Read the passage carefully and answer the questions.
Amines are considered to be derivatives of ammonia obtained by replacement of hydrogen atoms with alkyl or aryl groups. They are usually prepared from nitro compounds, halides, amides, imides etc. The presence of hydrogen bonding influences its physical properties. In alkylamines, a combination of electron releasing steric and hydrogen bonding factors influences the stability of substituted ammonium cations in protic polar solvents and thus affects the basic nature of amines. Alkyl amines are found to be stronger bases than ammonia which is more basic than water. Primary and secondary amines are engaged in intermolecular association due to hydrogen bonding. This intermolecular association is more in primary amines than in secondary amines. Tertiary amines do not have intermoleulcar association due to the absence of hydrogen atom available for hydrogen bonding. Thus, the boiling point of amines depends on hydrogen bonding. In aromatic amines, electron releasing and withdrawing groups increase and decrease their basic character. Aryl diazonium salts usually obtained from arylamine undergo replacement of the diazonium group with a variety of nucleophile. Their coupling reaction of aryl diazonium salts with phenol or arylamine results in formation of dyes.
The most basic species in the following is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \({NH_2}^-\)
A base accepts a proton. The weaker the conjugate acid, the stronger the base, because a species that holds its proton tightly is formed eagerly from the base. So compare the conjugate acids of the four species:
\( \ce{NH2-} \) → \( \ce{NH3} \): ammonia hardly gives up a proton at all; it is an extremely weak acid.
\( \ce{OH-} \) → \( \ce{H2O} \): water is a very weak acid, but much stronger than ammonia.
\( \ce{NH3} \) → \( \ce{NH4+} \): the ammonium ion is a weak acid.
\( \ce{H2O} \) → \( \ce{H3O+} \): the hydronium ion is a strong acid.
The order of basicity is therefore the reverse of the acid strengths of these conjugate acids:
\( \displaystyle \ce{NH2-} \gt \ce{OH-} \gt \ce{NH3} \gt \ce{H2O} \)
Between the two anions, nitrogen is less electronegative than oxygen, so the negative charge on N in \( \ce{NH2-} \) is held less firmly and its lone pairs are more available for a proton. So the most basic species is \( \ce{NH2-} \).
Why the other options are incorrect
Option (2) \(H_2O\) — Incorrect. Water is the weakest base of the four; its conjugate acid \( \ce{H3O+} \) is a strong acid.
Option (3) \(OH^-\) — Incorrect. Hydroxide is a strong base, but its conjugate acid, water, is a stronger acid than ammonia, so \( \ce{OH-} \) is weaker than \( \ce{NH2-} \); \( \ce{NH2-} \) even removes a proton from water: \( \ce{NH2- + H2O -> NH3 + OH-} \).
Option (4) \(NH_3\) — Incorrect. Ammonia is a neutral weak base; losing a proton to form the anion \( \ce{NH2-} \) makes the nitrogen far more basic.