The electron displacement in an organic molecule may take place under the influence of an atom or a substituent
group or in the presence of an appropriate attacking reagent
If any group has more than one p bond in conjugation, then only one p bond will take part in delocalisation
It is a temporary effect. The organic compound having a multiple bond ( double or triple bond)
show this effect in the presence of an attacking reagent only
Stability of reaction intermediates
Reactive, shortlived, high energy, unstable species, those are formed in the course of organic
reactions are called reaction intermediates
Homolysis fission of Covalent Bonds
Heterolytic fission of Covalent Bonds
Definition : A carbon intermediate which contain three bond pair & a positive charge on it is called carbocation.
Definition : A carbon intermediate which contain three bond pair and a negative charge on it, is called carbanion
Homolysis of covalent bond results into free radical intermediates possessing the unpaired electrons
Definition : There is a group of intermediates in which carbon forms only two bonds. These neutral divalent
carbon species are called carbenes. Most carbenes are highly unstable that are capable of only fleeting existence.
Soon after carbenes are formed, they usually react with another
Nitrenes
The benzene ring has one extra C – C p bond in benzyne
Protonation of the carbanion by water on the a-carbon gives back the carbonyl compound. Protonation on
oxygen gives the enol. Notice that the enolate ion is the conjugate base of both the carbonyl compound and the
enol.
(a) For monocarbonyl % Enol is very less
(b) Enol content increases with increase in the stability of enol by resonance, hyperconjugation, hydrogen
bonding etc
Exchange of a-hydrogen from deuterium as well as racemization at the a-carbon are catalyzed not only by
bases but also by acids due to the phenomenon of tautomerisation
An acid is a substance that can donate (or loose) a proton, and a base is a substance that can accept a proton.
Let us consider, an example of this concept, the reaction that occurs when gaseous hydrogen chloride dissolves
in water
Aliphatic bases
Aniline is less basic than Ammonia because presence of lone pair of electron at nitrogen atom conjugate with p-
electron of benzene ring and as a result electron density at nitrogen atom is decreased and hence the basic
character is also decreased
An acid is a substance that can donate (or loose) a proton, and a base is a substance that can accept a proton.
Let us consider, an example of this concept, the reaction that occurs when gaseous hydrogen chloride dissolves
in water :
Being most electronegative the sp hybridised carbon atom of ethyne polarizes its C–H bond to the greatest
extent causing its H to be most positive therefore ethyne is most acidic hydrocarbon
Conjugate base of carboxylic acid exists as two equivalent cannonical structures (A) and (B). This ion is
resonance stablised and resonance hybrid structure is (C).
Comparison between two geometrical isomers
Formic acid is more acidic than benzoic acid while phenyl acetic acid is more acidic than acetic acid.
It is common observation that generally ortho substituted benzoic acids are more acidic as compared to their
isomers and benzoic acid itself. This is called ortho effect (which is combined effect of steric hindrance,
crowding & electronic effect) in benzoic acid. However exceptions are seen.
The trend is not regular in the aqueous state as evident by their pKb values given in Table.
Nature of aqueous solution : Amines combine with water to form alkyl ammonium hydroxides.
This gives hydroxide ions in solution, thus the aqueous solution of amines is basic in nature.
Reaction of Acids with salts
