Water (H2O) Hydration, Hydrolysis & Solvolysis

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Water ($H_2O$)

The universal solvent, nucleophile, and agent of hydration and hydrolysis.

By chemca Team • Updated Oct 2026

Water ($H_2O$) is not just a solvent; it is an active participant in many organic reactions. Because it has lone pairs on oxygen, it acts as a weak nucleophile and a weak base. In the presence of acid or base catalysts, it is responsible for the hydration of multiple bonds and the hydrolysis of various functional groups.

1. Hydration of Alkenes and Alkynes

Adding $H-OH$ Across Pi Bonds

Pure water does not react with alkenes or alkynes. An acid catalyst (typically $H_2SO_4$ or $H_3PO_4$) is required to generate the electrophile ($H^+$).

A. Acid-Catalyzed Hydration of Alkenes

Water adds across the double bond following Markovnikov's Rule, forming an alcohol. Because the mechanism involves a carbocation intermediate, rearrangements (hydride or alkyl shifts) can occur.

$$ CH_3-CH=CH_2 + H_2O \xrightarrow{H^+} \underset{\text{Propan-2-ol}}{CH_3-CH(OH)-CH_3} $$

B. Hydration of Alkynes (Kucherov Reaction)

Alkynes are less reactive toward electrophiles. Hydration requires both an acid ($H_2SO_4$) and a heavy metal catalyst, typically Mercuric Sulfate ($HgSO_4$). The initial enol product rapidly tautomerizes into a ketone (or aldehyde if ethyne is used).

$$ R-C\equiv CH + H_2O \xrightarrow{H_2SO_4, \ HgSO_4} \left[ R-C(OH)=CH_2 \right] \rightleftharpoons \underset{\text{Ketone}}{R-CO-CH_3} $$

2. Hydrolysis of Functional Groups

Cleavage by Water

Hydrolysis involves the breaking of a bond via the addition of water. It is heavily utilized for carboxylic acid derivatives and nitriles.

1. Hydrolysis of Esters: Can be acid-catalyzed (reversible) or base-catalyzed (irreversible, called Saponification).

$$ R-COOR' + H_2O \xrightarrow{H^+} \underset{\text{Carboxylic Acid}}{R-COOH} + \underset{\text{Alcohol}}{R'-OH} $$

2. Complete Hydrolysis of Nitriles ($R-C\equiv N$): Heating a nitrile with aqueous acid or base completely hydrolyzes the $C\equiv N$ triple bond to yield a Carboxylic Acid and Ammonia ($NH_3$/$NH_4^+$).

$$ R-C\equiv N + 2 \ H_2O \xrightarrow{H^+ \text{ or } OH^-, \ \Delta} \underset{\text{Carboxylic Acid}}{R-COOH} + NH_3 $$

3. Hydrolysis of Amides: Requires strong heating with aqueous acid/base to form a carboxylic acid and an amine/ammonia.

$$ R-CONH_2 + H_2O \xrightarrow{H^+, \ \Delta} R-COOH + NH_4^+ $$

3. Solvolysis ($S_N1$ Mechanism)

Water as a Protic Solvent and Nucleophile

When tertiary ($3^\circ$) or secondary ($2^\circ$) alkyl halides are dissolved in water, the water acts as both the solvent and the nucleophile. This specific type of substitution is called Solvolysis.

Action: Water is a highly polar protic solvent. It strongly solvates the leaving group (like $Cl^-$ or $Br^-$), lowering the activation energy for the formation of a carbocation. The weak nucleophile ($H_2O$) then attacks the carbocation, yielding an alcohol after deprotonation.

$$ \underset{\text{tert-Butyl bromide}}{(CH_3)_3C-Br} + H_2O \xrightarrow{S_N1} \underset{\text{tert-Butyl alcohol}}{(CH_3)_3C-OH} + HBr $$

Stereochemistry Note: Because the $S_N1$ mechanism involves a planar carbocation intermediate, attack by water occurs from both faces, leading to a racemic mixture (though often with slight inversion due to ion-pair shielding).

4. Summary of Substrates and Transformations

Substrate	Reagent / Condition	Reaction Type	Major Product
Alkene	$H_2O$ / $H^+$	Hydration (Markovnikov)	Alcohol
Alkyne	$H_2O$ / $H^+$, $\mathbf{Hg^{2+}}$	Hydration (Kucherov)	Ketone (Aldehyde if Ethyne)
Nitrile ($R-CN$)	$H_2O$ (excess) / $H^+$, $\Delta$	Complete Hydrolysis	Carboxylic Acid
Ester ($R-COOR'$)	$H_2O$ / $OH^-$, $\Delta$	Hydrolysis (Saponification)	Carboxylate salt + Alcohol
$3^\circ$ Alkyl Halide	$H_2O$ (Solvent)	$S_N1$ Solvolysis	Tertiary Alcohol (Racemic)

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