Which of the following processes has the highest atom economy?
A) Making poly(ethene) from ethene.
B) Making ethene from eicosane, C20H42.
C) Making chloromethane from methane.
D) Making magnesium chloride from magnesium and hydrochloric acid
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Atom economy describes the conversion efficiency of a chemical process in the desired product actually produced, in terms of all atoms involved. While percent yield of a chemical reaction concentrates on the actual yield of the desired product, the atom economy approach focuses on the amount of atoms of reactant molecules that are incorporated in the desired product. That is why it is a better representation of overall efficiency (and economy) of a chemical reaction. The greater the atom economy, the fewer waste by-products created.
Reactions on which all atoms of the reactants are incorporated in the desired product have an atom economy 100%. For example, consider the reaction: H2 + CO = CH3OH.
Of the given examples, A) Making polyethene from ethene:
n(CH2=CH2) → (CH2–CH2)n
Here, no molecule or part of a molecule is expelled. This is an addition polymerization reaction having atom economy = 100%.
B) Making ethene from eicosane,
C20H42 → 10C2H4 + H2(g)
Here, one molecule of H2 is expelled as gas, and not incorporated in the desired product. So, atom economy is <100%.
C) Making chloromethane from methane:
CH4 + Cl2 → CH3Cl + HCl
One molecule of HCl is expelled, and not incorporated in the desired product. So, atom economy is again <100%.
D) Making magnesium chloride from magnesium and hydrochloric acid:
Mg + 2HCl → MgCl2 + H2 (g)
Here too, one molecule of gaseous H2 is expelled, and not incorporated in the desired product. So, atom economy is <100%.
Therefore, the process having highest atom economy is A) Making polyethene from ethene.
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