REACTION IN ORGANIC COMPOUNDS
COVALENT BOND CLEAVAGE / FISSION
• All chemical reactions involved bond-breaking and bond-forming .
• Reactions occur at the active site/functional group.
• Reactive intermediate occurs in the course of reaction
• Types of covalent bond cleavage:
a. Homolytic cleavage
b. Heterolytic cleavage
HOMOLYTIC CLEAVAGE
• Homolytic cleavage is a bond fission in which the two shared electrons of the covalent bond are split equally between the two atoms.
• Occurs in a non-polar bond involving two atoms of similar electronegativity.
• Free radicals (atoms/group of atoms contains an unpaired valence electron) are formed.
HETEROLYTIC CLEAVAGE
• Heterolytic cleavage is the bond-breakage in which both electrons remain on one of the atoms.
• Occurs in a polar bond involving two atoms of different electronegativities.
• Cation and anion are formed.
• The stability of carbocation & free radical increases with the number of alkyl groups present.
• The alkyl groups are electron-releasing, thus help to stabilize the +ve charge on the carbocation / free radical.
• This is called the inductive effect
Relative Stabilities of intermediates
• The alkyl group decreases the stability of a carbanion
• The greater the number of alkyl group, the less stable is the carbanion
Reagents and sites of organic reactions
a) Electrophile
• ..is an electron-deficient species which capable of accepting electron pairs.
• it attacks electron-rich areas of organic molecules.
• can be either neutral or positively charged.
b) Nucleophile
• ..is an electron-rich species which is capable of donating electron pairs.
• it attacks low electron density areas of organic molecules.
• can be either neutral or negatively charged.
Types of Organic Reactions
Four main types of organic reactions:
1. Addition reaction
2. Substitution reaction
3. Elimination reaction
4. Rearrangement reaction
Addition Reaction
• A reaction in which atoms or groups add to adjacent atoms of a multiple bond..
• 2 types of addition :-
a) Electrophilic Addition
•Initiated by an electrophile, which attacks a nucleophilic site of a molecule.
•Typical reaction of unsaturated compounds such as alkenes and alkynes.
b) Nucleophilic Addition
• Initiated by a nucleophile, which attacks an electrophilic site of a molecule.
– Typical reaction of carbonyl compounds.
Substitution Reaction
– A reaction in which an atom or group in a molecule is replaced by another atom or group.
– Three types of substitution :-
a) Free-radical Substitution
– Substitution which involves free radicals as intermediate species.
b) Electrophilic Substitution
• Typical reaction of aromatic compounds.
– The aromatic nucleus has high electron density, thus it is nucleophilic and is prone to electrophilic attack.
c) Nucleophilic Substitution
• Typical reaction of saturated organic compounds bearing polar bond as functional group, such as haloalkane and alcohol.
Elimination Reaction
• A reaction in which atoms or groups are removed from adjacent carbon atoms of a molecule to form a multiple bond (double or triple bond).
• Elimination reaction results in the formation of unsaturated molecules.
Rearrangement Reaction
• A reaction in which atoms or groups in a molecule change position to form a more stable structure.
