Pyridine (C5H5N) and pyrazine (C4H4N2) have structures similar to benzene

Pyridine (C5H5N) and pyrazine (C4H4N2) have structures similar to benzene. Both compounds have structures with 6 atoms in a ring. Draw Lewis structures for pyridine and pyrazine showing the two most common resonance forms. The N atoms in pyrazine are directly opposite each other in the ring with 2 carbons between them in either direction. Indicate resonance with a double-headed arrow between structures. Draw the Lewis structures with the formal charges minimized.

The correct answer and explanation is:

Sure! Here is the explanation and correct Lewis structures for pyridine and pyrazine including their two most common resonance forms.

Pyridine (C₅H₅N)

• Pyridine is a six-membered aromatic ring similar to benzene, but one carbon is replaced by a nitrogen atom.
• The ring contains 5 carbons and 1 nitrogen.
• The nitrogen atom contributes a lone pair in an sp² orbital, which is not part of the aromatic π-system.
• The resonance involves shifting the positions of the double bonds around the ring.
• Formal charges are minimized by having neutral atoms and the nitrogen keeps its lone pair.

Two main resonance forms for pyridine:

1. Double bonds at C2=C3, C4=C5, C6=N1 (N is position 1).
2. Double bonds at C1=C2, C3=C4, C5=N6 (rotated position of double bonds).

Each resonance form alternates the placement of double bonds around the ring while nitrogen maintains its lone pair.

Pyrazine (C₄H₄N₂)

• Pyrazine has a six-membered ring with two nitrogen atoms opposite each other (positions 1 and 4).
• The remaining 4 positions are carbons.
• Like pyridine, the nitrogens have lone pairs outside the aromatic π-system.
• The ring has alternating double bonds similar to benzene.
• Resonance structures shift double bonds around the ring while keeping nitrogen lone pairs intact.
• Formal charges are minimized in all major resonance contributors (neutral atoms).

Two main resonance forms for pyrazine:

1. Double bonds between C2=C3, C5=C6, and between N1-C6 and N4-C3.
2. Double bonds between C1=C2, C3=C4, and between N1-C2 and N4-C5 (shifted pattern).

Formal charges and resonance:

• In both molecules, nitrogen atoms generally do not carry formal charges because their lone pairs remain outside the π system.
• The resonance forms differ by the placement of double bonds.
• Aromaticity and electron delocalization stabilize both molecules.

Visuals (text-based Lewis structures):

Pyridine resonance forms:

Resonance form 1:
   N
  / \
 C=C C
 ||  ||
 C  C

Resonance form 2:
   N
  / \
 C  C=C
 ||  ||
 C=C C

Pyrazine resonance forms (N at opposite positions):

Resonance form 1:
 N     N
 |     |
C=C - C=C
||     ||
C     C

Resonance form 2:
 N     N
 |     |
C - C=C -
||    ||
C=C   C