What does atom economy mean
What is the atom economy for making hydrogen by reacting methane with steam according to the equation above? A low atom economy indicates that the reaction is efficient inefficient fast slow. Methane gas can be produced through microbial breakdown of organic waste. Which reaction should be used to produce hydrogen gas? The hydrogen, however, comes from the reaction between methane and water according to the equation below.
Hydrogen gas is produced from coal and steam in a controlled process to produce carbon monoxide and hydrogen gas. The equation to this chemical reaction is shown on the right. The atom economy of a reaction can be calculated:.
Note that the total mass of reactants can be substituted for products in this equation because in every reaction the total mass of products equals the total mass of reactants. This is shown below:. Principles 3 and 4 deal with the toxicity of all substances used in a reaction including the reactants and products.
Considering Equation 1 it is clear that the two hydrogens and one oxygen that go into the formation of water are wasted. However if a reaction is going to form a waste product, then water is about as environmentally benign nontoxic and presents no disposal problem if it is pure as can be imagined. However all the products and the reactants should be evaluated for their toxicity.
Excellent examples of the formation of less toxic products that have the same efficacy, are seen in new pesticides developed by Rohm and Haas for controlling insects and for controlling marine fouling organisms. Each of these new examples of pesticides have won a Presidential Green Chemistry Challenge award.
Principle 5 prompts the consideration of auxiliary substances solvents, separation agents, drying agents etc. Although water is used as the solvent an environmental plus 5 , in a typical experimental procedure 1 to carry out Equation 1, workup of the product 1-bromobutane after distillation requires 1 mL of concentrated sulfuric acid, 1 mL of 3M sodium hydroxide, anhydrous calcium chloride, 1 mL of ethanol, 1 mL of acetone and 2 mL of p-xylene all to isolate only It is thus clear that the waste generated from these auxiliary substances is significant and exceeds the amount of waste at least from a mass point of view that is generated directly from the reaction.
Many organic reactions utilize large amounts of organic solvents which are frequently toxic. These solvents often find their way into the water, soil and air resulting in significant pollution of the environment.
Efforts are underway to replace organic solvents with water, carbon dioxide and room temperature ionic liquids. In fact Joseph DeSimone of the University of North Carolina has been awarded a Presidential Green Chemistry Challenge award for his work in developing surfactants for liquid and supercritical carbon dioxide. As a result of Dr. DeSimone's efforts, a process using liquid carbon dioxide has been developed for the dry cleaning of clothes.
This process recycles the carbon dioxide, that is obtained as waste from other chemical procedures and allows for the replacement of perchloroethylene, the health effects of which have come into question.
Principle 6 considers the energy requirements of a reaction. This necessitates the use of some energy source and most often this energy source is a fossil fuel. In order to perform the reaction and isolate the 1-bromobutane formed in Equation 1, a reflux and two distillations are required.
Thus the energy requirements of this reaction are significant. Most organic starting materials, such as the 1-butanol in Equation 1, are ultimately derived from crude oil, a nonrenewable depleting resource.
Principle 7 urges us to consider whether these starting materials can be derived from renewable resources. Renewable resources generally means biological and plant based materials biomass. Carbon dioxide and methane are also generally considered to be renewable since they can be generated from both natural and synthetic methods.
Some may mistakenly refer to the reaction described in Equation 1 as an acid catalyzed reaction when in fact it is actually an acid promoted reaction. This is a result of the fact that the sulfuric acid in this reaction is required in stoichiometric, not catalytic amounts. As principle 9 indicates reagents used in catalytic amounts are preferable to reagents used in stoichiometric amounts.
Since one mole of sulfuric acid is required for the loss of every water molecule in this reaction, then only stoichiometric quantities of this reagent will suffice. Significant strides have recently been made to develop reactions that are promoted by nontoxic and recoverable catalysts. A biocatalytic process discovered and developed by Lilly Research Laboratories for producing a potential anticonvulsant drug has won a Presidential Green Chemistry Challenge award.
In all of the remaining discussions of the efficiency of a reaction, the discussion will be limited to the atom economy based on the stoichiometry of the reaction. When one encounters these reactions in the laboratory it may also be prudent to calculate the atom economy based on the quantities of the reagents used experimental atom economy.
In addition one may also want to consider matters such as toxicity, energy use, the use of auxiliary substances, catalytic versus stoichiometric reagents and renewable versus nonrenewable feedstocks. In the substitution reaction above Equation 1a it was revealed that the poor atom economy resulted from the fact that the atoms of the leaving group OH that is being replaced, the counterion sodium of our nucleophile bromide , and the sulfuric acid that is required for this reaction all are wasted in forming unwanted products in this reaction.
By virtue of the fact that elimination reactions require only the loss of atoms while gaining none from the reactant, means that elimination reactions are in general even worse, in terms of their atom economy, than substitution reactions. As an example consider the atoms of the following elimination reaction.
Base promoted dehydrohalogenation of alkyl halides is a common method of producing alkenes from alkyl halides via elimination. In Equation 2 the formation of methyl propene is accomplished by the reaction of 2-bromomethylpropane 7 with sodium ethoxide 8.
In this reaction, the atoms of the reactants that are incorporated into the desired product 9 and the atoms of the desired product are indicated in green, while the unutilized atoms of the reactants are shown in brown as are the atoms in the unwanted products of the reaction.
The poor atom. Table 5 Atom Economy Equation 2. Because addition reactions in general lead to the incorporation of all the atoms of the reactants into the final desired products, addition reactions result in high atom economy.
From an atom economy point of view, addition reactions are thus environmentally preferable to elimination and substitution reactions. As a case in point consider the following addition of hydrogen bromide to methyl propene. In this example all the atoms of the reactants 9 and 11 are shown in green since all of these atoms are utilized in the final desired product 7.
Table 6 Atom Economy Equation 3. Rearrangement reactions involve reorganization of the atoms of a molecule. As already mentioned, you can burn it as a fuel for power generation or heating a reactor vessel. BUT, carbon dioxide is not toxic as long as plenty of air is around! This greatly reduces the cost of purifying the hydrogen to use e. Also, the reaction between steam and carbon monoxide is exothermic, this reduces the energy needs of the overall process.
I'm only interested in atom economy here - see other notes on electrolysis for details. Process A. Electrolysis of aqueous sodium chloride solution brine. Very low atom economy and waste to deal with, BUT, sodium hydroxide and chlorine are useful saleable chemicals.
This has a much greater atom economy than process B and no waste products, no pollution. This is an ideal process, especially if you can use green energy e. BUT, both processes have their place in the chemical industry - from Process A we do need sodium hydroxide and chlorine to manufacture other products e. You can use either a hydrogen or b a hydrocarbon gas like methane to reduce the oxides of metals of low reactivity to obtain the metal itself.
BUT, would this be the preferential method used? Reaction a has the higher atom economy, BUT, hydrogen is probably more costly to produce than cheap methane gas from crude oil. Therefore method b is probably more economic. Rather than here, I've added more atom economy calculations to the Reacting mass ratio calculations of reactants and products from equations page.
Above is typical periodic table used in GCSE science-chemistry specifications in doing chemical calculations, and I've 'usually' used these values in my exemplar calculations to cover most syllabuses.
0コメント