Eh don't confuse yourself in the last few hours before each
paper lah! Don't worry so much lah, if Chan Kim Seng or George
Chong or CS Toh say "heat", then you write "heat" lor. In almost
all cases for H2 syllabus, just "heating" without "reflux" is
acceptable. But if Cambridge asks you questions about reflux, or
why / when it is needed, I wrote about it a few weeks ago :
You need to know how to draw, label and describe the use of the
reflux apparatus (when it came out in the Singapore A levels a few
years ago, most Singapore JC students couldn't do this question).
There are 2 main types of reflux setups that can be tested at A
levels :
simple reflux, and
reflux with distillation.
Heating is required for most organic chemistry reactions (other
than those whose reactants or products are vulnerable to thermal
decomposition or undesired side reactions, then room temperature or
warming is required instead of heating), as most organic chemistry
reactions involve breaking of covalent bonds and hence involve high
Ea.
However, heating alone without reflux (ie. the use of a condenser)
causes problems : organic reactants and solvents, being simple
covalent molecular with weak intermolecular van der Waals are
volatitle (ie. low boiling point). As such, either the solvent, or
one or more of reactant(s), may vaporize and escape, instead of
remaining in the reaction mixture, which sabotages the reaction by
preventing it from continuing and/or reaching equilibrium yield for
the product.
Hence heating under reflux (ie. with the use of a condenser) has 2
main purposes :
1) Ensuring the solvent does not vaporize away and cause the
reaction vessel to boil dry, stopping the reaction. In addition, as
a particular solvent has its own fixed boiling point, hence by
careful choice of solvent, you can control the temperature within a
very narrow range, to allow the reaction will proceed at the
temperature ideal for that particular reaction. The constant
boiling action also serves to continuously mix the solution,
ensuring the reactants can continue to effectively mix and collide
and thus reach equilibrium yield for the product.
2) Ensuring that the organic, volatile reactants (1 or more of
which may have boiling points below the solvent and products) do
not vaporize away and cause the reaction to slow down and stop,
which prevents reaching the maximum equilibrium yield possible.
Hydroysis of amide
Esterification of amine
Nu sub of Carboxylic acid to ester
Hydrolysis if esters
Nu sub if alcohol into ester
Oxdn of alcohol
Oxdn of carbonyl cpds
Hydrolysis of nitriles
Side chain oxdn of methylbenzene
Elimination , NuSub of halogenoalkanes into nitriles and
alcohol
Esub of NO2 into halogenoarene
Ive listed down all the organic reactions in H2 syllabus that
requires heat/ heat under reflux as (one of) its condition
One perennial qns that puzzles many:
When need use/write heat under reflux or
heat??
Pleaseee help!!!
For my own notes i wrote all as heat only, refering to kim seng
chan's book and the hwach's pam peng cheong summary book, none
stated HUR