IUPAC Nomenclature

IUPAC nomenclature uses the longest continuous chain of carbon atoms to determine the basic root name of the compound.The root name is then modified due to the presence of different functional groups which replace hydrogen or carbon atoms in the parent sturcture.

There are a number of different ways to modify the root name to indicate the functional groups present.

Substitutive : (most common) : the highest priority functional group modifies the suffix of the root name, while all other groups, or substituents, are added as prefixes to the root name.

Functional group : names the compound based on the highest priority functional group, i.e. as an alcohol, ketone, alkyl halide, etc.

Replacement : used to indicate when an atom, usually carbon, is replaced by another atom.

Conjunctive : used to combine named subunits (i.e. cyclohexanecarboxylic acid).

Common or trivial : due to widespread use, some compunds with simple names have been adopted into basic IUPAC nomenclature.

Remember: that organic molecules can in general be either chains (also known as acyclic)

or cyclic or a combination of both. In most cases this doesn't make a difference.  The general rules for cyclic systems will be developed for cycloalkanes and can be applied to other scenarios.

molecules are not restricted to a single functional group, they can have several functional groups. A common example are amino acids which have both an amine and a carboxylic acid present.

Basic rules

The IUPAC systematic name of an organic compound can be constructed based on a series of steps and rules:

Indentification of the principle functional group and substituents Indentification of the longest continuous chain containing the principle

functional group.

Assign locants (i.e. numbering) to the principle functional group and substituents.

The steps and rules are summarised below, more details are provided as the cases are encountered.

Principle Functional group

The principle functional group is used to define the class the compound belongs to e.g. an alcohol, ROH

The principle functional group is usually given the lowest locant possible.

Longest chain The longest continuous chain containing the principle functional group defines the root name. 

Other groups attached to this chain are called substituents.

If there are two chains of equal length, then the choice that gives the simplest substituents is chosen.

Numbering (i.e. assigning locants)

The numbers that define the positions of the principle functional group and substituents are called locants.

Compounds are numbered from one end of the longest continuous chain.

The locants are assigned such that the principle functional group gets the lowest possible locant.

If this results in a "tie" then the first point of difference rule is applied so that the first time a difference in numbering occurs, then the method that gives the lower number at this first difference is used.

In the event that there is no first point of difference then alphabetisation is used.

Basic IUPAC Organic Nomenclature

What's in a name?

The IUPAC name of an organic molecule is assembled from components that describe various features of the molecule.

Functional group suffixThis is added to the end of the name based on the principle functional group.

RootThis defines the number of atoms (usually carbon atoms) in the longest continuous chain that contains the principle functional group.

Substituent prefixAny groups other than the principle functional group are substituents and are added to the beginning of the name in alphabetical order.

MultiplierIf a group occurs more than once, a simple multiplier (e.g. di, tri, tetra, etc.)  is used

to indicate how many times it occurs.

LocantsLocants are numbers (or occasionally letters e.g. N-) that define the position of the principle functional group and substituents.  Typically there needs to be a locant for each functional groups and each substituent. Llocant for the principle functional group is placed before the functional group suffix, e.g. pentan-2-ol, see below.

The basic structure of the IUPAC name is shown schematically below :

Root names

C1 = meth- C2 = eth-

C3 = prop-

C4 = but-

C5 = pent-

C6 = hex-

C7 = hept-

C8 = oct-

C9 = non-

C10 = dec-

C11 = undec-

C12 = dodec-

C16 = hexadec-

C18 = octadec-

C20 = icos-

Functional Groups—Functional group is part of molecule which mainly responsible for its physical and chemical properties.

Some compound and their functional groups:

Table-1

S.N. General form of compounds Functional Group1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

R-X

R-F

-X halo

16

17

18

19

20

21

22

23

Table2

Name of some important functional groups as prefix and suffix

Group Formula Prefix Suffix

Table-3

Important functional groups arranged in decreasing priority order for a nomenclature perspective

1

2

3

4

5

6

7

8

9

Principle functional group Secondary suffix

-OH ol

-CHO al/ carbaldehyde

-COOH oic acid/Caboxylic acid

-COOR oate/carboxylate

-CONH2 amide/carboxamide

-COO- oate/carboxylate

-RSH thiol

Group Common name

Alkanes

Nomenclature Formula

Functional group suffix = -ane

Substituent name = alkyl

.Simple alkane chains without branches are named using the appropriate root name plus the suffix -ane:

Propane

  CH3CH2CH3

Simple cyclic alkanes  without branches are named using the prefix cyclo plus the appropriate root name plus the suffix -ane:

Cyclopropane

Simple Branched Alkanes

The substituent is named in a similar way to the parent alkane. It is named based on the number of carbon atoms in the branch plus the suffix -yl.  

CH3- methyl

CH3CH2- ethyl

CH3CH2CH2- propyl

CH3CH2CH2CH2- butyl

CH3CH2CH2CH2CH2- pentyl

2-methylpentane

CH3CH2CH2CH(CH3)2 

3-ethylpentane

CH3CH2CH(CH2CH3)CH2CH3

3,3-dimethylhexane

CH3CH2C(CH3)2CH2CH2CH3

2,3-dimethylhexane

(CH3)2CHCH(CH3)CH2CH2CH3

 4-ethyl-2-methylhexane

(CH3CH2)2CHCH2CH(CH3)2

Common names for alkyl substituents

Certain alkyl substituents are very common, and you should be able to recognise and name them quickly, these are listed below

Notes:

the the prefix "iso" is not hyphenated. the prefix "n-"denotes the straight chain substituent i.e. n-hexyl =

CH3CH2CH2CH2CH2CH2-

the symbol "R" is commonly used to generically represent an alkyl group, e.g. R-X

Structural formula

Name Generic line drawing

CH3- methyl-

CH3CH2- ethyl-

CH3CH2CH2-propyl-orn-propyl

(CH3)2CH- isopropyl-

CH3CH2CH2CH2-butyl-orn-butyl

CH3CHCH2CH3sec-butyl- or s-butyl

(CH3)2CHCH2- isobutyl

(CH3)3C-tert-butyl or t-butyl

Complex names for alkyl substituents

For the substituent: determine the point of attachment to the chain that defines the parent root (i.e.

the longest continuous chain) the first carbon in the substituent is regarded as the C1 of the substituent

from C1, find the longest continuous chain originating from C1 - this is the root for the substituent.

identify any substituents off this chain

list these alphabetically with appropriate multipliers

insert locants as required remembering that the point of attachment is defined as C1.

Now include this "complex" substituent in the overall name... the complex substituent appears in brackets proceeded by its locant the complex substituent is alphabetised based on the first letter of the name in

the bracket : this includes the multiplier e.g. di

this is because the term in brackets is the name of the whole complex substituent and not several substituents.

See Complex substituent name : 1,1-dimethylethyl

4-(1,1-dimethylethyl)-5-ethylnonane

Common alkyl substituents can also be named in this more systematic way.

Alkyl group, R- Common name Complex name (if different)

CH3- methyl-

CH3CH2- ethyl-

CH3CH2CH2- propyl-

(CH3)2CH- isopropyl- (1-methylethyl)-

CH3CH2CH2CH2- butyl-

CH3CH2CHCH3 sec-butyl- or s-butyl (1-methylpropyl)-

(CH3)2CHCH2- isobutyl (2-methylpropyl)-

(CH3)3C- tert-butyl or t-butyl (1,1-dimethylethyl)-

Remember that if a structure has two chains of equal length, then the choice that gives the simplest branches is chosen,

3-ethyl-2,4-dimethylhexane

Substituted Cycloalkanes

In principle, a substituted cycloalkane could be name in two ways, either as an alkyl substituted cycloalkane, or as a cycloalkyl substituted alkane

When the ring size (e.g. number of C atoms) is larger than the longest continuous chain, then the ring becomes the parent and hence the system is treated as a alkyl cycloalkane.

Other rules: If a ring is monosubstituted then no locant is required since the substituent

must be at C1. If a ring is polysubstituted then each substituent requires a locant to

unambiguously assign their relative positions.

2-cyclopropylbutane

ethylcyclohexane

1-ethyl-2-methylcyclohexane

Polycyclic alkanes

(1) Spiro ring systems: Spiro ring systems share a single common atom

spiro[2.2]pentane spiro[2.5]octane 5-methylspiro[3.5]nonane

(2) Fused ring systems: Fused ring systems share two or more common atoms,

bicyclo[1.1.0]butane

(3) Bridged ring systems: If there are more than two common atoms then the a "bridged" system is obtained.

bicyclo[2.2.1]heptane

Alkenes

Nomenclature Formula

Functional group suffix = -ene

Substituent name = alkenyl

 propene

CH3CH=CH2

 but-2-ene or 2-butene

 CH3CH=CHCH3

cyclohexene 4-methylpent-2-ene or 4-methyl-2-pentene

(CH3)2CHCH=CHCH3

 3-methylhex-3-ene or 3-methyl-3-hexene

Alkenes as substituents

In some cases, a group containing an alkene may need to be treated as a substituent. 

In these cases the substituent is named in a similar fashion to simple alkyl substituents.

The method is required when the alkene is not the priority group.

The substituent is named in a similar way to the parent alkene.

It is named based on the number of carbon atoms in the branch plus the suffix -yl. i.e. alkenyl

There are two common names that are widely used:

Alkenyl group Common name Systematic name

CH2=CH- vinyl- ethenyl

CH2=CHCH2- allyl- 2-propenyl

CH3CH=CH- 1-propenyl

CH3CH=CHCH(CH=CH2)2 3-ethenylhexa-1,4-diene

CH2=CHH=CH2

buta-1,3-diene or 1,3-butadieneCH3CH=CHCH=CH2

(E)-penta-1,3-diene or (E)-1,3-pentadiene

CH3C(CH3)=CHH=CH2

2-methylpenta-1,4-dieneor

2-methyl-1,4-pentadiene

Alkynes

Nomenclature Formula

Functional group suffix = -yne

Substituent prefix = alkynyl

CH3CH2C≡CHbut-1-yne or 1-butyne

CH3C≡CCH3

but-2-yne or 2-butyne

CH3C≡CCH2C≡CHhexa-1,4-diyne

or1,4-hexadiyne

Alcohols

Nomenclature Formula

Functional class name = alkyl alcohol e.g. ethyl alcohol

Substituent suffix = -ol e.g. ethanol

Substituent prefix = hydroxy-   e.g. hydroxyethane

CH3CH(OH)CH3

propan-2-ol or 2-propanol (or isopropanol)

CH2=CHCH2CH2OH

but-3-en-1-ol or 3-buten-1-ol

Primary amines Primary amines, NH2R may be named according to one of three methods as follows:

(a) by citing the name of the substituent group R as a prefix to the name of the parent hydride azane;

(b) by adding the suffix "-amine" to the name of the parent hydride RH;

(c) by adding "-amine" to the substituent name for the group R .

Examples to R-5.4.1

Ketones, thioketones, and their analogues R-5.6.2.1 Ketones. The generic term "ketone" refers to compounds containing a carbonyl group, >C=O, joined to two carbon atoms. Ketones are named substitutively by adding a suffix such as "-one", and "-dione" to the name of a parent hydride with elision of the final "e" of the parent hydride, if any, before "o". When a group having priority for citation as principal characteristic group is present, a ketone is described by the prefix "oxo-". Functional class names for monoketones and vicinal diketones, etc., are formed by citing the prefix names for the two groups attached to the carbonyl group(s) in alphabetical order followed by the class name "ketone", "diketone", etc., as a separate word.

Examples to R-5.6.2.1

Diketones derived from cyclic parent hydrides having the maximum number of noncumulative double bonds by conversion of two -CH= groups into >CO groups with rearrangement of double bonds to a quinonoid structure may be named alternatively by adding the suffix "-quinone" to the name of the aromatic parent hydride.

Example to R-5.6.2.1

Acyl derivatives of benzene or naphthalene have been named by changing the "-ic acid" or "-oic acid" ending of a trivial name of the acid corresponding to the acyl group to "-ophenone" or "-onaphthone". Only the names acetophenone, propiophenone, and benzophenone are retained in these recommendations (see R-9.1, Table 27(a)). Acyl derivatives of cyclic parent hydrides are named

by prefixing the substituent name derived from the cyclic parent hydride to the name of the acyclic ketone.

Example to R-5.6.2.1

Some trivial names are retained (see R-9.1, Table 27(a)).

R-5.6.2.2 Chalcogen analogues of ketones are named by using suffixes such as "-thione" and "-selone", and prefix names such as "thioxo-" and "selenoxo-". The use of prefixes such as "thio-" and "seleno-" with the trivial names of ketones, such as acetone, to indicate replacement of the ketonic oxygen atom with a chalcogen atom is not recommended.

Examples to R-5.6.2.2