Compound of Carbon Containing Halogen: Stability of Carbocation (For CBSE, ICSE, IAS, NET, NRA 2022)

Get unlimited access to the best preparation resource for competitive exams : get questions, notes, tests, video lectures and more- for all subjects of your exam.

Stability of Carbocation

Stability of Carbocation
  • A carbocation is classified as primary, secondary or tertiary depending upon whether the positively charged carbon atom is linked to one, two or three carbon atoms.
  • The number of alkyl groups attached to the positively charged carbon atoms increases, the stability of the carbocation also increases because alkyl group are electron releasing in nature and help in the stabilization of the positive charge on the carbon atom of carbocation.
  • Haloarenes are almost unreactive to reagents such as , , and under ordinary laboratory conditions but can show nucleophilic substitution reactions under drastic conditions.
Stability of Carbocation

The presence of electron withdrawing groups such as − groups at o- and p-position (but not a m-position) with respect to halogen activates the halogens towards nucleophilic displacement.

For example:

Stability of Carbocation
  • Haloarenes can also readily undergo substitution reactions in benzene ring
  • The benzene ring is an electron rich species. Therefore, it is attacked by an electron deficient species such as .
  • ortho and para positions in haloarenes are electron rich due to resonance, thus, the electrophilic substitution takes place mainly at these positions.
  • For example, the nitration of chlorobenzene, with a mixture of concentrated and , gives a mixture of 2-nitrochlorobenzene and 4-nitrochlorobenzene.
Stability of Carbocation

Haloalkanes are highly reactive compounds due to the presence of a polar carbon- halogen bond in their molecules.

The Bond Energy Values
  • These bond energy values show that C-I bond is the weakest bond and C-F bond is the strongest bond.
  • Thus, the order of reactivity of haloalkanes is

Elimination Reactions

When haloalkanes are heated with aqueous solution of potassium or sodium hydroxide, the major product formed is the alcohol, produced by nucleophilic displacement of the halogen atom by .

If a haloalkane is heated with concentrated alcoholic potassium hydroxide, the major product formed is an alkene due to the elimination of hydrogen halide. This is called elimination or dehydrohalogenation.

If the structure of alkyl halide is such that it can undergo elimination in two different ways, then the more highly substituted alkene (i.e.. having lesser number of hydrogen atoms on the doubly bonded carbon atoms) is the major product of elimination. This is known as Saytzeff՚s rule.

For example,

Elimination Reactions

Developed by: