Difference Between Carbocation and Carbanion

Carbocation vs Carbanion
 

The key difference between carbocation and carboanion is their charges; they are both organic molecular species with opposite charges. Carbocation is positively charged ion and carboanion is negatively charged ion. Their stability depends on several factors, and some of them are very important in the synthesis of other chemical compounds.                                                                                                    

What is Carbocation

A carbocation is a chemical species which carries a positive charge on a carbon atom. Its name gives the clear idea that it is a cation (a positive ion), and the word carbo refers to a carbon atom. Carbocation includes several categories; primary carbocation, secondary carbocation, and tertiary carbocation. They are classified according to the number of alkyl groups attached to the positively charged carbon atom. Their stability and the reactivity vary depending on these substituents.

Difference Between Carbocation and Carbanion

Carbocation Stability Trend

What is Carbanion

A carboanion is an organic molecular species with a negative electrical charge located on a carbon atom. In other words, it is an anion in which a carbon atom possess an unshared pair of electrons with three substituents. Its total number of valence electrons is equal to eight.  They are formed by removing positively charged groups or atoms from a neutral molecule. They are very important as chemical intermediates to synthesize other substances such as plastics and polyethene (or polyethylene). The smallest carbanoin is ‘methide ion’ (CH3-); formed from methane (CH4) by the loss of a proton (H-).

Main Difference - Carbocation vs Carbanion

What is the difference between Carbocation and Carbanion?

Characteristics of Carbocation and Carbanion

Carbocation: Carbocation is sp2 hybridized, and the vacant p-orbital lies perpendicular to the plane of three substituted groups. Therefore, it has a trigonal planar molecular structure. Carbocation requires one electron pair to complete the octet.   They can react with nucleophiles, can be deprotonated from a pi-bond and can have re-arrangements in the same species.

Carbanion: An alkyl carboanion has three bonding pairs and one lone pair; so its hybridization is sp3, and the geometry is pyramidal. The geometry of allyl or benzyl carboanion is planar, and the hybridization is sp2. The octet is complete in the outermost orbit of a carboanionic carbon atom and it behaves as a nucleophile to react with electrophiles.

Stability:

Carbocation: Carbocation stability depends on various factors. It’s more stable when more –R groups are attached to the positive carbon atom. Therefore, tertiary carbocation is comparatively stable than primary ones. Difference Between Carbocation and Carbanion - image 5Resonance structures also increase the stability.

Carbanion: The stability of carboanion depends on several factors; Electronegativity of carbanionic carbon, resonance effect, inductive effect caused from attached substituent and stabilization by >C=O, –NO2 and CN groups present on carbanionic carbon

Definitions:

Induct effect: It can be experimentally observable effect of the transmission of charge through a chain of atoms in a molecule, resulting in a permanent dipole in a bond.

Examples of Carbocation and Carbanion

Carbocation: 

Primary Carbocation:Difference Between Carbocation and Carbanion - image 1

In a primary (1°) carbocation, the positively charged carbon atom is attached to only one alkyl group and two hydrogen atoms.

Secondary Carbocation:Difference Between Carbocation and Carbanion - image 2

In a secondary (2°) carbocation, the positively charged carbon atom is attached to two other alkyl groups (which may be the same or different) and one hydrogen atom.

Tertiary Carbocation:Difference Between Carbocation and Carbanion - image 3

In a tertiary (3°) carbocation, the positive carbon atom is attached to three alkyl groups (which may be any combination of same or different), but no hydrogen atoms.

Carbanion:

Carboanion is also classified into three categories in the same way as in carbocation; primary carboanion, secondary carboanion, and tertiary carboanion. That is also done based on the number of –R groups attached to the anionic carbon atom. Difference Between Carbocation and Carbanion - image 4

References:

“Chemistry.” : Carbanion. Accessed June 09, 2016 from here
“Carbocation.” Wikipedia. Accessed June 09, 2016. from here
“Carbocations (or Carbonium Ions).” Chemguide. Accessed June 09, 2016. from here
“Carbocations”. Organice Chemistry Tutorials.  Accessed June 09, 2016.  from here
Image Courtesy:
“Carbocation Stability Trend” by Alatleephillips – Own work (CC BY-SA 4.0) via Commons Wikimedia
“Carbanion Structural Formulae V” by Jü – Own work  (CC0) via Commons Wikimedia