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Reaction Mechanism

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Reaction Mechanism - Lesson Summary

Reaction mechanism involves breaking and forming bonds, movement of electrons and rate follows sequential description of rate of reaction.

A covalent bond can get cleaved either by homolytic cleavage (or) by heterolytic cleavage. In homolytic cleavage a bond breaks in such a way that each broken part gets one of the two bonded electrons.

Homolytic cleavage results in the formation of neutral atoms (or) groups with one unpaired electron. These are known as free radicals. The free radicals thus formed are highly reactive due to the presence of unpaired electrons.
 

 
Ex: Formation chlorine free radicals from Chlorine molecule.
 

 
Depending on the number of alkyl groups directly attached to the carbon free radical alkyl free radicals are classified as primary, secondary and tertiary. With increase in number of alkyl groups the stability also increases. Thus primary alkyl radicals are the least stable while tertiary radicals are the most stable.
 


In heterolytic cleavage, bond cleaves in such a way that the shared electron pair goes to the more electronegative atom.



After heterolytic cleavage, the atom that loses the bonded electrons gets a sextet electronic structure and acquires a positive charge. 

Depending on the number of alkyl groups directly attached to the positively charged carbon, carbocations are classified as primary, secondary and tertiary carbocations.



Ex: Ethyl cation is a primary carbocation, while isopropyl carbocation and tert-butyl carbocations are secondary and tertiary carbocations.

A positively charged carbon with a sextet, called a carbocation or a carbonium ion. carbocations are very reactive as they have a strong tendency to complete their octet . 

Stability of the carbocations increases with increase in number of alkyl groups bonded to the carbocation. Hence, primary carbocations are highly unstable while tertiary carbocations are the most stable. 



All the organic reactions proceeded by heterolytic bond cleavage are known as ionic or hetero-polar or polar reactions.

Polar organic reactions are caused by two types of reagents: electrophiles and nucleophiles. An electrophile is an electron-seeking species that is electron deficient. A reaction initiated by an electrophile is called an electrophilic reaction. Electrophiles  always attack the substrate molecule at the site of maximum electron density.

Electrophiles may be positively charged molecules (or) neutral molecules.

An electron-rich reagent that donates electrons to the positive (or) electron-deficient site in a compound is called a nucleophile. A reaction initiated by a nucleophile is called a nucleophilic reaction. Nucleophiles always attack the substrate molecule at the site of minimum electron density. Nucleophiles may be negatively charged molecules or neutral molecules.

 

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