ISOMERISM TRICK TO CALCULATE THE NUMBER OF ALDEHYDE AND KETONE ISOMERS

ISOMERISM TRICK For ALDEHYDE AND KETONES

REMEMBER THIS TABLE FIRST

Carbon Number	No. of Isomers
1	1
2	1
3	2
4	4
5	8

 ALDEHYDES AND KETONES

If carbon and hydrogen are in this relation CnH_2nO then be sure the given molecular formula gives ALDEHYDE and KETONE isomers.

We know general formula for aldehyde is R-CHO and that of ether is R-CO-R
[Since aldehydes and ketones are functional isomers  so from the provided molecular formula both the isomers of aldehydes and ketones can be obtained.]
First you should place the provided molecular formula in the above format.

Eg.

a)  CH₂O
H-CHO
[You should omit the carbon of functional group] 
no carbon atoms, so total no. of aldehyde isomer =1.


To form ketone minimun 3 carbon atoms are required

Total isomer = 1 aldehyde isomer only.

b)  C₂H₄O
CH₃-CHO
[Don't count the carbon atom of functional group]
Carbon no. 1  so, 1 aldehyde isomer

Total no. of isomers = 1 aldehyde

 c)  C₃H₆O
C₂H₅-CHO
Carbon no. 2  so, 1 aldehyde isomer


Now, for ketone
CH₃-CO-CH₃
See carbon no. on R.H.S.
[keep one carbon on L.H.S, it is compulsary]

Carbon no. 1 so, 1 ketone isomer.

Total no. of isomers = 1 aldehyde + 1 ketone = 2 isomers

 d)  C₄H₈O
C₃H₇-CHO
Carbon no. 3  so, 2 aldehyde isomer


Now, for ketone
CH₃-CO-C₂H₅
See carbon no. on R.H.S.

Carbon no. 2 so, 1 ketone isomer.

Total no. of isomers = 2 aldehyde + 1 ketone = 3 isomers


 e)  C₅H₁₀O
C₄H₉-CHO
Carbon no. 4  so, 4 aldehyde isomer


Now, for ketone
CH₃-CO-C₃H₇
See carbon no. on R.H.S.

Carbon no. 3 so, 2 ketone isomer.

[Note: In case of such unsymmetrical alkyl group on R.H.S. and L.H.S. increase the L.H.S. by 1 carbon no. at a time as we discussed in alcohol isomers]
[there was unsymmetrical alyl group on  CH₃-CO-C₂H₅  but we didn't increase because on increasing it will give the same ketone as C₂H₅-CO-CH₃]

Increasing the carbon number we get,
C₂H₅-CO-C₂H₅
See carbon no. on R.H.S.
Carbon no. 2 so, 1 ketone isomer.

 Total ketone isomers = 2 + 1  = 3

Total no. of isomers = 4 aldehyde + 3 ketone = 7 isomers

Confused!! in which case we shall increase the carbon no. by step 1
Solution: You have to increase the carbon no as I did in ketone only in case of those functional groups which contains unequal no. of alkyl groups on their two sides.