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CLASSIFICATION AND NOMENCLATURE OF ORGANIC COMPOUNDS

CLASSIFICATION OF ORGANIC COMPOUNDS

The ability of carbon to combine with large number of elements especially O, N, S, X etc, to undergo catenation to form chains of varying lengths and shapes and existence of isomers has led to the formation of more than five million organic compounds. These have been classified into the following main groups

ACYCLIC OR OPEN CHAIN COMPOUNDS

CYCLIC OR CLOSED CHAIN COMPOUNDS

                   

HOMOCYCLIC COMPOUNDS

The ring system is made up of one type of atoms generally carbon
  • Alicyclic : The cyclic compounds resembling open chain aliphatic compounds. For example: Cycloalkanes
  • Aromatic : The benzene, naphthalene and their derivatives etc are homocyclic aromatic compounds         
            

HETEROCYCLIC COMPOUNDS

The ring system is made up of two or more than two types of atoms. They may be
  • Alicyclic
            
  • Aromatic
                       

CLASSIFICATION BASED ON FUNCTIONAL GROUPS

On the basis of functional groups which confer characteristic properties on them, the organic compounds have been classified as follows
Class
Functional group
Halides
X (Cl, Br, I) Halo
Esters
Olefins Alkenes
>C = C<
Acid halides
Acetylenes/ Alkynes
Anhydrides
Alcohols
—OH (Hydroxy)
Amines
—NH2
Aldehydes
Ketones                                   
Sulphonic acid
—SO3H
Acids
Amides

HOMOLOGOUS SERIES  

A group of a particular class of compounds where a preceding or succeeding member differ by one–CH2. The members of the series are known as homologues. The homologues
  • have the same general formula CnH2n+2 or  CnH2n+1 X
  • molecular weight differing by 14 of two successive members
  • can be prepared by general methods of preparation
  • have almost similar chemical properties
  • show regular gradation in physical properties such as mpt, bpt, density etc

NOMENCLATURE

The most widely accepted and the latest system of naming organic compounds is IUPAC (International Union of Pure and Applied Chemists) system, according to which the name essentially consists of three parts.

WORD ROOT

It indicates the nature of the basic carbon skeleton. From C1 to C4 common names have been retained and from C5 upwards Greek number roots have been used
Chain length
Word root
Chain length
Word root
C1
Meth-
C7
Hept-
C2
Eth-
C8
Oct-
C3
Prop-
C9
Non-
C4
But-
C10
Dec-
C5
Pent-
C11
Undec-
C6
Hex-
C12
Dodec-
The generic word root for any carbon chain is “alk”.

SUFFIX

These are of two types

PRIMARY SUFFIX
It is added to the word root to designate saturation or unsaturation in a carbon chain
Type of Carbon chain
Primary Suffix
Generic name
Saturated
- ane
Alkane
Unsaturated with one C=C
- ene
Alkene
Unsaturated with one C≡C
- yne
Alkyne

SECONDARY SUFFIX
It is added to indicate the functional group present in the compound. The terminal ‘e’ is dropped, if secondary suffix begins with a vowel (a, e, i, o, u, y) but it is retained if the secondary suffix begins with a consonant.
Functional Group
Secondary Suffix
Generic name
– OH
– ol
Alkanol
– CHO
– al
Alkanal
>C = O
– one
Alkanone
– COOH
– oic acid
Alkanoic acid
– COX
– oyl halide
Alkanoyl halide
– CONH2
– amide
Alkanamide
– COOR
– alkyl -- – oate
Alkyl alkanoate
– (CO)2O
– oic anhydride
Alkanoic anhydride
– CN
– nitrile
Alkane nitrile
– SH
– thiol
Alkanethiol
– NH2
– amine
Alkanamine

PREFIX

They are of two types

PRIMARY PREFIX
It  is for cyclic nature of the compound and primary prefix cyclo is used immediately before the word root. eg.:
Primary prefix
Word root
Prim. suffix
Sec. suffix
IUPAC name
Cyclo
hex
ane
–        
Cyclohexane
SECONDARY PREFIX
The certain atoms and groups which are not considered as functional groups but are treated as substituents are called secondary prefixes. They are added before the word root in case of acyclic compounds and before the primary prefix in case of cyclic compounds in alphabetical order.
The important secondary prefixes are
Substituent
Sec. prefix
– X (F, Cl, Br, I)
   Halo
– NO2
    Nitro
– NO Nitroso

                                 
Diazo
  • OR
Alkoxy
–R (CH3, C2H5, C3H7, etc)
Alkyl

Thus the complete IUPAC name of an organic compound consists of the following parts
  • Secondary prefix
  • Primary prefix
  • Word root
  • Primary suffix
  • Secondary suffix
For example:
IUPAC name is 4-Bromocyclohex-2-ene-1-ol   or 4-Bromo-2-Cyclohexenol
Primary prefix = Cyclo
Secondary prefix = 4-bromo
Word root = hex
Primary suffix = ene
Secondary suffix = ol

ALKYL GROUPS

Univalent groups formed by the removal of one hydrogen atom from an alkane are known as alkyl groups or alphyl groups. Their names are obtained by changing the suffix –ane of parent hydrocarbon by –yl.

Alkane
Group
Shorthand notation
IUPAC Name
Methane
Methyl CH3
Me
Methyl
Ethane
ethyl C2H5
Et
Ethyl
Propane
n-propyl  CH3CH2CH2
Isopropyl
n-Pr,   or Pr
Iso-
1-propyl
2-propyl
Butane
n-butyl  CH3CH2–CH2–CH2
s-butyl
Iso-butyl (CH3)2CH–CH2–  
t-butyl (CH3)3C–
–n-Bu, ,Bu
s-Bu,  or ,BuS    
Iso-Bu, or Bui
t-Bu, But
1-butyl
1-methyl propyl
2-methyl propyl
1,1-dimethyl ethyl

LINE ANGLE FORMULA

Bonds are represented by lines, carbon atoms are assumed to be present at the start and finish of a line. Nitrogen, oxygen and halogens are labelled, but hydrogens are only shown when bonded to a drawn atom. Each atom is assumed to have sufficient hydrogen atoms around it to make it neutral. For example:
n-Hexane CH3(CH2)4CH3
2-Cyclohexenone
But-1, 3-diene                              
3-methyl but-2-ol    

NOMENCLATURE OF COMPLEX HYDROCARBONS

The following rules are followed
  • Longest chain rule : The longest continuous chain of carbon atoms is picked up which forms the base name of the compound.
  • Numbering : The longest chain is numbered by arabic numerals beginning with the end nearest a substituent.
  • If two or more side chains are in equivalent positions, then the one cited first in the name is assigned the lower number.
  • If two or more of the same alkyl groups are present, use the prefixes di, tri etc to avoid repetition
  • Alphabetical order : The side chains are cited in alphabetical order
  • Longest chain with maximum number of side chains : If two or more chains of the same length are possible, choose the one with maximum number of side chains.
  • Locant sum : Sum of the locants must be minimum. But out of two sets of the sum of the locants, the set having the lowest number when compared by term is preferred. For example out of (2+6+7=15) and (3+4+7=14), the first set is correct.
  • The name of a complex radical is considered to begin with the first letter of its complete name i.e. including the numerical affix (di, tri, tetra etc are numerical affix) for alphabetical order.
  • When the side chains have the identical name the priority is given to side chain having lowest locant
  • The numerical prefixes bis, tris tetrakis are used to indicate the multiplicity of substituted substituent

NOMENCLATURE OF COMPLEX ALKENES AND ALKYNES

  • Selection of longest chain containing maximum number of double or triple bonds (sometimes longest chain rule is violated)
It contains longest chain of 7C atoms, but both the double bonds are not included. Hence longest chain of 6C catoms is picked up)
  • If both, the double and triple bonds, are present the compound is regarded as derivative of alkyne. In such cases the terminal 'e' of -ene is dropped if it is followed by suffix starting with a,i,o,u,y. For example :
  • If double and triple bonds are at equidistant from either side, the preference is given to double bond.
  • If the compound contains two or more double or triple bonds a terminal "a" is added to the word root.
       
  • The terminal 'a' is not added to the word root when the complete primary suffix do not start with a numerical affix
 
(Note that di, tri, tetra.. are numerical affix)
  • Side chains containing multiple bonds are named as follows
Allyl CH2= CH2=CH–CH2
Ethylidene CH3–CH= 
Vinyl CH2=CH–

NOMENCLATURE OF CYCLOALKANES (ALICYCLIC COMPOUNDS)

  • The base name is decided by the number of carbon atoms which the cyclic or acyclic portion contains. If the ring contains more or equal number of carbon atoms as alkyl then it is regarded as derivative of cycloalkane
  • Carbons are numbered to give lowest numbers to substituted carbons. For example
  • When there are more acyclic than cyclic carbons the cyclic part becomes cycloalkyl substituent
  • When acyclic portion contains a multiple bond or a functional group, the cyclic portion is treated as substituent.
  • In case when both contain the same functional group, the base name is decided by the number of c-atoms.
  • When both contain the different functional groups, the base name is decided by principal characteristic group
  • When the acylic ring is directly attached to benzene ring, it is named as derivative of benzene
  • Presence of certain groups
Functional gp.
Suffix
–COOH
Carboxylic acid
–COOR
Alkyl carboxylate
–COX
Carbonyl halide
–CONH2
Carboxamide
Carbonitrile
–CHO
Carbaldehyde

NOMENCLATURE OF POLYCYCLIC ALKANES

There are three ways that rings can be joined.
  • Fused rings         
  • Bridged rings
  • Spirocyclic compounds   

The carbon atoms common to both the rings are called bridge head atoms. The chain of carbon atoms connecting the bridge head atoms is called a bridge.

Numbering of C-atoms in fused rings and bridge rings : The numbering starts from bridge head carbon, proceeds along the longest bridge passing through the second bridge head atom, proceeds to the next longest bridge and completed along the shortest path.

Numbering of C-atoms in spiro compounds : The numbering starts from the carbon atom, next to spiro atom, present in the smaller ring giving minimum number to atoms containing functional groups.

FUSED RINGS

Fused rings share two adjacent carbon atoms and the bond between them eg. :

BRIDGED RINGS

These share two non adjacent carbon atoms (the bridge head carbons) and one or more carbon atoms between them
More examples of bridge Compounds

SPIROCYCLIC COMPOUNDS

The two rings share one carbon atom

More examples of spiro Compounds

NOMENCLATURE OF COMPOUNDS CONTAINING IDENTICAL CYCLIC UNITS JOINED BY A SINGLE BOND


No. of cyclic hydrocarbon units
Two
Three
Four
Prefix
bu
ter
quarter

Numbering of C-atoms : The numbering starts from the C-atom joining the rings.
     
   

NOMENCLATURE OF COMPOUNDS CONTAINING TERMINATING FUNCTIONAL GROUPS

If only one group such as –COOH, –CHO, –COOR, –CONH2, –COCl or – is present in the molecule it is always given number 1 and 1 is never written when there is no ambiguity.
2-Ethylhexanoic acid
  
2-Methylpropanamide
Methylpentanoate

NOMENCLATURE OF COMPOUNDS CONTAINING TWO OR MORE THAN TWO SIMILAR TERMINAL GROUPS

PRESENCE OF ONLY TWO SIMILAR TERMINAL GROUPS

The carbon atoms of such groups are included in the principal chain. For example

PRESENCE OF MORE THAN TWO SIMILAR TERMINAL GROUPS ATTACHED TO THE MAIN PRINCIPAL CHAIN

In this case special suffixes are used and carbons of terminal groups are not counted in the principal chain
Functional groups
Suffix
–COOH
- Carboxylic acid
–CHO
- Carbaldehyde
–COX
- Carbonylhalide
–CONH2
- Carboxamide
–COOR
- Alkyl carboxylate
–CN
- Carbonitrile

PRESENCE OF MORE THAN TWO SIMILAR TERMINAL GROUPS NOT DIRECTLY ATTACHED TO THE PRINCIPAL CHAIN

In such case the longest chain with two similar terminal groups is selected and carbons of groups are counted in the principal chain.

NOMENCLATURE OF COMPOUNDS CONTAINING SUBSTITUENTS (NOT REGARDED PRINCIPAL FUNCTIONAL GROUPS)

                   
2-Bromo-3-Chlorobutane                     3- Nitromethylhexane
(follow alphabetical order)

NOMENCLATURE OF COMPOUNDS CONTAINING MORE THAN ONE TYPE OF FUNCTIONAL GROUPS

In such case the compound is regarded as derivative of senior functional group and the other functional groups are regarded as substituents. The numbering of the parent chain is done in such a way so that the functional group of highest priority gets the lower number and the chain contains the maximum number of functional groups.

The seniority of functional groups (highest priority) follow the following order:-
Group
Prefix name
2ยบ Suffix name
– SO3H
Sulpho
Sulphonic acid
– COOH
Carboxy
Oic acid
– COOR
alkoxy carbonyl
Oate
– COX
Halo carbonyl/Halo formyl
–oyl halide
– CONH2
Carbamoyl
amide
– CHO
Aldo or formyl
al
– CN
Cyano
nitrile
– NC
Isocyano
Isonitrile
>C = O
Keto or oxo
one
– OH
Hydroxy
ol
– SH
Mercapto
thiol
– NH2
Amino
Amine
– OR
Alkoxy
Epoxy
> C = C <
ene
yne
– N = N
Azo
– NO2
Nitro
– NO
Nitroso
– X (Cl, Br, I)
Halo (Cl, Br, I)


The terminal e of the primary suffix is replaced by the suffix name of functional group.

Alphabetical order for substituents : These should be placed in alphabetical order.
Naming of substituted substituents : In this case the subsidiary substituents are named as prefixes. For example

NOMENCLATURE OF AROMATIC COMPOUNDS

Generally Benzene and its derivatives are known as aromatic compounds. They are of two types
  • Nuclear substituted : The functional group is directly attached to the benzene nucleus e.g. phenol, toluene, chlorobenzene etc.
  • Side chain substituted : The functional group is present in the side chain e.g. Benzyl alcohol, Benzylamine etc.
In the first case the compounds are named as derivatives of benzene and in the second case as derivatives of aliphatic compounds (except arenes).
The IUPAC name of benzene is cyclohex-1,3,5-triene, but now aromatic compounds have their popular common name adopted by IUPAC. In IUPAC system the position of functional groups are indicated by arabic numerals i.e. 1, 2, 3 instead of o, m and p.

AROMATIC HYDROCARBONS (ARENES)

CONTAINING ONE RING
CONTAINING MORE THAN ONE RING

ARYL GROUPS

HALOGEN DERIVATIVES

PHENOLS

Nuclear substituted hydroxy derivatives are known as phenols

Side chain substituted hydroxy derivatives are known as alcohols

AROMATIC ETHERS

AMINES

  

NITRO COMPOUNDS

ALDEHYDES

KETONES

ACIDS

ACID DERIVATIVES

SULPHONIC ACIDS



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Best Coaching Institute in Delhi for IIT JEE, NEET, AIIMS, PMT - Clear IIT Medical: Classification & Nomenclature of Organic Compounds | Chemistry Notes for IITJEE/NEET
Classification & Nomenclature of Organic Compounds | Chemistry Notes for IITJEE/NEET
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