|Class 12 Chemistry Aldehydes Ketones Acids||Chemical properties of aldehydes and ketones|
Chemical properties of aldehydes and ketones
Types of reactions shown
In this given structure
The nucleophile attack on the carbon atom and the reaction occur in a way as shown below:
This reaction is acid catalysed reaction .Due to which hydroxyl group is formed from aldehyde.
Relative strength of Aldehydes and ketones
If the magnitude of positive charge is more on carbon atom ,then more readily it shows nucleophilic addition reaction .So, out of all members HCHO shows this reaction more readily as it has no alkyl group( electron releasing group) present.
So, if we compare formaldehyde, ethanol and propanone, the order of their strength towards nucleophilic addition is:
More the bulky groups attached, less space will be available for nucleophile to attack. Therefore, reactivity will be less. So, this can also be the reason to explain the trend.
Let’s show some nucleophilic addition reactions:
(a)Addition of HCN (Cyanohydrins are formed)
If only HCN is used, the reaction is slow. So, the base is used that help in removing hydrogen ion from HCN and sets the nucleophile free. As a result, reaction becomes fast. The base reacts as shown below:
If we carry out hydrolysis of the above compound, 2-methyl 2-hydroxy propionitrile, then the lactic acid is produced due to hydrolysis of CN group as shown below:
If we heat lactic acid, in presence of sulphuric acid then the water molecule gets removed and acrylic acid is formed as shown:
The reaction involved is given below:
For aldehyde, the reaction goes in forward reaction and for ketone backside is favoured (as two methyl groups increase bulkiness and form unstable products).
Note: diethyl ketone, acetophenone and Benzophenone will not undergo these reactions because of bulkiness.
When Grignard reagent is added following reactions occur:
Please note: In this formaldehyde form primary alcohol, any other aldehyde form secondary alcohol and ketone forms tertiary alcohol.
(d)Addition of alcohols: In this we get two products that are hemi-acetals and acetals from aldehyde. From ketone we get, ketals.
The reaction occurs as shown:
Please note in this we use dry HCl