9. GCSE level chemistry exam revision notes on the Principles & Practice of Chemical Production - Synthesising Molecules

See also 7. Chemical & Pharmaceutical Industry Economics and Sustainability

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

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

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

Introduction

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 are the starting point raw materials?

Choosing the right 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.

e.g. you need sources of nitrogen and hydrogen to synthesise ammonia using the Haber Process,

or sulfur, oxygen and water to manufacture sulfuric acid by the Contact Process.

The phrase chemical feedstock means the actual reactant molecules that are fed into the reactor chamber e.g. hydrocarbons like octane or ethene, the alcohol ethanol, nitrogen from air, hydrogen from methane etc.

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.

There are several factors to be taken into consideration before choosing the starting raw materials - that is assuming there is a choice e.g.

The cost of extracting, separating and purifying the starting chemicals from which to make the product.

 e.g. is the raw material too costly to process and make a profitable product.

Some metal ores have a very low concentration of the desired metal and not economic to exploit - though new techniques like bioleaching and phytomining are extracting metals like copper from low grade ore.

Is the process economically viable and the product ultimately profitable.

Can any of the raw materials or energy be obtained from renewable sources e.g. the air (nitrogen), solar/wind power.

What are the energy costs like - can you operate the chemical process efficiently using the minimum energy - see section . Lower temperatures and pressure conditions use less energy and engineering costs are lowered too.

If it is an equilibrium reaction, how far does it go to the desired product side.

The reaction conditions chosen must be carefully controlled to allow an efficient rate of product production (economically acceptable rate) and maximise the yield of product e.g. by control of reactant concentrations, reactor pressure, reactor temperature and appropriate catalyst.

Choosing the right type of reaction to make the product

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

Carrying out a 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 warning symbols

Calculating the quantities of the reactant chemicals required to make the product

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, reaction conditions and yield

The type of chemical plant required and controlling the process e.g. in terms of temperature, pressure, concentrations and catalysts.

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 above-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.

 The diagram on the left shows the apparatus of 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.

Electric heaters would be used instead of bunsen burners!

The yield is how much of the product you make from the reactants - usually expressed as percentage yield.

See % reaction yield and theoretical yield calculations, why you can't get 100% yield

and atom economy calculations

The reaction conditions often determine the yield, especially if its an equilibrium reaction.

You want the equilibrium reaction to be as much to the right as possible i.e. maximum possible yield, but sometimes, as in the Haber synthesis of ammonia, its much more economic to get a low yield fast.

Be aware that conditions that give the fastest reaction rate, may not give the highest yield of desired product.

Reaction rate always increases with temperature, but often a compromise is made to get the highest yield as fast and as economic as possible.

 If its a biochemical reaction, like fermentation, the catalytic enzymes work best under optimum conditions e.g pH and temperature of reaction medium.

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'