Structure and Bonding:  The Behavior of Amines

Write the reaction for the neutralization of ammonia with hydrogen ion.

Examine the structures of ammonia and the three amines given below.  Write the formulas for all four substances.  How would you describe the geometry of the nitrogen atom in each molecule?

ammonia methyl amine dimethyl amine trimethyl amine


formula: formula: formula:
-- primary (1o) amine secondary (2o) amine tertiary (3o) amine
Kb = 1.8 x 10-5 Kb = 4.4 x 10-4 Kb = 5.3 x 10-4 Kb = 6.4 x 10-4

How is the type (primary, secondary, or tertiary) determined for an amine?

All four substances can act as bases.  Why?

Write a reaction to illustrate the base behavior of an amine.

How does the base behavior change as more methyl groups (-CH3) are substituted?

The salt of CH3NH2 is shown below.  Write its formula.  How would you describe the geometry of the nitrogen atom in the salt?

Ammonia and the amines have a trigonal pyramidal geometry with a lone pair of electrons (derived from tetrahedral geometry).  All four positions on the nitrogen of the amine are occupied in the salt form.  The salt has a charge of +1 since it accepted a hydrogen ion.  The salts of amines are tetrahedral due to the addition of the H+ onto the lone pair.  

The general reaction for the neutralization of an amine is shown below based on ammonia as a starting point where no R groups have been substituted.

RnNH3-n     +       H+                RnNH+4-n        n = 0, 1, 2, or 3

Are the structures given below an amine or the salt of an amine.  Circle the type of amine.

cocaine glucosamine amphetamine

amine    salt amine    salt amine    salt
1o    2o    3o 1o    2o    3o 1o    2o    3o

Using ammonia as an example, what would be present (NH3 or NH+4) at low pH?     at high pH?

Given the salt of ethylamine, CH3CH2NH+3, how would you recover the amine - by addition of acid or base?  Illustrate with a reaction.

View the structure of aniline, where the amine group occurs on benzene, given below.  What is unusual about the geometry of the nitrogen?

The amine group is planar with the benzene ring.  This is due to the lone pair on the nitrogen interacting with the p orbitals of the benzene ring.  The nitrogen of the amine is trigonal planar in aniline.  A movie illustrating the behavior of aniline (select the basicity of amines) can be found at Colby College's Chemistry website- click here. 

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