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

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 

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

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.