Hydration of Hex-1-ene (OCR A Level Chemistry A): Revision Note
Exam code: H432
PAG 5.4: Hydration of hex-1-ene
Hydration of an alkene
Hex-1-ene reacts with concentrated sulfuric acid, followed by water, to make hexan-2-ol:
CH3CH2CH2CH2CH=CH2 + H2O CH3CH2CH2CH2CH(OH)CH3
The hexan-2-ol is then separated, distilled and the product is tested
Preparation of hexan-2-ol
Prepare an ice bath in a 250 cm3 beaker
Measure 5 cm³ of hex-1-ene using a measuring cylinder and pour it into a boiling tube
Plug the boiling tube with mineral wool and place it in the ice bath
Measure 5 cm3 of concentrated sulfuric acid
After about 5 minutes, remove the wool and slowly add the acid to the hex-1-ene, keeping the boiling tube in the ice bath
Stir the mixture until it becomes one layer (homogeneous)
Keep the boiling tube in the ice bath and add 10 cm3 of distilled water
This separates the reaction mixture into two layers:
The top layer will contain hexan-2-ol
The bottom layer will contain unreacted acid
Separation and distillation of hexan-2-ol
Pour the contents of the boiling tube into a separating funnel
Shake gently several times and allow the layers to settle
Remove the stopper and run off the lower aqueous layer
Add 10 cm3 of distilled water to the funnel, shake again, and remove the lower layer
Place 25 g of anhydrous sodium carbonate into a 250 cm3 conical flask
Run the hexan-2-ol layer into this flask to dry it
Swirl gently, then filter the mixture into a clean container
Set up distillation apparatus and collect the fraction between 130 oC to 160 oC
Boiling point point of hexan-2-ol is 136 oC
Boiling point of hex-1-ene 63 oC

Hexan-2-ol has a higher boiling point than hex-1-ene because:
Hexan-2-ol has hydrogen bonding
Hex-1-ene has temporary induced dipole-dipole forces
Testing hex-1-ene and hexan-2-ol
ou can confirm that the product is hexan-2-ol by performing a series of qualitative tests
These tests show:
The absence of an alkene functional group
The presence of an alcohol functional group
Table of tests and results for hex-1-ene and hexan-2-ol
Test | Result for hex-1-ene | Result for hexan-2-ol |
---|---|---|
Shake with bromine water | Orange to colourless | No change |
Shake with acidified KMnO4 | Purple to colourless | No change |
Addition of sodium metal | No reaction | Fizzing seen |
Combustion | Smokey flame | Less smokey flame |
Practical skills reminder
This practical develops essential skills in the preparation, purification, and verification of a liquid organic product, including:
Using a separating funnel to remove aqueous impurities
Washing with water and drying with anhydrous sodium carbonate
Performing distillation to collect a pure fraction in a defined boiling range
Carrying out qualitative tests to confirm the presence of the alcohol group and absence of unreacted alkene (e.g. bromine water, KMnO₄, sodium)
Why is hexan-2-ol formed?
Hexan-2-ol is formed as the secondary carbocation that is formed in the mechanism is more stable than the primary carbocation
The mechanism proceeds in three main stages:
Stage 1: Forming the secondary carbocation
The double bond in hex-1-ene attacks a hydrogen from sulfuric acid
This forms a secondary carbocation on the more substituted carbon atom

Stage 2: Nucleophilic attack by water
A lone pair on a water molecule attacks the secondary carbocation
This forms a protonated alcohol intermediate

Stage 3: H2SO4 catalyst regeneration
The bond between the hydrogen (green) and oxygen ()undergoes heterolytic fission
Both the electrons from the bond move onto the blue oxygen
The lone pair on HSO4- attacks the green hydrogen
This regenerates the original sulfuric acid catalyst

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