9. The Principles & Practice of Chemical Production - Synthesising Molecules

 

See also 7. Chemical & Pharmaceutical Industry Economics & Sustainability

 

and 8. Products of the Chemical & Pharmaceutical Industries & Impact on Us

 

7. to 9. are all connected as a survey of the chemical and pharmaceutical industries, lots of overlap I'm afraid

Doc Brown's KS4 Science GCSE/IGCSE O Level Industrial Chemistry Revision Notes

Index of sections: 1. Limestone, lime - uses, thermal decomposition of carbonates, hydroxides and nitrates  *  2. Enzymes and Biotechnology  *  3. Contact Process, the importance of sulphuric acid  *  4. How can metals be made more useful? (alloys of Al, Fe, steel etc.) * 5. The importance of titanium  *  6. Instrumental Methods of Chemical Analysis * 7. Chemical & Pharmaceutical Industry Economics & Sustainability * 8. Products of the Chemical & Pharmaceutical Industries & impact on us * 9. The Principles & Practice of Chemical Production - Synthesising Molecules  and other web pages of industrial chemistry notes: Ammonia synthesis/uses/fertilisers * Oil Products * Extraction of MetalsHalogens - sodium chloride Electrolysis * Transition Metals * Extra Electrochemistry

9. The Principles & Practice of Chemical Production - Synthesising Molecules

Chemical synthesis is the term applied to describe the process of making a chemical compound which maybe a single stage or multi-stage series of chemical reactions.

You are essentially taking some raw material to produce, usually, relatively simple starter molecules and using them to synthesise a complex compound.

A synthesis involves many factors and stages e.g. methods and apparatus, calculation of amounts required, control of process, maximise yield, all in all to achieve by the most efficiently economic and profitable process - See 7. Chemical economics and sustainability

 

How do we synthesise compounds? What does synthesis involve? What is the starting point raw materials?

diagram to do?

The starting materials

You need to know and find the raw materials that you can convert into a chemical feedstock that becomes the starting point for the synthesis of the chemical compound - the desired product.

The phrase chemical feedstock means the actual reactant molecules that are fed into the reactor chamber e.g. ethene, ethanol, nitrogen, hydrogen.

The raw material usually requires processing e.g. purifying or chemically modifying it to produce a purer starting feedstock material.

Raw materials can be naturally occurring substances like crude oil, natural gas, coal, deliberately 'grown' biomass, water, oxygen and nitrogen from air, mineral ores.

Choosing the right type of reaction

There are lots to choose from! See Types of Chemical Reactions or Chemical Processes (index at top of this page) e.g. Addition * Contact Process * Cracking * Decomposition * Dehydration * Displacement * Double decomposition * Electrolysis * Esterification * Exothermic reaction * Fermentation * Galvanising * Haber Process * Hydration * Neutralisation * Polymerisation * Precipitation * Substitution  * Thermal decomposition

Risk assessment of the reaction

This involves trying to anticipate any dangers in the processes involved in the synthesis of the compound e.g. identifying ANY potential hazards - hazardous chemicals and their manipulation, risk of injury to personnel involved in the processes - chemical plant workers, how can risks be reduced (risk is unavoidable in the chemical industry!).

hazard signs Hazard warning symbols

Calculating the quantities of the reactant chemicals required

You need to calculate how much of the reactants and in what ratio to give the desired quantity of the product. The starting point is balanced symbol equation from which you can theoretically calculate the quantities of reactants needed. You can work in reacting masses or reacting moles and convert to masses. The links below offer pages on how to calculate the mass of products formed from a given mass of reactants. You do not want waste any of the raw materials and chemical feedstocks that go into the process of making the compound. Waste makes the process less economic, waste is money down the drain!, though I hope that's not where the waste goes!

See Reacting mass ratio calculations of reactants and products

and Calculation of how much of a reactant is needed?

and Connecting moles, mass and formula mass - the basis of reacting mole ratio calculations

Selecting the apparatus and reaction conditions

The reaction must be carried out in appropriately designed apparatus e.g. reactor chamber, distillation unit etc., depending on the type of synthesis reaction being carried out. The chemical plant equipment must be of the right size to accommodate the reactants and products needed for sustainable commercial production. If the reaction is very exothermic, heat exchangers may be needed, but this 'waste heat' can be used to preheat reactants or make hot water for heating offices, or steam to drive an electrical generator. If its an endothermic reaction that only occurs at elevated temperatures, a heating system is needed.

Decisions must be made on the reaction conditions such as temperature, pressure, concentration and a catalyst may be needed to give an efficient and safe speed of reaction. See 'industrial paragraphs' in Rates of Reaction). Quite often a compromise has to be made which I've discussed in detail for the Haber Synthesis of ammonia, the diagram is on the right is a generalised design for a chemical plant making ammonia. The separation of the ammonia product is done at the bottom of the reactor and unreacted gases recycled.

Left: The first stage of preparing an ester in a school or college laboratory are illustrated above. I'm afraid you will have to appreciate (without photographs) that in industry the equipment will be rather larger, but in principle what you see in the above diagram would be reproduced in a chemical plant by chemical engineers.

Extracting and isolating the product - dealing with by-products and waste too

The product needs to be extracted or isolated from the reaction mixture which may involve distillation, filtration, centrifuging, solvent extraction. Basically, you want to separate out the crude product (since will still be impure) and leave behind as much of the waste material as possible before doing the final purification. The waste products, if of no use, must be disposed of safely with no harm to workers, public and the environment - all of which is governed by strictly enforced legal regulations.

It is possible sometimes possible to recycle unused reactants - especially if it is a continuous process like ammonia production (and not a batch process). But, the 'waste' may include a useful by-product that can be sold, so further separate but parallel processing may go on alongside the main product production. All in all, all the products other than the main desired product to totally waste material of new use at all, must all be dealt with.

The 2nd stage, distilling the ester from the reaction mixture, the flask is simulating the 'reactor vessel' of a chemical plant.

Purifying the product

Purifying requires the last of any impurities to be removed from the isolated or extracted  'crude' product. This may involve drying with a desiccating agent  if water is the only impurity, if its a solid it may be recrystallised from a suitable solvent, it may involve an extra or multiple fractional distillation.

Further purification of the ester using a separating funnel.

The complete preparation of an insoluble compound, mixing (preparation), filtration (separation), washing and drying (purification).

Measuring the yield and purity - quality control

You never get 100% of the theoretical yield, but you do need to know how much product you have actually got from the mass of reactants used. The success of the process can be quantified as the actual percentage yield (100 x mass of actual yield / theoretical maximum mass of product).

You also need to check on the purity of the product for quality control and on every batch, or regular monitoring if the process is continuous. The % purity analysis tells you exactly how much of the product is actually the real product itself! Drugs must be especially pure to avoid chemical contamination and side-effects in the patient, which can be very serious (look up the infamous Thalidomide Case).

Different levels of purity are allowed if no hazard is involved (health, environment  etc.), so you may get cheaper impure grades or more expensive grades depending on the use of the chemical. Why waste money on 'over-purification' if a very pure product isn't required!

See Explaining and calculating % purity of a product

Explaining and calculating % reaction yield, reasons why never 100%

Explaining and calculating atom economy

and for more example calculations see Calculating % yield and theoretical yield, atom economy

AND this page has a section on 'how to check on the purity of a compound' scroll down to 1.1h 'PURE SUBSTANCE'

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