Solutions and answers to chemistry problems: Reasoning Questions

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REASONING QUESTIONS

Q. Tell which is the stronger base and why.

Ans

(a) Aniline or o-bromoaniline.
Aniline is a stronger base than o-bromoaniline. Aniline form anilinium ion by accepting a proton.

In general, electron-donating groups such as -CH₃, OCH₃, -NH₂ etc. increase the basecity by stabilizing the anilinium ion while electron withdrawing substituents such as - NO₂, -CN, -X (halogens) decrease the basicity of amines by destabilizing the anilinium ion. But o-substituted anilines are usually weaker bases than anilines regardless of the nature of the sulistituent (electron withdrawing or donating). This is due to Detho-effect which is due to combination of steric and electronic factors.
In case of halogens (-Cl, -Br, -I), electron withdrawing inductive effect dominates so it is expected that o-bromoaniline should be weaker base than aniline

(b) aniline or cyclohexylamine
Cyclohexylamine is a stronger base than aniline. This is because lone pair of electrons on the N- atom of aniline is declocalized over the benzene ring.

However in cyclhexylamine the lone pair of electrons on N- atom are not conjugated and are therefore more readily available for donation or protonation than that on the N- atom of aniline. Further N- atom inaniline is sp² hybridized and in cyclohexylamine it is wp³ hybridized. Since sp² has greater electronegative character than sp³, the electrons are more tightly held in case of aniline whereas they are losely held and readily available for protonation in cyclohexylamine.

Q. Illustrate the amphoteric nature of amino acids by writing an equation for the reaction of proline in its dipolar ion form with an equivalent of

Ans

(a) hydrochloric acid. HCl

(b) sodium hydroxide, NaOH

In general, in dipolan ion form amino acid can accept a proton from acid and exist as cation and can donate a proton to base and exist as anion.
This shows its amphoteric behaviour.