Nomenclature of Organic Compounds

• According to IUPAC system of nomenclature, name of an organic compound consists of following three parts:

1. Word root
2. Suffix
3. Prefix

Word Root

• Word root is the basic unit of name, and represents the number of carbon atoms in the parent chain.
• Parent chain is selected as per the longest chain rule.
• Parent chain is the longest continuous chain of C-atoms including functional group and multiple bonds, if any.
• General word root for any carbon chain is alk.

Suffix

• There are two types of suffixes — Primary suffix and Secondary suffix.

Primary Suffix

• Primary suffix indicates the nature of linkages (bonding) between carbon atoms.
• 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.

• Use of primary suffix on appropriate word root for some typical compounds is illustrated below

• Extra ‘a’ has been added to the word root since the primary suffix, diene or diyne begins with a consonant, i.e., ‘d’ instead of a vowel.

Secondary Suffix

• Secondary suffix is used to indicate a functional group in the organic compound.
• Secondary suffixes for various functional groups are given in table below;

• Secondary suffix is added to primary suffix, (written after primary suffix).
• While adding secondary suffix to primary suffix, following rules should be followed:

1. If the secondary suffix begins with a vowel, then the terminal ‘e’ of the primary suffix is dropped before adding a secondary suffix.
2. If the secondary suffix begins with a consonant, then the last (terminal) ‘e’ is retained and the secondary suffix is added after ‘e’.
3. 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.

Prefix

• Prefix is a part of a name which appears before the word root.

• There are two types of prefixes — Primary prefix and Secondary prefix.

Primary Prefix

• Primary prefix is used to distinguish alicyclic (aliphatic cyclic compounds) from acyclic (open chain) compounds.
• If the given compound is alicyclic, then prefix cyclo is used just before the word root. For example,

Secondary Prefix

• In the IUPAC system of nomenclature, certain characteristic groups are not considered functional groups. Such characteristic groups are considered as substituents and are denoted by secondary prefixes.
• While adding secondary prefix, following rules should be followed:

1. In case of alicyclic (aliphatic cyclic) compounds, secondary prefix is added before primary prefix, (before word cyclo), in alphabetical order.
2. In case of open-chain (acyclic) compounds, a secondary prefix is added just before word root in alphabetical order.
3. Name of the organic compound can be obtained by writing word root, prefixes and suffixes in the following order.

Secondary Prefix + Primary Prefix + word Root + Primary Suffix +Secondary Prefix

Alkanes

General formula: CnH2n+2   Suffix: ane

• Alkanes are open chain hydrocarbons in which carbon atoms are linked with one another by single bonds.
• Earlier these compounds were called paraffins (meaning with no affinity).

Classification of Carbon Atoms in Alkanes

• Depending on number of carbon atoms to which a carbon atom is bonded, it can be classified as follows:

1. Primary carbon atom (1°).Carbon atom is attached to one carbon atom only.
2. Secondary carbon atom (2°).Carbon atom is attached to two other carbon atoms.
3. Tertiary carbon atom (3°).Carbon atom is attached to three other carbon atoms in.
4. Quaternary carbon atom (4°). Carbon atoms are attached to four carbon atoms.
5. Hydrogen atoms attached to primary (1°), secondary (2°) and tertiary (3o) carbon atoms are referred to as primary, secondary and tertiary hydrogens respectively.

Alkyl Groups

• Alkyl group is represented by the general formula CnH2n+1 and denoted by R (an alkane is denoted as RH).
• Alkyl groups are named after their parent alkanes by replacing the suffix -ane by -yl.

Name of the alkane – ane + yl → Name of the alkyl Group

• Nature of the alkyl group depends on the nature of hydrogen atoms lost from the hydrocarbon chain.
• Accordingly, the following cases become possible.

Alkenes

General formula: CnH2n   Suffix: ene

• Alkene is structurally obtained by removing two hydrogens from corresponding alkane.
• Common name of an alkene is obtained by replacing ending –ane from the name of its parent alkane with -ylene.

Alkane –  ane  +  ylene  → Alkylene

• IUPAC name of an alkene is derived by replacing -ane of parent alkane by -ene.

Alkane –  ane  + ene  →  Alkene

• In higher alkenes, the position of the double bond is indicated by the serial number of the first carbon of the double bond.
• Numbering of carbon atoms in the chain is done by the lowest number rule.

Alkenyl Group

• Groups derived by removing hydrogen atoms from an alkene molecule are called alkenyl groups.
• IUPAC name is obtained by replacing – e of alkene by suffix -yl.
• Numbering of group-chains starts with carbon directly attached to the parent chain.
• Few alkenyl groups have trivial names which are commonly used.

Alkynes

General formula: CnH2n-2    Suffix: -yne

• Unsaturated hydrocarbons containing carbon-carbon triple bond (−C≡C−) are called alkynes (earlier acetylenes).
• Position of the triple bond is indicated by the serial number of the first carbon.
• IUPAC name of an alkyne is obtained by replacing ending -ane from the name of parent alkane with -yne.

Name of the alkane – ane + yne → Name of the alkyne

Haloalkanes (or Alkyl Halides)

General formula: CnHn2+1 X or RX Prefix: Halo

• Haloalkanes (alkyl halides) are monohalogen derivatives of alkanes. These were earlier called alkyl halides.
• Haloalkanes are obtained by replacing one hydrogen atom of alkane molecule by one halogen (X) atom

RH  −  H  +  X (= F, Cl, Br, I) →  RX

Common name of haloalkane is written by adding the word halide to the alkyl group attached to the halogen atom.

Name of the alkyl halide = Name of the alkyl group + Halide

• IUPAC name of monohalogen derivative of alkane is obtained by adding prefix halo (fluoro, chloro, bromo or iodo) to the name of parent alkane.

Halo + Name of parent alkane = Haloalkane

• For example, the monochloro derivative of methane is called chloromethane, (by prefixing chloro to methane).
• Carbon atoms are -numbered, if necessary, to show the position of the halogen atom.

Polyhaloalkanes

• Halogen derivatives of alkanes containing more than one halogen atom are termed polyhaloalkanes.
• Compounds containing two halogen atoms are termed dihalo-, and those containing three halogen atoms trihalo-, and so on.
• General formula of dihalogen derivatives is CnH2nX2.
• In the IUPAC system, the dihalogen derivative is named as the dihalogen derivative of parent alkane, and positions of halogen atoms are indicated as usual.

Alkanols, (Saturated Monohydric Alcohols)

General formula CnH2n+1OH Functional group: -OH Suffix: ol

• Alkanols (alcohols) are mono-hydroxy derivatives of alkanes, represented by the general formula CnH2n+1OH (R−OH).
• Alkanols are further classified as primary, secondary and tertiary alcohols according to the nature of the carbon atom to which −OH group is attached.
• Common names of monohydric alcohols are obtained from the name of the alkyl group attached to the −OH group by adding the word alcohol to it.

Name of the alkyl group + alcohol → Name of the alcohol

• IUPAC name of monohydric alcohol is obtained by replacing terminal -e from the name of alkane with -ol.

Name of alkane – e + ol → Name of the alkanol (alcohol)

Alkoxy alkanes (or Ethers)

General formula: CnH2n+1−O−CmH2m+1   Functional group: R−O− Prefix: alkoxy

• Ethers are alkoxy derivatives of alkanes, represented by the general formula R−O−R’, where R and R’ are alkyl groups.
• When R & R’ are the same then the ether is termed as simple ether.
• When R & R’ are different, ether is termed as mixed ether.
• Alkoxy alkanes are obtained by replacing one hydrogen of alkane by an alkoxy group
• In the common system of nomenclature, ethers are named after alkyl groups attached to oxygen atoms. Alkyl groups are named in alphabetical order.
• In IUPAC system of nomenclature, larger alkyl group is considered as residual alkane and smaller alkyl group with oxygen atom is considered as alkoxy group.

Alkanals (or, Aldehydes)

• Common name of an aldehyde is obtained from the common name of acid which it gives on oxidation.
• This is done by replacing ending -ic acid in the name of acid by the word aldehyde.
• In the IUPAC system, the name of an aldehyde is obtained by replacing terminal ‘e’ of parent alkane by suffix -al.

IUPAC name of the aldehyde = Name of the parent alkane – e + al = Alkanal

• In aldehydes, −CHO group is always present at the end of the chain. So, there is no need to designate its position as 1.

• While counting carbon atoms in the parent chain, carbon of the −CHO group is also counted.
• For example, parent chain in CH3CH2CH2CHO consists of four C atoms; it should be named as butanal and not butan-1-al 

Alkanones (or, Ketones)

• Two alkyl groups (R and R’) may be the same or different.
• When two alkyl groups (R and R’) are same, the ketone is called a simple ketone, when the two alkyl groups are different then the ketone is called a mixed ketone.
• Common name of an alkanone (ketone) is based on the names of alkyl groups attached to the carbonyl group. Alkyl groups are named in alphabetical order.
• IUPAC name of an alkane is obtained by replacing last -e from the name of parent hydrocarbon with suffix –one.
• Position of the carbonyl group is indicated by a numeral referring to the serial number of carbon atoms of the carbonyl group in the chain. Numbering is done so as to give the carbonyl group the lowest number.

IUPAC name of the ketone = Name of the parent alkane – e + one → Alkanone

Conclusion

It is a method of naming organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). The primary function of chemical nomenclature is to ensure that a spoken or written chemical name leaves no ambiguity concerning which chemical compound the name refers to: each chemical name should refer to a single substance.