CH105  Introductory Chemistry II

Lab Exercise:  Isomers around Double Bonds

Week 6

 

Introduction:

 

Recall from Chapter 12 that isomers are different compounds that have the same molecular formula.  In alkanes, 2 isomers will have different carbon to carbon connections.  This type of isomer is sometimes called a constitutional isomer. 

 

In alkenes another possibility sometimes exists.  Because double bonds lock carbons into particular arrangements, the specific arrangement of carbons on either side of the double bond can vary.

 

These isomers are called geometric isomers, or cis-trans isomers.  The cis isomer is the one that has the longest carbon chain traveling into and out of the double bond on the same side of the double bond.  The trans isomer is the compound with the longest carbon chain traveling into and out of the double bond on opposite sides.

 

Although they look really similar, isomers are different compounds.  They will have differing properties.  In today’s experiment we will be examining two geometric isomers with the molecular formula C4H4O4.  They are fumaric and maleic acids.  Both are presented to you as “unknowns,” labeled A and B. 

 

First we will explore the physical properties of these substances.  As you investigate them, seek out and note their differences.  In the second part, we will perform a chemical reaction to change one isomer into the other.  We will break the double bond, then allow it to reform.  Since one isomer is naturally more stable than the other, the less stable isomer is converted by this process into the more stable isomer.

 

Fill out the attached data sheet as you work and turn it in as this week’s lab report.

 

Procedure:

 

Part A:  physical properties of Fumaric and Maleic acids

 

  1. Use the CRC Handbook of Chemistry and Physics or the Merck Index (both available in the lab) to look up the physical properties of the two isomers you are working with today.  Record the properties that are different, and explain how these differences might be used to distinguish a sample of fumaric acid from one of maleic acid.

 

  1. Test the solubility of each of the acids in water.  Add 3 mL of water to each of two test tubes.  Add a portion of acid to each one and shake, to note differences in their solubilities.  Record these differences and guess which acid is which based on your experiment.

 

  1. Test the melting points of the acids.  Use a capillary tube and MelTemp apparatus as instructed.  Record the melting temperature ranges for each acid, and guess which acid is which based on this test.

 

Part B:  Changing the less stable “cis” isomer into the more stable “trans” isomer.

 

In this reaction sequence, we will first break the double bond, which allows rotation around the carbons that were formerly stuck in position.  Reforming the double bond is likely to find the carbons in the trans conformation, because the trans arrangement is more stable (lower energy) than the cis arrangement.  We will attempt to change both substances A and B.  Whichever one exhibits a change in properties after the reactions must have originally been the cis isomer, since the trans isomer will be unchanged by our manipulations.

 

To make the switch:

 

1.      Weigh out 1.0 gram of the acid and put it into a large test tube.

 

2.      Add 10 mL of water.

 

3.      Move to the hood and heat the test tube to boiling.

 

4.      Add 10 mL of concentrated HCl to the flask very slowly (over a period of about a minute).  Be cautious!  This is strong and concentrated (about 12 M) acid and will damage your clothes and skin.  Wear your safety glasses for sure and gloves if you wish.  If you spill please let me know right away.

 

5.      Heat to boiling for one minute.  Remove the flask from heat and allow to cool for about 10 minutes.

 

6.      Prepare an ice bath (add 4 or 5 ice cubes to water in a 250 mL beaker) and cool the test tube an additional 10 minutes.  Also cool another test tube containing 25 mL of distilled water.

 

7.      Suction filter the solid on a Buchner funnel attached to an aspirator.  Wash the solid on the filter paper by dribbling 25 mL of ice-cold water over the solid with the aspirator running.

 

8.      When the solid is quite dry, remove from the aspirator.  Collect small samples of each and test their solubilities as you did in part A.  Which one appears to have undergone a change? 

 

 

CH105  Introductory Chemistry II

Lab Exercise:  Isomers around Double Bonds

Week 6  Data Sheet

 

1.  Draw structural or condensed structural formulas of:

 

Maleic Acid:                                                                             Fumaric Acid:

 

 

 

2.  Physical properties (from CRC or Merck Index):

 

Maleic Acid:                                                                             Fumaric Acid:

 

 

 

3.  What properties listed above differ between the two acids?

 

 

 

4.  How could you use these differences to identify a sample as maleic vs. fumaric acid?

 

 

 

5.  Record solubility observations below and guess the identity of each sample:

 

 

 

 

6.  Record melting point temperature ranges and guess the identity of each sample:

 

 

 

 

7.  Does your answer from melting points agree with solubility data?

 

 

 

8.  Which sample changed properties after the cis/trans conversion?

 

 

9.  Identify the two original samples based on the cis/trans conversion: