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NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals

NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals

Free Radicals

Free Radicals

These are highly reactive, neutral and electron deficient species.
The order of the stability of free radicals is
NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 1

Carbocation

Carbon containing chemical species bearing a positive charge on carbon and carry 6 electrons in its valence shell are called carhocations.
The carbocations follow the following order of stability
NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 2
Less stable carbocations (l°/2°) can be converted into more stable carbocations (2°/3°) through either 1, 2-hydride shift or 1, 2-methyl shift.

Carbanions

Carbon containing chemical species bearing a negative charge on carbon atom and carrying 8 electrons in its valence shell is called carbanion.
Simple alkyl carbaflions are pyramidal while carbanions which are stabilised by resonance are planar. Methyl carbanion is isostructural (pyramidal) and isoelectronic (10e) with ammonia.
The order of stability of carbanions is as:
NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 3

Carbenes

The neutral divalent carbon species in which two non-bonding electrons are present along with 2 bonding pairs are called carbenes. Carbenes act as electrophiles as these are electron deficient species:
Carbenes are of two types:
NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 4
Triplet carbene acts as diradical & it is always more stable than a singlet carbene

Electrophiles or Electron deficient Species

  •  All non-metal cations and metal cations which have vacant d-orbitals are electrophiles.
    e.g. Cl+, NO2+, CH3CO+.
  • Lewis acids (incomplete octet), e.g. BF3, ZnCl2 (anhydrous),FeCl3 (anhydrous), AlCl3 (anhydrous), CH2 etc., are electrophiles.
  • Non-metal acidic oxides, e.g. CO2, S02, etc., are electrophiles.

Nucleophiles or Electron Rich Species

  • All anions, NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 5
  • Lewis bases, NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 6
  • Benzene, alkenes, etc.
  • Nucleophilicity order is
    NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 7
  •  Generally, strong base is strong nucleophile.

Electronic Displacement in a Covalent Bond

Presence of some atom or group in a molecule or presence of attacking reagent may lead to electronic displacement in a covalent bond. As a consequence of which, centres of different electron densities are generated. These centres are susceptible to attack by the reagents. The factors that create the centres of different electron densities are discussed below:

Inductive Effect

Inductive effect is just like shifting of shared pair of electrons in polar covalent molecules. If shared pair is more shifted towards the more electronegative atom. The less electronegative atom acquire slight positive charge and more electronegative atom acquire partial negative charge, NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 8
This is a permanent effect and propagates through carbon chain,
e.g. NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 9
Here, Cl has -I effect and alkyl group has +I effect. Greater the number of C-atoms in alkyl groups, greater would be its
+I effect
NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 10
+I effect of alkyl groups enhances -I effect of the halogen atoms. 3°-dkyl halide will be most reactive due. to more +I effect.
NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 11

 Electromeric Effect

It is defined as the polarity produced in a multiple bonded compound as a reagent approaches it. In the presence of attacking reagent, the two NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 21 electrons are completely transferred to any of the one atom. This effect is temporary.
NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 12
 Resonance Effect

It involves delocalisation of NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 21electrons. This effect may be of +R type or -R type. Electron donating group with respect to conjugate system shows +R effect, e.g. halogens, —OH, — OR, — OCOR, — NH2,— NHCOR etc.
NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 13
Electron donating groups producing +R effect are ortho and para directing. They activate the benzene ring towards the electrophilic substitution reactions pxcept halogens. Halogens slightly deactivate the benzene ring towards the electrophilic substitution reaction. More the EDG more is the basic nature. Electron withdrawing groups with respect to conjugate system shows -R effect, e.g. —COOH, — COOR, — CHO, — CN, — N02, etc.
NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 14
Electron withdrawing groups (EWG) producing – R effect are meta directing. They deactivate the benzene ring towards the electrophilic substitution reaction. More the EWG, more is the acidic nature

Resonance energy :

  • Number of NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 15 NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 16 contributing structures oc resonance energyNEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 16 stability
  •  In benzene, resonance energy is 36 kcal/mol.

Relation between Resonance and Bond Order

In compounds exhibiting resonance, bond order can be given by the formula
NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 17
NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 18
Hyperconjugation

It involves delocalisation of a-electrons of a C—H bond of an alkyl group attached directly to an atom of unsaturated system or to an atom with an unshared p-orbital.
NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 19

Condition for Hyperconjugation
These are as follows:

  • Compounds should have at least one sp2  hybrid carbon of either alkene, alkyl carbocation or alkyl free radical.
  •  a-carbon with respect to sp2 hybrid carbon should have at least one hydrogen. More the number of H—C bonds attached to the unsaturated system, more stable will be the alkene

Applications of Hyperconjugation

The following are the important applications of hyper conjugation.
Stability of alkenes More the number of a-hydrogen atoms, more stable is the alkene.
NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals 20
Stability of carbocation Greater the number of alkyl groups attached to a positively charged carbon atom, greater is the stability.

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