IIT JEE , AIEEE Forum - Chemistry , Organic Chemistry

sonalisapal

if methyl grp is attached to 1degree,2 degree,3 degree amine the order of basicity is given as 2 degree >1>3>NH3 wheras when ethyl grp attached order of basicity is given as 2>1>NH3>3...why??????

which is more basic benzyl amine or aliphatic 2 degree amine and why?

shubham_sachdeva

q1. steric hindrance.
Q2. ALIPHATIC 2 DEGREE AMINE.

sonalisapal

but phenyl of C6H5-CH2-NH2 will exert +R then it shud be more basic than 2 degree amine?

shubham_sachdeva

PHENYL GP WILL SHOW +i EFFECT LESS THEN ALKYL GPS. ALSO BENZENE RING IS STABLIZED DUE TO RESONANCE...

harsha_27

ans to the first qn is the steric hindrance
the ans to the 2nd qn. is 2*amine because in benzyl amine the lone pair on N is not involved in the +M effect as in aniline however phenyl group exerts weak +I effect on N but that effect is less than the methyl group so 2*amine has more e* density and more basic...............

madhusudan.chavan

The basicity of an amine in aqueous solution primarily depends upon the stability of the ammonium cation or the conjugate acid formed by accepting a proton from water. It depends on following three factors: i) +I effect of the alkyl groups, ii) Extent of H-bonding with water molecules, iii) steric effects of the alkyl groups. All these three factors are favourable for sec. amine and hence sec. amines are strongest bases. If the alkyl group is methyl then there is no steric hinderance to H-bonding. The stability due to H-bonding predominates over the stability due to +I effect of the methyl group, hence methyl amine is stronger base than trimethylamine.
However, if the slkyl group is bigger than methyl,i.e., ethyl, propyl,etc. there will be some steric hinderance to H-bonding. As a result stability due to + I effect predominates over the stability due to H-bonding and hence tert. amines become more basic than prim amines.
All classes of aliphatic amines are stronger bases than ammonia.
Due to - I effect of the aryl group, the unshared electron pair on nitrogen in benzyl amine is less available than in alkyl amines for sharing with a proton. As a result simple aralkyl amines are weaker bases than alkyl amines.

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