The order of reactivity of alcohols with hydrogen halides (HX, like HCl, HBr, HI) depends mainly on how easily the C–OH bond can be broken and replaced by halogen.
Key points:
- The reaction proceeds either via carbocation formation (SN1) or direct displacement (SN2) depending on the substrate.
- Carbocation stability (tertiary > secondary > primary) governs the rate in SN1 cases.
- Resonance-stabilized carbocations (benzylic, allylic) are even more reactive than tertiary.
Reactivity order:
Benzylic alcohol > Allylic alcohol > Tertiary alcohol > Secondary alcohol > Primary alcohol
Explanation:
- Benzylic alcohols → form benzylic carbocations stabilized by resonance with aromatic ring → very fast.
- Allylic alcohols → form allylic carbocations stabilized by resonance with adjacent double bond → also very fast.
- Tertiary alcohols → form stable 3° carbocations → faster than 2° or 1°.
- Secondary alcohols → less stable carbocation, slower.
- Primary alcohols → do not form stable carbocations easily, react only by SN2 mechanism with HX, so they are slowest.
Final order:
Benzyl > Allyl > 3° > 2° > 1°
Sarvan sir- Chemistry For All 