Which statement about the ground states of the simplest carbenes is correct?

A $: CH X_{2}$ has a singlet ground state, while $: CCl X_{2}$ has a triplet ground state
BBoth $: CH X_{2}$ and $: CCl X_{2}$ have triplet ground states
CBoth have singlet ground states, because carbenes cannot have unpaired electrons
D $: CH X_{2}$ has a triplet ground state, while $: CCl X_{2}$ has a singlet ground state

Answer: D

For methylene the singlet-triplet gap is small and the triplet (two parallel unpaired electrons, nearly linear) is the lower in energy. In

: CCl X_{2}

the chlorine lone pairs donate into the empty

p

orbital, which stabilises the singlet state strongly and makes it the ground state. This is why dichlorocarbene adds stereospecifically.