hey guys, for today's problem we want to be able to draw out 2-chlorohexane
that's it we just want to be able to draw out to 2-chlorohexane, so if this seems
confusing you're gonna want to go back and review my tutorials on nomenclature
as well as the Cahn-Ingold-Prelog convention in terms of being able to
assign R and S
ok guys so when we draw structures from names what we want to do is first take a
look at what the parent molecule is so clearly we see that this is a hexane so
hex is the prefix that means six carbons and then ane is the suffix that means it
is an alkane which is a fully saturated hydrocarbon so we know that we are going
to have a six carbon chain and we also know that on carbon 2 there will be a
chloro group. so I've drawn the two possibilities for 2-chlorohexane
and so what we also want to be able to do is draw the one that has R
stereochemistry and so the way we're going to do that, first let's
understand these are the two possibilities because we can either have it on
it's on carbon 2 but it can be either a wedge bond or a dash bond, now we have to
choose the one that that exhibits R stereochemistry, so let's take a look
here, what we want to be able to do by Cahn-Ingold-Prelog is assign the
priorities of the four, remember that Cahn-Ingolg-Prelog
goes by atomic mass and we go one carbon atom a time so what are the
four atoms that this carbon is attached to? this carbon is the chiral
center and so we're going to want to take a look, we know that chlorine for
sure is the heaviest so that's number one, we know that hydrogen is for sure
the lightest so that's priority four. now over here we have a carbon and carbon
now those two carbons tie and we should also make a note that is very tempting
to look at this entire big group and say
this beats this methyl and in this case we might be right but you will be led
astray at some point if you think that way, we really do have to put on our
blinders and go one at a time. so what we do is we say carbon and carbon well
that's a tie, so what items are those carbons attached to
well this carbon is attached to hydrogen, hydrogen, hydrogen, while this one is
attached to hydrogen, hydrogen, carbon. right, we have the two implied hydrogens
there as well as this carbon whereas this methyl carbon is simply attached to
three hydrogens. now the way that works is it's essentially like
these hydrogens cancel, those hydrogens cancel, but then this carbon is gonna
beat that hydrogen, by mass, and so in this case it does occur that
this will be priority two, and this will be priority three. now once again in order
to assess R and S we have to have the lowest priority group away from us. now
fortunately in this case that is already happening because the dash bond hydrogen
which again is going away from us, past the board, is the farthest away from us
so we can simply assess from 1 to 2 to 3, we see that from 1 to 2 to 3 we are going in
clockwise fashion so that is and R stereocenter. ok so this did end up
being the correct one and then for this one, we know that this one is going to be
S because it is the opposite stereochemistry and if we want to do, if
we want to assess R and S on ones like these we have two options, we can either
imagine yourself behind the board and so right now on the other side of the
molecule from where you're standing I can look at the chlorine that's coming
towards me, the rest of the molecule going that way and then the methyl here
and I see that as counterclockwise which is S. if you have a hard time with spatial reasoning
then instead we could pretend that this is a wedge, so if we pretend that
this is a wedge we're going to assess that as R the way we just did but then
you have to reverse it to S because anytime we swap two groups as we would
the chlorine and hydrogen, swapping them to make hydrogen be away from us, you
are inverting the stereocenter so if you invert the stereocenter to make it
easier for yourself to assess R and S, you must be
then invert your answer back to reflect the original molecule.
thanks for watching guys, subscribe to my channel for more tutorials, and as always feel free to email me