Organic Chemistry - Part II

3. Naming of hydrocarbons
International Union of Pure and Applied Chemistry (IUPAC) has standardized names of organic compounds. Since organic compounds are many in numbers and are quite complicated, IUPAC names are always used. In the discussion below, we will see how hydrocarbons are named.

The number of carbon atoms in a molecule, where it is situated, in a chain or a branch, is what one has to first see. The table below shows the suffixes are used for the number of carbon atoms present.

Number of carbon atoms	Suffix used
1	meth
2	eth
3	prop
4	but (pronounced as bute)
5	pent
6	hex
7	hept
8	oct
9	non
10	dec

Alkyl Groups
In inorganic chemistry we have seen what radicals are. To recapitulate, radicals are atoms or groups of atoms that always stay together in a reaction. For example OH^-, SO₄^--, etc. are radicals. In organic chemistry, radicals exist but in the form of groups of carbon and hydrogen atoms. A group formed by removal of one hydrogen atom from an alkane molecule is called an alkyl group. For example methyl group is denoted by CH₃ , ethyl group is represented as C₂H₅  . The structures of methyl and ethyl groups are shown below.  

Knowing the names of suffixes, the alkyl groups and the fact that hydrocarbons derive their names from ”ane” (single bond), “ene” (double bond) and “yne” (triple bond), we can now see how the names of hydrocarbons are standardized by IUPAC.  

Naming of saturated hydrocarbons

I. IUPAC nomenclature for straight chain saturated hydrocarbons : 

1. CH₄ :  The compound contains 1 carbon atom. Hence the suffix is “meth”. It has saturated bond, that is carbon bound to 4 hydrogen atoms in a sigma bond. There are no double or triple bonds. Hence this compound is an alkane. The IUPAC name, thus is a combination of “meth” and “ane” that is methane.
The structural formula of CH₄  is shown below.  

2. C₂H₆ : The compound contain 2 carbon atoms. Hence the suffix is “eth”. The structural formula is shown below.  

The carbon atoms are saturated, with each having four bonds attached to different atoms. Thus it is an alkane. The IUPAC name is a combination of “eth” and “ane”. Which is ethane.

3. C₃H₈ : The compound contain 3 carbon atoms. Hence the suffix is “prop”. The structural formula is shown below.  

The carbon atoms are saturated, with each having four bonds attached to different atoms. Thus it is an alkane. The IUPAC name is a combination of “prop” and “ane”. Which is propane.

4. C₄H₁₀ : The compound contain 4 carbon atoms. Hence the suffix is “but”. The structural formula is shown below.  

The carbon atoms are saturated, with each having four bonds attached to different atoms. Thus it is an alkane. The IUPAC name is a combination of “but” and “ane”. Which is butane. We have seen in the earlier section, that butane can have a branch also. To distinguish between a straight chain and a branched chain, the structural formula with a straight chain is commonly known as n-butane (n for normal). But IUPAC does not give any adjectives, so in the IUPAC nomenclature the straight chain butane is called just butane. The branched butane is given a different name, as we shall see later in the chapter.

5. C₅H₁₂ : The compound contain 5 carbon atoms. Hence the suffix is “pent”. The structural formula is shown below.  

The carbon atoms are saturated, with each having four bonds attached to different atoms. Thus it is an alkane. The IUPAC name is a combination of “pent” and “ane”. Which is pentane. Just like butane, pentane can have branches too. To distinguish between a straight chain and a branched chain, the structural formula with a straight chain is commonly known as n-pentane (n for normal). But IUPAC does not give any adjectives, so in the IUPAC nomenclature the straight chain pentane is called just pentane. The branched pentane is given a different name, as we shall see later in the chapter. 

Examples below will clarify the way the IUPAC nomenclature is used.  

Example 1 :  Consider the molecule C₄H₁₀, which as a straight chain in n-butane. Now a branched chain with 4 carbon atoms can also be made.  

To write the IUPAC  name, first consider the straight chain. It has 3 carbon atoms, hence it is a propane molecule. Number the carbon atoms as 1, 2 and 3 from left to right. The alkyl group is a methyl group (CH₃) attached to carbon atom number 3.  

The IUPAC name of this molecule is 2-methy-propane. This is a relatively simple structure. If you number the carbon atoms from right to left, again the molecule will be named as 2-methyl-propane.

Example 2 : Consider the molecule C₅H₁₂. The simple branched chain will have 4 carbon atoms in a straight chain and a methyl  group attached to one of the carbon atoms in the straight chain.  

The straight chain is called butane. Number the carbon atom so that the alkyl group branch is attached to the lower numbered carbon atom.  

The IUPAC name of this molecule is 2-methyl-butane. The common name of this molecule is isopentane.

There is yet another way the structure of the hydrocarbon C₅H₁₂ may be present. This has two branches.    

The common name for this compound is neopentane as it contains 5 carbon atoms. But the IUPAC name is 2,2,dimethyl-propane.  

The reason for this can be appreciated from the rules given earlier. The straight chain has 3 carbon atom, hence the parent hydrocarbon is a propane. There are two methyl groups (CH₃)  attached to carbon atom number 2. Hence the name 2,2, dimethyl-propane.

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