Main Difference – Carbocation vs Carbanion
Carbocation and carbanion are two terms that are frequently used in organic chemistry. These are organic chemical species bearing an electrical charge on a carbon atom. Carbocations and carbanions are often found as intermediates of some reactions. The main difference between carbocation and carbanion is that carbocation contains a carbon atom bearing a positive charge whereas carbanion contains a carbon atom bearing a negative charge.
Key Areas Covered
1. What is Carbocation
– Definition, Types, Formation, Reactions with Examples
2. What is Carbanion
– Definition, Types, Formation, Reactions with Examples
3. What is the Difference Between Carbocation and Carbanion
– Comparison of Key Differences
Key Terms: Carbocation, Carbanion, Electrophilic Addition, Intermediates, Methyl Carbanion, Methyl Carbocation, Nucleophilic Addition, Primary Carbanion, Primary Carbocation, Secondary Carbanion, Secondary Carbocation, Tertiary Carbanion, Tertiary Carbocation,
Trigonal Planar, Pyramidal
What is Carbocation
The term carbocation can be defined as an ion containing a positively charged carbon atom. Carbocation refers to the whole molecule, not only the positively charged carbon atom. A carbocation may have one or more positive charges. These carbocations are generally unstable because p orbitals of the carbon atom are free due to loss of electrons. Therefore, carbocations are very often reactive. This favors the reaction between a carbocation and a nucleophile. Carbocations are paramagnetic due to incomplete electron pairing. Typically, carbocations show sp2 hybridization. This is because a carbon atom with a positive charge can have only three bonds around it. The geometry around this carbon is trigonal planar.
Generally, carbocations are divided into four groups according to the number of carbon atoms that the positively charged carbon atom is attached to.
Types of Carbocation
These carbocations contain a positively charged carbon atom that is not attached to any other carbon atoms.
Here, the positively charged carbon atom in the carbocation is connected to another carbon atom through a covalent bond. This type of carbocations are stable than methyl carbocations but are less stable than other carbocations.
The positively charged carbon atom is bonded to two other carbon atoms. These carbocations are stable than primary carbocations.
The positively charged carbon atom is attached to three other carbon atoms. This form is very stable.
Formation of Carbonation
Breaking the Bond between a Leaving Group and Carbon Atom
If the organic molecule has a good leaving group, it can leave the molecule through ionization. This ionization gives the bonding electron pair to the leaving group, resulting in a positive charge on the carbon atom.
Due to the high reactivity of carbocations, they undergo chemical reactions very easily.
Reactions of Carbocations
A nucleophile is a chemical species rich with electrons. It can donate electrons to the positively charged carbon atom of the carbocation by forming a covalent bond with the carbon atom.
The carbocation can be rearranged forming a stable carbocation other than the existing carbocation by exchanging the bonding electrons with adjacent bonds.
The above image shows rearrangement of a carbocation. There the positive charge is moved from one carbon atom to the other. But the new structure is stable because it is a secondary carbocation. Initial ion was a primary carbocation.
What is Carbanion
A carbanion is an ion that contains a negatively charged carbon atom. Unlike a carbocation, a carbon atom bearing the negative charge is sp3 hybridized and the geometry is pyramidal (except for benzyl carbanion). The outermost orbitals of the carbon atom obey the octet rule, having eight electrons. A carbanion almost always acts as a nucleophile. Therefore, it can react with electrophiles. Carbanions are diamagnetic due to the completion of electron pairing.
Various Types of Carbanions
The negatively charged carbon atom is not bonded to any other carbon atom.
Here the negatively charged carbon atom in the carbanion is connected to another carbon atom through a covalent bond.
The negatively charged carbon atom is bonded to two other carbon atoms.
The negatively charged carbon atom is attached to three other carbon atoms.
A carbanion is formed when a group or an atom leaves the bonding electrons.
Carbanions mainly undergo electrophilic addition reactions as they can act as nucleophiles. Therefore, they react with electrophiles.
Major Reactions of Carbanions
The structure of carbanions can be changed in order to obtain the most stable structure. There the bonding electron pairs can be moved. This cause the carbanion to become a normal ion, not a carbanion.
Difference Between Carbocation and Carbanion
Carbocation: Carbocation is an ion containing a positively charged carbon atom.
Carbanion: Carbanion is an ion containing a negative charged carbon atom.
Carbocation: The carbon atom bearing the positive charge is sp2 hybridized in Carbocation.
Carbanion: The carbon atom bearing the negative charge is sp3 hybridized in Carbanion.
Carbocation: The geometry of the carbon atom is trigonal planar in Carbocation.
Carbanion: The geometry of the carbon atom is pyramidal in Carbanion.
Carbocation: Carbocation is paramagnetic.
Carbanion: Carbanion is diamagnetic.
Carbocation: Carbocation act as an electrophile in chemical reactions.
Carbanion: Carbanion act as a nucleophile in chemical reactions.
Carbocation and carbanion refer to organic chemical species bearing an electrical charge on a carbon atom. The main difference between carbocation and carbanion is that carbocation contains a carbon atom bearing a positive charge whereas carbanion contains a carbon atom bearing a negative charge.
1.Agrawal, Ravin. “Carbanions(Structure,Stability,Formation).” CHEMISTRY. Ravin agrawal, 25 Nov. 2016. Web. Available here. 05 July 2017.
2.”Carbocations.” Chemistry LibreTexts. N.p., 21 July 2016. Web. Available here. 05 July 2017.
1. “Methyl cation” By Wickey-nl – Own work (Public Domain) via Commons Wikimedia
2.”ElectrophilicAdditionmechanism” By V8rik at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
3.”NS1 reaction part2 recombination carbocation nucleophile” By V8rik at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
4.”Carbocation rearrangment” By FlyScienceGuy – Own work (CC BY-SA 4.0) via Commons Wikimedia
5.” Réaction Gabriel du malonate d’éthyle-carbanion” By henry3bis – Own work (GFDL) via Commons Wikimedia
6.” OMPDC Carbanion Mechanism” By Shareef164 –(CC BY-SA 3.0) via Commons Wikimedia
7. “Substituted Carbanions V.1″ By Jü – Own work (CC BY-SA 3.0) via Commons Wikimedia