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Mechanism – Heterolytic nucleophilic addition, e.g. HCN with carbonyls.
Mechanism – Free Radical (Homolytic) substitution.
Addition reactions occur when two substances join together without any side products.
Free radicals are species that have an unpaired electron, which makes them very reactive.
Homolytic bond breaking occurs when each atom in the bond which breaks leaves with
one electron each.
Substitution means an atom or group of atoms in a molecule is replaced by another atom
or group of atoms.
Reagents and conditions
Heat under reflux with a mixture of sulphuric acid and potassium cyanide, in a solvent
such as aqueous ethanol.
Overall equation
CH3CHO + HCN
→
Initiation step
Cl-Cl →
2Cl. ,ultra violet light provides energy to break the bond.
Propagation steps
.
.
CH3-H + Cl
→
CH3 + H-Cl
.
→
CH3-Cl + Cl.
CH3 + Cl-Cl
Termination steps
.
.
→
CH3-CH3
CH3 + CH3
.
.
Cl + Cl
→
Cl-Cl
.
.
CH3 + Cl
→
CH3-Cl
→
CH3Cl + HCl
Overall equation:
CH4 + Cl2
CH3CH(OH)CN
-
Attack by CN nucleophile on carbonyl group…
followed by protonation/hydrolysis by H
+
Reagents and conditions
Mix the reactants in the presence of u.v. light, heat if necessary.
Mechanism – Heterolytic nucleophilic addition, e.g. HCN with carbonyls continued.
Mechanism – Free Radical (Homolytic) substitution continued.
Why is a mixture of H 2SO4 and KCN used rather than HCN itself?
Free radical substitution reactions are quite random, generally any of the C-H bonds can
become a C-Cl bond. Products containing more than one halogen atom can be formed by
further substitution on molecules that already contain a halogen atom.
HCN is an extremely weak acid, not enough CN- nucleophiles would be present to
start the reaction.
CH3Cl + Cl2
→
CH2Cl2 + HCl
Use the mechanism to explain why the reaction is inhibited at high and low pH.
-
At low pH the concentration of protons is very high, all CN ions would be converted
to HCN, therefore there would be insufficient CN- nucleophiles for the first step.
As a mixture of products with similar boiling points is always formed, fractional
distillation must be used to separate them. To maximise the amount of CH3Cl formed, use
excess methane. Alternatively, to get mostly CCl4, use excess chlorine.
At high pH the concentration of OH - is very high, so insufficient H+ ions available
+
for the second step as H ions react with OH to make H2O.
Sometimes specific C-H bonds can be targeted, such as…
CH3CH2CO2H + Cl2
→
CH3CHClCO2H + HCl
Note that the above reaction is useful in the organic synthesis of hydroxy acids and amino
acids.
C6H5CH3 + Cl2
→
C 6H5 CH2Cl + HCl
RCHO → RCH(OH)CN → RCH(OH)CO2H → RCH(NH2)CO2-NH4+ → RCH(NH2)CO 2H
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(C-H bonds next to –CO2 H
are weaker than others)
(Benzene ring is more
stable than the side group)
Free radical substitution is an example of a chain reaction, that is where a product each
step is a reactant in the next. There is no need for the irradiation to be carried
continuously, in fact these reactions can be explosive due to their chain nature.
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