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in this video we're going to focus on
naming organic compounds particularly
alkanes
so let's start with a simple example
how can we name this alkane
well we need to identify the number of
carbons and the parent chain
and so because it has six carbons
this is called hexane
now keep in mind
one carbon is methane two carbons is
ethane three is for propane
four is for butane
five carbons is pentane
seven is heptane
eight is octane nine is nine ten is
decane
now what about this example
how can we name this particular alkane
the first thing you want to do is count
the number of carbon atoms
in the parent chain or the longest chain
now should we count it in this direction
or
should we count it
in this direction
the methyl group that we see here
is called a substituent
and you want to count in such a way that
the substituents have
a lower number
and 2
is lower than 4
so this is the correct way to account it
so how can we name it
we have a method group on carbon two
and
there's five carbons in the parent chain
so this is going to be called
2-methyl
pentane
you need to use a hyphen to separate a
number from a word
now what about this example
so clearly we need to number it from
left to right rather than right to left
because we want the substituents to be 2
3
instead of
4 and 5 if we counted it in this
direction
so two and three are lower than four and
five so
we're going to use
this example
now we have a methyl group
on
okay this computer is having issues now
so we have a method group on carbon 2
and on carbon 3.
so how can we name it in this case
so the parent chain is hexane
because it has six carbons
and there's two methyl groups so how can
we write this
so you need to write two comma three
dash
dimethyl
dies for two tries for three tetras of
four
so it's dimethyl hexane
if you have multiple substituents
then you need to use a comma to separate
the numbers
by the way when you get a chance check
out the description section of this
video
i'm going to post a few links
particularly my patreon page where you
can access
the organic chemistry one and two final
exam preparation videos if you're
currently studying for that
so feel free to take a look at that when
you get a chance i'm going to post some
other links to some other videos as well
that you might find helpful too
now what about this example
go ahead and name
that particular
alkane
so we're going to number it from left to
right because it's going to give us the
lower numbers
and this time
we have three methyl groups
as opposed to two
so the parent chain is still hexane
and so let's put it all together so we
have a methyl group on carbon two and we
have two methyl groups on carbon three
so we have to write three three you
don't wanna write the three just once
you need to write it twice because
there's two of them on carbon three
and then we need a hyphen
trimethyl because there's three methyl
groups
and then the parent name hexane
so that's how we can name this
particular organic compound
here's another example for you
go ahead and try that problem
so we need to number it from left to
right
and it looks like we have a methyl group
on carbon 3
and on carbon 4 we have a substituent
with two carbons so that's an ethyl
group
now how should we name this how can we
put this together
should we name it as
3-methyl
dash
4-ethyl
and the parent chain has seven carbons
so
that's heptane
or
should we say
for ethyl
dash three methyl heptane
so which of these two names
is correct
now looking at the first example
the numbers are arranged in increasing
order now looking at the second example
we could see that it's arranged in
alphabetical order e
comes before m
so which one should we give more
priority to
arranging the numbers in the senate
order or putting the letters in
alphabetical order
it turns out in this situation
you need to alphabetize
the substituents so the ethyl group
must come before the methyl group
regardless if the numbers are increasing
or decreasing
so this is the right answer
now let's move on to our next example
so notice that we have a propyl
substituent there's three carbons
and we have an ethyl substituent
how do you think we should name this
particular molecule if you want to pause
the video and try the problem feel free
to do so
now what direction
should we count this molecule
so let's count it both ways i'm going to
redraw
this structure
so in the first example let's count it
from left to right
and for the second example
let's count it the other way
now based on the last example we know
that we have to put the substituents in
alphabetical order
so ethyl comes before methyl i mean not
methyl but well it does come before
methyl but in this example ethyl comes
before propyl
so name in
this molecule based on the way it's
counted it's going to be called
5
ethyl dash
4-propyl
and there's eight carbons in the parent
chain so that's going to be octane
now for this molecule at the bottom
the ethyl is on carbon four so it's four
ethyl
and then the propyl is on carbon five
so five propyl
and then
octane
so looking at these two examples
it doesn't matter which way we number it
in terms of the numbers that we have
because we do get the same numbers
here we have five and four
and four and five
so
going left to right or right to left
we still get the same exact numbers but
notice that
the way the numbers are arranged when we
name it
in both cases the substituents are
placed in alphabetical order but in the
first example the numbers are in
decrease in order it's five then four
and in the second example it's in
ascendant order
so if you get to the situation where
it's already placed in alphabetical
order then
the first group should have the lower
number
or in other words you want to place the
numbers in ascendant order
going from low to high so this
is the right way to name it
now let's move on to our next example
feel free to try that problem
so we need to number it
from
left to right
if we do it the other way
notice that
we won't get
the lower numbers
going from left to right
we have two methyls on carbon two and an
ethyl on carbon four
from right to left we have an ethylene
carbon three two methyls and carbon five
so two two four is less than
three three i mean three five five so we
want to
number it from left to right
now let's go ahead and name it
so we have a ethel on carbon four
and
two methyls on carbon two so that's a
dimethyl
now how should we name this
should we say it's four ethyl
dash
2 comma 2 dimethyl
hexane
or
should we say it's 2 comma 2
dimethyl
dash 4-ethyl
hexane
which is the right way to name it
now looking at the second example
the numbers appear to be an ascendant
order so that seems like a good sign
and also
d comes before e
so that looks like
the second answer should be the right
answer
but it turns out that it's not
and this
is the right answer
and the reason is
that when
you're placing the substituents
in alphabetical order
you don't take into account the prefixes
so you need to ignore the prefix di tri
tetra and things like that
so you're comparing
e
and m
so therefore
ethyl comes before methyl so we need to
put it in alphabetical order before
looking at the way the numbers are
arranged
when you write your final answer
so placing it in alphabetical order has
more priority than making sure that the
numbers are in ascendant order
