Thanks to Alchemie for sponsoring this video and making it possible to learn
organic chemistry in an interactive way. Here are some important keywords to help
you understand this mechanism and the others that follow. We'll start with the
difference between a nucleophile and electrophile. You know something is a
nucleophile because it has electrons or a negative charge, while an electrophile
will have a partial or full positive charge. A nucleophile will donate a pair
of electrons while an electrophile will accept a pair of electrons. Remember that
electrons flow from nucleophile to electrophile. Protonation is the addition
of a proton. A proton refers to H+ and a PI bond simply refers to a double bond.
Here's a generic hydrohalogenation mechanism where X represents one of
these halogens. The reaction starts with the nucleophile which is our PI bond
grabbing the electrophile which is our hydrogen since it has a partial positive
charge and electrons flow from nucleophile to electrophile. Our hydrogen
now goes to one of the carbons that was a part of the double bond and the other
carbon is positively charged since we lost electrons. This positively charged
carbon is known as a carbocation. Since the hydrogen is taken, the electrons in
this bond will now go to the halogen which results in a negative charge
since the halogen just gained electrons. Since this alkene was symmetrical, it
does not matter where the hydrogen or carbocation goes whether it's this
carbon or the other. But if an alkene is not symmetrical, then it does matter
where the hydrogen and carbocation goes. This video which I'll link up right now
goes over what to do when an alkyne is not symmetrical. Continuing on, what happens
next in this reaction is called a nucleophilic attack. This is where our
new nucleophile, which is the halogen, attacks the carbocation and attaches
onto that carbon to give us our product. Here's a common hydrohalogenation
example. This double bond or PI bond is our nucleophile and
the hydrogen is our electrophile. Where students make the most mistakes is by
not understanding how electrons move and what's happening to each atom.
That's why we'll be using the mechanisms app
because it's literally designed to help
you avoid these mistakes. If you would like to follow along with me you can, the
link to download this app is in the description. For this example that we're
doing, it is in the essentials pack which is free and it's under the third section
under addition, it's number one. So step one, protonation. This is where the
electrons in the double bond reach out and grab the hydrogen. Since the hydrogen
is being taken away from the bromine, the electrons in this bond now go to bromine.
For the carbons that have the double bond, one carbon is now the carbocation
and the other has the hydrogen. Step two is a nucleophilic attack on the carbocation
Our bromine is now negatively charged and will act as our new
nucleophile. The electrons from the bromine will attack the carbo cation and
form a new bond which results in our product for this reaction. The mechanisms
app even shows you the steps you need to take for each type of reaction. Here we
see there are only two steps. One was protonation of the carbon in the double
bond, or we can state this as the deprotonation of HBr, meaning we removed
the hydrogen from HBr. The second one was our nucleophilic attack on the carbocation.
Now that you know the basics, let's get to the questions that will be
on your tests you will most likely be tested on a carbon chain that isn't
symmetrical. Before you click on the next video, try this question out on the
mechanisms app. The link to download this app is in the description. And this
question is part of their free essentials pack, it is in the third
section under addition it's number five. And I'll see you in the next video where
we'll cover what to do with a non-symmetrical alkene and understand
Markovnikov's rule.