Chemistry - A Molecular Science

bond. The bond is polar with the negative

end on oxygen and the positive end on carbon.

The partial positive charge and only three el

ectron regions make the carbon atom Lewis

acidic. The lone pair on the nitrogen atom ma

kes it Lewis basic. The attack is from the

lone pair on N to the C atom in the C=O bond. The lone pair on the N atom is used to form a C-N bond (Arrow a below). The two additional electrons in the C-N bond would put 10 electrons around the C atom, so the

π electrons in the C=O bond are converted into a

lone pair on the oxygen atom (Arrow b).

HCC^3

H

O

NH

H

CH

3

HCC^3

H

O

CH

(^3) N
H
H

• a
  b
  The positive formal charge on the N atom makes
  the H atoms attached to it acidic, so the
  formal charge is eliminated by a solvent-assist
  ed proton transfer similar to that shown in
  the reaction of SO
  and water. The proton shown in red below is transferred from N to a 3
  water molecule as the N-H bonding pair is conv
  erted to a lone pair on N. Another proton
  (blue) is transferred from a water molecule to
  the O atom with negative formal charge. The
  resulting product has no formal charge.
  HCC^3
  H
  O
  N
  H
  H
  H
  CH
  3
  H O
  H
  HCC^3
  H
  OH
  NH CH
  3


• 
  

  H O
  The above reaction is an example of organi
  c chemistry, which is introduced in the
  following chapter. Organic chemistry has many Lewis acid-base reactions with mechanisms that are almost identical to the one shown here; the only difference is the identity of the acid and the base. To summarize, a Lewis acid-base reaction results in the formation of a coordinate
  covalent bond between an acid and a base. Th
  e bonding pair comes from the lone pair of
  the base. The acid must have an empty orbital
  that can overlap with the filled orbital on the
  base to form the bond. If the very broad classifi
  cation of Lewis is used to define acid-base
  reactions, then
  a very large number of chemical reactions can be classified as either acid-
  base or redox reactions. In one, a base
  shares
  its electrons with an acid; in the other, a
  reductant
  transfers
  its electrons to an oxidant
  .*

  
  

• There are many similarities in
  the treatment of redox and acid-
  base chemistry that will be indicated in the margin as we proceed throu
  gh this cha
  pter.
  Chapter 12 Acid-Base Chemistry
  © by
  North
  Carolina
  State
  University