Please give me an detailed explanation of the rules of IUPAC nomenclature of the Hydrocarbons including all the functional groups given in the syllabus with suitable examples.

IUPAC System

IUPAC nomenclature of aliphatic compounds: 

According to the IUPAC system of nomenclature, the name of an organic compound consists of the following three parts:

• Word root
• Suffix
• Prefix

Word Root

The word root represents the number of carbon atoms in the parent chain and it is the basic unit of the name. The parent chain is selected as per the longest chain rule i.e., the parent chain is the longest continuous chain of C-atoms including the functional group and the multiple bonds, if any.

For the carbon chains up to four carbon atoms, special word roots based on their common names are used. For chains consisting of five or more carbon atoms, the word roots are derived from the Greek numerals as shown in table. The general word root for any carbon chain is alk. The word roots for the chains consisting of different number of carbon atoms are listed below:

Word roots for different lengths of carbon chains

Chain length	Word root *	Chain length	Word root *
C1	Meth	C8	Oct
C2	Eth	C9	Non
C3	Prop	C10	Dec
C4	But	C11	Undec
C5	Pent	C12	Dodec
C6	Hex	C13	Tridec
C7	Hept	C18	Octadec

 

* an extra 'a' is added to the word root if the primary suffix to be added to the word root begins with a consonant

Suffix: 

There are two types of suffixes.

Primary suffix

A primary suffix is added to the word root (i.e., at the end of the word root) to indicate whether the parent chain is a saturated or unsaturated. Some primary suffixes are given below.

• Saturated - ane
• Unsaturated with: one double bond - ene

two double bonds - diene

three double bounds - triene

• Unsaturated with: one triple bond - yne

Two triple bond - diyene

Primary suffix on appropriate word root for some typical compounds are:

Formula of the organic compound	‘C’ - atoms in parent chain	Word root	Primary suffix	IUPAC name
CH3CH2CH2CH3	4	But	ane	Butane
CH3CH= CH2	3	Prop	ene	Propene
CH≡ CH	2	Eth	yne	Ethyne
CH2 = CH - CH = CH2	4	Buta*	diene	Butadiene
HC≡ C - C≡ CH	4	Buta*	diyne	Butadiyne

 

* An extra 'a' has been added to the word root since the primary suffix i.e., diene or diyne begins with consonant i.e., 'd' instead of a vowel as is the case in the first three cases.

Secondary suffix:

Secondary suffix is used to indicate the function group in the organic compound. Secondary suffixes for various functional groups are given in the table below.

Family of organic compounds			Functional group	Secondary suffix
IUPAC names	Trivial names
Alkanols	Alcohols	(ROH)	- OH	-ol
Alkanals	Aldehydes	(RCHO)	- CHO	-al
Alkanones	Ketones	(RCOR')	>C = O	-one
Alkanoic acids	Carboxylic acids	(RCOOH)	- COOH	-oic acid
Alkanmides	Acid amides	(RCONH2)	- CONH2	-amide
Alkanoyl chlorides	Acid chlorides	(RCOCl)	- COCl	-oyl chloride
Alkyl alkanoates	Esters	(RCOOR')	- COOR	-alkyl...oate
Alkanenitriles	Cyanides	(RCN)	- CN	nitrile
Alkanthiols	Thioalcohols	(RSH)	- SH	thiol
Alkanarnines	Amines	(RNH2)	- NH2	amine

Secondary suffix is written after the primary suffix. While doing so, the following rules should be followed.

If the secondary suffix begins with a vowel, then the terminal e of the primary suffix is dropped before adding the secondary suffix.

If the secondary suffix begins with a consonant, then the last (terminal) e is retained and the secondary suffix is added after e.

If the secondary suffix has some numerical prefix, such as, di, tri etc., before it then the terminal e of the primary suffix is retained.

The addition of the secondary suffix to the primary suffix is illustrated as:

Formula of compound	C - atoms (parent chain)	Word root	Primary suffix	Secondary suffix	IUPAC name	Remarks
CH3CH2CH2COOH	4	But	ane	Oic acid	Butaroic acid	'e' of primary suffix removed
CH3CH2CN	3	Prop	ane	Nitrile	Propenenitrile	'e' of primary suffix not removed
CH2 - OH    |   CH2 - OH	2	etn	ane	Diol	Etharediol	'e' of primary suffix not removed

 

Prefix:

Prefix is a part of the name, which appears before the word root. There are two types prefixes:

Primary prefix:

A primary prefix is used to distinguish alicyclic (aliphatic cyclic compounds) from acylic compounds. A prefix cyclo is used before the word root if the compound is an alicyclic compound. For example,

Cyclopentane:

  

If the prefix cyclo is absent, in an acylic compound it indicates an open chain compound.

Secondary prefix:

In the IUPAC system of nomenclature, groups are not considered as functional groups (which are indicated by secondary suffixes) but as substituents. Each such group (or substituent) is denoted by a secondary prefix. Some typical secondary prefixes are:

Substituent group	Secondary prefix
- F	Fluoro
- Cl	Chloro
- Br	Bromo
- I	lodo
CH3 -	Methyl
C2H5	Ethyl
CH3CH2CH3 -	n-propyl
(CH3)2CH -	iso-propyl
(CH3)3C -	t-butyl
- NO2	nito
- NH2	amino
- NO	nitroso
- N ≡ N +	diazo
- OCH3	methoxy
- OC2H5	ethoxy
- OR	alkoxy

 

Groups such as -NO₂, -OR, -F, -Cl, -Br and -I are considered as substituents and are indicated by the prefixes. Groups such as -CHO, -CO, -COOH, -COCl, -CONH₂, -COOR, -NH₂, -CN and -OH are considered as functional groups and are indicated by suffixes.

The following rules should be observed while adding a secondary prefix.

• In the case of alicylic compounds (aliphatic cyclic), the secondary prefix is added before the primary prefix (before the word cyclo), in the alphabetical order.

 

 

 

• In the case of open chain compounds (acyclic), the secondary prefix is added just before the word root in the alphabetical order.

Nitromethane : CH₃NO₂

Thus, the name of an organic compound can be obtained by writing the word root, prefixes and suffixes in the following order as per rules described.

Secondary prefix + Primary prefix + Word root + Primary suffix + Secondary suffix.