The key difference between singlet and triplet carbene is that the singlet carbenes are spin-paired whereas the triplet carbenes have two unpaired electrons.
A carbene is a functional group in organic chemistry. It has two valence shell electrons that do not participate in any bonding. The general formula for this group is R-(C:)-R’ or R=C: in which the “R” is either a hydrogen atom or alkyl substituents. We can classify carbenes into two groups as singlets or triplets depending on the electronic structure.
CONTENTS
1. Overview and Key Difference
2. What is Singlet Carbene
3. What is Triplet Carbene
4. Side by Side Comparison – Singlet vs Triplet Carbene in Tabular Form
5. Summary
What is Singlet Carbene?
Singlet carbene is a form of carbene group that has no unpaired electrons. Hence, we name it as “spin-paired carbene”. The total spin of these groups is zero. This type of carbene group has sp2 hybrid structure. These are diamagnetic because there are no unpaired electrons. Moreover, the singlet carbene group has 102° bond angle. These groups more often occur in aqueous media because these are not stable in a gaseous state.
Figure 01: The simplest Carbene is Methylene
When considering the reactivity of the singlet carbene group, they generally participate in cheletropic reactions via acting as either electrophile or nucleophiles. Therefore, the reactions of them are stereospecific.
What is Triplet Carbene?
Triplet carbene is a form of carbene group that has two unpaired electrons. The geometry of this group can be either linear or bent. If it is linear geometry, then it has sp hybrid structure. But if it is bent geometry, then it has sp2 hybrid structure. However, most of the times, the triplet carbene has non-linear geometry except for those with nitrogen, oxygen, or sulfur atoms. Moreover, the bond angle for these groups is 125-140°.
Figure 02: Difference Between Singlet and Triplet Carbene Geometries
Due to the presence of triplet carbene, they are paramagnetic. Therefore, we can observe them via electron spin resonance spectroscopy. The total spin of these carbenes is one. Generally, these groups are stable at their gaseous state. When considering the reactivity of triplet carbene, they act as diradicals and can participate in step-wise radical reactions. However, unlike singlet carbene, these triplet carbenes have to go through an intermediate with two unpaired electrons. The reactions that they undergo are stereoselective.
What is the Difference Between Singlet and Triplet Carbene?
Carbenes are organic functional groups that have the chemical formula R-(C:)-R’ or R=C: There are two forms of carbenes as singlet carbene and triplet carbene depending on the electronic structure. Therefore, the key difference between singlet and triplet carbene is that singlet carbenes are spin-paired whereas triplet carbenes have two unpaired electrons. There are many other differences between singlet carbene and triplet carbene such as the differences in their geometry, hybrid structures, total spin, bond angles, magnetic properties, etc.
The below inforgraphic presents more information on the difference between singlet and triplet carbene in tabular form.
Summary – Singlet vs Triplet Carbene
Carbene is in two major types as singlet carbene and triplet carbene according to their electronic structure. The key difference between singlet and triplet carbene is that the singlet carbenes are spin-paired whereas the triplet carbenes have two unpaired electrons.
Reference:
1. “Carbenes.” Chemistry LibreTexts, Libretexts, 21 July 2016. Available here
2. “Carbene.” Wikipedia, Wikimedia Foundation, 24 Sept. 2018. Available here
Image Courtesy:
1.”Carbene”By Hbf878 – Own work, (CC0) via Commons Wikimedia
2.”Carbenes”By Hbf878 – Own work, (CC0) via Commons Wikimedia