A big thanks to alchemie for sponsoring
this video and making it possible to
understand organic chemistry through
their awesome games. To get the most
stable carbocation sometimes we have to
rearrange our structure. The two types of
rearrangements we'll see, is a hydride
shift or a methyl shift. A methyl shift
can also be referred to as an alkyl
shift. A hydride shift is where we move a
hydrogen to a different carbon to give
us a better carbocation. While a methyl
shift or alkyl shift is when we move an
entire methyl group or substituent to a
different carbon to give us a better
carbocation. Here's how you can tell
when to use either shift by just looking
at the initial structure. Your clue to do
a hydride shift is when a double bond is
next to a tertiary carbon. We know a
carbon is tertiary if it is attached to
three different carbons. Your clue
to do a methyl shift or alkyl shift is
when a double bond is next to a
quaternary carbon. A quaternary carbon is
when a carbon is attached to four
different carbons. To really see how
electrons move I'll be using the
mechanisms app. If you would like to
follow along with me you can download
the app using the link in the
description. Our example is in the
essentials pack which is free and it's
under the third section under addition,
it's number nine. Our first step is
protonation, this is where the electrons
in the double bond reach out and grab
the hydrogen. Electrons from this bond
now go to the chlorine and our chlorine
is now negatively charged since it
gained electrons. Now we get a secondary
carbocation but we always want to make
sure we have the most stable carbocation possible. In this case we need to
do a hydride shift to give us a more
stable carbocation. We know to do this
because our clue here was that if a
double bond is next to a tertiary carbon
we will do a hydride shift. There is a
hydrogen here on our tertiary carbon and
we will move that here to give us a
better carbocation.
So we went from a secondary carbocation
to a tertiary carbocation. Now that we
have the most stable carbocation this
second step is to have our nucleophile
of chlorine attack the carbocation.
Here's our product with the hydrogen we
attached during the first step of
protonation and this is what it would
look like if we didn't show the
hydrogens for the final product you
typically do not show the hydrogens. If
you're following along on the mechanisms
app the next example of a methyl shift
is number seven under the third section
under addition.
Step one is protonation so our double
bond is our nucleophile and it reaches
out and grabs our electrophile which is
hydrogen, the electrons in this bond now
go to the chlorine making it have a
negative charge. Once again we want the
most stable carbocation but we
currently have a secondary carbo cation
so we must do a methyl shift. We know to
do this because our original structure
had a double bond next to a quaternary
carbon. We'll move this methyl group here
and our new carbocation is tertiary. Now
that we have the most stable carbocation, we can have our nucleophile of
chlorine attack the carbocation which
results to our product and here's our
product with the hydrogen we attached
during the first step of protonation and
this is what it would look like if we
didn't show the hydrogens. If you need
practice problems check the links in the
description there you'll find a study
guide on hydride shifts and alkyl shifts
and if you would like more help with
hydrohalogenation and then click on this
video.
