Explain order of Kb in aquas solution of Triethylamine, Diethylamine and Ethylamine.

(Et)₂NH     >     (Et)₃N     >      (Et)NH₃             {Et = Ethyl group (-CH₂CH₃)}

Answer:-

-> (Et)₂NH is more basic than (Et)₃N because of less steric hindrance and more hydration energy of (Et)₂NH.

-> (Et)₃N is more basic than (Et)NH₃ because of more +I Effect in (Et)₃N.

Why Dimethylamine (2' amonia) is more basic than Trimethylamine (3' amonia) ?

Answer:-

-> Less steric hindrance in (CH₃)₂NH than (CH₃)₃N.

-> More hydration energy of (CH₃)₂NH than (CH₃)₃N.


Example:
  
Order of basic strength:

(CH₃)₂-NH           >           (CH₃)-NH₂            >        (CH₃)₃-N

{Secondary amine}       {Primary amine}            {Tertiary amine}

*Basic strength depends on:-

                                                  -> Inductive effect

                                                  -> Steric hindrance

                                                  -> Hydration energy

So, Dimethylamine is more basic than Trimethylamine in aqueous solution.

Reason:-

a. Less steric hindrance of Dimethylamine than Trimethylamine.

b. More hydration energy of Dimethylamine than Trimethylamine.

Which of the following have maximum PKa value ?

Options:

a. CH₃-CF-COOH                                                 b.  F-CH₂-CH₂-COOH

c. CH₃-CHBr-COOH                                             d.  CH₂Br-CH₂-COOH

Answer: (d)

Reason:

              Minimum –I Effect

              Minimum acidic strength

              Minimum K^a

              Maximum P^ka

Basic strength depends on

*Basic strength depends on:-

                                                  -> Inductive effect

                                                  -> Steric hindrance

                                                  -> Hydration energy

So, Dimethylamine is more basic than Trimethylamine in aqueous solution.

Reason:-

a. Less steric hindrance of Dimethylamine than Trimethylamine.

b. More hydration energy of Dimethylamine than Trimethylamine.

Example:-
a. CH₃-NH₂             b. (CH₃)₂-NH             c. (CH₃)₃-N

                                                                 -> More +I Effect

                                                                 -> More electron density

                                                                 -> More tendency to donate lone pair

                                                                     but H⁺ less acceptable lone pair

                                                                     due to Steric hindrance.

Order of basic strength:

(CH₃)₂-NH           >           (CH₃)-NH₂            >        (CH₃)₃-N

{Secondary amine}       {Primary amine}            {Tertiary amine}

Acidic strength is directly proportional ot stability of conjugate base.

Note:  *Acidic strength  stability of conjugate base

i.e. More stable conjugate base more acidic character of corresponding acid.

Example:-

1.  H₃C-CH₃         <     H₂C=CH₂       <      HC≡CH

-> H₃C-C^-H₂              H₂C=C^-H                HC≡C^-         {Conjugate base}

-> -ve on less EN (SP³ C)                      -> -ve on more EN (SP C)

-> less stable conjugate base                -> more stable conjugate base

2. H₃C-CH₂-OH     >     (CH₃)₂-CH-OH      >     (CH₃)₃-C-OH

-> H₃C-CH₂-O^-              (CH₃)₂-CH-O^-               (CH₃)₃-C-O^-     {Conjugate base}

-> Less +I effect of ethyl group                       -> more +I effect of butyl group

-> Less –ve charge on O-atom                        -> more –ve charge on O-atom

-> more stable conjugate base                        -> less stable conjugate base

3. NO₂-CH₂-COOH   >  H-CH₂-COOH   >   CH₃-CH₂-COOH

-> NO₂-CH₂-COO^-        H-CH₂-COO^-         CH₃-CH₂-COO^-     {Conjugate base}

                                                                   -> More +I effect

                                                                   -> More –ve charge

                                                                   -> Less stable conjugate base

                                                                    So, cross pounding acid is less acidic

Acidic strength is directly proportional to -I effect and inversion-ally proportional to +I effect.

Example:

a. H₃C-CH₂-COOH      >       (CH₃)₂-CH-COOH       >       (CH₃)₃-C-COOH

                               +I Effect increases

          ========================================>

                             So, acidic strength decreases

b.   Cl₃C-CH₂-COOH      >     Cl₂HC-CH-COOH      >      ClH₂C-C-COOH

                               -I Effect decreases

         =========================================>

                         So, acidic strength decreases

c. CH₃-CH₂-CHF-COOH     >     CH₃-CHF-CH₂-COOH   >   F-(CH₂)₃-COOH

             Distance of Fluorine atom increases so, -I Effect decreases

         ==========================================>

                         That’s why acidic strength decreases

d. COOH-COOH    >    COOH-CH₂-COOH   >   COOH-CH₂-CH₂-COOH

   {Oxalic acid}               {Malonic acid}                 {Succinic acid}

         ==========================================>

              +I Effect increases so, acidic strength decreases

e. CH₃-(CH₂)₂-COOH   <    CH₂=CH-CH₃-COOH   <   CH≡C-CH₂-COOH

          ===================================================>

                   -I Effect increases

                S-Character increases 

                So, EN increases 

                So, -ve charge decreases 

                   That’s why stability increases

Negetive charge on more EN atom is more stable and Positive charge on less EN atom is more stable

Note: *Negative charge on more EN-Atom is more stable.

Example:-

=> HO^-   is more stable than    H₂C^-

-> 8 protons                          -> 6 protons

-> more attraction          

    So, more stable

Note: *Positive charge on less EN-Atom is more stable.

Example:-

=> H₃C⁺    is more stable than     HO⁺

-> 6 protons                              -> 8 protons

                                                 -> more repulsion

                                                      So, less stable