Percentage Yield and Atom Economy In a chemical reaction which is totally efficient all the REACTANTS are converted into products. This will give 100% yield. Most reactions, particularly organic reactions give low yields. Possible reasons: Impure reactants. Product is lost during purification. Side reactions. Equilibrium reaction means that a reaction is never completed.

Perform calculations to determine the percentage yield of a reaction. Week 19 © Pearson Education Ltd 2008 This document may have been altered from the original

Definitions Know that: The theoretical yield is the maximum mass of products which would be obtained from the balanced equation. The actual yield is the mass of products obtained. The percentage yield = Actual yield x 100% Theoretical yield Limiting reactant is the substance present in lowest quantity which determines the actual yield. Excess – more than the mass determined by the balanced equation is used to maximise product obtained.

Examples 1. Ethene, C 2 H 4 can be obtained from the dehydration of ethanol. In an experiment 0.5g of ethene was obtained from 5.0g ethanol. What is the theoretical yield and the percentage yield of ethene? 2. A student prepared a sample of 1-bromobutane from 10.0g butanol (other reagents being in excess). After purification she found she had made 12.0g of bromobutane. What is the theoretical yield and what is the percentage yield of bromobutane?

Try this! The following is an industrial conversion of ethene into ethyl ethanoate in 3 steps: C 2 H 4 → C 2 H 5 OH → CH 3 CO 2 H → CH 3 CO 2 C 2 H 5 a) For each step: i) Write a balanced equation. ii) Quote reagents and conditions required. b) If 70g of ethene were used in the first step i) What is the theoretical yield of ethanol? ii) If 90g of ethanol were made what is the percentage yield for the first step in the synthesis?

ci) If all of the 90g of ethanol is used in the second step what is the theoretical yield of ethanoic acid made in the second reaction? ii) If 100g of acid is actually made what is the percentage yield in the second step. di) If all the 100g of acid is used in the third step what is the theoretical yield of ester? ii)If 73g of ester are actually made what is the percentage yield of ester actually made. iii) Can you account for this low percentage yield in this reaction?

Define the atom economy of a reaction and describe the benefits of developing chemical processes with a high atom economy. Explain that addition reactions have an atom economy of 100%, whereas substitution reactions are less efficient. Carry out calculations to determine the atom economy of a reaction. Explain that a reaction may have a high percentage yield but a low atom economy. Week 19 © Pearson Education Ltd 2008 This document may have been altered from the original

Atom Economy Many reactant atoms do not end up in the desired product. This leads to a huge waste of material. Scientists are increasingly aware of the need to conserve resources and reduce waste in industrial processes. Atom economy is used in determining the overall efficiency of a chemical process. A high atom economy for a reaction means that less resources are wasted Where the desired product is just one of several products of a reaction it is possible to have a high percentage yield for that reaction but poor atom economy.

Atom Economy Atom economy = Mass of atoms in desired product x 100 sum of the masses of all products e.g. Poly(1,1-dichloromethane) is used as a plastic wrapping material. i)What is the name and structure of the monomer? The monomer is made by heating ethane with chlorine gas: C 2 H 6 + 3Cl 2 → C 2 H 2 Cl 2 + 4HCl Calculate the atom economy for this reaction.

More Examples 1. The monomer tetrafluoroethene (C 2 F 4 ) can be used to make non-stick linings for pans. Calculate the atom economy of the following reaction used to make the monmer: 2CHF 3 → C 2 F 4 + 2HF Why will any substitution reaction have an atom economy of less than 100% whereas an addition reaction can have an atom economy of 100%?

2.,The equation for the fermentation of glucose to produce ethanol is: C 6 H 12 O 6 (aq) → 2C 2 H 5 OH (aq) + 2CO 2 What is the atom economy of this process? 3i).Calculate the atom economy in each conversion step in the manufacture of ethyl ethanoate from ethene. ii)Which is the most efficient step in this reaction sequence?