Q76. - Organic Chemistry

Question

Question:

a. What is the hybridization of each N atom in nicotine? 

b. What is the geometry around each N atom? 

c. In what type of orbital does the lone pair on each N atom reside?

d. Draw a constitutional isomer of nicotine. 

e. Draw a resonance structure of nicotine.

                                          

Step-by-Step Solution

Answer

Answer

1. The N atom that belongs to the six-membered ring is  sp² hybridized, and the one belonging to the five-membered ring is  sp³ hybridized.
2. The N atom present in the six-membered and five-membered rings has trigonal planar and tetrahedral geometry, respectively.
3. The lone pair of electrons on N (six-membered ring) belongs to the sp² -hybridized orbital, and the one on N (five-membered ring) belongs to the  sp³  -hybridized orbital.
4. The constitutional isomer of nicotine is shown be

                                                   

      e. The two resonance structures of nicotine are as follows:

                           

Step-by-Step Solution Step 1: Hybridization of nitrogen

The hybridization (mixing of orbitals) of an atom is determined by considering the number of atomic orbitals involved/participating in hybridization.

The nitrogen atom that is bonded to three other atoms and has a lone pair on it or is bonded to four other atoms is said to be ${\mathrm{sp}}^3$hybridized.

The nitrogen atom attached to two other groups and bears a lone pair is said to be ${\mathrm{sp}}^2$  hybridized.

Step 2: Hybridization of the nitrogen atom in nicotine

                                                                       

                                             Representation of the hybridization of nitrogen atoms in nicotine 

 

The N atom in the nicotine molecule present on the six-membered ring is connected to two carbon atoms and contains a lone pair of electrons. Therefore, it is ${\mathrm{sp}}^2$ hybridized, and thus, the lone pair of electrons reside in the ${\mathrm{sp}}^2$ -hybridized orbital.

 

The N atom present on the five-membered ring is bonded to three carbon atoms and contains a lone pair of electrons on it, and therefore, it will be ${\mathrm{sp}}^3$ hybridized.

The lone pair on this nitrogen atom resides in the ${\mathrm{sp}}^3$-hybridized orbital.

 

Step 3: Geometry and Hybridization

• ${\mathrm{sp}}^3$  hybridization: Tetrahedral geometry.
•  ${\mathrm{sp}}^2$ hybridization: Trigonal planar geometry.
• $\mathrm{sp}$  hybridization:  Linear geometry.

 The ${\mathrm{sp}}^2$hybridized N atom has trigonal planar geometry, and the ${\mathrm{sp}}^3$  hybridized nitrogen atom has tetrahedral geometry.

Step 4: Constitutional isomer of nicotine

The molecules that bear the same molecular formula but adopt different structures are named constitutional isomers.

 Interchanging the position of the methyl group present on  ${\mathrm{sp}}^3$-hybridized nitrogen atom with the hydrogen atom present adjacent to it gives the structure of the constitutional isomer of nicotine.

                                                                   

                                                                    The constitutional isomer of nicotine 

Step 5: Resonance structure of nicotine

The different structures used to describe the bonding in a molecule that cannot be described adequately using a single Lewis structure are termed resonance structures.

 

Nicotine has two resonance structures, which are given below:

                           

                                                             Resonance structures of nicotine