Carbon ring structures are common in organic chemistry. Draw a Lewis structure for each carbon ring structure, including any necessary resonance structures. a. C4H8 b. C4H4 c. C6H12 d. C6H6
The Correct Answer and Explanation is:
Lewis (Kekulé) structures
a. C₄H₈ – cyclobutane
H H
| |
H – C – C – H
| |
H – C – C – H
| |
H H
All four C–C bonds are σ-bonds; each carbon is sp³ and bears two hydrogens.
b. C₄H₄ – cyclobutadiene
Two equivalent resonance contributors:
Form A Form B
H H H H
\ / | |
C=C C C
/ \ || ||
C C C C
\ / | |
C=C C C
/ \ | |
H H H H
(The π bonds shift, exchanging single and double bonds on every edge of the square.)
c. C₆H₁₂ – cyclohexane
H H
/ \
H – C C – H
| | | |
H C C H
| | | |
H – C C – H
\ /
H H
Six sp³ carbons linked by σ-bonds; each carbon carries two hydrogens.
d. C₆H₆ – benzene
Kekulé resonance pair (or circle notation for delocalisation):
Form 1 Form 2
/ \ \ /
H C C H H C C H
|| || || ||
H C C H H C C H
\ / / \
Alternating double bonds exchange positions, delocalising six π electrons around the ring.
explanation
Carbon rings illustrate how the hydrogen count drops as π bonds or additional rings are introduced. The saturated rule CₙH₂ₙ applies to simple monocyclic alkanes, and each extra degree of unsaturation (a π bond or an extra ring fusion) removes two hydrogens.
(1) C₄H₈ matches CₙH₂ₙ, so the only monocycle possible is cyclobutane. Four sp³ carbons form a square joined by four C–C σ-bonds; every carbon also bears two C–H σ-bonds directed outward. The octet is satisfied locally, and because the electrons are confined in σ-frameworks no resonance structures are required.
(2) C₄H₄ possesses two additional degrees of unsaturation. A four-membered ring with two C=C double bonds—cyclobutadiene—fits the formula. Drawing the first Lewis structure places double bonds between C1=C2 and C3=C4, leaving single bonds on the remaining edges. Shifting the π electrons by one carbon produces a second, equivalent Kekulé picture with the pattern reversed. These two Lewis forms are blended in resonance; nevertheless the molecule is anti-aromatic because only four π electrons circulate, so it is highly reactive and usually observed only in matrices at very low temperature.
(3) C₆H₁₂ again fits CₙH₂ₙ, giving cyclohexane. In a Lewis diagram the six sp³ carbons form a hexagon of single bonds and each carries two hydrogens. Although the lowest-energy chair conformation is puckered rather than planar, conformation does not alter the Lewis picture. No resonance contributors are needed because all bonds are σ.
(4) C₆H₆ is three unsaturations above a saturated ring. The structure that fulfils every valence is benzene: a planar hexagon of sp² carbons where alternating C=C bonds can be drawn in two Kekulé arrangements. Resonance between them delocalises six π electrons evenly around the ring—often depicted as a circle—conferring aromatic stability (~150 kJ mol⁻¹). Each carbon retains one hydrogen, completing the classical benzene Lewis structure. This comparison highlights how formula guides structure and reactivity trends.
