Enantiomers in 1,2 dimethyl cyclohexanes Video Lecture - Organic Chemistry

hi this is the triack and in this video
we'll talk about stereoisomerism and
chair conformation now for cyclohexanes
we have two stereoisomers we have the
cysts and the trends if we take the
example of one to die methylcyclohexane
we can see that the two methyls could be
facing the same side forward in this
example or it could be backward and this
is the cysts confirmation or they could
be facing opposite sides also this is
one and two and in this case this is
going to be the trans conformation or
the trans stereoisomer now the question
is how can we represent these stereo
isomers using chair conformation so for
that we will take a look on the chair
and we'll start by drawing the assist
conformation so that's going to be the
SIS now for sis conformation we have to
keep in mind that the two methyl groups
they have to be facing the same side if
you recall the video on axial and
equatorial bonds we set four up carbons
the axials are facing up and for the
down carbon the axial are facing down
now for equatorial on up carbon the
equatorial are facing down and on a Down
carbon the equatorial are facing up so
now for me to have the methyl groups
facing the same side if I choose to
place the ch3 on an axial and here now
the axial here is facing down the
equatorial is facing up therefore on the
second carbon the ch3 has to be on an
equatorial so for cysts four one two
diamond
cyclohexane forces it has to be axial
equatorial or we can consider another
chair conformation we are now if we draw
the axial and equatorial so the axial
and equatorial so if we choose to place
the ch3 on an equatorial in here
therefore the ch3 on the second carbon
will be on the axial in here now for
trans stereoisomer in a similar way if I
draw the axial and equatorial bonds so
these are the axial and the equatorial
bonds in this case if I place the ch3 on
an axial on the first carbon it has to
be on an axial on the second carbon
because the axial group bonds are facing
opposite sides or it could be
or in this case it could be both ch3
czar or both ch3 groups are on
equatorial bonds since one equatorial is
facing up and the other equatorial is
facing down now in this case looking at
the trends it's either axial axial or
equatorial equatorial however you should
not be thinking using axial and
equatorial bonds when answering a
question about cysts and trans
confirmations however you should be
thinking same side for sis and opposite
sides for trans now consider the
following example for 1 3 dimethyl
cyclohexane if we choose to number the
carbons on this chair conformation so if
we chose this one as 1 this is the
second carbon and this is the third
carbon now drawing the axial and
equatorial bonds on only 1 & 3 position
so this would be the axial in here and
the axial in here since these are both
of them are up carbons now equatorial
will be both facing down now if we place
the ch3 group on an axial on the carbon
number 1 for assist conformation it has
to be also on an axial for a the carbon
number 3 and so this is the assist and
we can notice here that says in this
case is axial and axial or the flipped
chair conformation where we will have
the axial becomes equatorial and
equatorial becomes axial we can have in
this case both ch3 is on equatorial
bonds
so this is assists and in this case its
equatorial equatorial and this is the
main reason why I said you should not be
thinking axial and equatorial when you
are assigning or when you are answering
question about cysts and trends because
it depends on the position of your
substituent's in the case of one two is
different than in the case of one three
four trends it's going to be similar to
what we have explained previously now
you have to consider the X axial and
equatorial and in this case if you
choose to place one ch3 on axial now the
second ch3 on the carbon number three
has to be on equatorial since axial and
equatorial are facing opposite sides so
this would be the trans conformation
where you have it axial and equatorial
so this indicates that there is no
general rule that relates sis and trans
to axial and equatorial positions the
last example we can discuss here is the
one for dimethyl cyclohexane which we
will see that it's going to be different
than the one three dimethyl cyclohexane
but it's similar to one to die
methylcyclohexane since this stereo
isomer here this is when we have the ch3
groups on an axial and equatorial in
both cases for sis so we have two
different examples or trans has to be
either bath on axial or bath on
equatorial so this takes us back to the
example one to die methylcyclohexane
that was discussed earlier in this video