Thanks to alchemie for sponsoring this
video and making it possible to practice
organic chemistry examples in an
effective and playful way. You can follow
along with me and we'll try this
question together using the mechanisms
app. The link to download this app is in
the description and this question is
part of their essentials pack which is
free it's under the third section of
addition, it's number five. If you need a
refresher on the general mechanism of
hydrohalogenation then check out my
previous video. Onto our example of an
alkene that is not symmetrical.
Step one 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 bromine and our
bromine is now negatively charged since
it gained electrons. Okay so where does
the hydrogen go?
Does it go to this carbon or does it go
to the other carbon ? Only one is going to
be correct. This is known as being
regioselective meaning the hydrogen is
selective or just picky as to which
carbon it attaches to. If we were to have
the hydrogen go to this first carbon it
would be wrong and thankfully the
mechanisms app can help you avoid that
mistake thanks to their hint feature.
This carbocation is not the most stable,
remember that a tertiary carbocation is
the most stable and most favorable then
a secondary carbocation follows and
primary and last is a methyl carbocation. We know a carbocation is
tertiary if there are three carbons
attached to the carbocation,
two carbons attached to the carbocation
is secondary, one carbon is primary and
no carbons attached to the carbocation
would be the methyl. So in this case our
tertiary carbocation is the most stable
so the hydrogen would favor the least
substituted carbon. Now that we have the
correct carbocation. Step two is to have
our nucleophile which is bromine attack
the carbocation to form our product. If
you notice the hydrogen went to the
least substituted carbon and the halogen
went to the most
substituted carbon, this is markovnikov's
rule. Another way to remember this is we
always want the most stable carbocation.
So first make sure to get the most
stable carbocation so a tertiary carbocation
if possible then do a
nucleophilic attack to form our product,
which in this case was a tertiary alkyl
bromide. Do this next question on your
own using the mechanisms app there are
hints to help you avoid mistakes and in
the essentials pack there's even a video
to guide you step by step. The link to
download this app is in the description
and to find this exact question click on
the essentials pack then it's the third
topic under addition and it's number 8.
I'll see you in the next video where
we'll cover when to use a hydride and
alkyl shift.
