Percent Yield and Atom Economy

Yield

Chemists need to measure the efficiency of chemical reactions in order to compare alternative routes to products and their associated economic and environmental costs. Percentage yield has long been used for this purpose, as it compares the expected product quantity with the actual obtained.

Percent yield

Measures the success of an experiment

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Found in the lab

Based on stoichiometric calculation

% yield = actual amount of product x 100 theoretical amount of product

The problem with Yield

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A high yield is desirable, but is not the whole story. The reaction once used to manufacture phenol from benzene generates 1 mole of sodium sulphite (126g), for every mole of phenol (88g) produced. This may be acceptable if there is sufficient demand for the sodium sulphite, but if not, it presents a serious waste management problem, and adds significantly to costs.

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It is much better to reduce the amount of waste generated in the first place, rather than treat it at a later date, and this is an important aim of green chemistry. A yield calculation does not indicate how efficiently the reactants have been used in generating the desired product, and so does not indicate how effectively waste is being reduced.

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Atom Economy

Measures the efficiency of a reaction

%AE = M_r of desired product x 100 M_r of all reactants

High atom economy means reduced waste.

Ex. Benzene can be oxidized to produce maleic anhydride

%AE = 98.06 x 100

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Find the percent atom economy of this reaction.

C₆H₆

C₄H₂O₃

% AE = 44%

Molecular formula

(78.12+ 144.00)