Organic Chemistry

56 Substitution and Elimination Reaction Mechanisms

56.1 Nucleophilic Substitution Reactions

Nucleophilic substitution reactions (SN1 andSN2 ) are very closely related to theE1andE2
elimination reactions, discussed later in this section, and it is generally a good idea to learn
the reactions together, as there are parallels in reaction mechanism, preferred substrates,
and the reactions sometimes compete with each other.

It’s important to understand that substitution and elimination reactions are not associated
with a specific compound or mixture so much as they’re a representation of how certain
reactions take place. At times, combinations of these mechanisms may occur together in
the same reaction or may compete against each other, with influences such as solvent or
nucleophile choice being the determining factor as to which reaction will dominate.

NoteIn the notationSN1 andSN2,

Sstands forsubstitution(something takes the place of something else)

N: stands fornucleophilic(anucleophiledisplaces another nucleophile)

1 : stands forunimolecular(the concentration of only one kind of molecule determines the rate of

the reaction)

2 : stands forbimolecular(the concentration of two types of molecules determine the rate of the

reaction)

Innucleophilicsubstitution, anucleophileattacksamoleculeandtakestheplaceofanother
nucleophile, which then leaves. The nucleophile that leaves is called theleaving group.

Nucleophilic substitutionsrequire

1.anucleophile(such as a Lewis base)

2.anelectrophilewith aleaving group

Aleaving groupis a charged or neutral moiety (group) which breaks free.

56.1.1 SN1 vs SN 2

One of the main differences betweenSN1 andSN2 is that theSN1 reaction is a 2-step
reaction, initiated by disassociation of the leaving group. TheSN2 reaction, on the other
hand, is a 1-step reaction where the attacking nucleophile, because of its higher affinity for
and stronger bonding with the carbon, forces the leaving group to leave. These two things
happen in a single step.