Acid Catalyzed Dehydration of Alcohols

Conditions:

• Acidic solution (H₂SO₄, H₃PO₄)
• HEAT

Alcohols can be eliminated in the presence of acids.  For our purposes, we will write acids as HA where “H” is the proton and “A” represents the base.  Think of it as a variable to represent whatever acid is being discussed in the example.

For example:

HA = H₂SO₄

H = H⁺

A = HSO₄⁻

Why is there “-“ and “+”?  Remember that a proton joined with its conjugate base has a neutral charge.  When separated, they are charged species. 

So, alcohols are protonated by strong acids (the “H” in HA).

• Acidic solution (H₂SO₄, H₃PO₄)

How do they protonate?  The lone pair on the oxygen molecule grabs the proton.  Remember that when drawing arrows, the path traced is for the electrons not the proton

We now have an oxonium ion.  Nifty, right?

Oxonium is a neat way to say oxygen cation with three bonds to it.

“Ox” like the animal – “Own” – “Knee” – “Um”

Whether you have a primary, secondary, or even tertiary alcohol, the first step is still the same.

Protonate the alcohol – Do this first.

From here, we need the dehydration in dehydration reaction to count.

Dehydration – remove water

• HEAT (Δ)

Heat favors eliminations.  When you see the delta (Δ) in the reaciton arrows means that heat is being added to this reaction, which tends to favor elimination over substitution.

Elimination – we are removing something. 

Primary – Only goes E2, carbocation formation would be way too unstable.

Secondary, Tertiary – E1, acidic solution allows for a stable carbocation