Question

Which of the following statements are correct?
(1) A triplet carbene is more stable than a singlet carbene
(2) The carbocation, $F _{3}- \overset{+}{C} $is less stable than the carbocation, $F _{3} \overset{+}{C}$
(3) Methyl carbanion is both isostructural and isoelectronic with ammonia
(4) A singlet carbene is a paramagnetic species

• A. 1, 2 and 3 are correct
• B. 1 and 2 are correct
• C. 2 and 4 are correct
• D. 1 and 3 are correct

Answer 1

Answer: (A)

Triplet carbene is generally more stable than the singlet carbene.
2. $F _{3} C - C ^{+}$is less stable than $F _{3} C ^{+}$. This is because of the fact that lone pairs of electrons on each of the three $F$-atoms in $\overset{+}{C} F_{3}$overlap with the empty $p$-orbital of the carbocation carbon. As a result, dispersal of positive charge occurs. In contrast, the strong $-I$ effect of the three $F$ - atoms in $F _{3} C - C ^{+}$intensifies the positive charge and thus, it is less stable than $CF _{3}^{+}$
3. $CH _{3}^{-} \Rightarrow 3 b p+1 l p=$ pyramidal geometry
Number of electrons $=6+3+1=10$
$NH _{3} \Rightarrow 3 b p+1 l p=$ pyramidal geometry
Number of electrons $=7+3=10$
Thus, methyl carbanion is isostructural and isoelectronic with ammonia.