Ab‐initio study of the mechanism for the reaction of a primary amine with an aldehyde group

The interaction of a primary amine with an aldehyde group to form the enolamine RHN—C(OH)HR' is investigated for the simplified system of ammonia and formaldehyde, using the sto‐3 G method. The usually accepted approach in which the NH 3 lone electron pair points toward the electrophilic aldehydic C atom in a direction perpendicular to the plane of formaldehyde is found to be associated with a strong repulsive potential for N—C separation R 1 , less than ∼3 Å. A concomitant tunneling of a proton from a solution hydronium ion to the carbonyl oxygen, which increases as R 1 decreases, is shown to result in an energywise downhill closer approach of NH 3 to R 1 = 1.6 Å, thus leading to the formation of the enolamine. The electronic charge transferred from NH 3 in this system is also discussed.