Alkanes And Its IUPAC Names

Alkanes are the hydrocarbons containing Carbon and Hydrogen attached by a single bond.

Alkanes are known as paraffins (Parum =less, affinity = reactivity).

General formula of Alkane is C_nH_2n+2

IF n = 1 then C₁H_2x1+2 = CH₄ Methane

IF n = 2 then C₂H_2x2+2 = C₂H₆ Ethane

IF n = 3 then C₂H_2x2+2 = C₃H₈ Propane and so on.....

IUPAC names of first 20 straight chain alkanes are as follows,

Number of C atoms	Molecular Formula	Name of straight chain	Structural Formula
1	CH4	methane	CH4
2	C2H6	ethane	CH3-CH3
3	C3H8	propane	CH3-CH2-CH3
4	C4H10	n-butane	CH3-CH2-CH2-CH3
5	C5H12	n-pentane	CH3CH2CH2CH2CH3
6	C6H14	n-hexane	CH3CH2CH2CH2CH2CH3
7	C7H16	n-heptane	CH3CH2CH2CH2CH2CH2CH3
8	C8H18	n-octane	CH3(CH2)6CH3
9	C9H20	n-nonane	CH3(CH2)7CH3
10	C10H22	n-decane	CH3(CH2)8CH3
11	C11H24	n-undecane	CH3(CH2)9CH3
12	C12H26	n-dodecane	CH3(CH2)10CH3
13	C13H28	n-tridecane	CH3(CH2)11CH3
14	C14H30	n-tetradecane	CH3(CH2)12CH3
15	C15H32	n-pentadecane	CH3(CH2)13CH3
16	C16H34	n-hexadecane	CH3(CH2)14CH3
17	C17H36	n-heptadecane	CH3(CH2)15CH3
18	C18H38	n-octadecane	CH3(CH2)16CH3
19	C19H40	n-nonadecane	CH3(CH2)17CH3
20	C20H42	n-icosane	CH3(CH2)18CH3

 

To understand IUPAC names of alkanes containing branches within the structure, first we need to understand Alkyl Group.

What is an Alkyl group?

•      An alkyl group is formed by removing one hydrogen atom from an alkane. They are part of alkanes but generally known as substituents.

 

International Union Of Pure and Applied Chemistry (IUPAC) of Alkanes: To write the IUPAC of alkanes we need to understand some of rules which are as follows,

 

Rule 1.  Select the longest continuous carbon chain.

Identify the longest (maximum number) carbon chain in the molecule. This chain is called the parent chain.

Rule 2. Name the longest chain.

The longest carbon chain is chosen as the basis for the name. The names are given as per number of carbon atom present in parent ring. These are the Parent Names.

Rule 3. Number the longest chain The carbon atoms in the longest chain are numbered. The numbering should be started from that end which will give lowest number to the carbons carrying substituents.

Rule 4. Identify the substituent. Name the substituent. Indicate its position by the number of the carbon atom to which it is attached.

 

 

 

 

Rule 5. Prefix the position number and name of substituent onto the parent name. The whole name is written as one single word.

Rule 6. When the same substituents are present two or more times in the molecules, prefix di (for two), tri (for three), tetra (for four), penta (for five), etc. are used.

While writing IUPAC names, the numbers are separated by (,) commas, and a hyphen (-) should be written between a number and a alphabet.

In the given example we can count the parent chain both side, as methyl groups are present at 2^nd and 3^rd positions a comma (,) should be added between them and a hyphen (-) to separate number and alphabet.

Rule 7. When two or more different substituents are present, their names are arranged in alphabetic order.

In the following example two substituents are present first is ethyl which is present on 5^th  position and second is methyl which is present on 4^th position of the parent chain but as letter "e" of ethyl comes first in alphabetical order than "m" of methyl, it should be written before methyl while writing the IUPAC name. 

Some of the examples are as follows,

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