NEET Chemistry Notes Some Basic Principles and Techniques -Free Radicals
These are highly reactive, neutral and electron deficient species.
The order of the stability of free radicals is
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
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.
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:
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:
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,
- Lewis bases,
- Benzene, alkenes, etc.
- Nucleophilicity order is
- 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 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,
This is a permanent effect and propagates through carbon chain,
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 of alkyl groups enhances -I effect of the halogen atoms. 3°-dkyl halide will be most reactive due. to more +I 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 electrons are completely transferred to any of the one atom. This effect is temporary.
It involves delocalisation of electrons. 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.
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.
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 contributing structures oc resonance energy 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
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.
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.
Stability of carbocation Greater the number of alkyl groups attached to a positively charged carbon atom, greater is the stability.