Question

Arrange the following compounds in increasing order of their property as indicated:
(A) $\text{ Acetaldehyde, Acetone, Di-tert-butyl ketone, Methyl tert-butyl ketone }$ (reactivity towards$\text{ HCN }$)
(B) $\text{C}{{\text{H}}_{\text{3}}}\text{C}{{\text{H}}_{\text{2}}}\text{COOH, C}{{\text{H}}_{\text{3}}}\text{CH}\left( \text{Br} \right)\text{C}{{\text{H}}_{\text{2}}}\text{COOH, }{{\left( \text{C}{{\text{H}}_{\text{3}}} \right)}_{\text{2}}}\text{CHCOOH, C}{{\text{H}}_{\text{3}}}\text{C}{{\text{H}}_{\text{2}}}\text{CH}\left( \text{Br} \right)\text{COOH }$ (Acid strength)
(C) $\text{ Benzoic acid, 4-Nitrobenzoic acid, 3, 4-Dinitrobenzoic acid, 4-Methoxybenzoic acid }$ (Acid strength)

Answer 1

The inductive effect is stated as the effect on electron density in the part of the molecule due to the electron-withdrawing or electron-donating group present on the molecule. There are two types of inductive effects which are +I when the chemical species tend to donate an electron and –I effect when the chemical species tend to withdraw the electron.

Complete Solution:
We know that the inductive effect is the effect associated with the electron density of the molecule. It is found to be related to the ability of the substituents to either donate or withdraw the electron density to the attached carbon. Based on it let's find out the order of compounds:

(A) The cyanide is nucleophilic. It attacks the carbonyl group of the compounds. Steric hindrance at the carbonyl groups increases with the increase in the +I effect of the alkyl group.This creates a steric crowding for the cyanide attack and hence the reactivity of compounds towards the nucleophilic attack of $\text{ CN }$ decreases. Hence, the increasing order of the reactivity towards $\text{ HCN }$ is as follows:
$\text{ Di-tert-butyl ketone }>\text{ Methyl tert-butyl ketone }>\text{ Acetone }>\text{ Acetaldehyde }$

(B) The electron-donating group increases the electron density around the oxygen of the hydroxyl group which results in the destabilization of charge and it decreases the rate of protonation.Hence, the +I effect decreases the acidity.
However, the electron-withdrawing group stabilizes the negative charge formed on the oxygen after deprotonation. Thus.-I the effect increases the acidity of the compound. In short acidic strength increases because of the –I effect and decreases due to+ I effect.
Let's have a look at the compound. The Isopropyl group shows a higher +I effect compared to a propyl group. Thus, butanoic acid is a stronger acid than 2-methyl propionic acid.
The inductive effect depends on the distance. Increase in distance causes the decrease in -I effect. Therefore, the acidic strength of 3-bromobutanoic acid is less than that of 2-bromobutanoic acid. Therefore, the increasing order of acid strength is given as follows:
$\text{ }{{\left( \text{C}{{\text{H}}_{\text{3}}} \right)}_{\text{2}}}\text{CHCOOH }<\text{ C}{{\text{H}}_{\text{3}}}\text{C}{{\text{H}}_{\text{2}}}\text{C}{{\text{H}}_{\text{2}}}\text{COOH }<\text{ C}{{\text{H}}_{\text{3}}}\text{CH}\left( \text{Br} \right)\text{C}{{\text{H}}_{\text{2}}}\text{COOH }<\text{ C}{{\text{H}}_{\text{3}}}\text{C}{{\text{H}}_{\text{2}}}\text{CH}\left( \text{Br} \right)\text{COOH }$

(C) We are interested to arrange the compounds in increasing order of their acidity. The acidic strength of the compound depends on how easily it loses its proton. Therefore, acid strength increases due to the electron-withdrawing group and decreases due to the electron-donating group.
Electron donating groups like methyl, methoxy, etc. produced +I effect. Therefore, 4-methoxy benzoic acid is a weaker acid as compared to benzoic acid.
- Electron withdrawing groups like nitro, cyano, etc. produce –I effect and increase the acidity. Therefore, the acid strength of 4-nitrobenzoic acid and 3, 4-dinitrobenzoic acid is greater than the benzoic acid strength.
- As the 3, 4-nitro benzoic acid has two nitro groups in the Meta position to the carboxylic acid it has higher acid strength than that of 4-nitrobenzoic acid.
The increasing order of acid strength is given as follows,
$\text{ 4-methoxy benzoic acid }<\text{ benzoic acid }<\text{ 4-nitrobenzoic acid }<\text{ 3, 4-dinitrobenzoic acid }$

Note: Before solving such a type of question always remember to draw the chemical structure of the compound. Chemical structure gives an idea about the effects of acting in the compound. The inductive effect is mostly observed in organic compounds. Try to link the properties with each other.
-For example, the inductive effect (+I) depends on the electron-donating group. It donates electron density which increases the electron density around the hydroxyl group of carboxylic acid group and more negative charge on oxygen which destabilizes carboxylate ion and therefore decreases in acid strength.