Q29E - Organic Chemistry (Mcmurry)

Question

The enamine prepared from acetone and dimethylamine is shown in its

lowest-energy form.

(a) What is the geometry and hybridization of the nitrogen atom?

(b) What orbital on nitrogen holds the lone pair of electrons?

(c) What is the geometric relationship between the p orbitals of the

double bond and the nitrogen orbital that holds the lone pair? Why

do you think this geometry represents the minimum energy?

Step-by-Step Solution

Answer

1. The hybridization and geometry of nitrogen are ${\text{sp}}^{\text{2}}$and planar.
2. The orbital holding the lone pair of an electron in nitrogen is the p orbital.
3. The p orbitals of the double bond and the nitrogen orbital that holds the lone pair are in conjugation with each other.

Step 1: Nitrogen geometry and hybridization

Enamine prepared from acetone and dimethylamine has nitrogen, making the three a single bond with the carbon, and a lone pair.

The lone pair present in the nitrogen, on resonance, forms double bonds. Thus, nitrogen in the enamine makes three single bonds and has one lone pair/double bond; therefore, hybridization is ${\mathbf{sp}}^2$ for this hybridization; the molecule's geometry is "planar."

Step 2: Orbital of Nitrogen holding lone pair electrons

The nitrogen of the enamine is ${\text{sp}}^{\text{2}}$ and is of planar geometry; the nitrogen of the enamine can perform resonance (delocalization of electron).

Due to resonance, the nitrogen can participate in the pie bond, which forms by p orbitals; therefore, the lone pair of enamine nitrogen is in the "p orbital."

Step 3: Relation between the p orbital of nitrogen and the p orbital of the double bond

The p orbital holding the lone-pair electrons of nitrogen is aligned for overlap with the pie electron of the enamine double bond. With this geometry between the p orbital of nitrogen (holding lone pair) and the p orbital of pie bond, the nitrogen lone pair electron can be conjugated with the double bond, lowering the energy (resonance stabilize).

 

The conjugation between the lone pair electron of nitrogen and the double bond is shown below.

Resonating structure