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A Level Organic Chemistry - Nomenclature

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Nomenclature - Introduction

The names of simple organic compounds are composed of two parts -

(i) a prefix which comes from the number of carbon atoms in the longest straight chain in the molecule:

1 carbon atom : meth___ 19 carbon atoms : nonadec___
2 carbon atoms : eth___ 20 carbon atoms : eicos___
3 carbon atoms : prop___ 21 carbon atoms : heneicos___
4 carbon atoms : but___ 22 carbon atoms : docos___
5 carbon atoms : pent___ 23 carbon atoms : tricos___
6 carbon atoms : hex___ 24 carbon atoms : tetracos___
7 carbon atoms : hept___ 25 carbon atoms : pentacos___
8 carbon atoms : oct___ 26 carbon atoms : hexacos___
9 carbon atoms : non___ 27 carbon atoms : heptacos___
10 carbon atoms : dec___ 28 carbon atoms : octacos___
11 carbon atoms : undec___ 29 carbon atoms : nonacos___
12 carbon atoms : dodec___ 30 carbon atoms : triacont___
13 carbon atoms : tridec___ 31 carbon atoms : hentriacont___
14 carbon atoms : tetradec___ 32 carbon atoms : dotriacont___
15 carbon atoms : pentadec___ 33 carbon atoms : tritriacont___
16 carbon atoms : hexadec___ 34 carbon atoms : tetratriacont___
17 carbon atoms : heptadec___ 35 carbon atoms : pentatriacont___
18 carbon atoms : octadec___ 36 carbon atoms : hexatriacont___

(ii) a suffix which shows which type of different organic compound it is.

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Nomenclature - Naming Alkanes

With alkanes this suffix is ___ane.

Examples -

ethane : displayed formula of ethane space-filling diagram of ethane
octane : displayed formula of octane space-filling diagram of octane

Nomenclature - Naming Alkenes

With alkenes this suffix is ___ene.

Examples -

ethene : displayed formula of ethene space-filling diagram of ethene
but1ene : displayed formula of but1ene space-filling diagram of but1ene
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Nomenclature - Naming Haloalkanes

With haloalkanes the basis for the name is the parent organic group (e.g. alkane, alkene, etc.)

(i) Position number -

Count the number of carbon atoms from the nearest chain end to where the halogen is attached.

(ii) Prefix -

Second comes the prefix based on which halogen element is present,

fluorine - fluoro__
chlorine - chloro__
bromine - bromo__
iodine - iodo__

(iii) Suffix -

Simply add the normal unchanged alkane (or alkene) name onto the suffix (see naming alkanes in the alkanes page).

Examples -

bromoethane : displayed formula of bromoethane (CH3CH2Br) space filling diagram of bromoethane (CH3CH2Br)
1-chlorobutane : displayed formula of 1chlorobutane (CH3CH2CH2CH2Cl) space filling diagram of 1chlorobutane (CH3CH2CH2CH2Cl)

Some examples to try out -

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Nomenclature - Naming Alcohols

With alcohols this suffix is ___anol.

Examples -

ethanol : displayed formula of ethanol (CH3CH2OH) space filling diagram of ethanol (CH3CH2OH)
propan-1-ol : displayed formula of propan1ol (CH3CHOHCH3) space filling diagram of propan1ol (CH3CHOHCH3)

Nomenclature - Naming Aldehydes and Ketones

(1) Aldehydes :

With aldehydes this suffix is ___anal.

Examples -

ethanal : displayed formula of ethanal (CH3CHO) space filling diagram of ethanal (CH3CHO)
butanal : displayed formula of butanal (CH3CH2CH2CHO) space filling diagram of butanal (CH3CH2CH2CHO)

(2) Ketones :

With ketones this suffix is ___anone.

Examples -

(i) prefix

- this depends on the number of carbon atoms in the longest chain

(ii) position number

- count from the nearest chain end to the carbonyl carbon atom

(iii) suffix

- add __anone around the position number at the end of the prefix

Exemplar compounds -

propanone : displayed formula of propanone space-filling diagram of propanone
pentan-2-one : displayed formula of pentan-2-one space-filling diagram of pentan-2-one

Nomenclature - Naming Carboxylic Acids

With carboxylic acids this suffix is ___anoic acid.

Examples -

ethanoic acid :
3-methylpentanoic acid :

With esters this suffix is ___anoate.

Examples -

ethyl ethanoate : displayed formula of ethyl ethanoate
1-methylethyl propanoate : displayed formula of 1-methylethyl propanoate

Nomenclature - Naming Cyclic Compounds

(1) Cycloalkanes :

They are called alkanes as they do not have any carbon-carbon double bonds. The name is based on the alkane with the same number of carbon atoms. Cyclo__ is added as a prefix to the whole name.

Examples -

cyclopropane : displayed formula of cyclopropane (C3H6) space filling diagram of cyclopropane (C3H6)
cyclopentane : displayed formula of cyclopentane (C5H10) space filling diagram of cyclopentane (C5H10)

(2) Cycloalkenes :

They are called alkenes as they have a carbon-carbon double bond. The name is based on the alkene with the same number of carbon atoms. Cyclo__ is added as a prefix to the whole name.

Examples -

cyclopropene : displayed formula of cyclopropene (C3H4) space filling diagram of cyclopropene (C3H4)
cyclohexene : displayed formula of cyclohexene (C5H8) space filling diagram of cyclohexene (C5H8)

(3) Cyclic Esters (Lactones) :

Examples -

oxiran-2-one : displayed formula of alactone (C2H2O2) space filling diagram of alactone (C2H2O2)
dihydrofuran-2-one : displayed formula of glactone (C4H6O2) space filling diagram of glactone (C4H6O2)

Nomenclature - Structural Isomers

(1) Chain Isomers :

This is also known as branching in a molecule.

When the alkane, or any other organic molecule, is not just a simple straight chain of carbon atoms joined together the names become a little more complex.

The longest connected chain of carbon atoms must be found as before and the alkane (alkene, alcohol, etc.) name generated as usual. For example, with 5 carbon atoms and 12 hydrogen atoms :

5 carbon atoms long is a pentane 4 carbon atoms long is a butane 3 carbon atoms long is a propane

Then the name for the pendent group is found, again by counting the number of carbon atoms present, and used as a prefix.

CH3- group : methyl___
CH3CH2- group : ethyl___
CH3CH2CH2- group : propyl___
CH3CH2CH2CH2- group : butyl___

The pendent group is itself prefixed by a number denoting which carbon atom it is attached to, followed by a hyphen.

If there is more than one of the same pendent group then each group is numbered, with a comma used to separate the numbers. The group is also prefixed by di__ to denote two of them, tri__ to denote three of them, tetra__ to denote four of them, penta__ to denote five of them, etc.

Continuing the previous examples :

No pendent groups = pentane 1 pendent methyl group on the 2nd carbon = 2-methylbutane 2 pendent methyl groups on the 2nd carbon = 2,2-dimethylpropane

N.B.: The numbers used to indicate the positions of the pendent groups must be the lowest numbers possible, so always check them from both ends of the molecule.

Further examples -

2,3-dimethylbutane 3-methylbut-1-ene 2-methylbutan-1-ol

Some examples to try out -


(2) Positional Isomers :


(3) Functional Group Isomers :


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Nomenclature - Stereoisomers

(1) Geometric Isomers :


(2) Optical Isomers :


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Skeletal formulae :

There are a number of different ways of representing an organic molecule, and all of them will probably examined in AS and A2 question papers.

The simplest is a molecular formula, which is simply the number of each type of atom totaled up.

A structural formula separates the atoms into groups, such as CH3 and CH2 parts of a molecule.

A displayed structure shows all the bonds between the atoms, including all the bonds going to hydrogen atoms.

A skeletal structure is similar to the displayed structure, though all the hydrogens are removed, including any bonds to hydrogen atoms, and the labels for carbon atoms are removed as well. Labels for atoms other than carbon are kept (see the nomenclature sections of other homologous series for examples of their skeletal formulae).

e.g. for propane,

molecular formula : C3H8
structural formula : CH3CH2CH3
displayed formula :
skeletal formula :

and for 3-ethylpentane the skeletal formula would look like this -

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Nomenclature - Summary Table

Homologous Series General Functional Exemplar Compound
Name Formula Group Name Formula Structure

Alkanes

CnH2n+2 carbon carbon single bond methane CH4 displayed formula of methane

Alkenes

CnH2n carbon carbon double bond ethene CH2CH2 displayed formula of ethene

Haloalkanes

CnH2n+1X carbon halogen single bond 1-chloropropane CH3CH2CH2Cl displayed formula of 4-chlorooctane

Amines

CnH2n+3N primary amine propylamine CH3CH2CH2NH2 displayed formula of propylamine

Nitriles

CnH2n-1N carbon nitrogen triple bond propanenitrile CH3CH2CN displayed formula of propanenitrile

Alcohols

CnH2n+2O carbon oxygen single bond butan-1-ol CH3CH2CH2CH2OH displayed formula of propan-1-ol

Aldehydes

CnH2nO pentanal CH3CH2CH2CH2CHO displayed formula of butanal

Ketones

CnH2nO pentan-2-one CH3COCH2CH2CH3 displayed formula of pentan-2-one

Carboxylic Acids

CnH2nO2 hexanoic acid CH3CH2CH2CH2CH2COOH displayed formula of hexanoic acid

Acid Chlorides

CnH2n-1ClO acyl chloride group hexanoyl chloride CH3CH2CH2CH2CH2CClO displayed formula of hexanoyl chloride

Acid Anhydrides

(CnH2n-1O)2O acid anhydride group propanoic anhydride (C2H5CO)2O displayed formula of propanoic anhydride

Esters

CnH2nO2 propyl butanoate CH3CH2CH2COOCH2CH2CH3 displayed formula of propyl butanoate

Amides

CnH2n+1NO heptanamide CH3CH2CH2CH2CH2CH2CONH2 displayed formula of heptanamide
N-propyl-butanamide CH3CH2CH2CONHCH2CH2CH3

Cycloalkanes

CnH2n cyclononane C9H18 displayed formula of cyclononane

Cycloalkenes

CnH2n-2 cyclodecene C10H18 displayed formula of cyclononane

Cyclic Esters (Lactones)

CnH2nO2 oxacycloundecan-2-one C9H18COO displayed formula of cyclononane
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written by Dr Richard Clarkson : © Saturday, 1 November 1997

Updated : Monday 22nd October, 2012

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