Question

How might you use 2,4-dinitrophenylhydrazine to determine if the crude product obtained from the Grignard reaction between phenyl magnesium bromide and benzaldehyde contains unreacted benzaldehyde as an impurity? Use an equation to illustrate your explanation and describe what you would expect to observe.  

Answer 1

http://www.chemguide.co.uk/organicprops/carbonyls/addelim.html

The reaction has two uses in testing for aldehydes and ketones.

• First, you can just use it to test for the presence of the carbon-oxygen double bond. You only get an orange or yellow precipitate from a carbon-oxygen double bond in an aldehyde or ketone.

• Secondly, you can use it to help to identify the specific aldehyde or ketone.

  The precipitate is filtered and washed with, for example, methanol and then recrystallised from a suitable solvent which will vary depending on the nature of the aldehyde or ketone. For example, you can recrystallise the products from the small aldehydes and ketones from a mixture of ethanol and water.

  The crystals are dissolved in the minimum quantity of hot solvent. When the solution cools, the crystals are re-precipitated and can be filtered, washed with a small amount of solvent and dried. They should then be pure.

  If you then find the melting point of the crystals, you can compare it with tables of the melting points of 2,4-dinitrophenylhydrazones of all the common aldehydes and ketones to find out which one you are likely to have got.