Hydrocarbons - Constitutional Isomers of C5H12 Video Lecture - Class 11

hey guys it's Casey from the Montreal
student Network and today we are going
to be looking at the 5 carbon
constitutional isomers for alkanes so
like we said last video constitutional
isomers they have the same chemical
formula so when you sum up the amount of
carbons and hydrogen's since that's all
there is inside an alkane they'll have
these same numbers but since it's an
isomer they will have different atomic
arrangements so those carbons will be
arranged differently and the hydrogen's
you know we'll just be placed to
completely 4 carbon you don't need to I
mean the four bonds on the carbon you
don't need to put too much thought into
placing the hydrogen's and I'll show you
guys exactly how that works right now so
there are three constitutional isomers
for five carbon alkanes we have the
standard one that you can imagine a
straight chain and remember straight
chains are always diagonal because they
do have the tetrahedral arrangement so
this is what you'll most likely see if
it did actually you know occur in real
life but it is a straight chain so we'll
draw this out in a bit of an expanded
form just so you guys can see what you
know just you can count your carbons i'd
recommend when you're just learning
constitutional isomers draw this out
with the bond line so you can make sure
you you have all your hydrogens are all
accounted for and you can make sure you
get the idea that the carbons need to
have four bonds and once see this one in
the middle they have two bonds to carbon
so they'll have two extra bonds two
hydrogen's to complete that four four
bonds and once you get that idea down
you know pie once you get to the next
chapter you can stop drawing this
expanded form because it takes a lot of
time and you don't have a lot of time to
draw something like this on the on a
test now if you're going to draw the
chemical formula for this we can see we
can count 1 2 3 4 5 6 7 8 9 10 11 12 1 2
3 4 5 all our heart hydrogen's and
carbons are accounted for so this is the
correct bond line formula the chemical
formula for it would be ch3 ch2 3 since
there'll be 3
ch2 units in the middle we can count
those 1 2 3 and one more ch3 on the end
so that is the correct condensed formula
and this is a correct bond line formula
now what if we
arranged differently what if we take
this branch over here and place it over
here so we're pretty much moving it
right over here and I'll give us
something that looks like this
now make sure you know that this is
equal to this they have the exact same
structure this is identical they're not
isomers of each other it's not different
atomic arrangements this they are
identical you essentially just move this
one around so if you rotate this like
this it would turn out to be what you
have right over there and this is equal
to this so don't get confused a lot of
times you might think there are more
isomers than there actually is and you
need to know that these are identical
compounds but once you get into naming
which we'll do in a couple videos you'll
be able to just name both compounds
realize they have the same exact name no
matter which way you look at it and
there you go you don't you won't get
confused so let's draw the expanded
formula for this one I'll redraw the
bond lines so we have it down here
the expanded will be like this now we
can draw these ones on the end we'll
have three hydrogen's on a pure two this
one over here three hydrogen's this
one's attached to two carbons so I have
two hydrogen's and right there you can
count them all up and see that it has 12
1 2 3 4 5 6 7 8 9 10 11 12 and how would
we draw the condensed formula for
something like this well you can just
take it slowly draw a ch3 representing
this guy over here it'll be connected to
a CH ch2 unit right there you can just
draw expand it out like this ch2 and ch3
that's the correct way of representing
it it works on a test most teachers will
want the bond line but if they ask for
the condensed structural formula this is
completely acceptable as well now we see
how that's arranged differently and it
has the same chemical formula so it is
a constitutional isomer of what we saw
right here now this if you want to give
it a name the first one the the name you
can use to differentiate this n-pentane
but in normal situations you just call
it pentane when we go down to look at
the one here that has one branch the
other or one of the other constitutional
isomers we will associate the name izo
pentane just like in the previous video
when we had one branch we called it
isobutane it's just an extra name there
is another one for the third
constitutional isomer which we'll look
at so it's good to know these these
common names in case they ask you for a
common name but on a test
there is a naming system to you PAC
naming system which will make it much
simple you don't really need to memorize
all these extra common names so and the
last one we have let's get this the
structure we had it before what happens
if we take this branch over here and put
it in there we know we still have an
extra bond right there so we're gonna
have a carbon that's attached to four
other carbons and we're gonna get a
structure like this and that is like the
peace sign number I lost three I said
there's the structure it looks like a
peace sign this one really looks like a
peace sign without the circle around it
so anyways we're gonna draw the
structural the expanded to structural
formula just take a look and we're gonna
try to have that kind of tetrahedral
arrangement you know it's something that
you teach your PI will not take off
marks for if you don't do but a it's
better you can understand a bit more of
how science science works just by
looking at the tetrahedral arrangement
with the it's 109 points five whatever
bond bond degree angles and you see
right that we have we have four ch3
groups so four times three we know there
is 12 hydrogens and when you can Scout
are what's at five carbons so there we
do obey the chemical formula and clearly
there's a different atomic arrangement
it's not similar to the one we have up
here and it's not
similar to the one we have up here with
n-pentane we're going to assign the
chemical the condensed structural
formula to this will be C ch3 4 so it's
a bit different than what we've seen so
far it's one carbon the carbon the
middle or has already attached the four
of the carbon so it will not have any
hydrogen's and it will have four ch3
groups surrounding it one two three four
so you can condense it like this or
obviously if you want to do the long way
you can just write something like this
really it's not a problem with your
teachers but once again there's a lot of
stuff to do in an organic chemistry test
and you might want to just save time and
once you can identify something like
this if you get this condensed formula
you don't need to take the time to write
it out you can automatically figure out
what it is and finally we are going to
assign the name neo pentane to this
derivative of pentane how it's going to
have two branches instead of one if it
has one branch
it's isopentane if it has no branches
it's n-pentane so just take one quick
look at what we've seen so far we have n
paint in it but there with no branches
we have new or isopentane here with one
branch and then finally with two
branches we will have where is it
right down there we have neopentane and
if you try to arrange this in any other
way you'll know that there is really no
other way of arranging it so there's no
other constitutional isomers for example
you might think that you can take this
branch here let's use a different color
let's use blue didn't use any other
colors this video let's say you take
this branch and bring it all the way
down here instead you're gonna get a
structure that looks like this but in
reality this structure is exactly the
same as this there's no difference you
know you just got to look at it a bit
differently and you can see that okay
here is carbon one two three and four
obviously we should remember in the
other way you always want a number so
your branches have the lowest number on
them so here's one two three and four
one two three and four we see that you
know
the odd branch out is on carbon number
two in both cases so they're identical
you really want to make sure you should
know how many eyes of isomers each type
of carbon has are each carbon count has
just so you don't run into a problem
like this on the test and say there's
more carbon and constitutional isomers
and there are so that's the video for
today if you have any comments or
questions feel free to leave a comments
and I will answer as soon as possible
thank you very much for watching my name
once again is Casey and have a great day