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As I said, there's different ways to represent these molecules and these
structural formulas and the somewhat abbreviated condensed formulas are the
common ways of doing that
but as we learned in chapter one these molecules really three-dimensional
objects so sometimes we want to look at these molecules in a way that
reflects that
this next slide does that for the same four molecules here
it shows ball and stick models and space filling models
remember for an atom making for single bonds, it's going to have that tetrahedral
shape around it
and that's what's determining the bond angles in the bond the ball and stick models
and they give us a a little bit better idea of what these alkanes really look
like
sometimes these are referred to as straight chain molecules, but for
something like butane, you can see it's really kind of a zig-zag chain
but each carbon again is bonding to
the number of hydrogens needed to give it four bonds
The space filling models, we won't use these as much but they do show up
sometimes
they are a little bit closer to the truth is far as the actual shape of the
molecule
but you can see some of the atoms get obscured
because they're hiding behind other atoms
both of these remind us that these are three-dimensional
types of molecules and sometimes we need to appreciate that
but these names as i say have become part of a system
that allows us to not only identify these molecules themselves but
also to identify them when we make certain modifications to them
and we'll be talking about that as we go along
those first four you need to know those names and need to know these
alkanes here
as well
with five up to ten carbons
and those names there identify them as having those particular numbers of
carbons
pentane, just like a pentagon, has five sides
pentane has five carbons
hex is the
term for six carbon molecules
just like a hexagon has six sides, hexane has six carbons
hep- is the word for seven heptane, octane just like a
octopus has eight tentacles, octane eight carbons
nonane means nine and decane means ten carbons, just like a decade
is ten years
and there alkanes that go beyond this with twelve carbons twenty carbons
thirty or forty
and they have their own names, but it's only the first ten that i want you to keep
up with as far as being able to put a name with the structure
most common alkanes do feature no more than ten carbons in a chain, so
these first ten here, that's a good group to keep track of.
the main reason there's so many alkanes is not simply because we can keep adding
carbons in the chain, but because they don't always have to be constructed so
that the carbons are all in sequence one after the other
and that brings us to this next slide
and this is really why the alkane the family is so large
because they often times exist as groups of "structural isomers"
if you look at butane, the way i showed it to you just now, it's the four carbons in
sequence like you see on the left
but for four carbons and ten hydrogens
there's another way to put them together so that the octet rule is satisfied and
all the atoms are happy,
and that's what you see on the right hand side
three carbons in sequence and then one that said to be branching off the middle
carbon
so these two forms of butane are considered structural isomers of each
other
and the book uses the term constitutional isomers
the way they are put together is different
even though they share the same molecular formula
and so that's a very common and important phenomenon
in the world of organic chemistry
butane is the first alkane for which that
is a possibility
so there you see the definition for what structural isomers are
because they're different in connectivity they will have different names
and one way to distinguish these is just to stick the
prefix "iso" in front of the one on the right to call it isobutane.
the fact that they have different boiling points means they do have
different properties in general and so they should have a difference in the way
we refer to them
and we'll see some more examples and the next segment