Preparation of Propanone

Authors Avatar

EXP4 – Preparation of Propanone

Aim:

The purpose of this experiment is to synthesis propanone (58 g/mol)  from propan-2-ol (60 g/mol).

Principle:

This synthesis is an oxidation reaction of secondary alcohol. Secondary can be oxidized by either acidified potassium dichromate (VI) or acidified potassium permanganate (VII).

In our synthesis, we oxidize propan-2-ol which is the smallest secondary alcohol available; its oxidation will form a propanone (commonly called acetone) according to the below equation. The most common reagent used for oxidation of secondary alcohols to ketones is chromic acid, H2CrO4.

Chromic acid is produced in situ by reaction of potassium dichromate (VI), sulphuric acid and water.      

K2Cr2O7 + H2O + 2H2SO4  2 H2CrO4 + 2 NaHSO4

The oxidation mechanism is outlined below, the propan-2-ol and chromic acid produce a chromate ester, which then reductively eliminates the Chromium species and produce the propanone product.


Method:

The experiment is a oxidation reaction where a secondary alcohol
(propan-2-ol) is oxidized by acidified potassium dichromate. The reaction readily occurs in room conditions. The product is propanone and the reaction stops at that stage, no catalyst is needed for the reaction and the reaction completes within a few minutes.

The reaction is started by adding acidified potassium dichromate to the alcohol. The colour of the oxidizer is initially orange and the colour changes to dirty green upon mixing. The oxidation reaction produce a large amount of heat which heats up the reaction mixture. The flask used for the reaction is situated in an ice bath to cool down the mixture to prevent reactants or products being boiled away. After addition of the oxidizer, the products are collected by simple distillation.  

To increase the yields of reaction, the oxidizer is used in excess to make sure reaction is complete. Distillates of boiling point ranged from 54 °C to 58 °C should be collected as products. The resulting product is wet and may be treated with inert dehydrating agents (eg: anhydrous calcium chloride) and re-distillation for purification.

The ketone could be tested with a melting point pointing point test after I has reacted with a hydrazine to form a hydrazone. The hydrazone has a sharp and distinct melting point in which the ketone structure could be found when results are compared with data book values.

Join now!

Apparatus and chemicals:

  • Propan-2-ol
  • Concentrated sulphuric acid
  • Potassium dichromate (VI)
  • 2,4-dinitrophenylhydrazine
  • Ice bath
  • Boiling tubes
  • Droppers
  • Measuring cylinder 10cm3 
  • Quick fit apparatus for simple distillation.
  • Thermometer

Experiment Procedure and Observations

  1. Approximately 3 gram of potassium dichromate and 10cm3 of distilled water was added to a boiling tube and shook to dissolve the powder. The resulting solution is bright orange with a little un-dissolved powder.
  2. Approximately 3 cm3 (accurately 5.35gram) of concentrated sulphuric acid were slowly added to the aqueous potassium dichromate solution using a dropper. Heat was generated and warmed up ...

This is a preview of the whole essay