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X
This represents the general reaction that corresponds to an elimination
y and z don't have to be single atoms they can be
groups of atoms but we're going to see that there are a few common examples
of what Y
corresponds to you and what Z corresponds to oftentimes one of those
is a hydrogen atom and the other is well something else
and you can see that reaction still leaves each of those carbons with
4 bonds but now it's a double and two singles
and so we have made an alkene from something that
did not have that double bond initially and these are the three most
important types of the elimination reactions
they all begin with the letters DE
because in chemistry that means you are removing something
dehydrogenation is the first one so you can see we're taking
hydrogens from adjacent carbons and that's a general method for
turning alkanes and alkene. dehydration
just like you already know that word to mean it means a lack of water
and so if you remove water from
an alcohol, -OH rom one carbon and hydrogen an from adjacent one,
you can see water is a byproduct but you also get
an alkene. that's a very common way for introducing a carbon carbon double bond
because alcohols are
plentiful and pretty much all of them can be dehydrated
this last one is the same kind of thing except this X would be one of the
halogens
and especially when it is chlorine or bromine that's
a very common thing to remove that halogen along with a
hydrogen from next door so dehydro halogenation's the fancy term for
doing that. in the laboratory
dehydration is probably the most common of those
three types of things and we will be dehydrating an alcohol in one of our labs
and you can see all of these are essentially involving the same process
we take away the -OH group but we're also removing a hydrogen from
adjacent to that carbon
and so that gives us our double bond and in every case water as a byproduct
these conditions tell us that we need a catalyst to make this happen
and surfuric acid is a very common one. In chapter 4
we were talking about using things like HCl or HBr
to react with alcohols and in that case the outcome was oftentimes
substitution but sulfate is not a very good nucleophile so that's why we tend
to get a lot of the alkene
but with any acid you're bound to get
a little bit of substitution and at least a little bit of dehydration
and so those processes compete with one another
and when we get less than a hundred percent yield when we're trying to do a
substitution
it's often because we have done an elimination whether we wanted to
or not. the fact that this third one only involves mild heat instead of these
greater temperatures means that
this tertiary alcohol dehydrates more readily
so just as tertiary alcohols were more
quick to undergo substitution reactions we find that they are also quick to
undergo
dehydration reactions. turns out it's for the same reason
if we're not using sulfuric acid then
this is phosphoric acid down on the bottom H3PO4
that's a common catalyst phosphate is also a
bad nucleophile so we normally get eliminations as opposed to substitution
and this is a potassium hydrogen sulfate salt
that works much the same way that the
sulfuric acid does, it's just an alternative for
again encouraging these reactions but what all these have in common is that there's
only one possible alkene
in the middle one here the cyclohexanol --I could just as easily imagine the
double bond forming up here towards the top of the ring
I just chose to draw that double bond along the side
but either way you make cyclohexene if you dehydrate
cyclohexanol but
oftentimes when we have an alcohol depending on which hydrogen gets removed
along with the -OH
we can have a mixture of possible alkenes and that's what's coming up
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