dehydration of aldol reaction products

dehydration of aldol reaction products

November 8, 2019 0 By Jose Scott


Welcome to Chem Help ASAP. Now that we know how the aldol reaction works
and makes beta-hydroxyketones, let’s see what we can do with them. [turn page]
At the top of the screen is a typical aldol reaction. The process starts with a ketone. Step one is making the enolate with LDA. Step two is adding an aldehyde to get a tetrahydral
intermediate. Step three is protonation with water to form
the beta-hydroxyketone. This product is fine in its own right, but
we can do more with this product, the beta-hydroxyketone. If we treat this product with either acid
or base (but not both at the same time), the molecule loses water to form a new alkene. This product has its own special name, an
alpha,beta-unsaturated ketone. The term “unsaturated” is a reference
to the pi bond in the molecule, and alpha,beta tells you the carbons that are part of the
pi bond. First, how does this happen? The mechanism depends on whether you are using
acid or base. For organic I classes, I normally just cover
the base reaction. The base for this reaction is normally sodium
hydroxide or an alkoxide like sodium methoxide. Let’s use methoxide, in which case, the
solvent will be methanol. Methoxide removes an alpha-proton from our
beta-hydroxyketone to form a new enolate. Alkoxides aren’t great at this reaction,
but they work well enough. Once the enolate forms, it kicks out a hydroxyl
as a leaving group. Normally, when we say “leaving group”,
we’re thinking of SN2 leaving groups, like bromides or tosylates. This is not an SN2 reaction, so we need to
be open to different leaving groups. Once the hydroxide is gone, we form a new
pi bond. (As an aside, I’m not going through the
acid mechanism, which is longer and less connected to our enolate chemistry). We have performed an elimination reaction. We’ve made a pi bond. This is not a simple E1 or an E2 reaction. If you really want to impress your friends
on the social media outlet of your choice, tell them you learned an E1cb reaction. It will get lots of “likes” and “shares”
I’m sure. I don’t think it’s important to know the
details of the E1cb, but we should quickly recognize that this transformation is definitely
some type of elimination because we’ve made a pi bond. The beauty of this elimination is that we
access this new unsaturated ketone, our alpha,beta-unsaturated ketone. The C-C double bond can be used in all sorts
of alkene reactions, including many of the addition reactions that are covered in the
alkene addition reaction playlist. So, with a simple addition of base at the
end of an aldol reaction, we can convert the beta-hydroxyketone to an alpha,beta-unsaturated
ketone, which contains a potentially useful C-C pi bond for additional reactions.