8.6 Acid-Catalyzed Hydration
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92 Terms
What is hydration in alkene chemistry?
Addition of H and OH across a double bond
How many methods for hydration will be discused?
Three methods
How many of these methods give Markovnikov addition?
Two methods
How many give anti-Markovnikov addition
One method
What will this section focus on?
The first method for hydration (a Markovnikov method)
What functional group does hydration produce?
An alcohol
What is acid-catalyzed hydration?
Addition of water across a double bond in the presence of an acid
What are the net groups added in acid-catalyzed hydration?
H and OH
Where does the OH group add in acid-catalyzed hydration?
To the more substituted carbon (Markovnikov addition)
What does acid-catalyzed hydration require?
An alkene, water, and an acid catalyst (often H₃O⁺ or dilute acid)
What is the stereochemical outcome of acid-catalyzed hydration?
Depends on the carbocation intermediate (often racemic if a chiral center forms)
What product is formed from acid-catalyzed hydration?
An alcohol
What does the reagent H₃O⁺ represent?
Water plus an acid source (often H₂SO₄)
Why is the acid shown in brackets?
Because the acid is not consumed; it acts as a catalyst
What type of reaction is this?
Acid-catalyzed hydration
Why is hydration called acid-catalyzed?
The acid provides H⁺ to start the reaction but is regenerated at the end
Is the acid used up in the reaction?
No, it is a catalyst
What factor strongly affects the rate of acid-catalyzed hydration?
The structure of the alkene
Why does alkene structure matter?
Because the reaction proceeds through a carbocation intermediate
Which alkenes react fastest in acid-catalyzed hydration?
More substituted alkenes
Which alkenes react slowest?
Less substituted (or unsubstituted) alkenes.
Why do more substituted alkenes react faster?
They form more stable carbocations.
What is the general rate trend for acid-catalyzed hydration?
Tertiary > Secondary > Primary.
What factor strongly affects the rate of acid-catalyzed hydration?
The structure of the alkene
Why does alkene structure matter?
Because the reaction proceeds through a carbocation intermediate
Which alkenes react fastest in acid-catalyzed hydration?
More substituted alkenes.
Which alkenes react slowest?
Less substituted (or unsubstituted) alkenes
Why do more substituted alkenes react faster?
They form more stable carbocations.
What is the general rate trend for acid-catalyzed hydration?
Tertiary > Secondary > Primary
How does the rate of acid-catalyzed hydration change with more alkyl groups on the alkene?
The rate increases dramatically with each additional alkyl group
Which alkene reacts fastest in acid-catalyzed hydration?
The most substituted alkene (usually tertiary)
Which alkene reacts slowest?
The least substituted alkene (primary or unsubstituted).
At which carbon is the OH group installed during acid-catalyzed hydration?
At the more substituted carbon
What rule does this follow?
Markovnikov’s rule
Why does increased substitution increase the reaction rate?
Because more substituted alkenes form more stable carbocations.
What key intermediate explains both the rate and regioselectivity of acid-catalyzed hydration?
A carbocation intermediate.
Why does the OH attach to the more substituted carbon?
Because the protonation step forms the more stable carbocation on that carbon.
What two major observations does Mechanism 8.2 explain?
Markovnikov regiochemistry and the effect of alkene substitution on reaction rate
What is the first step of acid-catalyzed hydration?
Protonation of the alkene to form a carbocation
What is the second step?
Nucleophilic attack on the carbocation
What is the nucleophile in acid-catalyzed hydration?
Water (H₂O), a neutral nucleophile
What intermediate forms after water attacks the carbocation?
An oxonium ion
What is an oxonium ion?
A species where oxygen carries a positive charge after bonding to three atoms
Why does nucleophilic attack by water create a charged intermediate?
Because water is neutral, so adding it to a carbocation produces a positively charged species
What must happen to the oxonium ion to yield the final alcohol?
It must be deprotonated
What acts as the base to remove the proton?
Water (H₂O)
Why doesn’t hydroxide (HO⁻) act as the base?
Because under acidic conditions, hydroxide concentration is extremely low.
Why is water the most likely base in acidic solution?
Water is present in large excess
What must a proposed mechanism always match?
The reaction conditions
Why is water (H₂O) used as the base in the final step of acid-catalyzed hydration?
Because hydroxide concentration is negligible under acidic conditions
What does the acronym P.A.D. stand for?
Protonate, Attack, Deprotonate
When is the P.A.D. pattern commonly seen?
In reactions that occur under acidic conditions.
What are the three steps of acid-catalyzed hydration?
Protonation → Nucleophilic attack → Deprotonation
Why does acid-catalyzed hydration follow Markovnikov addition?
Because protonation occurs in a way that forms the more stable carbocation.
Where does the proton add in the rate-determining step?
To the carbon of the double bond that has more hydrogens
Where does the positive charge end up after protonation?
On the carbon with more alkyl groups (more substituted)
Why do more substituted alkenes react faster?
They form more stable carbocations
What is usually the slow step (rate-determining step)?
Protonation to form the carbocation
Why are equilibrium arrows (⇌) used in the mechanism for acid-catalyzed hydration?
Because the reaction is reversible and can proceed in both directions.
What is the reverse of acid-catalyzed hydration?
Acid-catalyzed dehydration (an E1 elimination)
What type of reaction converts an alcohol into an alkene under acidic conditions?
Acid-catalyzed dehydration.
Do most organic reactions involve equilibria?
Yes, but equilibrium arrows are only shown when chemists can easily shift the equilibrium
Why are equilibrium arrows shown for hydration/dehydration?
Why are equilibrium arrows shown for hydration/dehydration?
Which process is favored at low temperature?
Addition (hydration)
Which process is favored at high temperature?
Elimination (dehydration)
Besides temperature, what else affects the equilibrium between hydration and dehydration?
The concentration of water.
What conditions favor hydration (formation of alcohol)?
Dilute acid (a large amount of water present)
What conditions favor dehydration (formation of alkene)?
Concentrated acid (little water present)
Why does dilute acid favor hydration?
Excess water drives the equilibrium toward alcohol formation
Why does concentrated acid favor dehydration?
Low water conditions shift the equilibrium toward alkene formation
What does Le Châtelier’s principle state?
A system at equilibrium will shift to minimize any stress applied to it
In the hydration/dehydration equilibrium, where does water appear?
On the side of the alcohol formation reaction (the reactant side of hydration)
What happens when more water is added to the system?
The equilibrium shifts toward producing more alcohol.
What reaction conditions favor hydration (alkene → alcohol)?
Dilute acid (large amount of water)
Why does dilute acid favor hydration?
Excess water pushes the equilibrium toward alcohol formation.
What happens if water is removed from the system?
The equilibrium shifts toward forming the alkene.
What reaction conditions favor dehydration (alcohol → alkene)?
Concentrated acid (very little water)
How else can water be removed to favor dehydration?
By distillation or other chemical methods
What is a key factor in determining the outcome of hydration vs. dehydration?
Careful choice of reaction conditions (water concentration and temperature).
What principle helps predict and control these outcomes?
Le Châtelier’s principle
What determines the stereochemical outcome of acid-catalyzed hydration?
The carbocation intermediate.
What is the geometry of the carbocation formed during hydration?
Trigonal planar
How can water attack the carbocation?
From either face of the planar intermediate.
What stereochemical result does this lead to if a new chiral center is formed?
A racemic mixture.
Why are both enantiomers formed in equal amounts?
Because the nucleophile attacks from both sides with equal probability.
Is acid-catalyzed hydration stereoselective?
No, because of the planar carbocation intermediate.
What happens when acid-catalyzed hydration creates a new chiral center?
A racemic mixture is formed.
Why is the product racemic?
The planar carbocation can be attacked equally from either side.
What two products form when a new chiral center is generated?
A pair of enantiomers.
In what ratio are the enantiomers produced?
1:1.
Is the reaction stereoselective?
No, it produces both enantiomers equally.
Note 
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