IUPAC NOMENCLATURE part 2

NOMENCLATURE PART 1 

IUPAC system of Nomenclature 

• The basic criterion for naming a structure by IUPAC system is choice of a parent name of the basic carbon skeleton.
• Nomenclature of alkanes is fundamental to naming whole class of organic compounds because it helps us identify the basic carbon skeleton.

General rules for IUPAC nomenclature :

The IUPAC system is the most rational and widely used system of nomenclature in organic chemistry. The most important feature of this system is that any given molecular structure has only one IUPAC name and any given IUPAC name denotes only one molecular structure.

The IUPAC name of any organic compound essentially consists of five parts, i.e.

1. Word root 
2. Primary Suffix 
3. Secondary Suffix 
4. Primary Prefix 
5. Secondary Prefix

Thus, a complete IUPAC name of an organic compound consists of the following parts : Secondary prefix +Primary prefix + Word root + Primary suffix + Secondary suffix

1. Word root : It is the basic unit of the name. It denotes the number of carbon atoms present in the principal chain (the longest possible continuous chain of carbon atoms including the functional group and based upon the common names of alkanes) of the organic molecules. 

Root word : According to number of carbon's in parent C–chain. 

 

Number of carbon	Root word	Number of carbon	Root word
1	Meth	6	Hex
2	Eth	7	Hept
3	Prop	8	Oct
4	But	9	Non
5	Pent	10	Dec

2.Primary Suffix : A primary suffix is always added to the word root to indicate whether the carbon chain is saturated or unsaturated. The three basic primary suffixes are given below :

S.No.	Types of carbon chain	Primary suffix	General name
1	saturated	-ane	Alkane
2	unsaturated with one double bond	-ene	Alkene
3	unsaturated with one triple bond	-yne	Alkyne

 If the parent carbon chain contain two, three or more double or triple bond, numerical prefix such as di (for two), tri (for three), tetra (for four) etc. are added to the primary suffix. For example.

S.No	Types of carbon chain	primary suffix	General name
1	unsaturated with two double bond	-diene	alkadiene
2	unsaturated with two triple  bond	-diyne	alkadiyne

 

 3.Secondary Suffix : A secondary suffix is always added to the primary suffix to indicate the nature of the functional group present in the organic compounds. Secondary suffix of some important functional groups are given below.

S.No.	class of organic compounds	functional group	seconadary suffix
1	Alcohols	-OH	-ol
2	Aldehydes	-CHO	-al
3	Ketones	>C=O	-one
4	carboxylic acid	-COOH	-oic acid
5	Acid amides	-CONH2	-amide
6	Acid chlorides	-COX	-oly halide
7	Esters	-COOR	alkanoate
8	Nitriles	-CN	-nitrile
9	Thioalcohols	-SH	-thiol

10

Amines

- NH2

-amine

 

The following examples illustrate the use of word root, primary suffix and secondary suffix in naming of organic compounds.

S,NO.	Organic compounds	Word root	Primary suffix	secondary suffix	IUPAC name
1	CH3CH2OH	Eth	an(e)	ol	Ethanol
2	CH3CH2CH2NH2	Prop	an(e)	Amine	Propanamine
3	CH3CH2CH2COOH	But	an(e)	Oic acid	Butanoic acid
4	CH3CH2CN	Prop	an(e)	Nitrile	Propanenitrile
5	CH2=CHCHO	Prop	en(e)	al	Propenal
6	HC=CCOOH	Prop	yne(e)	oic acid	Propynoic acid

4. Primary prefix : A primary prefix is used simply to distinguish cyclic from acyclic compounds. For example, in case of carbocyclic compounds.(cyclic compounds containing only carbon atoms in the ring.), a primary prefix, cyclo is used immediately before the word root.  Thus.

 Cyclo               + Pent         + ane                =  Cyclopentane 
 

Primary prefix + word root +primary suffix = IUPAC 

If the prefix cyclo is not used, it simply indicates that the compound is acyclic or open chain.

 5.Secondary Prefix : In IUPAC system of nomenclature, certain groups are not considered as functional groups but instead are treated as substituents. These are called secondary prefixes and are added immediately before the word root (or the primary prefix in case of carbocyclic compounds) in alphabetical order to denote the side chains or substituent groups. The secondary prefixes for some groups which are always treated as substituents groups (regardless of the fact whether the organic compound is monofunctional or polyfunctional) are given below :

S.NO	Organic compounds	Secondary prefix	Word root	word root	IUPAC name
1	CH3CH2-Br	Bromo	eth	ane	Bromoethane
2	CH3­­-NO2	Nitro	meth	ane	Nitromethane
3	C2H5-OC2H5	Ethoxy	eth	ane	Ethoxyethane

 In case of carbocylic compounds, primary prefixes are also used

 

4Bromo    +  Cyclo  + hex    + an (e)   +1-ol  

secondary   primary  word     primary    secondary   

prefix          prefix      root      suffix         suffix 

E x . Write the IUPAC name of the compound

Sol.  2 - Isopropyl-5-methylcyclohexanol or 2-(1-methylethyl)-5-methyl cyclohexanol

Here 

Secondary prefix  = 2 - Isoprypyl 5–methyl 

Prim aryprefix     = Cyclo 

Word root            = hex

 Primary suffix    = an (e) 

Secondary suffix = ol

IUPAC Nomenclature of Branched/Complex Alkanes :

(1)

• Select the longest chain of carbon atoms in the molecule.
• Count the number of carbon atoms in that chain and name according to the following rules.

Example :

Longest chain has 7 carbons.

It is a hept (word root)  + ane (primary suffix)

When chains of equal lengths are competing for selection, that chain is selected which has more number of subtituents. 

Here the chain shown is selected.   

(2)Carbon atoms in the longest chain selected as above in numbered consecutively form one end to the other such that the substituents attached get the lowest number.

In the above example, according to this rule, the numbering will be done as

By this numbering, locant (substituents) get the number 2, 3 and 4 compared to 4, 5 and 6 if numbering is done from other end.

(3)Each substituent, which obviously, is an alkyl group is named according to number of carbon atoms present in it and it is prefixed by the number to which it is located in the main chain. In the above example, substituents are as following: 

– CH3 (methyl) group at carbon NO. 2                  ⇒2-methyl

 – C2H5 (ethyl) group at carbon NO. 3                  ⇒3-ethyl

 – CH2CH2CH3 (propyl) group at carbon NO. 4   ⇒4-propyl 

Hence, the above compound is named as :

              3-Ethyl-2-methyl-4-propylheptane

( 4 ) If the same substituent occurs more than once in the molecule, the prefix di (for two), tri (for three), etc. used to indicate how many times it appears. The above example can be written with a little modification as :

Example :

Methyl at No. 2, 3, Ethyl at no. 3, propyl at no.4 

This will be named as : 3-Ethyl-2,3-dimethyl-4-propylheptane

 

 

( 5 ) The name of the compound is composed in such a manner that each substituent with its number and name is written alphabetically just before the parent name. Prefixes di, tri, tetra etc. are not considered in deciding alphabetical order.

Ethyl will be written before methyl which will be written before propyl. Note that in the above examples, this pattern has been compiled with. 

*Also, as per convention, 

1. numbers are separated each other by commas.
2. numbers are separated from words by hyphens and
3. write the name of the compound as a single word (with no space between) 

E x . Write the IUPAC name of

1. Primary suffix is ane as all are single bonds.
2. Chain is numbered as shown.
3.  Root word is hex
4. Prefixes methyl appears twice ∴It is 2, 4-dimethyl and 3-ethyl 
5. While arranging in alphabetical order Replicators di, tri, tetra, are not considered.

 ∴3-Ethyl-2,4-dimethylhexane

IUPAC NOMENCLATURE OF ALKENES 

Functional group : –C=C–

(1) Select the longest chain containing carbon-carbon double bond. This need not be the longest chain in the compound as a whole. Parent name will be alkene corresponding to number of carbon atoms in the longest chain. 

 Longest chain is as shown above it has 6 atoms ⇒ hexene = parent name

 (2) Carbon atoms in the longest chain is numbered so that doubly bonded carbon atom gets the lowest number. The position of double bond is indicated by the smaller of the numbers assigned to two carbon atoms of double bond. 

The above example can be rewritten as,

Position of double bond will be indicated as no. 1. 

Hence, name will be, 3-Ethyl–3–Methylhex–1–ene

 

IUPAC nomenclature of alkynes (– C≡C – group)

Numbering of longest chain is exactly same as that for naming alkenes.

Example  

CH₃C≡C–CH₃ 

But–2–yne

 IUPAC nomenclature of hydrocarbons containing both double and triple bonds occurring only once :

1. Such hydrocarbon is named as alkenyne (not alkynene).
2. Numbering is done in a manner that double and triple bonds get the lowest possible number. In case of a choice, the double bond is given preference over triple bond.

Example : CH₃–CH=CH–C≡CH

                   5       4      3    2    1         ⇒  multiple bond at (1) and (3)

note            1      2       3    4     5        ⇒multiple bond at   (2) and (4)

Name will be Pent–3–en–1–yne

IUPAC NOMENCLATURE PART 3