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If you wanted to make a cake you would add flour, eggs, water, baking soda and your favourite flavours.
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You may add more ingredients but the important idea here is that all the ingredients will be included in the final product - your cake.
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Following on the same idea, reactions with a high atom economy
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is one where all of the atoms in the reactants are included in the final, desired product.
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Such a reaction would have little, if any, waste produced.
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According to law of conservation of mass no atoms are created or destroyed in the reaction.
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The atoms from the reactants are simply rearranged to form products.
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So why not maximise a certain reaction so that no atoms are wasted as side products?
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Going back to the cake example, it wouldn't be a good idea to add all those ingredients but then find that one of them is not baked into your final cake.
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Here's a quick challenge for you.
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When an alkene such as hex-2-ene is hydrogenated in the presence of a metal catalyst what is the product?
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Pause, think, and resume when ready.
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The answer is hexane, this hydrogenation reaction has a hundred percent atom economy.
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The final product hexane has all the atoms from the reactants, hex-2-ene and hydrogen.
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No atoms are wasted as unwanted byproducts.
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Furthermore, the use of a catalyst makes this process very efficient and the catalyst can be reused.