Extraction is one of the most common and often used techniques in organic chemistry.  It is a common way of separating a desired compound from a mixture.  The separation itself is usually based on polarity differences between the molecule you are interested in isolating and the other components of the mixture.  Often times a chemical reaction is needed to alter the polarity of one of the components of the mixture.  This type of separation is called a chemically active extraction.  For more information on the theory of extraction see Chapter 37 in the OCLSM.  The principles governing the actual technique can be found beginning with page 126 in the OCLSM.


benzoic acid mixture in ether on steam bath  suction filtration after boiling  chemically active extraction  completing the chemically active extraction


Goal:  The goal of this lab is to separate benzoic acid from a mixture containing benzoic acid, cellulose (a natural polymer of glucose) and methyl orange (a common acid/base indicator).  You will use a chemically active extraction to convert the water insoluble benzoic acid into its water soluble salt by treating the carboxylic acid with base.  Finally the benzoic acid will be precipitated by adding strong acid to the carboxylate salt solution.  The following extraction schemes show the methodology you will use.



PROCEDURE:


1) Weigh 4 g of the crude benzoic acid mixture using the analytical balance and place it in a 125 mL Erlenmeyer flask.  Proper weighing procedure is covered on pages 55-56 of the OCLSM.  Note: We do not use vials in the lab so a paper towel or watch glass will have to be substituted.


2) Add 50 mL of diethyl ether to the flask. See page 3 in the OCLSM for proper procedures when dealing with the transfer of volatile liquids and use of the hood.


3) Add boiling stones (page 144 in the OCLSM) and heat the mixture on a steam bath until the ether begins to boil.  Proper use of a steam bath is found on page 147 of the OCLSM.


4) Vacuum filter the mixture using a Buchner funnel and a filter flask (page 109 in the OCLSM).    Note: The Buchner funnel may be used to separate any solid from a liquid.  It is not just for the separation of crystals.  Observations: Which components remain in the ether?  Which have been filtered out?  Why?


5) Place the filtrate (ether) in a 500 mL separatory funnel and extract twice with two 30 mL portions of 1 M sodium hydroxide (NaOH).  Collect and keep the aqueous layers from both extractions.  Combine the two aqueous layers in a 250 mL beaker or a flask.  Chapter 15 of the OCLSM covers this topic in great detail.

Formation of two layers after the addition of NaOH to ether
How to shake the separatory funnel
Venting process
Collection of aqueous layers


6) Cool the combined aqueous extracts on ice.  Acidify with 50 mL of 6M HCl.  Cool on ice again. Addition of HCl to the aqueous layers.  Observations: During acidification the solution should turn a bright pink.  Why?


7) Collect the benzoic acid by suction filtration and save it on a watch glass.  You will recrystallize it during the next laboratory period.  Observation: Comment on the color, shape and quantity of your benzoic acid.



CHEMICAL INFORMATION
Name Structure (2-D)
benzoic acid benzoic acid
cellulose cellulose
methyl orange methyl orange
diethyl ether diethyl ether