Nomenclature and Isomerism

Nomenclature

The IUPAC nomenclature system is a set of logical rules devised and used by organic chemists to circumvent problems caused by arbitrary nomenclature.

In general, an IUPAC name will have three essential features:

• A root or base indicating a major chain or ring of carbon atoms found in the molecular structure.
  • A suffix or other element(s) designating functional groups that may be present in the compound.
  • Names of substituent groups, other than hydrogen, that complete the molecular structure.

Here is a simple list of rules to follow. Some examples are given at the end of the list.

1. Firstly, the longest chain is to be identified which is known as parent chain
2. Identification of all substituents groups is to be done
3. Number the carbons of the parent chain from the end that gives the substituents the lowest numbers. When comparing a series of numbers, the series that is the "lowest" is the one which contains the lowest number at the occasion of the first difference. If two or more side chains are in equivalent positions, assign the lowest number to the one which will come first in the name.
4. If the same appending group occurs more than once, the location of each point on which the appendix group is given. Repetition of an appending group makes prefix occur di, tri, tetra etc.
5. If there are two or more different appending groups then, alphabetical order is used to enlist them. The prefix used for the appending group in alphabetical order is iso. The prefixes secondary and tertiary are used when compared.
6. If chain of equal length is to be made parent chain, then, following points aka priority:

The greatest number of side chains.
Substituents having the lowest- numbers.
Having the greatest number of carbon atoms in the smaller side chain.
Having least branched side chains

7. The prefix cyclo is directly used for cyclic hydrocarbons.

In summary, the name of the compound is written out with the substituents in alphabetical order followed by the base name (derived from the number of carbons in the parent chain). Commas are used between numbers and dashes are used between letters and numbers. There are no spaces in the name.

CH₄ → methane

C₂H₆ → Ethane

CH₃ —CH₃ → Ethane

C₃H₈ → Propane

CH₃ —CH₂ —CH₃ → n-propane

CH₃ —CH₂ — CH₂ —CH₂— CH₃ → n-pentane

₂

→ Isohexane OR 2 – methyl pentane (IUPAC)

→ Isobutane OR 2 − methyl propane

IUPAC Nomenclature

Prefix + word root + primary suffix + secondary suffix

Prefix → 3-methyl

Word root → Hex

Primary suffix → ane

Therefore the IUPAC name is 3-methyl hexane

Prefix → 2,2 – dimethyl

Word root → prop

Primary suffix → ane

Prefix →3,6,6_trimethyl-5,5-diethyl-4-propyl

Word root → Non

Primary suffix → ane

The IUPAC name is3,6,6_trimethyl-5,5-diethyl-4-propyl-nonane.

Prefix

-CH₃ → Methyl

-C₂H₅ → Ethyl

-Cl → Chloro

-Br → Bromo

-I → Iodo

-F → Fluoro

-CO → Keto

-CHO → Aldo

-NH₂ → Amino

-OH → Hydroxyl

-CN → Cyano

-NO₂ → Nitro

CH₃ Br Cl

I I I

CH₃ — C— C— C— CH₂— CH₃

I I I

Cl I Cl

Prefix → 3-bromo 2, 4, 4 –trichloro 3-iodo 2-methyl

Word root → Hex

Primary suffix → ane

The IUPAC name is 3-bromo 2, 4, 4 –trichloro 3-iodo 2-methyl hexane.

Note:

R-X → alkyl halide or haloalkane

X = Cl, Br, I, F

CH₃-Cl

Chloromethane OR Methyl Chloride

CH₃-CH₂-CH₂-Br

Bromo Propane OR Propyl bromide

Prefix → 3 – bromo 2 – chloro 3, 4 – diethyl 4 – methyl 2 – nitro

Word root → Hept

Primary suffix → ane

The IUPAC name is 3 – bromo 2 – chloro 3, 4 – diethyl 4 – methyl 2 – nitro Heptane

Prefix → 3, 3, 7 – tribromo 2, 2 – dichloro 4,4 – diethyl 6 – iodo

Word root → oct

Primary suffix → ane

The IUPAC name is 3, 3, 7 – tribromo 2, 2 – dichloro 4,4 – diethyl 6 – iodo Octane

Isomerism

The organic compounds having same molecular formula but different structural formula are called isomers and the phenomenon is called isomerism.

From butane isomers, occur i.e. methane, ethane, propane does not have isomers.

E.g. Butane (C₄H₁₀)

CH₃ – CH₂ – CH₂ – CH₃ → n – butane

CH₃ – CH – CH₃ → 2 – methylpropane

I

CH₃

E.g. Pentane (C₅H₁₂₎

E.g. Hexane (C₆H₁₄)

E.g. Heptane(C₇H₁₆)

Reference:

Pulse, Tracy. Introduction to chemistry. u.s.a: flexbook, 2010.

Pathak, Sita Karki. The Text Book of Chemistry. Kathmandu: Vidhyarthi Pustak Bhandar, 2012.