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Chapter 8: Nucleophilic Substitution
Nucleophilicity versus Basicity |
Leaving Groups |
Solvent Effects |
Substitution versus Elimination |
Substitution reactions involving Alcohols |
Substitution reactions of Alkyl Halides |
Self Assessment |
"nucleus loving" which describes the tendency of an electron rich
species to be attracted to the positive nuclear charge of an electron poor species,
the electrophile .
expresses the ability of the nucleophile to react in this fashion.
In general terms this can
be appreciated by considering the availability of the electrons in the nucleophile.
The more available the electrons, the more nucleophilic the system.
Hence the first step should be to locate the nucleophilic center. At this
point we will be considering Nu that contain
lone pairs and may be anionic, however the high electron density of a
C=C is also a nucleophile.
of important nucleophiles are shown to the left.
(compared with basicity)
Here is a table of relative
nucleophilicities as measured in methanol (CH3OH):
- Across a row in the
periodic table nucleophilicity (lone pair donation) C- >
N- > O- > F- since increasing electronegativity
decreases the lone pair availability. This is the same order as for
- If one is comparing
the same central atom, higher electron density will increase the nucleophilicity,
e.g. an anion will be a better Nu (lone pair donor) than a neutral atom
such as HO- > H2O. This is the same order
as for basicity.
- Within a group in the
periodic table, increasing polarization of the nucleophile as you go
down a group enhances the ability to form the new C-X bond and increases the
nucleophilicity, so I- > Br- > Cl- > F-.
The electron density of larger atoms is more readily distorted i.e.
polarized, since the electrons are further from the nucleus.
Note that is the opposite order to basicity (acidity increases down a
group) where polarisability is much less important for bond formation to the
very small proton.
RO-, NC-, N3-
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