Properties of Aldehydes and Ketones
Physical Properties
of Aldehydes and Ketones
− Boiling Points
of Aldehydes and Ketones
The boiling point of aldehydes and ketones is higher than that of non-polar compounds (hydrocarbons) but lower than those of corresponding alcohols and carboxylic acids as aldehydes and ketones do not form H-bonds with themselves.
− Solubility
of Aldehydes and Ketones
The lower members (up to 4 carbons) of aldehydes and ketones are soluble in water due to H-bonding. The higher members do not dissolve in water because the hydrocarbon part is larger and resists the formation of hydrogen bonds with water molecules.
Chemical Properties
of Aldehydes and Ketones
Both aldehydes and ketones contain carbonyl group, therefore they undergo same reactions like nucleophilic addition reactions, oxidation, reduction, halogenation etc.
− Nucleophilic Addition Reactions
in Aldehydes and Ketones
Aldehydes are more susceptible to nucleophilic attack than ketones as described below :
Reactivity of Aliphatic Aldehydes and Ketones
Aliphatic aldehydes are more reactive than ketones because of the following reasons :
- Inductive effect : Alkyl groups are electron donating in nature (i.e., show
+I-Effect). Hence, the presence of alkyl groups attached to
the carbonyl group increases the electron density on the carbonyl carbon thereby decreases its reactivity towards nucleophilic addition
reactions. Therefore, the order of reactivity is :
- Steric hinderance : The more the number and the bigger the size of the alkyl group, the more difficult for a nucleophile to
attack due to steric hinderance (crowding). Hence, the order of reactivity is :
Reactivity of Aromatic Aldehydes and Ketones
Aromatic aldehydes and ketones exhibit electron donating resonance which increases the electron density on the carbonyl carbon.
Because of this reason, the carbonyl carbon becomes less electrophilic, and hence is less susceptible to nucleophilic attack.
Aromatic aldehydes, however, are more reactive than aromatic ketones. The reactivity of aromatic aldehydes and ketones follows the order :
C6H5CHO > C6H5COCH3 > C6H5COC6H5
− Oxidation Reaction
Aldehydes can be easily oxidised to carboxylic acids due to the presence of a hydrogen atom on carbonyl group which can be easily converted to OH group. Since ketones do not have any hydrogen atom attached to the carbonyl group, they cannot be oxidised easily, and therefore, strong oxidising agents are required for this purpose.
− Reduction of Aldehydes and Ketones
Aldehydes and ketones can be reduced to a variety of compounds such as alcohols, hydrocarbons etc.
− Other Reactions of Aldehydes and Ketones
Aldehydes and ketones undergo many other reactions that we will discover in later chapters.
Questions
Question : Arrange the following compounds in increasing order of their boiling points
CH3CHO, CH3CH2OH, CH3OCH3, CH3CH2CH3
Answer : The above compounds have comparable molecular masses. Out of them CH3CH2OH has the highest boiling point because alcohol molecules tend to form intermolecular H-bonds with each other.
Since the >C=O group present in aldehydes is quite polar whereas ethers are slightly polar, the dipole moment of CH3CHO is higher than that of CH3OCH3. Therefore, dipole-dipole interactions are stronger in CH3CHO than in CH3OCH3; hence, the boiling point of CH3CHO is higher than that of CH3OCH3.
CH3CH2CH3 molecules are only held by weak van der Waals' forces. Hence, they have the least boiling point among the given compounds.
Hence, the order of their boiling points is : CH3CH2CH3 < CH3OCH3 < CH3CHO < CH3CH2OH
Question : Arrange the following compounds in increasing order of their reactivity in nucleophilic
addition reactions
(i) Ethanal, Propanal, Propanone, Butanone.
(ii) Benzaldehyde, p-tolualdehyde, p-nitrobenzaldehyde, acetophenone.
Answer :
(i) The increasing order of reactivity is: butanone < propanone < propanal < ethanal. The reasons are given below:
- Aldehydes are more reactive than ketones because the >C=O group in aldehydes is less crowded.
- The +I-effect of alkyl groups increases in the order : methyl < ethyl.....
(ii) Acetophenone being a ketone is the least reactive among the given compounds.
The presence of an electron releasing group -CH3 at p-position in p-tolualdehyde makes it less reactive than benzaldehyde. Conversely, the presence of an electron withdrawing group -NO2 at p-position in p-nitrobenzaldehyde makes it more reactive than benzaldehyde.
Hence, the order is: Acetophenone < p-tolualdehyde < benzaldehyde < p-nitrobenzaldehyde.