Introduction - Hydrocarbons, Chemistry, Class 11 Video Lecture

okay this video is going to give you
some basic information about
hydrocarbons so when I say that term
hydrocarbon I'm talking about a
substance that contains only carbon and
hydrogen now if we assume that the
simplest form of hydrocarbon would be
methane it makes sense that you have one
carbon and four hydrogen's it gets more
complicated when you start adding
carbons to the chain so the simplest
hydrocarbon is ch4 and it's called
methane two carbons is called ethane
c2h6 three carbons is called propane
c3h8 and four carbons is called butane
c4h10 what do these all have in common
besides the fact that they are
hydrocarbons they have all single bonds
their name ends with ane and the formula
for the number of carbons and hydrogen's
follows the following pattern cnh2n plus
two now this hydrocarbon is called an
alkane when it contains all single bonds
so we talked about methane ethane
propane and butane after you get to five
carbons once you get to five carbons the
names will sound like you are in
geometry class we use the prefixes that
come from the Greek so pentane c5h12
again notice the pattern 2 n plus 2 for
the number of hydrogens 6 carbons is
called hexane c6h14 7 carbons is called
heptane c 7h 16 and of course 8 carbons
would be octane
c8h18 and you can go on from there no
nein decane things like that now if your
hydrocarbon contains a ring well then
the formula is a little bit different
instead of 2 n plus 2
it's just cnh2n and the reason why you
lose those two hydrogen's is because
when the chain closes on itself those
two hydrogen's on the end will go away
so the formula for a three membered ring
is not c3h8 but c3h6 and the name
propane is still there but we call it
cyclopropane so the prefix cyclo is
added when you have a ring of course
four carbons is butane but this is cyclo
butane c4h10 ring is called cyclopentane
c 5h 10 and you could have cyclohexane
cyclohexane cyclooctane etc another way
to change the structure of a hydrocarbon
is to have a double bond so we talked
about alkanes where the ending was ane
now these would be called alkenes where
the ending is E&E so there is no such
thing as methane because you have to
have at least two carbons to make a
double bond but there is ethene it's
also known as ethylene in the formula is
c2h4 when you have three carbons and a
double bond it's called propane also
known as propylene c3h6 now after that
would be four carbons remember that the
name would be butte but not butane
butene now here we have a little bit of
an issue with where the double bond is
located in these two examples you can
see that the double bond is either on
the end or the double bond is in the
middle now if there's any confusion
about the location of the double bond it
is numbered the carbon chain is numbered
in order and the number refers to the
lowest number of carbon atoms so when
the double bond is on the end we call it
1 butene when the double bond is
the middle we call it 2 butene and you
might be wondering is there a separate
molecule known as three butene well
hopefully you can see why there is no
such thing as three butene we always use
the lowest number possible so by
flipping this molecule around it would
become 1 butene so the molecule 3 butane
does not exist now to check for
understanding let's start with a formula
a structural formula for c4h8 this is
the molecular formula
c4h8 well if we begin with double bonds
we already know that you could have the
double bond on the end so 1 butene and 2
butene are two examples and we know
there's no such thing as 3 butene so you
might wonder what else is there oh and
by the way this particular molecule 2
butene actually has two different
isomers that are called sis and trans
and those are geometric isomers but I'm
not going to teach that right now in
this tutorial well instead of having 4
carbons in a row you could have 3
carbons in a row with a double bond and
this would be the molecule you come up
with
it's called propane when we have 3
carbons in a row and we have a double
bond now I have to put a branch on the
carbon chain and this particular branch
ch3 is known as a methyl group now
hopefully you can understand why we're
not allowed to put the methyl group on
the end because that would extend the
length of the carbon chain from 3 to 4
if the methyl group were located here
that would be called butene so the only
place I can put a methyl group on
propane and not extend the carbon chain
would be to put it on carbon number 2 so
2 methyl propane is a unique molecule
that we haven't seen so far well that's
pretty much it for examples of carbons
that contain the carbon chain with a
double bond don't forget we can also
have cyclo alkane
so what are different cycloalkanes you
could have for c4h8 well there is the
most obvious cyclobutane but you could
also have a three carbon ring with a
methyl group
on the side and the name of that
molecule would be called methyl
cyclopropane and now that's it
all of those are the only examples you
could think of for drawing c4h8 alright
let's take a look at another example C 5
H 10 and we'll start with double bonds
you could have five carbons in a row
with the double bond on the end and the
name is 1-pentene if you put the double
bond on carbon two that would be called
two pentane now is there such thing as
three pentane hopefully you can see no
that would not exist because you'd
simply number the carbon chain from the
other side so there is no such thing as
three pentene well in general we've
already put four carbon with five
carbons in a row let's try four carbons
in a row this would be called butene and
because the double bond is on the end
this would be called 1-butene and our
methyl group would be on either carbon 2
or carbon 3 and these are unique
molecules that have different names so 2
methyl 1 butene and then now with the
methyl group attached to carbon 3 3
methyl 1 butene is there any other type
of butene molecule yes there's 2 butene
and we're going to put the methyl group
on carbon 2 and that's the only other
possibility of having double bonds now
on to cyclo alkanes
there's cyclopentane with 5 carbons
connected in a ring if instead of five
carbons you have four carbons in a ring
you could have one methyl group and the
name of that would be methyl cyclo
butane
instead of four carbons in a ring you
could have a three carbon ring which is
called cyclo propane but now you have to
have either two methyl groups or
possibly an ethyl group let's take a
look at the two methyl groups if the two
methyl groups are on the same carbon we
call it one one dimethyl cyclopropane if
the two methyl groups are on adjacent
carbons we call it one two dimethyl
cyclopropane and then finally you could
have a cyclopropane with a two carbon
branch coming off of it and that is
known as an ethyl group so the last
example would be ethyl cyclopropane well
this is not an exhaustive list of all
the different types of organic compounds
that are out there this just gets you
started thinking about how to draw
structural formulas for simple
hydrocarbons if you have any further
questions please see me