Identifying an Unknown Carbonyl Compound

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Name:  Chow Ching Yue (7)   Group:         4            Grade:               

Date:  07/09/2010

Identifying an Unknown Carbonyl Compound

Aim

The purpose of this experiment is to classify a carbonyl compound by a simple test and to identity it by preparation of a derivative.

Introduction

In this experiment, we are provided with a sample of compound X, which is known to be a ketone from a given list.  In the first part of the experiment, preparation of a derivative of the compound with 2,4-dinitrophenylhydrazine is required.

Owing to the high eleconegativity of oxygen, the carbonyl group in the compound is strongly polarized. The carbonyl carbon is electron deficient and is easily attacked by nucleophiles, forming precipitate with 2,4-dinitrophenylhydrazine by addition-elimination reaction.  The precipitate formed, 2,4-dinitrophenylhydrazone, is a colored crystal which has sharp melting point:

Before having the second part of experiment, the precipitate formed has undergone recrystalisation, which can increase the purity of the crystals.  Thus, crystals with higher purity can give a more accurate result in the melting point determination.

In second part of experiment, by determinating the melting point and comparing with the data book, we can identify the particular ketone.

In this experiment, the variables are:

Hazard Warning

As all organic compounds have harmful vapors and con be toxic by absorption through the skin.  Some are flammable.  We must:

  1. keep the stoppers in the bottles as much as possible
  2. keep the bottles away from flames
  3. wear safety spectacles and gloves
  4. carry out experiment in good ventilation

Requirements – Part A

Procedure – Part B

  1. 30 drops of the unknown compound was put into boiling tube.  15cm3 of the 2,4-dinitrophenylhydrazine solution was added and stirred.

  1. 2cm3 of dilute sulphuric acid was added to increase the amount of precipitation.

     

  1. Suction filtration apparatus in Fig. 8.4 was used to filter the precipitate.
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  1. Suction was stopped by switching off the bump, and the precipitate was soaked in about 3.0 cm3 of methanol.

  1. Suction was resumed and the crystals were dried by drawing air through them for a few minutes.

  1. The solid was recrystallised by using the following procedures.

  1. The crystal was transferred to a boiling tube standing on a steam bath.

  1. The crystal was dissolve in the minimum amount of hot ethanol

  1. When the crystals had have dissolved, the solution was cooled in an ice-water mixture until crystal ...

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