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A final round-up of some advanced aspects of radical reactivity,
this page aims to collate some themes of radical reactions not
already covered to aid the reader in the tackling of further
problems in this area.
The Nucleophilicity and Electrophilicity of Radicals
Thinking back to the radical addition to alkenes on the previous
page, where would the radical add if the double bond had an
electron-donating/withdrawing substituent? Clearly a different
product will be given depending on which end of the double bond
is attacked by the radical, but how is this determined?
The solution is to classify your radical as either a nucleophilic
or electrophilic radical - then
deciding where it attacks is simple - it will attack the δ+
site if it is nucleophilic, and the δ-
site if electrophilic. It could be said that a radical is halfway
between nucleophile and electrophile, and thus cannot be either
one alone. However, the radical in question will probably be
more stable with, say, negative charge rather than positive
charge - or vice versa.
For example - consider the Cl3C.
radical as Cl3C+ and Cl3C-
...... which one is the more stable? Answer - the Cl3C-
one because the negative charge is inductively withdrawn and
therefore stabilised by the three chlorines. So the Cl3C.
radical would rather have more electrons
than less - it is electrophilic! This same thing can be applied
to most radicals, thus identifying them as nucleophilic or electrophilic
radicals. The example to the right shows these principles in action; |
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Preference for 5-Membered Ring Closure
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For stereoelectronic reasons (commonly
from computer based energy calculations), radicals prefer to
close rings as 5-membered rather than 6-membered when the option
is available, as the geometry for orbital overlap is more favourable
in this case. eg; |
The precise term for this ring closure is '5-exo' - the disfavoured
option that is not taken is '6-endo'. There are however a few
occasions when 6-endo is preferred - these should be kept in
mind!
The reason for this 6-endo process being favoured over 5-exo
is as follows;
The initial tertiary radical is so stabilised by the two electron
withdrawing groups adjacent to it, that closing a five membered
ring and forming a completely un-stabilised primary radical
is a very disfavoured process, even taking stereoelectronics
into account - instead the radical would rather form a secondary
radical in the 6-endo process, so this is how the reactions
proceeds.
Isocyanides
This appear to have very odd reactivity, but in general it can
be noted that the primary action always takes place at the carbon
end! The following example contains many important features
of radical chemistry - look out for H atom abstraction and 5-exo
ring closure in particular.
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