cyc addition reactions are pericyclic
reactions in which two Pi systems
combined to form a single ring structure
now there are two different types of cyc
addition reactions we have cyc addition
reactions that take place under thermal
conditions and cyc addition reactions
that take place under photochemical
conditions and let's begin by recalling
what a thermal condition is so a ther
Thal cyc addition reaction is a cyc
addition reaction in which the energy
source is heat the energy comes from
heat now in a thermal reaction in a
thermal cyc addition reaction we can
generalize the following two important
points if the reactant molecules have a
total of four n Pi electrons where n is
simply a number that begins with zero
excuse me it could be 1 2 3 4 and so
forth so basically if we have 0 4 8 12
16 20 and so forth P electrons in our
reacting molecules then the thermal CYO
addition reaction will not take place
there will be no rotation that is
possible to basically undergo a
concerted one-step mechanism for example
if we have two Ethan molecules reacting
on the thermal conditions to produce the
cyclobutane no thermal cyc addition
reaction will actually take place
because we have 1 2 3 4 we have four Pi
electrons in our system because four
electrons is equal to 4 multiplied by 1
where n is equal to 1 so the above
thermal cyc addition reaction will will
not take place since the number of pi
electrons is equal to 4 = 4 * 1 where n
is equal to 1 however if the reacting
systems have let's say 2 6 10 14 18 22
and so forth basically 4 n + 2 p
electrons in our reactant system then
the thermal cyc addition reaction will
take place and the prototypical example
of such a thermal reaction is the deal's
old the reaction in which we have our
Aline our dienophile reacting with our
13 bead our dine on the thermal
conditions to produce the cyclohexene so
because we have 2 four six we have six P
electrons in our system and 6 is equal
to 4 * 1 + 2 so our n is equal to 1
since this fits our Criterion then we
see that the thermal reaction will in
fact take place now the exact opposite
is true when we are under photochemical
conditions so basically a photochemical
cyc addition reaction is a reaction
where the energy doesn't come from heat
but it comes from light so basically
what happens in photochemical conditions
we have our light that shines onto our
molecule bumping an electron to a higher
orbital and that reverses the results we
just discussed so basically for
photochemical Cy addition reactions we
see that if we have four n p electrons
in our system then our photochemical cyc
addition reaction will take place but if
we had four n p electrons on the thermal
conditions we saw the reaction did not
take place so in this case these two
ethenes could not react in a one-step
mechanism to produce the cyc buttine but
in this case uh cyc butane but in this
case this reaction will take place as
long as our conditions are photochemical
conditions now on the other hand if we
try to react our two molecules that
deals all the reaction under photo
chemical conditions this will not take
place because we have 4 n + 2 number of
electrons so we can generalize these
results into the following table so uh
this basically this column designates
the number of electrons so we have 4 n
or 4 n + 2 where n is 0 1 2 3 so for
example if n is equal to zero this
becomes zero this becomes two if n is
equal to 1 this is four this is six if f
is two this is eight this is this is 10
and so forth so we see that for the
thermal case if we have 4 n electrons no
thermal reaction will take place if we
have 4 n plus2 a reaction will take
place under thermal conditions what and
the photochemical case
4N our reaction will take place as in
this case and 4 n plus 2 as in this case
no reaction takes place under
photochemical conditions