Amines are basic in nature due to the presence of lone pair of electrons on N atom of `-NH_2` group. The basic strength of amines can be expressed by their dissociation constant, `K_b` or `pK_b` .
`RNH_2+H_2O rarr RNH_3^(+) +OH^(-)`
`K_b = ([RNH_3^(+)][OH^(-)])/([RNH_2])` and `pK_b= -log K_b` Greater the `K_b` value or smaller the `pK_b` value, more is the basic strength of amine. Aliphatic amines are stronger bases than ammonia due to the electron releasing effect of alkyl groups. The basic strength among amines decreases as : `2^@>1^@>3^@` Aryl amines such as aniline are less basic than aliphatic amines due to the involvement of lone pair of electrons on N atom with the resonance in benzene. In derivatives of aniline, the electron releasing groups increase the basic strength while electron withdrawing groups decrease the basic strength. The base weakening effect of electron withdrawing group and base strengthening effect of electron releasing group is more marked at p-position than at m-position. Every o- substituted aniline is less basic than aniline due to ortho effect. Answer the following 5 questions : Which of the following has lowest `pK_b` value ?

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