What is the order of the rate of dehydration of alcohol?

January 9, 2026 •

2 min read

According to organic chemistry principles, the rate of a dehydration reaction is directly tied to the stability of its intermediate carbocation. This critical factor determines the specific hierarchy for what is the order of the rate of dehydration of alcohol.

Quick Summary

The rate of dehydration of alcohols follows the order: tertiary > secondary > primary. This reactivity is primarily governed by the stability of the intermediate carbocation formed during the elimination reaction, with tertiary carbocations being the most stable and primary the least.

Key Points

Reactivity Order: The rate of dehydration of alcohols follows the sequence: tertiary > secondary > primary.
Carbocation Stability: This order is determined by the stability of the intermediate carbocation formed during the reaction, with tertiary carbocations being the most stable.
E1 vs. E2 Mechanisms: Tertiary and secondary alcohols dehydrate via an E1 mechanism involving a carbocation intermediate, while primary alcohols use an E2 mechanism to avoid an unstable intermediate.
Reaction Conditions: Harsher conditions, including higher temperatures and stronger acid catalysts, are necessary for dehydrating less reactive alcohols, such as primary ones.
Regioselectivity: For alcohols that can form multiple alkenes, the more substituted alkene is typically the major product, a principle known as Zaitsev's rule.

Understanding Alcohol Dehydration

Dehydration is an elimination reaction in which an alcohol loses a water molecule ($H_2O$) to form an alkene. This reaction typically requires an acid catalyst like concentrated sulfuric acid ($H_2SO_4$) or phosphoric acid ($H_3PO_4$) and heat. The rate of dehydration varies with the alcohol's structure, specifically whether it is primary (1°), secondary (2°), or tertiary (3°).

The Mechanism: E1 vs. E2

The dehydration of alcohols can proceed via the E1 or E2 mechanism. Tertiary (3°) and secondary (2°) alcohols generally react through the E1 pathway, which involves the formation of a carbocation intermediate after the protonated hydroxyl group leaves as water. The rate-determining step is the formation of this carbocation. Primary (1°) alcohols, however, react through the E2 mechanism. This concerted mechanism avoids the formation of an unstable primary carbocation, with the removal of a proton and the departure of the leaving group occurring simultaneously to form the alkene.

Carbocation Stability and Reaction Rate

Carbocation stability is the key factor determining the rate of E1 dehydration reactions. More stable carbocations form more readily, lowering the activation energy and increasing the reaction rate. Tertiary carbocations are the most stable due to electron donation from three alkyl groups (hyperconjugation). Secondary carbocations, with two alkyl groups, are less stable than tertiary but more stable than primary ones. Primary carbocations are the least stable and are not formed in E1 reactions.

Comparison of Alcohol Dehydration Rates


Feature	Tertiary (3°) Alcohol	Secondary (2°) Alcohol	Primary (1°) Alcohol
Rate of Dehydration	Fastest	Intermediate	Slowest
Governing Mechanism	E1	E1	E2
Carbocation Stability	Most Stable	Intermediate Stability	Least Stable (Not formed)
Reaction Conditions	Mild conditions (e.g., dilute $H_2SO_4$, 25–80°C)	Moderate conditions (e.g., more concentrated acid, 100–140°C)	Harsh conditions (e.g., conc. $H_2SO_4$, 170–180°C)
Intermediate	Tertiary carbocation	Secondary carbocation	No carbocation (concerted reaction)

Illustrative Examples

As an illustration, consider the dehydration of butanol isomers. Tert-butanol (3°) dehydrates quickly under mild conditions (e.g., 20% $H_2SO_4$, 85°C) to yield 2-methylpropene. 2-Butanol (2°) requires harsher conditions (e.g., 75% $H_2SO_4$, 100–140°C) to form a secondary carbocation, producing a mix of 2-butene and 1-butene (following Zaitsev's rule). 1-Butanol (1°) needs the harshest conditions (e.g., conc. $H_2SO_4$, 170–180°C) for E2 elimination, yielding primarily 1-butene.

Factors Influencing the Reaction

Factors affecting alcohol dehydration include temperature, which generally increases the reaction rate. The acid catalyst is crucial for protonating the hydroxyl group. Carbocation rearrangements (hydride or alkyl shifts) can occur in E1 reactions, leading to different alkene products. The solvent can also influence the reaction mechanism.

Conclusion

The rate of dehydration of alcohol is dictated by its structural classification and the stability of the intermediate carbocation. The established order of reactivity is tertiary > secondary > primary, directly reflecting the ease of forming the carbocation intermediate. Tertiary alcohols, forming stable carbocations, react fastest via the E1 mechanism. Primary alcohols, unable to form stable carbocations, react slowest through the concerted E2 mechanism. This relationship highlights the significance of carbocation stability in organic reaction kinetics.

Frequently Asked Questions

The defining factor is the stability of the carbocation intermediate. The more stable the carbocation, the faster the rate of dehydration.

Tertiary alcohols form the most stable carbocations during the E1 reaction mechanism due to the electron-donating effect of three alkyl groups. Primary alcohols, by contrast, form the least stable carbocations, forcing them to use the slower E2 mechanism.

Yes, in E1 reactions involving secondary and tertiary alcohols, carbocation rearrangements (hydride or alkyl shifts) can occur to form a more stable carbocation. This can lead to a mixture of alkene products.

A primary carbocation is highly unstable and does not form under standard dehydration conditions. This is why primary alcohols bypass the E1 pathway and instead undergo an E2 mechanism.

The reaction requires an acid catalyst (e.g., concentrated $H_2SO_4$ or $H_3PO_4$) and heat. The specific temperature and acid concentration vary depending on whether the alcohol is primary, secondary, or tertiary.

The product is an alkene (a hydrocarbon with a carbon-carbon double bond) and water.

Yes, when multiple alkene products are possible, the reaction typically favors the formation of the more substituted (more stable) alkene, which is known as Zaitsev's rule.