KPK Board 12th class Chemistry Ch 19 Aldehydes and Ketones questions answers
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Aldehydes can be easily oxidized even by a mild oxidizing agent due to presence of hydrogen atom directly attached with carbonyl carbon. While no such hydrogen is there in all other carbonyl compounds this is characteristic hydrogen of aldehydes.
Nucleophilic addition reaction is a catalyzed by either by a acid or by a base,
Mechanism: In acid catalyzed addition reaction the addition process is initiated by the proton of the acid which combines with carbonyl oxygen atom to form oxonium ion. This increase the electrophilic nature of carbonyl carbon by inducing more positive charge on it and thus increase its ability to be attack by a weak nucleophiles.
In base catalyzed addition reaction the base react with attacking reagent and generate the nucleophile. This nucleophile attacked the carbon atom of carbonyl group and the pi electron of C=O bond move to the oxygen atom because of its high electronegativity.
Aldehydes and ketones undergo nucleophilic addition reactions whereas alkene,undergoes electrophilic addition reaction. This is because in case of aldehyde and ketones the =C=O bond is more polar due to greater electronegativity of oxygen atom which creates an electrophilic carbon therefore a nucleophile can easily attack the electrophilic carbon. Whereas double bond is alkene (C = C) is not that much polar due to which electron density is equally distributed between the two carbon atom hence an electrophile can be easily attacked hence they undergo electrophilic addition reactions easily rather than nucleophilic addition reaction.
Aldehydes can be easily oxidized because of presence of hydrogen atom directly attached with carbonyl carbon due to which an aldehyde can be easily oxidized as compare to ketone.
Ketones are less reactive than aldehydes towards nucleophile. The less reactivity of the ketones is due to the following two reasons;
- Steric Hindrance:
The carbonyl carbon of ketone is more crowded as compared to the aldehyde. Thus the approach of the nucleophile toward the positive charge carbon of carbonyl group is hindrance.
- Electronic effect of Alkyl group:
Another factor that decreases the reactivity of ketone is the electron donating effect of alkyl group attached to carbonyl carbon. Because alkyl group reduce the partial positive charge on carbonyl carbon.
The following two tests are used to distinguish between aldehydes and ketones.
(i) Tollen’s test (ii) Fehling Solution Test
1 Tollen’s Test:
Reagents: AgNO,/NH,OH
CH3-CO-H + 2AgNO3 + NH4OH àCH3-CO—NH+4 + 2Ag + H2O
This test is not given by ketones.
(2) Fehling Solution Tests: Reagent: Cu(OH)2 / NaOH
CH3-CO-H + Cu(OH)2 + NaOH + CH3-CO–-Na+ + Cu2O + H2O
This test is also not given by ketones.
