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Considering the basic strength of amines in aqueous solution, which one has the smallest pKb value ?

Option: 1

\mathrm{\left(C H_{3}\right)_{2} N H}

Option: 2

\mathrm{CH _{3} NH _{2}}

Option: 3

\mathrm{\left(C H_{3}\right)_{3} N}

Option: 4

\mathrm{C _{6} H _{5} NH _{2}}

Answers (1)


Comparison of Basic nature in aliphatic amines in aqueous phase. -

The basic strength of an amine in an aqueous medium is determined not only on the basic of +I effect of the alkyl groups attached to the nitrogen atom but also the solvation of alkyl ammonium ion formed by uptake of the proton. 

Both the  +I  effect and solvation of amine make basic order of amines in an aqueous medium as

\mathrm{2\degree>1\degree>3\degree>NH_{3}} (when R= CH_3)

The order of basic strength of Amine

\mathrm{(CH_{3})_{2} \:\: NH > CH_{3}-NH_{2}> (CH_{3})_{3}\:\: N> C_{6}H_{5}-NH_{2}}

The higher the basic strength, the smaller will be the pKb value.

So, \mathrm{\left( CH _{3}\right)_{2} NH}  has the smallest pKb value among the given species.

The correct option is 1.

Posted by

Gautam harsolia

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