Introduction to Sigmatropic Shifts Video Lecture - Organic Chemistry

a sigmatropic shift is nothing more than
an intramolecular reaction in which an H
atom moves from one position on a
molecule to a different position on that
same molecule and to see what we mean by
that let's take a look at the following
molecule so let's say we take one 3
pentadiene as shown and we place it
under thermal conditions at first glance
no reaction actually seems to take place
because the products that we form is
exactly the same as the reactants that
we begin with so we begin with 1 3
pentadiene
and we end up with what with 1 3 penta
diene as well now what actually is taken
place is a reaction and intramolecular
reaction but we can't see it in this
case because our molecule is not labeled
isotopically so the fact that we have an
unlabeled reaction that hides the fact
that a reaction is in fact taking place
if we replace these 3 H atoms with 3d
Tyrian atoms we see that the product is
different isotopically then our react
and so we see that one of the H atoms
moves to this H atom and that shifts
these two pi bonds onto different
positions so we see that if we label the
hydrogen's isotopically this will reveal
an intramolecular reaction in which the
H atom basically moves from one carbon
to a different carbon and this is an
example of a sigmatropic reaction a
sigma tropic shift now the carbon atom
where the h island begins on is known as
this starting carbon that atom is given
the number 1 and the last carbon where
our atom where our H atom ends up on is
known as the terminal carbon atom and to
determine the position the number on the
terminal carbon we simply begin counting
with our starting carbon and go all the
way on all the way up to
our terminal carbon and to designate
this particular sigmatropic reaction we
use those two values we take the values
we place them into a bracket and we
place that in front of our Sigma tropic
shift so X basically means the starting
carbon and Y means our final carbon the
terminal carbon so for this particular
case our H atom in this case DT reom
basically begins on this carbon and so
we give it the value 1 and then we begin
counting all the way up to the carbon
where our D basically moves on to so
that is the 5th carbon so we have 1
comma 5 Sigma tropic shift taking place
now what exactly is the era formulism or
the reaction mechanism for this
particular reaction so let's take a look
at the following section now we have to
point out the fact that this is not a
polar reaction it's not a reaction
between a Lewis acid Lewis base and
that's exactly why the arrow formulism
for the reaction mechanism can go both
ways so we can either have this Sigma
bond breaking off carrying this electron
this H atom all the way to this carbon
when that bond is formed this pi bond
breaks off forms a pi bond here which
breaks off and forms a PI bond here and
we and so we form this product in a
single concerted step now we can also go
in the other direction so here we go
clockwise but here we go
counterclockwise this pi bond can also
take off the H atom breaking this bond
which forms a PI bond breaking this bond
forming a PI bond here so the error
formulism
is correct in both cases and that's
because our Sigma tropic shift is not
really a polar reaction so this
basically implies that because our
reaction is not a polar reaction we
don't form any type of polar into
mediate within our reaction this
reaction is not affected by the type of
solvent that we use it's also not
affected by the acidity or the basicity
of our reactions and so it doesn't
matter if we use a polar solvent a
non-polar solvent this reaction will not
be a fact that in fact this reaction
takes place readily in the gas phase in
the gas state now something that does
affect the type of sigmatropic reaction
that takes place is the conditions under
which the energy comes from so if we use
a thermal reaction or a photochemical
reaction if we change the source of
energy the Sigma tropic shift will also
change so we said that if we take the
1:3 pen to dine and we place it on the
thermal conditions where the energy
source is heats our one 5 Sigma tropic
shift will take place however if the
energy source comes from light if we are
under photochemical conditions and we
take the same 1/3 pencil dyeing molecule
and reactant under light conditions then
we form a different product we form this
product that is that has undergone a
different type of Sigma tropic shift in
this case a 1 3 sigmatropic shift where
the H atom basically moved from the
first carbon to our third carbon so
we're going to discuss why this actually
takes place in the next lecture