Organic Reactions:

Organic Reactions: Pathways to new products Chapter 10

Reactions of alkanes • An alkane molecule such as ethane has a ‘backbone’ consisting of a chain of single C-C bonds. • An alkane molecule is non-polar as carbon and hydrogen have similar electronegativity. • Therefore they are insoluble in water, but are soluble in non-polar solvents. • The stability of the C-C bonds and the non-polar nature means that alkanes are quite non-reactive. • Most reactions involving alkanes are either combustion or substitution reactions.

Combustion • Alkanes can be used as fuel. • Combustion reactions involving alkanes release large amounts of heat energy. • Combustion of methane • CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + energy • Combustion of methane • 2C8H18(g) + 25O2(g) → 16CO2(g) + 18H2O(g) + energy

Substitution Reactions • One or more hydrogen atoms in an alkane is replaced by a different atom or functional group. • This involves breaking the C-H bonds and making new bonds with the substituting atom or group. • Chloroethane is a gas at room temperature and is used in a local anaesthetic spray. • CH3CH3(g) + Cl2(g) CH3CH2Cl(g) + HCl(g) Heat or light

Your Turn • Page 146 • Questions 1 and 2

Reactions of alkenes • Alkenes: • Are unsaturated • Are non-polar • Are insoluble in water • Participate in addition reactions • Polymerises to produce polymers.

Addition reactions of alkenes • The double covalent bond in ethene molecules has a significant effect on its chemical properties. • Ethene reacts more readily, and with more chemicals than ethane. • The reactions of ethene usually involve addition of a small molecule to produce a single product. • For example it reacts with bromine solution.

Addition Reactions • Involve the C=C bond being converted to a single bond. • Ethanol can be produced by an addition reaction of ethene and water using a catalyst to speed up the reaction.

Addition Polymerisation • A type of addition reaction of ethene is involved in making polyethene. • The number n in this reaction is very large (several thousand or more). • A molecule made by linking a large number of small molecules, is called a polymer. • The small molecule (in this case ethene) is called a monomer. • This type of reaction is known as addition polymerisation

Addition Polymerisation • When the polymer is being formed the ethene molecules add to the end of growing polymer chains. • Ethene is used as a base for other addition polymerisation reactions. For example to make PVC and polystyrene.

Your Turn • Page 149 • Question 1-3

Reactions of chloroalkanes • Once a more electronegative atom such as chlorine has been substituted for a hydrogen atom in an alkane, the molecule becomes polar. • Electrons in the carbon-chlorine bond are attracted towards the more electronegative chlorine atom. • This makes the carbon atom at the other end of the bond susceptible to attack by negatively charge ions.

Reactions of chloroalkanes • For example chloromethane is converted to methanol when it is reacted with hydroxide ions. • The chlorine atom is substituted by an OH functional group to form methanol.

Reactions of alkanols • Alkanols can be produced by addition reactions of alkenes or substitution reactions of chloroalkanes. • Ethanol has very different properties from ethane or chloroethane. • It is liquid at room temp. • It is widely used as a solvent in cosmetics and pharmaceuticals • It is the active ingredient in alcoholic drinks • It can act as a depressant on the human body, slowing reactions and responses. • Excess ethanol consumption also blocks the production of antidiuretic hormones, increasing urination and resulting in dehydration.

Reactions of alkanols • Ethanol is soluble in water as a consequence of its highly polar OH group, which readily forms hydrogen bonds with water molecules. • Alkanols can be turned into amide groups by reacting ethanol with ammonia. • CH3CH2OH(g) + NH3(g) CH3CH2NH2(g) + H2O(g) • What type of reaction is this? alumina 400°C

Reactions of alkanols • Alkanols can also be oxidised to form carboxylic acids: • CH3CH2OH(aq) CH3COOH(aq) • Not all alkanols will oxidise to form carboxylic acids. Carboxylic acid synthesise only occurs from primary alkanols. O2 (g) Primary Secondary Tertiary

Reactions of carboxylic acids • Carboxylic acids are weak acids, reacting with water to form weak acidic solutions. • CH3COOH(aq) + H2O(l)↔ CH3COO-(aq) + H3O+(aq)

Your Turn • Page 151 • Questions 8, 9 and 10

Esters • Esters are a group of organic compounds responsible for some of the natural and synthetic flavours and smells in ice-creams, lollies, flowers and fruits. • Esters composed of small molecules are volatile and smelly. Esters of larger molecular size are oils and waxes

Esters • Esters are made by a condensation reaction between carboxylic acids and an alkanol. • Reactions that involve the combination of two reactions and the elimination of a small molecule, such as water, are called condensation reactions.

Esters • Gently heating a mixture of ethanol and pure ethanoic acid, with a trace amount of sulfuric acid as a catalyst, produces an ester (ethyl ethanoate) and water. • Ethyl ethanoate is more commonly known as ethyl acetate, it is used as a solvent in paints and nail varnish

Esters • Below is the general equation for the esterification reaction involving a carboxylic acid and an alkanol.

Naming Esters • Esters have two-part names. • The first part derived from the name of the alkanol from which it is made • The ‘anol’ part is replace with ‘yl’ • Ethanol becomes ethyl • The second part comes from the carboxylic acid. • Where ‘ic acid’ is replace with ‘ate’ • Ethanoic acid becomes ethanoate. • Therefore we have ethyl ethanoate

Your Turn • Read pages 153 – 155 on polyesters • Page 156 • Questions 11 and 12

Reaction pathways • Which pathway is the most effective to make ethanol???

Reaction pathways • Production chemists need to find the most efficient pathway for making certain materials. • To do this there are certain areas to consider: • How readily available is the starting material • The yield (how much will it produce) • The purity of the final product • Can they minimise any unwanted side products • Can they minimise waste materials • Cost • How long will it take.

Example: synthesis of ethyl propanoate • What is ethyl propanoate made out of? • Suppose we only had alkanes and alkenes on hand how could we make ethyl propanoate?

Synthesis of ethyl propanoate • Ethanol is a two carbon compound that can be synthesised directly from ethene, or from ethene via the intermediate product chloroethane.

Synthesis of ethyl propanoate • Propanoic acid is a carboxylic acid containing 3 carbon atoms. • It is prepared by the oxidation of the primary alkanol propan-1-ol. • This in turn can be formed by the reaction of 1-chloropropane with NaOH. • 1-chloropropane is formed by reacting propane with chlorine. • A number of products will be formed which are separated by fractional distillation.

Synthesis of ethyl propanoate • The substitution reaction of propane is chosen rather than an addition reaction of propene because the addition of HCl to propene will result in the formation of unwanted 2-chloropropane. • Having synthesised ethanol and propanoic acid we can now prepare the ester using a condensation reaction.

Reaction pathway for the preparation of ethyl propanoate

Considerations • The purity of the product needs to be evaluated. For this a lot of companies will use some of the analysis techniques looked at in first term. • The yield must be taken into account, as not all of the reactants are necessarily converted to product Actual mass of product obtained % Yield = Theoretical mass of product

Your Turn • Page 159 • Question 15, 17 and 18

Fractional Distillation • A technique used to separate liquids that have different boiling points. • Commonly used in a laboratory to separate volatile liquids from a reaction mixture. • Industrial application of fractional distillation include: • Separation of the fractions from crude oil • Production of oxygen and nitrogen by the fractional distillation of liquid air • Extraction of ethanol from water in the fermentation of sugar.

Fractional distillation • The column is packed with glass beads or has glass shelves, providing a large surface area upon which the vapours condense. • There is a temparature gradient up the fractionating column; the column is cooler at the top than at the bottom.

Fractional distillation • Looking at our example of synthesis of the ester ethyl ethanoate. • Pure ethyl ethanoate can be extracted from the reaction mixture by fractional distillation. Look at the boiling points of components in the reaction mixture

Fractional distillation • The reaction mixture is heated in the distillation flask. • The vapour rises up the fractionating column. • The temperature at the top of the column slowing increases until it stabilises at about 57°C, which is the boiling point of ethyl ethanoate. • The fraction condensing over a small range of temperature near the boiling point of ethyl ethanoate, 55°C - 59°C is collected.

Your Turn • Page 161 • Question 19

Aspirin (chapter 14) • Pharmaceutical products are often developed from substances found in a plant that has been used as a traditional medicine. • Aspirin is one such substance. • Its origins are from a naturally occurring substance called salicin found in the leaves and bark of willow trees and in the herb medowsweet. • As long ago as 400BC people have recommended ‘an infusion of willow leaves and bark to relieve aches, pains, inflammation and fever.’

Aspirin • The body converts salicin into salicylic acid and this is the active substance that helps to reduce fever and acts as a pain killer. • Salicylic acid is more effective than salicin and by 1870 doctors were prescribing salicylic acid directly. • A lot of people could not tolerate salicylic acid directly and it tasted bad.

Aspirin • In 1897, Felix Hoffmann, synthesised an improved modification of salicylic acid. • Once he had made salicylic acid he replaced the hydroxy functional group with an ester functional group to form acetylsalicylic acid. • This is the compound known commercially as aspirin.

Aspirin • To make aspirin we could add a carboxylic acid and an alcohol. • This would form acetylsalicylic acid • However this is a slow reaction, with a low yield, as the water formed tends to drive the reaction backwards.

Aspirin • In an alternative reaction pathway, which is faster and produces higher yields, the ethanoic acid is replaced with ethanoic anhydride (acetic anhydride). • This is the preferred pathway for aspirin synthesis

Aspirin Synthesis • The products, acetylsalicylic acid and acetic acid have to be separated and the product purified before it can be put into tablet form and packaged for sale.

Soluble aspirin • Although aspirin has a –COOH functional group, pure acetylsalicylic acid is not very soluble in water. • Converting the carboxylic acid functional group into the sodium salt changes the molecule into an ion and makes it much more soluble. • It is used in many headache and cold remedies in this form.

PolyAspirin • A recent development is to make a polymer structure using a condensation reaction between salicylic acid and 1,8-octanedioic acid • Pretty much a polymer of aspirin which has a number of potential advantages: • It can be used as a controlled-release pain killer because the polymer breaks down slowly • Because it is a polymer with a similar molecular structure to polyesters it can be made into thread and used to stitch cuts or wounds together. • It has the potential to be used as a plastic coating for an injured bone or joint.

Your Turn • Page 225 • Question 1

Organic Reactions: