SOME BASIC PRINCIPLE OF ORGANIC CHEMISTRY
Hybridisation in Organic Compounds
The process of mixing atomic orbitals to form a set of new equivalent orbitals is termed as hybridisation.
|Number of or bitals|
|sp3||1s and 3p||Four -σ||109.5°||Tetrahedral|
|sp2||1s and 2p||Three -σ|
One – π
|sp||1s and 1p||Two – σ|
Inductive effect or Transmission effect
When an electron withdrawing (X) or electron-releasing (Y) group is attached to a carbon chain, polarity is induced on the carbon atom and on the substituent attached to it. This permanent polarity is due to displacement of shared electron of a covalent bond towards a more electronegative atom. This is called inductive effect or simply as I – effect.
Carbon-hydrogen bond is taken as a standard of inductive effect. Zero effect is assumed for this bond. Atoms or groups which have a greater electron withdrawing capacity than hydrogen are said to have–I effect whereas atoms or groups which have a greater electron releasing power are said to have +I effect.
N H 3 > NO 2 > CN SO3 H > CHO >CO > COOH > COCl >COOR
CONH 2 > F > Cl > Br > I > OH > OR > NH 2 > C6H5 > H
–I power of groups in decreasing order with respect to the reference H
ter. alkyl > sec. alkyl > pri. alkyl > CH3 >H
+ I power in decreasing order with respect to the reference H
+I power ∝ number of carbon in the same type of alkyl
CH3- CH2 – CH2 -CH2- > CH3 -CH2 -CH2- > CH3 -CH2 –
+ I power in decreasing order in same type of alkyl groups
CH 3– CH 3CH 2– (CH 3 )2CH– > (CH 3 )3 C–
Resonance effect or mesomeric effect
The effect in which π electrons are transferred from a multiple bond to an atom, or from a multiple bond to a single covalent bond or lone pair (s) of electrons from an atom to the adjacent single covalent bond is called mesomeric effect or simply as M-effect. In case of the compound with conjugated system of double bonds, the mesomeric effect is transmitted through whole of the conjugated system and thus the effect may better be known as conjugative effect.
(a) The groups which donate electrons to the double bond or to a conjugated system are said to have +M effect or +R effect.
+M effect groups:
(b) The groups which withdraw electrons from the double bond or from a conjugated system towards itself due to resonance are said to have -M effect or -R effect.
-M effect groups :
(3) The inductive and mesomeric effects, when present together, may act in the same direction or oppose each other. The mesomeric effect is more powerful than the former. For example, in vinyl chloride due to – I effect the chlorine atom should develop a negative charge but on account of mesomeric effect it has positive charge.
When a H – C bond is attached to an unsaturated system such as double bond or a benzene ring, the sigma (σ) electrons of the H – C bond interact or enter into conjugation with the unsaturated system. The interactions between the electrons of π systems (multiple bonds) and the adjacent σ bonds (single H – C bonds) of the substituent groups in organic compounds is called hyperconjugation. The concept of hyperconjugation was developed by Baker and Nathan and is also known as Baker and Nathan effect.
Inductomeric effect is the temporary effect which enhances the inductive effect and it accounts only in the presence of an attacking reagent.
The phenomenon of movement of electrons from one atom to another in multibonded atoms is electromeric effect.
There are basically two types of reagents used in organic chemistry, the electrophiles and nucleophiles.
(a) Electrophiles(Electron- loving) – Electrophilic reagents or electrophiles are the electron- deficient species which tend to attack the substrate at a position (or positions) of high electron density, e.g.
(b) Nucleophiles (Nucleus- loving)- Nucleophilic reagents or nucleophiles are the electron – rich reagents which tend to attack the substrate at a position (or positions) of low electron density.
The star indicates the atom that accepts electrons from or donates electrons to the substrate, depending on the case.
Geometrical or cis-trans isomerism
Geometric isomers are molecules that are locked into their spatial positions with respect to one another due to a double bond or a ring structure. The isomer in which same groups or atoms are on the same side of the double bond is known as cis form and the isomer in which same groups or atoms are on the opposite side is called trans-isomer.
Conditions for geometrical isomerism :
(i) It must contain a carbon-carbon double bond in the molecule.
(ii) Two unlike atoms or groups must be linked to each doubly bonded carbon atoms.