]
9. TWO OR MORE DIFFERENT PRINCIPAL GROUPS
The order of priority of principal groups is: -COOH > -CHO > =CO > OH > C=C 
The group with highest priority is given the lowest locant:

CH2=CH-CH2-CH2OH	but-3-en-1-ol
CH3-CH=CH-COOH	but-2-enoic acid

The group with least priority is usually relegated and given a prefix name like a minor group:
-OH	hydroxy...
=O	oxo....

CH3-CHOH-COOH	2-hydroxypropanoic acid
CH3-CO-COOH		2-oxopropanoic acid


[Rules Section 10]
10. AMINES AND HYDRAZINES

These compounds pose a particular problem. It is first necessary to decide whether the skeleton is (a) the hydrocarbon component or (b) the ammonia NH3 or hydrazine NH2NH2 component:

(a) The hydrocarbon as the skeleton:
Where there are more than one of these groups or a principal group is also present, they are treated as minor groups attached to a hydrocarbon skeleton.
amines		-NH2	amino...
hydrazines	-NHNH2	hydrazino...

CH2NH2-COOH		aminoethanoic acid 
CH2NH2-CH2NH2	1,2-diaminoethane
Note: the name ethane-1,2-diamine may be seen. This treats the group as a principal group, which it isn't, and therefore not recommended in school chemistry.

(b) Ammonia or hydrazine as the skeleton:
Where there is only one group or its salt, the hydrocarbon component is treated as the side-chain:
CH3-CH2-NH2		ethylamine
CH3-CH2-NH-NH2	ethylhydrazine
CH3-CH2-NH3Cl		ethylammonium chloride

There may be more than one alkyl group:
CH3CH2-NH-CH2-CH3	diethylamine
CH3CH2-NH-CH3		N-ethylmethylamine
Note: the use of the locant 'N' to show that the ethyl group is attached to the nitrogen atom and not to the methyl group.

There may be another minor group present on the hydrocarbon side-chain:
CH3-CHBr-CH2NH2	(2-bromopropyl)amine
CH3-CH(CH3)-NH2	(1-methylethyl)amine


[Rules Section 11]
11. SALTS AND ESTERS

Like inorganic salts the names of these compounds are derived by changing the ending 'ic' in the name of the acid to 'ate' and dividing the name into two parts separated by a space. This may signify the separation of the actual molecules into two parts (ions) as in the case of salts:

CH3COOH  + NaOH   =  CH3COONa  +  H2O
ethanoic acid + sodium hydroxide = sodium ethanoate + water

The break in the continuity of the hydrocarbon chain through the oxygen atom in esters is shown similarly by a break in the name:

CH3COOH + CH3OH = CH3-CO-O-CH3 + H2O
ethanoic acid + methanol = methyl ethanoate + water


[Rules Section 12]
12. DERIVATIVES OF BENZENE

As with amines, it is necessary to decide whether the benzene ring is the skeleton or only a side-chain.

(a) Benzene as a side-chain:
A benzene ring attached to an open hydrocarbon chain which also contains a principal group or double bond is treated as a minor side-chain and denoted by the prefix: phenyl...

<C6H5>-CH2OH		phenylmethanol
<C6H5>-CH=CH2	phenylethene
<C6H5>-CH2-COOH	phenylethanoic acid

Also, where there is more than one benzene ring or one amine group:
<C6H5>-CH2-<C6H5>	diphenylmethane
<C6H5>-NH2		phenylamine

(b) Benzene as the skeleton:
The benzene ring takes priority over any open hydrocarbon chain which does not contain a multiple bond or principalgroup. All previous rules apply except that one group on its own or a principal group always has position 1 and therefore does not need a locant:

<C6H5>-CH3		methylbenzene 
<C6H5>-CH2Cl		(chloromethyl)benzene
Cl-<C6H5>-CH3		1-chloro-4-methylbenzene
Cl-<C6H5>-Cl		1,4-dichlorobenzene
<C6H5>-OH		benzenol (but always called 'phenol', except 
				where there are 2 or more OH groups:)
Cl-<C6H5>-OH		4-chlorophenol
HO-<C6H5>-OH		benzene-1,4-diol
<C6H5>-SO3H		benzenesulphonic acid
<C6H5>-COOH		benzenecarboxylic acid (but usually called benzoic acid, except where there are 2 or more identical groups. Notice that the 'ene' not just the 'e' is dropped and that the carbon atom in the COOH group is not accounted for)
Cl-<C6H5>-COOH	4-chlorobenzoic acid
HOOC-<C6H5>-COOH	benzene-1,4-dicarboxylic acid

The alternative names for principal groups when the carbon atom can not be accounted for in the skeletal name are:
-COOH	...carboxylic acid
-COCl	...carboxyl chloride
-CONH2	...carboxamide
-CHO	...carbaldehyde
-CN	...carbonitrile


[EOF]
