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here are those rules for naming
alkanes that we saw in the last segment
and I'm gonna show you and demonstrate the use of these for putting names with
structures here in a few minutes
and there are many examples in your textbook and
a lot of the exercises at the end of chapter two that are assigned
are practice using these rules to put names with structures
There's also some worksheets, a part if
this second chapter that you can use for extra practice
as well and as I said before we're gonna
use these rules going forward into other chapters as well
as we encounter different substituents that are important in organic chemistry
right now since we're just dealing with hydrocarbons
the substituents also just contain those elements and so we
need to use these rules to distinguish those kinds of carbons from the ones
that are part of this parent chain
that's really the key to this naming system
this next slide here shows
some common substituents for
alkane molecules. They're termed alkyl group because their names end in -yl
and because they just have carbon and hydrogen they can show up
a lot of times in the types of compounds we see in this chapter
remember ch4 is methane so if you take away
a hydrogen from methane we call this fragment a methyl
so this little dash here is not just a hyphen, it's intended to be a
bond because methyl wouldn't be happy by itself it's gonna have to be attached to
a parent chain
somewhere we just call it methyl because it is so
similar to methane. if you take away a hydrogen from
ethane you get an ethyl group ch3 ch2
and this carbon with two hydrogens would need
another bond to be a happy set of atoms
and so ethyl groups can also dangle off of parent chains
propane is the three carbon alkane so propyl
is the three carbon fragment and there are others that we'll see
in a little bit as we go along
here's a fourth rule to add because we're going to have to number those carbons in
the chain to locate substituents
and we want to number in the direction that gives us the smallest numbers in
our name
so that doesn't always mean we number from left to right
I we number from whichever and again will
give us the lowest number the idea is that these rules should lead everyone to
the
exact same name for a compound
even using the same numbers. Now the way these
isomers of C5H12 are drawn
we are gonna essentially number them from left to right
and when all five carbons are in sequence we say there are no branches or
substituents
and that ones regular old pentane but the one in the middle notice it doesn't have
the word pentane in its name because only four those five carbons are
part of that parent chain
so its name is a derivative of butane it's butane with a methyl group hanging
off the second carbon
thus we call it 2-methylbutane, all one word
and we just separate the number from the letters with a hyphen
this one that I had previously identified as neopentane
That is a valid name although it's not very systematic
This is the systematic name:2,2-dimethylpropane
because only three of those carbons are in sequence and there's two branches
both off the second carbon so dimethyl means there are two these methyl groups
and we use the twos twice to identify the location of both methyl
groups
even though they are coming off the same carbon.
and so for other alkanes they may have
more carbons in the parent chain they may have more branches
but we can distinguish those and put a name with them
just using these simple sets of rules here
The point of this slide is that we don't have to draw that
parent chain from left to right we don't have to number that way
it can zigzag up and down the page it says here all of these can be
numbered in a way to identify them as the exact same molecule
2-methylpentane. So it takes a little bit of trial and error sometimes
to know what should be the number one carbon
we have to start it one end or the other but it's not always the and we might
guess to begin with
but with naming these we can quickly come up with
the way to number it so that we can use a number two
to identify the location of that methyl group
and so that's something to watch out for in general