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.

Asked by mababu65 | 30th Nov, 2010, 08:29: PM

Expert Answer:

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.

Answered by  | 1st Dec, 2010, 01:30: AM

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