Question

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 \(HCN\))\((ii) CH_3CH_2CH(Br)COOH,CH_3CH(Br)CH_2COOH,\)\((CH_3)_2CHCOOH,CH_3CH_2CH_2COOH\)(acid strength)\((iii)Benzoic\space acid,4-Nitrobenzoic \space acid\),\(3,4-Dinitrobenzoic \space acid,4-Methoxybenzoic \space acid\)(acid strength).
 

Answer 1

(i)When \(HCN\) reacts with a compound, the attacking species is a nucleophile,\(CN^{-}\). Therefore, as the negative charge on the compound increases, its reactivity with HCN decreases. In the given compounds, the\( +I \) 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:\(Di-tert-butyl \space ketone<Methyl \space tert-butyl \space ketone<Acetone<Acetaldehyde\)

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(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.\( +I \) effect will decrease the strength of the acids and groups having \(- I\) effect will increase the strength of the acids.In the given compounds,\(-CH_3\) group has \(+I \) effect and \(Br-\) group has \(-I\) effect.Thus,acids containing. \(Br-\) are stronger.
Now, the \(+I\) effect of isopropyl group is more than that of \(n-propyl \) group.Hence,\((CH_3)_2CHCOOH\) is a weaker acid than \(CH_3CH_2CH_2COOH\).
 

Also,the \(-I\) effect grows weaker as distance increases.

Hence, \(CH_3CH(Br)CH_2COOH\)  is a weaker acid than \(CH_3CH_2CH(Br)COOH\).Hence,the strengths of the given acids increase as:\((CH_3)_2CHCOOH<CH_3CH_2CH_2COOH<CH_3CH(Br)CH_2COOH<CH_3CH_2CH(Br)COOH\)

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(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,\(4-methoxybenzoic \space acid\)  is a weaker acid than benzoic acid. Nitro group is an electron-withdrawing group and will increase the strengths of acids. As \(3,4-dinitrobenzoic \space acid\) contains two nitro groups, it is a slightly stronger acid than\( 4-nitrobenzoic \space acid\).Hence,the strengths of the given acids increase as:

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