Nomenclature of Alkenes Video Lecture | Organic Chemistry for NEET

we've already seen how to name alkanes
for example this alkane is five carbons
one two three four five
we know it's called pentane so what do
we do for trying to name alkenes so this
compound has a double bonds presence but
there's also five carbons one two three
four and five so we start with pent
indicating five carbons present but
since there's a double bond we need to
change our ending from a n e to e and E
so let me write in here e and E and we
call this molecule pentene we want to
number it to give the lowest number
possible to our double bond so we start
from the left this would be carbon one
this would be carbon two three four and
five so our double bond starts at carbon
one and we could write one pentene if
you leave the one off it's implied that
the double bond starts at carbon one
what if we tried to number this starting
from the right so if we said that this
was carbon one that would make this
carbon two carbon three carbon four and
carbon five so you're saying the double
bond would start at carbon four and this
would be for pentene that's incorrect
because our goal is to give the double
bond the lowest number possible so we
call it 1-pentene or just pentene let's
look at this next example here so our
goal is to find the longest carbon chain
that includes a double bond and we want
to give the double bond the lowest
number possible so the longest carbon
chain that includes the double bonds we
would have to make this carbon one and
this would be carbon two this would be
carbon three
this is 4 and this is 5 so a five carbon
alkyne would be pentene you could put
the 1 in there or you could leave it off
next you think about the substituents in
this case we have only one group coming
off our carbon chain it's a two carbon
alkyl group and we call that ethyl so
with an ethyl group coming off carbon 2
we would call this 2 ethyl pentene for
the name
but that's not the longest carbon chains
that someone might look at this molecule
and say wait a second shouldn't this be
carbon one and then this 2 3 4 5 & 6
because that's six carbons and six
carbons is of course more than five the
problem with this is the double bond the
alkyne is not a part of our longest
carbon chain and that's one of our rules
we need to make the longest carbon chain
that includes a double bond and that's
why we name this as a pentane derivative
and not numbering it according to the
way I did in the right here so this is
incorrect so we find the longest carbon
chain that includes our double bond and
give the lowest number possible to your
double bond that's true even if you have
some alkyl substituents so let's look at
this next example and let's name it this
compound so first let's try numbering
from the left to the right this would be
carbon one this would be carbon two this
is carbon three carbon four carbon five
and carbon six a six carbon alkyne would
be called hexene let me write that in
here so hexing and our double bond
starts at carbon two so here we can see
our double bond starting at carbon two
so i'm going to put a two in front of
this so two hexene next let's think
about these substituents coming off of
that longest carbon chain including the
double bond we have a methyl group
coming off carbon to another methyl
group on carbon 4 and another methyl
group on carbon 5 so three methyl groups
would be called tri methyl let me write
that in here so we have tri methyl and
those methyl groups are on 2 4 & 5 so 2
4 5
tri methyl 2 hexene would be the name
for this compound there's another way to
name this molecule instead of putting
this two in front of the hexene you can
put the two between the hex and the een
so let me write out that way so we'd
still have methyl groups at 2 4 5 so 2 4
5 tri methyl tri methyl let's write hex
and then we'll put the 2 and
your hex to e so again the two right
here indicates the double bond starts at
carbon two this first way of naming it
two four five tri methyl two hexene it's
more of the old-school way of naming
this alkene two four five tri methyl hex
chewing is more of the newer way but
currently most people would accept both
ways to name this alkene what if you try
to number this from the right to the
left if you said that this was carbon
one and this is carbon two and this is
carbon three carbon four carbon five and
carbon six you might think this would be
correct because this would give you
methyl groups at two three and five and
thinking about the first point of
difference rule right we have 2 here and
a 2 here but we have a 3 here versus a 4
and we know that 3 is of course a lower
number so you might think that the alkyl
groups would lead you to number it
starting from the right the problem with
this way is this would give you an
alkene that starts at carbon four a
double bond that starts at carbon four
and our goal is to give the lowest
number possible to the double bond and
we want to give the double bond the
lowest number possible that's all for
any alkyl groups which is why we number
it the way we did and 2 4 5 try methyl 2
hexene is the correct aiyoo pack name
for this compound next let's look at
cyclic examples so down here on the Left
we already know this is cyclohexane with
an A and E ending if we put a double
bond again we have cyclohexene so let me
write that in here this is called cyclo
hex een so E and E so either one of
these carbons could be carbon one so
this carbon or this carbon either one of
them could be let's just say this one is
if that's carbon one then this must be
carbon 2 and so on around the ring three
four five and six
what if you have substituents let's look
at this example down here on the Left
first so we want to give our substituent
the lowest number possible so here's our
double bond so we could make this one
carbon one or the one below it carbon
to give this methyl group the lowest
number possible we make this top carbon
here carbon one so that's carbon one and
this is carbon two and so on around the
ring so this is a cyclohexane ring let
me write this down here so cyclo hexane
and we have this methyl group coming off
carbon one we would write one methyl
cyclo hexane what about this one right
here so again we have a cyclohexane ring
so let me start by writing that in so
cyclohexene as our parent name here and
we have a choice here's our double bond
we could make this one carbon one or we
could make this one carbon one well to
make the bottom one carbon one that
gives this substituents a lower number
right so this would be carbon one we
have to follow the double bond around
the ring so if that's carbon one this is
carbon two and that makes this carbon
three so let's think about those
substituents at carbon three we have two
methyl groups so that's a dimethyl so
let's write that in here so the die and
methyl cyclohexene and those two methyl
groups are both on carbon 3 so 3 3
dimethyl and then we have this ethyl
group here coming off of carbon 1 so
this is an ethyl group so let's write
that in so we have one ethyl
so one Ethel 3 3 dimethyl cyclohexene
and don't forget about the alphabet rule
so ethyl comes before methyl right E
comes before M which is why we put the
ethyl first in the name so 1 Sol 3 3
dimethyl cyclohexene