Question

The Carbon atom in singlet and triplet carbene is ____________ and ____________ - hybridized, respectively.

• A. $sp^2$ and $sp^2$
• B. $sp$ and $sp$
• C. $sp^2$ and $sp$
• D. $sp$ and $sp^2$

Hint:

Remember the pairing: Singlet = Paired electrons in one orbital = $sp^2$ hybridization (bent shape). Triplet = Unpaired electrons in two separate orbitals = $sp$ hybridization (linear shape for minimum repulsion).

Answer 1

The Correct Option is C

Step 1: Concept:
The question asks for the hybridization states of the central carbon atom in two different electronic states of carbenes: singlet and triplet.

Step 2: Key Formula or Approach:
Carbenes ($:CR_2$) are neutral reactive intermediates containing a divalent carbon atom with six valence electrons. Their geometry and hybridization depend on how the two non-bonding electrons are distributed.
- Singlet Carbene: The two non-bonding electrons are paired in a single orbital with opposite spins.
- Triplet Carbene: The two non-bonding electrons occupy two separate orthogonal orbitals with parallel spins.

Step 3: Step-by-step Explanation:

• Singlet Carbene: To accommodate a lone pair and two bonding pairs, the carbon atom utilizes three $sp^2$ hybridized orbitals. The paired non-bonding electrons occupy one $sp^2$ orbital, while the remaining unhybridized $p$-orbital is empty. This results in a bent geometry with a bond angle usually between $100^\circ$ and $110^\circ$. Therefore, the hybridization is $sp^2$.

• Triplet Carbene: To keep the two unpaired electrons as far apart as possible (Hund's Rule) to minimize electron-electron repulsion, the molecule prefers a linear geometry. In the simplest idealized model, the central carbon is $sp$ hybridized, forming two linear bonds, and leaving two orthogonal, unhybridized $p$-orbitals to house the two unpaired electrons.

• Note: While high-level computational chemistry shows that some ground-state triplet carbenes (like $:CH_2$) are actually bent with a wider angle (roughly $130^\circ - 150^\circ$, exhibiting sp$^2$-like character), standard undergraduate organic chemistry curricula traditionally teach that singlet carbenes are $sp^2$ hybridized (bent) and triplet carbenes are $sp$ hybridized (linear) as a simplifying model.

• Looking at the options provided, the standard textbook answer aligns with option (C).

Step 4: Final Answer:
The central carbon in a singlet carbene is $sp^2$ hybridized, and in a triplet carbene, it is traditionally considered $sp$ hybridized. This matches option (C).