Dehydration of Alcohols
Dehydration means removing a molecule of water from an alcohol. When an alcohol is heated with a strong acid (like concentrated H2SO4) it loses water and forms an alkene. This is called an α,β-elimination reaction.
Mechanism of Dehydration
The reaction takes place through a carbocation intermediate. It occurs in three steps:
1. Protonation of Alcohol
The –OH group of the alcohol picks up a proton (H⁺) from the acid and becomes –OH₂⁺, a very good leaving group.
2. Formation of Carbocation
The protonated alcohol loses water and forms a carbocation. This is the slow, rate-determining step.
3. Formation of Alkene
A base removes a β-hydrogen, forming a double bond. This gives the alkene.
Reactivity of Alcohols
The order of dehydration depends on carbocation stability:
- 3° alcohols dehydrate fastest
- 2° alcohols — moderate
- 1° alcohols — slowest
This is because 3° carbocations are the most stable.
Orientation of Product (Saytzeff Rule)
If more than one alkene can form, the major product is the more substituted alkene. More substituted alkenes are more stable due to hyperconjugation.
Saytzeff Rule: The alkene with more alkyl groups attached to the double bond is the main product.
Ease of Dehydration
The ease of dehydration follows:
3° > 2° > 1°
Tertiary alcohols easily lose water because they form stable carbocations. Primary alcohols require harsher conditions.
Acid Catalysis
Dehydration is acid-catalysed. The acid helps convert –OH (a poor leaving group) into –OH₂⁺ (a strong leaving group). The acid is not consumed in the reaction — it is regenerated.
Reversibility
Dehydration is reversible because alcohols can form alkenes, and alkenes can form alcohols (hydration). Under dehydration conditions, the alkene escapes as vapor, pushing the reaction forward.
Rearrangement
Carbocations may rearrange to form more stable carbocations (like hydride or alkyl shifts). Therefore, the final alkene may form from the rearranged carbocation.
Orientation Summary
During elimination, the proton is removed from the β-carbon that leads to formation of the more substituted (more stable) alkene.
This completes the simplified explanation of Dehydration of Alcohols.
Detailed Notes:
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PATH: PHARMD/PHARMD NOTES/ PHARMD FIRST YEAR NOTES/ ORGANIC CHEMISTRY/ PHARMACEUTICAL ORGANIC CHEMISTRY/ DEHYDRATION OF ALCOHOL, EASE OF DEHYDRATION, ACID CATALYSIS, REVERSIBILITY, ORIENTATION.