A-Level/ Advanced Higher Chemistry course notes. Concise notes summarising the topic of organic synthesis (part 2). Notes are easy to follow and contain a summary of the topic with highlighted key-points and supportive examples where appropriate.
SYNTHESIS
BOND FISSION
The reaction mechanisms for the addition of a halogen can be represented using curly arrows and
showing the cyclic ion intermediate.
Bond fission is the process of breaking bonds There are two types of bond
fission homolytic and heterolytic
Homolytic Fission Heterolytic Fission
Bond breaks I e moves to each Bonds break 2 e move to only
species free radicals formed one species ions are formed
Results in the formation
radicals
of two neutral Results in the formation of two
oppositely charged ions
occurs when each atom retains one Occurs when one atom retains both
electron
The reactionfrom the 0
mechanism the additionbond
for covalent electrons
of a hydrogen halide and from the co covalent
the acid-catalysed bond
addition and
of water
canand the bond
be represented breaks
using evenlyand showing thethe
curry arrows bond breaks
intermediate carbocation. The inductive
unevenly
A can be used to the products
stabilisation of intermediate carbocations formed during these reactions
A B
formed.
w A t B A B At t B
polar covalent B A
Normally occurs when non
bonds are broken and can be
Aj t B
initiated by UV light Normally occurs when polar covalent
bonds are broken
Reactions involving homolytic fission tend
to result in the formation of very Reactions involving heterolylic fissions
complex mixtures of products making tend to result in far fewer products
them unsuitable for organic synthesis than reactions involving homolytic fission
and so are better suited for organic
synthesis
The movement of electrons during bond fission and bond making can be
represented using curly arrow notation
Fish hook Arrow Curly Arrow
single electron source electron pairsource
i
TilEi 2e I
single electron destination electron pair destination
, ELECTROPHILES NUCLEOPHILES
In reactions involving heterolytic bond fission attacking groups are classified as
nucleophiles or electrophiles
Nucleophiles are negatively charged ions or neutral molecules that are electron rich
They have a pair of electrons readily available to donate to an
electron pair acceptor to form a covalent bond
Bff
They are attracted to atoms bearing a partial St or full positive charge
4 capable of donating an electron pair to form a new covalent bond
Electrophiles are positively charged ions or neutral molecules that are electron
deficient
They are capable of accepting a pair of electrons from an
eg Hf electron pair donor to form a covalent bond
503
They are attracted towards atoms bearing a partial 8 or
full negative charge
Capable of accepting an electron pair to form a new covalent bond
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