Question

1. What is the atom economy? Describe the atom economy of the rearrangement, addition, substitution, and elimination reactions. Give one example for each. 

2. What are the limitations of the atom economy?(4marks)

3. Which are the best types of reactions according to the atom economy?(4marks)

4. Calculate the E‐factor and atom economy for the following reactions:

Answer 1

Ans 1. The atom economy, also known as atom efficiency, is defined as the efficiency of a chemical reaction in terms of the atoms involved in the reactants and products desired. It is one of the methods designed by the chemists to measure the greenness of a reaction. Mathematically, it is defined as:

Atom economy= ((Formula weight of desired products)/(Formula weight of starting materials))*100%

A. E. in 1.) Rearrangement reactions: As these reactions involve rearrangement of same no. of atoms, the A.E for these reactions is 100%. For example: Electrocyclic rearrangement of 1,3-butadiene to cyclobutene (Figure).

2.) Addition reactions: In these reactions, one of the reactant adds to another forming a single product which has all the atoms of the reactants. ex: Hydrogenation of alkenes, Diels-alder reaction. (Figure)

3.) Substitution reactions: these dont have 100% efficiency as the desired product lacks certain atoms of the reactant molecules. Substitution of aryl halides (figure)

4.) Elimination reactions: these reactions proceed via elimination of small molecules, so the desired product lacks those atoms. So the A.E. is not 100%. (figure)

Ans 2: Limitations of A.E. are: Atom economies are theoretical and don't give any information about the yield or selectivity of the reaction. It only takes into account the stiochiometry and dont tell anything about toxicity of the reaction.

Ans 3. The best type of reactions according to atom economy are addittion and rearrangement reactions because they give 100 % atom economy, as explained above.

Ans 4.   E-factor of a reaction is calculated as (weight of the waste produced)/(weight of the desired product). E- factors and atom economies for the given reactions are calculated in the figure attached below.