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

PAG 5.4: Hydration of Hex-1-ene

Hydration of an alkene

• Hex-1-ene, CH₃CH₂CH₂CH₂CH=CH₂, can be reacted with concentrated sulfuric acid, followed by water, to make hexan-2-ol, CH₃CH₂CH₂CH₂CH(OH)CH₃

• The overall equation is:

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 

• Using a measuring cylinder, measure out 5 cm³ of hex-1-ene and pour this into a boiling tube

• Plug the boling tube with mineral wool and cool in the ice bath

• Using a measuring cylinder measure out 5 cm³ of concentrated sulfuric acid 

• Once the hex-1-ene has been cooling for about 5 minutes, remove the mineral wool bung and slowly add the acid to the hex-1-ene

  • Make sure the boiling tube is kept in the ice bath during this addition.

• Stir the mixture until it becomes one layer (homogenous)

• Keep the boiling tube in the ice bath and add 10 cm³ of distilled water which will separate 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 several times and allow the contents to settle

• Remove the stopper and allow the lower layer to run off

• Add 10 cm³ of distilled water to the funnel and shake, remove the lower layer once again

• Add 25 g of anhydrous sodium carbonate in a 250 cm³ conical flask and run the hexan-2-ol layer into this flask 

  • This will dry the hexan-2-ol

  • This could also neutralise any unreacted acid, however, this should have already been removed when the aqueous layers were removed

• Filter the contents of the flask

• Set up distillation apparatus and distil collecting the fraction between 130 °C to 160 °C

  • Boiling point point of hexan-2-ol is 136 °C

  • Boiling point of hex-1-ene 63 °C

Distillation apparatus 

• Hexan-2-ol has hydrogen bonding between its molecules giving it a much higher boiling point than hex-1-ene, which just has temporary induced dipole-dipole forces

Testing hex-1-ene and hexan-2-ol

Table of tests and results for hex-1-ene and hexan-2-ol 

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 is as follows:

Stage 1: 

A secondary carbocation is formed when the double bond in hex-1-ene attacks the H atom in sulfuric acid

Stage 2:

The bond between the green hydrogen and the blue oxygen undergoes heterolytic cleavage, and both the electrons from the bond move onto the blue oxygen. The now negatively-charged strong acid picks up the green electrophilic hydrogen reforming the original sulfuric acid catalyst.