Ketones : Structure and Nomenclature

In this article we will learn about Structure and Nomenclature of Ketones. 

Ketones are the organic compounds with carbon –oxygen double bond (C=O; carbonyl group). They have general formula (RCOR).

Key words: Ketone, Carbonyl compound, Common names and IUPAC nomenclature of ketones.

Introduction

The ketones have general formula as R-C(O)-R ; where R can be alkyl or aromatic. If the carbonyl group is bonded to two identical groups, then the ketone is termed as symmetrical ketone. And the ketone with two different groups is called as unsymmetrical ketone. 

Common Nomenclature

The ketones are named by adding the alkyl / aromatic groups in alphabetical order followed by the word “ketone”. When the both the groups are similar then prefix “di” is used in the names of ketone.

Let’s see few examples;

The compound 1 has two groups attached to carbonyl group these are ethyl (CH₃-CH₂) and methyl (CH₃). Now write the groups in alphabetical order followed by the word “ketone”. Therefore name of the compound 1 is ethyl methyl ketone.

The compound 2 has ethyl (CH₃-CH₂) and propyl (CH₃-CH₂-CH₂) bonded to the carbonyl group. Hence the name of the compound 2 is ethyl propyl ketone.

Similarly; compound 3 is named as methyl phenyl ketone, since it has phenyl and methyl groups bonded to the carbonyl group. 

Now consider compound 4 and 5 which has both the groups similar. In this situation we need to use prefix “di” in the name of ketone.

The compound 4 has two ethyl groups bonded to carbonyl groups so it is named as diethyl ketone. Likewise compound 5 is named as diphenyl ketone.

IUPAC Nomenclature

According to the IUPAC nomenclature system, ketones are considered as derivatives of analogous alkanes. There are certain steps we need to follow for assigning name of the compounds.

Step 1: Select longest carbon chain which includes carbonyl group.

Step 2: From the number of carbon atoms assign the name of alkane,

Step 3: Numbers to each carbon atom. Start the numbering in such a way that carbonyl carbon gets least possible number.

Step 4: Now remove “e” from the root name and add carbonyl number along with suffix “one”. This will be the root name of ketone.

Step 5: write the substituents in alphabetical order. 

For example; write the IUPAC name of the compound 6.

Give the numbers to each carbon in longest carbon chain. Also carbonyl carbon should get least possible number. Here numbering with carbonyl carbon 2 is correct. The numbering with carbonyl carbon 4 is not correct.
Here the longest chain is having 5 carbon atoms. Hence name of alkane is pentane. Now omit “e” and add 2-one. So correct name of compound 6 is pentan-2-one

Consider the compound 7 shown below and write its IUPAC name. 

The longest chain contains 6 carbon atoms. So the alkane name is hexane. Give the numbers to each carbon in longest carbon chain. The numbering with carbonyl carbon 3 is correct. Therefore root name of the ketone is hexan-3-one
There is methyl group present at carbon number 4. Hence write prefix “4-methyl”. So correct name of compound 6 is 4-methyl-hexan-3-one.

Let’s see the compound 8, what will be its IUPAC name?

The longest chain contains 4 carbon atoms. So the alkane name is butane. The numbering with carbonyl carbon 1 is correct. Therefore root name of the ketone is butan-1-one.
Consider phenyl group as substituent. Hence write prefix “1-phenyl”. So correct name of compound 6 is 1-phenyl-butan-1-one.

Now write IUPAC name of compound 9?

The cyclic chain contains 6 carbon atoms. So the alkane name is cyclohexane. The numbering should be start from carbonyl carbon. Therefore root name of the ketone is cyclohexan-1-one.
Here methyl group is a substituent at carbon number 2. Hence write prefix “2-methyl”. So correct name of compound 9 is 2-methyl-cyclohexan-1-one.

 Examples for practice:

Sr. No.	Structure	IUPAC Name
1.		-----------------------------------
2.		-----------------------------------
3.		-----------------------------------
4.		-----------------------------------

That's all for this topic. If you have any questions please feel free to ask me in the comment box. Thank you..!