Hydration of Hex-1-ene (OCR A Level Chemistry A): Revision Note

Exam code: H432

Philippa Platt

Written by: Philippa Platt

Reviewed by: Richard Boole

Updated on

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 rightwards arrow with straight H to the power of plus on top 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

Diagram of a distillation apparatus with a flask, heat source, thermometer, condenser with water in and out, and a distillate-collecting flask.
Distillation apparatus 
  • 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

Mechanism diagram showing electrophilic addition of H⁺ from sulfuric acid to hex-1-ene, resulting in formation of a secondary carbocation.
Electrophilic addition of H⁺ to hex-1-ene forms a more stable secondary carbocation

Stage 2: Nucleophilic attack by water

  • A lone pair on a water molecule attacks the secondary carbocation

  • This forms a protonated alcohol intermediate

Mechanism diagram showing electrophilic addition of H⁺ from sulfuric acid to hex-1-ene, resulting in formation of a secondary carbocation.
Electrophilic addition of H⁺ to hex-1-ene forms a more stable secondary carbocation

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

Diagram showing the alcohol intermediate transferring a proton (H⁺) to HOSO₃⁻, reforming H₂SO₄ and completing the catalytic cycle
Regeneration of sulfuric acid by proton transfer

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Philippa Platt

Author: Philippa Platt

Expertise: Chemistry Content Creator

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener

Richard Boole

Reviewer: Richard Boole

Expertise: Chemistry Content Creator

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.