Addition and Elimination Reactions

Addition reaction is nothing but an organic reaction, the combination of two or more molecules to a form a bigger one. In other words, a reaction where by a molecule reacts with another molecule having one or more multiple covalent bonds so as to form a molecule whose molecular mass is the total of the molecular masses of the reacting modules.

The mechanisms and functions of addition reaction

These addition reactions are restricted to chemical compounds that have multiply bonded atoms, such as molecules with carbon-carbon double bonds that are alkenes, or with triple bonds that is alkynes. It also contains molecules of carbon in hetero double bonds like those with carbonyl(C=O) groups or those with imine (C=N) groups. Polar addition reaction can be classified into two main types such as electrophilic addition and nucleophilic addition. A non-polar addition reaction exists as well known as free radical addition.

Meanwhile an addition reaction is the opposite of an elimination reaction. For example the hydration reaction of an alkene and the dehydration of an alcohol are addition and elimination reaction pairs. Addition reactions are also encountered in polymerizations and called addition polymerization.

The mechanism and functions of elimination reaction

Elimination reaction is yet another kind of organic reaction in which two substitutes are detached from a molecule in either a one or two-step mechanism. Either the unsaturation of the molecule increases or the valence of an atom in the molecule reduces by two, a process known as reductive elimination. A vital class of elimination reactions is those containing alkyl halides, or alkanes in general, with good leaving groups, reacting with a Lewis base to form an alkene in the reverse of an addition reaction. When the substrate is asymmetric, regioselectivity is decided by Zaitsev’s rule. The two step mechanisms are named and called as E2 reaction and E1 reaction.

And the E1cB elimination is an extraordinary kind of elimination containing carbanions. When it comes to addition-elimination reaction elimination occurs after a primary addition reaction and in the EI mechanism both substituent leave concurrently in a syn addition. In the related addition elimination reaction an addition reaction is succeeded by elimination reaction. In the majority of reactions it involves addition of nuclephiles to carbonyl compounds in what is called nucleophilic acyl substitution.

The other addition and elimination reactions are the reaction of an aliphatic amine to an imine and an aromatic amine to a scihiff base in alkylimino-de-oxo-bisubstitution. The hydrolysis of nitriles to carboxylic acids is also a form of addition-elimination.   

Questions:

• What is addition and elimination reaction?
• State the significance of addition reaction.