Question

Postlab Questions Answer the following questions on a separate sheet or in your lab notebook. Make sure that the copy page is readable! D) Calculate the percentage yield for both reactions. Show all calculations. 2) What is the color of the reaction at the beginning of reaction? What causes the color? 3) Why do you dissolve maleic acid in diethyl ether and fumaric acid in water? 4) Give the complete reaction mechanisms for the formation of meso and racemic 2,3- dibromosuccinic acid, using condensed structures. 5) Reaction products. If all bromination approaches are considered, four stereoisomeric products are possible from this reaction. Draw all possible stereoisomers and determine whether the products are (R) or (S). 6) Which form(s) of 2,3-dibromosuccinic acid isare) optically active? 7) Look up the melting points of meso and racemic 2,3-dibromosuccinic acid and compare to melting points of your products.

Answer 1

1.

% yield = actual yield /theoretical yield *100

Theoretical yield :

(a) 0.200 g of Maleic acid : 0.200g /116 g/mol = 1.72 *10^-3 moles

1 mol Maleic acid -------> 1 mol of dibromosuccinic acid

so, Theoretical yield of dibromosuccinic acid= 1.72 *10^-3 moles*276 g/mol = 0.475 g

(b)

0.200 g of Fumaric acid : 0.200g /116 g/mol = 1.72 *10^-3 moles

1 mol Fumaric acid -------> 1 mol of meso-dibromosuccinic acid

so, Theoretical yield of meso-dibromosuccinic acid= 1.72 *10^-3 moles*276 g/mol = 0.475 g

(use you yield ).

2. Color at beginning of reaction is brown , it is color of Bromine.

3. Because Maleic acid is soluble in water, and sparingly soluble in ether ; while opposite is true for fumaric acid . This reaction is addition reaction , we have to ensure that Br^- be in excess than reactant in solution so, water does not add to intermediate brominium ion.

4/5/6 . Mechanism via cyclic Brominium intermediate , which can be attacked on both carbons equally, maleic acid gives achiral intermediate which on attack of bromide ion gives racemic mixture of 2R,3R and 2S, 3S-Dibromo-succinic acid.

Fumaric acid gives chiral intermediate which gives 2S,3R and 2R, 3S-Dibromo-succinic acid, which meso , which due to internal compensation becomes optically inactive.

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