Carbocation

Carbocation refers to the intermediate of carbon-containing positive charge. A carbocation contains a carbon atom bearing three bonds and a positive charge. They are generally unstable as they have 6 valence electrons instead of 8 along with one open valence shell therefore, they do not  satisfy the octet rule.  Formerly, it was known as carbonium ion . Carbocation occurs in an atom with even-electron cation that holds a significant positive charge .

 

 

 

Types of carbocation

Depending upon the number of open valence shells, carbocation is classified into 4 types as; methyl carbocation, primary carbocation, secondary carbocation, and tertiary carbocation.  A methyl carbocation is the one in which the open valence shell carbon is not bonded to any carbon groups.  A primary carbocation is the one in which a single carbon group is attached to the carbon bearing the positive charge. In a secondary carbocation, two carbons are attached to the carbon bearing the positive charge. Similarly,  a tertiary carbocation is the one in which 3 carbons are attached to the carbon bearing the positive charge as shown in the picture below.

 

 

The geometry and hybridization of carbocation

The carbocation is sp2 hybridized with a trigonal geometry is shown below.  Additionally, there is a vacant p orbital indicating its electron-deficient nature. The carbon has 6 electrons in its valence shell resulting in electron-deficient status hence it is called electrophile. Geometrically, A carbocation is generally observed in an SN1 reaction, elimination reaction, etc.

 

 

Formation of carbocation

The carbocations are formed by either Cleavage of bond of carbon or by Electrophilic addition. If there is cleavage of the bond of carbon and its attached atoms, the leaving group takes away the shared electrons. Therefore, there is just one deficient electron. Consequently, a positive charge is developed by forming a carbocation. The higher the tendency of cleavage of bond or formation of a more stable carbocation the lower is the activation energy.  In the electrophilic addition, an electrophile attacks on unsaturated double or triple bonds resulting in the breaking of the pi-bond which results in the formation of a carbocation.

 

Formation of a Pi Bond

The carbocation receives the electrons from its nearby hydrogen to remove its positive charge thus completing its octet. As a result, a new pi bond is formed. The hydrogen atom is removed by any base. Because of the high reactivity of the carbocations, even a weak base like water or iodide ion can also be able to facilitate the deprotonation. During deprotonation, 2 types of products are formed.

 

 

Rearrangement of a Pi Bond

The bonding electrons of a carbocation are moved between adjacent atoms, as a result,  more stable carbocation can be formed. For instance, rearrangement will be highly possible if the primary carbocation is converted into the secondary carbocation which is more stable.

 

Stability of carbocation

The stability of the carbocation depends upon the resonance, hyperconjugation and the electronegativity. The stability of carbocations increases as the resonance number is raising.  In other words, more the number of resonating structures more is the stability of the carbocation. The reason for this is the delocalization of the positive charge.  Similarly, increasing substitution increases the hyperconjugation and as a result, there is more stability: More the hyperconjugation more is the stability.

 

Carbanions

Carbanions refer to the intermediate of carbon that is made up of negative charge. A carbanion contains eight electrons in the valence shell.

 

The geometry and hybridization of carbocation

Carbanions are sp3 hybridized with a trigonal pyramidal geometry as shown below

 

The Grignard reagent as a nucleophile

In some cases, the Grignard reagent reacts with carbonyl substances to yield alkanols. This is one of the techniques used to synthesize alkanols.

 

 

 

Read More

1.     

versatile nature of carbon

2.

Carbon and carbon compounds

3.

Carbona and its reactions

4.

Important trends and anomalous behaviour of carbon

 

 

 

Check your understanding

  1. What is carbocation?

  2. What is secondary carbocation and write the differences between the primary carbocation and the secondary carbocation?

  3. How to achieve a stable carbocation?

 



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