Q.a.

Ketones have two alkyl groups around the carbonyl carbon versus one in aldehydes — both electronically and sterically this makes them less reactive to nucleophiles.

Acetone, $(CH_3)_2C=O$, is a ketone with two alkyl (methyl) groups attached to the carbonyl carbon, whereas ethanal, $CH_3CHO$, is an aldehyde with only one alkyl group (the other position being a hydrogen).

• Electronic reason: alkyl groups are electron-donating (+I effect); with two such groups, acetone's carbonyl carbon has its electrophilicity (positive character) reduced more than ethanal's, making it a less attractive target for an incoming nucleophile. …

An aldehyde has only one +I/bulky alkyl group on the carbonyl carbon, so that carbon is more electrophilic and less crowded than a ketone's carbonyl carbon, hence more reactive to nucleophiles.

Less steric crowding and a more electron-poor carbonyl carbon make methanal react with HCN faster than acetone does — both a steric and an electronic effect favouring methanal.

Cyanohydrin formation is a nucleophilic addition reaction at the carbonyl carbon, and its rate depends on how accessible and how electrophilic that carbon is.

Steric factor: Methanal, $HCHO$, has only two small hydrogen atoms attached to the carbonyl carbon, leaving it wide open for the incoming $CN^-$ nucleophile to attack. Acetone, $(CH_3)_2C=O$, has two bulkier methyl groups that crowd the carbonyl carbon and hinder the approach of $CN^-$.

Electronic factor: The methyl groups in acetone are electron-releasing (+I effect), which pushes electron density onto the carbonyl carbon, making it less positively charged and hence less attractive to the nucleophile. Methanal, lacking any alkyl substituent, keeps its carbonyl carbon strongly electrophilic. …

Nucleophilic addition to a carbonyl compound is favoured by a more electrophilic, less hindered carbonyl carbon — so aldehydes beat ketones, and within each class, smaller alkyl substituents beat bulkier ones.

Two factors govern reactivity toward nucleophilic addition:

1. Electronic (+I) effect: alkyl groups attached to the carbonyl carbon push electron density onto it, making the carbon less electron-deficient and less attractive to an incoming nucleophile. More/bulkier alkyl groups $\Rightarrow$ lower reactivity.
2. Steric effect: bulkier alkyl groups around the carbonyl carbon physically hinder the approach of a nucleophile. More/bulkier groups $\Rightarrow$ lower reactivity.

Applying this to the given compounds:

• Ethanal ($CH_3CHO$) has only one small methyl group on the carbonyl carbon — least hindered, most electrophilic $\Rightarrow$ most reactive.
• Propanal ($CH_3CH_2CHO$) has one, slightly bulkier ethyl group — still an aldehyde (only one substituent), so more reactive than any ketone, but less reactive than ethanal. …

Two bulky, electron-releasing groups flank a ketone's carbonyl carbon (vs. one group + a small H for an aldehyde), lowering both its electrophilicity and its accessibility to a nucleophile.

Nucleophilic addition to a carbonyl group depends on the electrophilicity of the carbonyl carbon and how sterically accessible it is. In a ketone, the carbonyl carbon bears two alkyl/aryl groups, both of which are electron-donating (+I / hyperconjugative), reducing the positive character (electrophilicity) of the carbonyl carbon; they are also bulkier, sterically hindering the approach of a nucleophile. In an aldehyde, only one such group is present (the other position is a small hydrogen atom), so the carbonyl carbo …