Question

.247 grams maleic acid (molecular weight 144.13 g/mol) was mixed with .25 mL HCl, .07 grams of crystals resulted - which shoud be fumaric acid.

1. Was the major product cis or trans isomer?

2. What is the percent yeild of each isomer? (The weight of the other isomer is the difference between the mass of the product and the mas of the starting material)

3. Explain why formation of one geometric isomer is favored over another (...my guess would be stability)

Answer 1

.247 grams maleic acid (molecular weight 144.13 g/mol) was mixed with .25 mL HCl, .07 grams of crystals resulted - which shoud be fumaric acid.

1. Was the major product cis or trans isomer?

2. What is the percent yeild of each isomer? (The weight of the other isomer is the difference between the mass of the product and the mas of the starting material)

3. Explain why formation of one geometric isomer is favored over another (...my guess would be stability

Given data:

Maleic acid (cis isomer) and Fumaric acid (trans isomer)

0.247 g is the weight of mixture of these two acids.

On recrystallization only Fumaric acid crystalize out and weight of Fumaric acid (crystals) = 0.07 g.

We have,

Wt. of Maleic acid + Wt. of Fumaric acid = 0.247 g.

Wt. of Maleic acid + 0.07 g = 0.247 g.

Wt. of Maleic acid = 0.247 – 0.07 = 0.177 g.

It’s clear than Maleic acid is major product as it is formed in more amount.

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2. %Yield of isomer = [Yield of isomer in gram / Total yield (i.e. of all isomers)] x 100

So,

% Yield of Maleic acid = [Wt. of Maleic acid / Total wt.] x 100

                                          = [0.177/0.247] x 100

% Yield of Maleic acid = 71.70%

As only two isomers are there, we can write,

% Yield of Fumaric acid = 100 – 71.70 = 28.3%

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3.

Formation of cis isomer i.e. Maleic acid is favored because of the stability of this isomer.

In maleic acid –O-H group of one carboxylic part forms strong H-Bonding with O of O=C of anathor carboxylic group. This is a kind of an intramolecular H bonding involving 7 membered ring systems.

Such Intramolecular H bonding is not possible in Fumaric acid case.

Hence the formation of Maleic acid (cis- isomer) as major product.

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