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Carbocation: A Carbocation (previously known as carbonium ion) is basically an ion with a positively charged C atom. The charged carbon atom in a Carbonium ion is a “sextet”, i.e. it has only six electrons in its outer shell or outermost energy level.

Properties of Carbocation:

  1. It is positively charged species
  2. It has sextet of electron and that is why it is diamagnetic
  3. It is formed by Heterolysis
  4. It is generally formed due to Polar Solven

Structure:
(sp2) Triangular planer

Stability:
Its stability can be determined with the help of Inductive effect, Hyper Conjugation and Resonance effect.

Stability of Carbocation:

Charge ∝ 1/Stability

Stability of carbocation can also be determined by Hyper Conjugation (No Bond Resonance)

Allylic Carbocation:

An allylic Carbocation is basically a resonance-stabilized carbonium ion which has the two resonance structures, in which the formal charge of +1 is on an allylic carbon.

Benzylic Carbocation:

Ex.1 Compare the stability of the following Carbocation:

⇒More electronegativity
⇒Positive charge on more electronegative element is symbol of instability.
a > b > c

Ex.2 Compare the stability of the following compounds

Sol.  d > c > b > a

F being most electrons attracting group decreases the e- density from positively charged C-atom and decreases the charge density and makes the Carbocation less stable.


Ex.3 Compare the stability of the following Carbocation:

Sol. Due to greater size of Iodine, its lone pair will not be available for co-ordinate bond. Therefore lone pair would not stabilize Carbocation.

In case of F due to its small size its lone pair can easily coordinated to making it most stable
a > b > c > d          (Stability)

* By coordination the carbocation completes its octet and structure having complete octet of its atom is supposed to be most stable.

Note: In Resonating Structure of Ph3C+ at least one C gets sextet of e- and hence less stable than coordinated compound.

Ex.4 Compare the stabilities of the following Carbocation

  1. C+H2NH2
  2. C+H2OH
  3. C+H2HF

Sol. N, O, F belongs to same period
In period Electro negativity of the atom is deciding factor
F being most electronegative holds its e- pair very firmly.
Its L.P. will not be easily available for coordination.
Stability by it will be minimum.
a > b > c

Ex.5 Compare the following Carbocation in order of their stability.

  1. C+H2Cl
  2. C+H2OH

Sol. If periods of atoms which have to donate their electrons for coordination (for stability) is different then atomic size will be deciding factor. The atom whose size is greater will be unable to make it e- pair available for coordination.
b > a

Ex.6 Compare the stability of the following compounds

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