Difference Between Carbocation and Carbanion

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

Methyl Carbocation

These carbocations contain a positively charged carbon atom that is not attached to any other carbon atoms.

Figure 01: Methyl Carbocation

Primary Carbocation

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.

Figure 02: Primary carbocation. Here a –HH3 group is attached to the positively charged carbon atom.

Secondary Carbocation

The positively charged carbon atom is bonded to two other carbon atoms. These carbocations are stable than primary carbocations.

Figure 03: A secondary Carbocation. Here, the positively charged Carbon atom is bonded to two other carbon atoms. These two carbon atoms are shown in red circles.

Tertiary Carbocation

The positively charged carbon atom is attached to three other carbon atoms. This form is very stable.

Figure 04: Tertiary Carbocation

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.

Electrophilic Addition

An electrophile can attack a pi bond and make a covalent bond with one of the vinyl carbon atoms. This causes the other vinyl carbon atom to get a positive charge due to the lack of electrons.

Figure 06: Electrophilic addition of “X”

Due to the high reactivity of carbocations, they undergo chemical reactions very easily.

Reactions of Carbocations

Nucleophilic Addition

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.

Figure 07: The electron pairs of the oxygen atom in H2O can be donated to a carbocation

Rearrangement

The carbocation can be rearranged forming a stable carbocation other than the existing carbocation by exchanging the bonding electrons with adjacent bonds.

Figure 08: Rearrangement of carbocations

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 

Methyl Carbanion

The negatively charged carbon atom is not bonded to any other carbon atom.

Figure 09: The Methyl Carbanion

Primary Carbanion

Here the negatively charged carbon atom in the carbanion is connected to another carbon atom through a covalent bond.

Figure 10: A primary Carbanion

Secondary Carbanion

The negatively charged carbon atom is bonded to two other carbon atoms.

Figure 11: A Secondary Carbanion

Tertiary Carbanion

The negatively charged carbon atom is attached to three other carbon atoms.

Figure 12: Tertiary Carbanion

 A carbanion is formed when a group or an atom leaves the bonding electrons.

Figure 13: The H atom leaves as a proton, giving the bonding electron pair to the carbon atom

Carbanions mainly undergo electrophilic addition reactions as they can act as nucleophiles.  Therefore, they react with electrophiles.

Major Reactions of Carbanions 

Addition Reactions

Figure 14: In benzyl carbanion, the negatively charged carbon atom is sp2 hybridized and has planar geometry. (this is an exception as mentioned above)

Rearrangement

Figure 15: The resonance structures 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

Definition

Carbocation: Carbocation is an ion containing a positively charged carbon atom.

Carbanion: Carbanion is an ion containing a negative charged carbon atom.

Hybridization

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.

Geometry

Carbocation: The geometry of the carbon atom is trigonal planar in Carbocation.

Carbanion: The geometry of the carbon atom is pyramidal in Carbanion.

Magnetic Properties

Carbocation: Carbocation is paramagnetic.

Carbanion: Carbanion is diamagnetic.

Reactions

Carbocation: Carbocation act as an electrophile in chemical reactions.

Carbanion: Carbanion act as a nucleophile in chemical reactions.

Conclusion

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.

References:

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.

Image Courtesy:

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