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Chemistry Question on Aldehydes, Ketones and Carboxylic Acids

Arrange the following compounds in increasing order of their property as indicated:(i)Acetaldehyde,Acetone,$$Di-tert-butyl ketone,Methyl tert-butyl ketone(reactivity towards HCNHCN)(ii) CH_3CH_2CH(Br)COOH,CH_3CH(Br)CH_2COOH,$$(CH_3)_2CHCOOH,CH_3CH_2CH_2COOH(acid strength)(iii)Benzoic  acid,4−Nitrobenzoic  acid(iii)Benzoic\space acid,4-Nitrobenzoic \space acid,3,4−Dinitrobenzoic  acid,4−Methoxybenzoic  acid3,4-Dinitrobenzoic \space acid,4-Methoxybenzoic \space acid(acid strength).

Answer

(i) When HCNHCN reacts with a compound, the attacking species is a nucleophile,CN−CN^{-}. Therefore, as the negative charge on the compound increases, its reactivity with HCN decreases. In the given compounds, the+I+I effect increases as shown below. It can be observed that steric hindrance also increases in the same.

Hence, the given compounds can be arranged according to their increasing reactivities toward HCN as:Di−tert−butyl  ketone<Methyl  tert−butyl  ketone<Acetone<AcetaldehydeDi-tert-butyl \space ketone<Methyl \space tert-butyl \space ketone<Acetone<Acetaldehyde

(ii) After losing a proton, carboxylic acids gain a negative charge as shown:

Now, any group that will help stabilize the negative charge will increase the stability of the carboxyl ion and as a result, will increase the strength of the acid. Thus, groups having.+I+I effect will decrease the strength of the acids and groups having −I- I effect will increase the strength of the acids.In the given compounds,−CH3-CH_3 group has +I+I effect and Br−Br- group has −I-I effect.Thus,acids containing. Br−Br- are stronger.
Now, the +I+I effect of isopropyl group is more than that of n−propyln-propyl group.Hence,(CH3)2CHCOOH(CH_3)_2CHCOOH is a weaker acid than CH3CH2CH2COOHCH_3CH_2CH_2COOH.

Also,the −I-I effect grows weaker as distance increases.

Hence, CH3CH(Br)CH2COOHCH_3CH(Br)CH_2COOH is a weaker acid than CH3CH2CH(Br)COOHCH_3CH_2CH(Br)COOH.Hence,the strengths of the given acids increase as:(CH3)2CHCOOH<CH3CH2CH2COOH<CH3CH(Br)CH2COOH<CH3CH2CH(Br)COOH(CH_3)_2CHCOOH<CH_3CH_2CH_2COOH<CH_3CH(Br)CH_2COOH<CH_3CH_2CH(Br)COOH

(iii) As we have seen in the previous case, electron-donating groups decrease the strengths of acids, while electron-withdrawing groups increase the strengths of acids. As methoxy group is an electron-donating group,4−methoxybenzoic  acid4-methoxybenzoic \space acid is a weaker acid than benzoic acid. Nitro group is an electron-withdrawing group and will increase the strengths of acids. As 3,4−dinitrobenzoic  acid3,4-dinitrobenzoic \space acid contains two nitro groups, it is a slightly stronger acid than4−nitrobenzoic  acid 4-nitrobenzoic \space acid.Hence,the strengths of the given acids increase as:

4−Methoxybenzoic  acid<Benzoic  acid<4−Nitrobenzoic  acid<3,4−Dinitrobenzoic  acid.4-Methoxybenzoic\space acid<Benzoic\space acid<4-Nitrobenzoic \space acid< 3,4-Dinitrobenzoic\space acid.