Download Elimination reactions under acidic conditions

Elimination reactions under acidic conditions
Major concepts
 Alcohols may be dehydrated under acid catalyzed conditions to give alkenes. This is an example
of an elimination reaction.
 The mechanism of the dehydration of an alcohol to form an alkene is exactly the opposite of the
addition of water to an alkene to form an alcohol (microscopic reversibility)
 Sulfuric acid pulls water away from the reaction and makes the elimination thermodynamically
favorable.
 The more stable alkene is formed preferentially, which is generally the more substituted alkene
(Zaitsev product)or the conjugated alkene.
Vocabulary
 Elimination reactions
 Dehydration
 Microscopic reversibility
 Zaitsev product
Students should be able to:
 Draw a mechanism and energy diagram for elimination of an alcohol under acidic conditions
 Explain how additions of water to an alkene and elimination of an alcohol are opposite
mechanisms
 Describe how to shift equilibrium in favor of elimination or addition
 Predict the major product according to alkene stability
Daily Problems
1. Provide a mechanism for these elimination reactions of alcohols under acidic conditions.
2. Draw an energy diagram for this elimination. Explain the role of sulfuric acid in making it downhill
overall.
E
rxn coord
3. In which direction is this reaction favored? Explain your reasoning.
The reaction is favored towards the left. Since there is water added to the reaction, the more favorable
product is produced more, which is the left most product due to the maximum amount of sigma bonds.
4. In each of these reactions, there is more than one alkene that could form. Draw out all possible
products, and then circle the one you would predict to be the major product. (Don’t worry about
cis/trans isomers.)
5. In this reaction, the less substituted alkene is the major product. Explain.
The less substituted alkene is the major product because the double bond formed is in conjugation with
the existing double bond from the starting materials. Conjugation is very stabilizing!
Cumulative problems You have learned three types of reactions: acid/base, addition, and elimination.
Can you think about them all at the same time?
6. Predict the products of these reactions. (Hint: What type of reaction is it?)
elimination
elimination
7. Provide the reagents. (Hint: What type of reaction is it?)
H-I
addition
H2SO4
elimination
H2O/ H+
addition
Extension problem
8. Eliminations can also happen under basic conditions, but only if there is a carbonyl appropriately
placed. Here is an example reaction and a mechanism.
Here is an example that doesn’t work, with a mechanism.
The difference is the first step of each reaction. Explain why the first step works in the top reaction but
not the bottom reaction. (Hint: Think about the stability of the intermediate.)
The first acid/base reaction works because the anion made is stabilized by resonance and the final
product of the reaction is conjugated. The second does not work because OH is a bad leaving group and
the breaking of two sigma bonds and making of one pi and one sigma bond is unfavorable.