Chapter 16Alcohols and Carboxylic Acids Alcohols You should be able to:• Write general and molecular formulae for members of the alcohol homologous series.• Write fully displayed structures and names of alcohols,• Identify alcohols by their functional groups,• Relate the properties of alcohols to their functional groups,• Describe the reactions of ethanol, and• Describe the fermentation process by which ethanol is produced from carbohydrates.
Chapter 16Alcohols and Carboxylic Acids Alcohols• All alcohols belong to a family of organic compounds which contain the –OH group of atoms.• This group of atoms is called the hydroxyl group. It gives the alcohol its specific chemical properties and is the functional group of alcohols.• Most alcohols are colourless liquids with a strong smell. They have low boiling and melting points.• The members of the alcohol homologous series have the following features: 1. They have the general formula: CnH2n+1OH, where n is an integer. 2. They have a hydroxyl functional group, –OH. 3. Their names end with ‘-ol’. 4. The formula of each member differs from the previous one by –CH2.
Chapter 16Alcohols and Carboxylic Acids Alcohols Name Molecular Relative molecular Structural formula mass formulaMethanol CH3OH 32Ethanol C2H5OH 46Propanol C3H7OH 60Butanol C4H9OH 74
Chapter 16Alcohols and Carboxylic Acids Properties of Ethanol• Ethanol is the most important alcohol in the homologous series.• Only ethanol can be consumed.• Ethanol is a colourless liquid with a strong smell.• It has a boiling point of 78 °C and is volatile.• It mixes readily with water.• It is neutral to litmus.
Chapter 16Alcohols and Carboxylic AcidsQuick Check 11. (a) Name an alcohol with three carbon atoms. (b) State its (i) molecular formula. (ii) structural formula.2. (a) State the general formula of alcohols. (b) State the formula of an alcohol with 20 carbon atoms. (c) Do you think it is a liquid, solid or gas at room temperature and pressure? Solution
Chapter 16Alcohols and Carboxylic AcidsSolutions to Quick Check 11. (a) Propanol (b) (i) C3H7OH (ii)2. (a) CnH2n+1OH (b) C20H41OH (c) Solid, its melting point is high because it has very large molecular mass. Return
Chapter 16Alcohols and Carboxylic AcidsChemical Reactions1. Reaction with a reactive metal like sodium• When a small piece of sodium is dropped into some ethanol, it reacts steadily to give off bubbles of hydrogen gas leaving behind a colourless solution of sodium ethoxide, CH3CH2ONa.• On evaporating the solution to dryness, a white sodium ethoxide solid is obtained.• The reaction is similar to the reaction between water and sodium as in both reactions, the –OH group is replaced by Na+ ion.
Chapter 16Alcohols and Carboxylic Acids 2. Combustion• Ethanol burns in air (oxygen) to form carbon dioxide and water. The reaction is exothermic and heat energy is given out. Hence, ethanol can be used as a fuel. Ethanol + Oxygen Carbon dioxide + Water C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(l)
Chapter 16Alcohols and Carboxylic Acids3. Oxidation to Carboxylic Acids• (i) Ethanol is oxidised by hot acidified potassium dichromate(VI) solution into ethanoic acid: H+/K2Cr2O7 C2H5OH + 2[O] CH3COOH + H2O H+/K2Cr2O7 + 2[O] H H + ODuring the reaction, the potassium dichromate(VI) solution isreduced and the solution turns from orange to green incolour.
Chapter 16Alcohols and Carboxylic Acids4. Oxidation to Carboxylic Acids• (ii) The oxidation of ethanol can also be carried out by using air and bacteria.• When wine, beer or any liquid containing ethanol is left in the open for a few days, it becomes sour.• Airborne bacteria (called Acetobacter aceti) converts the alcohol into vinegar, which is a solution of ethanoic acid.• This method is used in the industry to make vinegar. Ethanol + Oxygen Ethanoic acid + Water C2H5OH +2[O] CH3COOH + H2O Barrels of vinegar made by oxidation
Chapter 16 Alcohols and Carboxylic Acids 5. Formation of Esters• Ethanol reacts with carboxylic acids in the presence of concentrated HCl or H2SO4 to form organic compounds called esters. For example, ethanol reacts with ethanoic acid to form an ester called ethyl ethanoate: C2H5OH + CH3COOH CH3COOC2H5 + H2O ethanol ethanoic acid ethyl ethanoate• Esters are sweet smelling liquids which can be used for making perfumes and flavouring agents.
Chapter 16Alcohols and Carboxylic Acids6. Dehydration to Alkene• Ethanol reacts with dehydrating agents such as concentrated sulphuric acid or anhydrous aluminium oxide to form alkenes. This is a dehydration reaction where water is lost across two adjacent carbon atoms in the ethanol. H H O In this process, ethanol is heated with excess concentrated sulphuric acid at 170 °C to form ethene. Alternatively, ethanol vapour can be passed over activated alumina (Al2O3) at 450 °C.
Chapter 16Alcohols and Carboxylic Acids Quick Check 21. (a) State the products formed by the combustion of alcohols in oxygen. (b) Construct a balanced equation for the combustion of propanol.2. (a) What is formed when ethanol is oxidised? (b) State two different ways in which oxidation of ethanol can be carried out. Solution
Chapter 16Alcohols and Carboxylic AcidsSolutions to Quick Check 21. (a) Carbon dioxide and water (b) 2C3H7OH(l) + 9O2(g) 6CO2(g) + 8H2O(l)2. (a) Ethanoic acid (b) Ethanol can be oxidised (i) by hot acidified potassium dichromate(VI) solution (into ethanoic acid) or (ii) by using air and bacteria. Return
Chapter 16Alcohols and Carboxylic AcidsMaking Ethanol by Fermentationof Carbohydrates• Large quantities of ethanol are made for consumption by fermentation.• Depending on the raw materials used, the ethanol is produced in different kinds of alcoholic drinks such as wine, whisky, brandy, and beer. Fermentation is carried out in large tanks• Carbohydrates such as starch and sugar are mixed with water and yeast, a unicellular fungi, and a source of enzymes.• The mixture is left to ferment without air for a few weeks.
Chapter 16Alcohols and Carboxylic AcidsMaking Ethanol by Fermentationof Carbohydrates• The temperature of the mixture should be kept around 37 oC for the yeast to work best.• The reaction only proceeds under anaerobic conditions, i.e. absence of oxygen.• During fermentation, the yeast feeds on the sugar and changes it into glucose and then into ethanol.• The ethanol obtained is distilled and made into wine or used as pure ethanol. The carbon dioxide produced in the fermentation is a by-product and can be made into dry ice.Step 1: C12H22O11 + H2O invertase C6H12O6 + C6H12O6 sucrose glucose fructoseStep 2: C6H12O6 zymase 2C2H5OH + 2CO2 glucose ethanol
Chapter 16Alcohols and Carboxylic AcidsMaking Ethanol from Ethene• We have learned in the previous chapter that alkenes undergo an addition reaction with steam to form alcohols.• This is the industrial method of making ethanol which is much cheaper than by the fermentation method.• Ethanol is manufactured by reacting ethene with steam using phosphoric(V) acid as a catalyst at a temperature of about 300 oC.• The reaction is reversible, and the formation of the ethanol is exothermic. CH2=CH2(g) +H2O(g) CH3CH2OH(g) ΔH = –45 kJ/mol
Chapter 16Alcohols and Carboxylic AcidsAlcoholic Beverages and Their Sources• The table below lists the types of alcoholic drinks and their sources of starch used. Drink Source of % volume Method of starch of ethanol preparation Beer Barley 3–8 Fermentation Wine Grapes / rice 8 – 18 Whisky Barley 30 – 60 Fermentation Brandy Grapes and fractional Vodka Rye / potatoes distillation
Chapter 16Alcohols and Carboxylic AcidsUses of Ethanol• Ethanol is a constituent of alcoholic beverages such as wines and whisky.• Ethanol is used widely as a solvent for paints, varnishes, liquid soap and other toiletries.• Ethanol is blended with petrol and used as a fuel for motorcars in many countries.• Ethanol can be oxidised into ethanoic acid which is used to manufacture plastics and drugs.
Chapter 16Alcohols and Carboxylic Acids Quick Check 31. What raw materials are required for the production of ethanol by fermentation?2. Construct chemical equations for the fermentation of sugar. Solution
Chapter 16Alcohols and Carboxylic AcidsCarboxylic Acids• The general formula is CnH2n+1COOH, where n is an integer.• They have an acid functional group:• Their names end with ‘-oic’, derived from the corresponding alkane by replacing the letter ‘e’ in ‘-ane’ with ‘-oic’. E.g. methane methanoic acid; ethane ethanoic acid.• Each member of the homologous series differs from the previous one by an extra –CH2 group.
Chapter 16Alcohols and Carboxylic AcidsStructure of Ethanoic acid• The molecular model and structural formula of a typical carboxylic acid, such as ethanoic acid, is as shown: Structure of ethanoic acid Molecular Structural Name formula formula Methanoic acid HCOOH Ethanoic acid CH3COOH Propanoic acid C2H5COOH Butanoic acid C3H7COOH
Chapter 16 Alcohols and Carboxylic Acids Properties of Ethanoic Acid• It is a colourless liquid with a strong sour smell.• It mixes readily with water and is commonly called acetic acid.• It turns blue litmus red. Chemical reactions• Carboxylic acids are weak monobasic acids.• Carboxylic acids show typical reactions of mineral acids. They react less vigorously as they are weak acids and dissociate only partially in water to form hydrogen ions.1. Reaction with MetalsEthanoic acid reacts with more reactive metals like sodium,calcium and magnesium to form a salt and liberate hydrogen gas. 2Na(s) + 2CH3COOH(aq) 2CH3COONa(aq) + H2(g)
Chapter 16 Alcohols and Carboxylic Acids 2. Reaction with Metallic Oxides and Hydroxides• Ethanoic acid reacts with the oxides and hydroxides of metals to form salt and water. Here, the reaction is a neutralisation reaction. MgO(s) + 2CH3COOH(aq) (CH3COO)2Mg(aq) + H2O(l) NaOH(aq) + CH3COOH(aq) CH3COONa(aq) + H2O(l) 3. Reaction with Carbonates• Ethanoic acid reacts with carbonates to produce salt, water and carbon dioxide. 2CH3COOH(aq) + Na2CO3(s) 2CH3COONa(aq) + H2O(l) + CO2(g) CH3COOH(aq) + NaHCO3(s) CH3COONa(aq) + H2O(l) + CO2(g)
Chapter 16 Alcohols and Carboxylic Acids4. Reaction with Alcohols• Ethanoic acid reacts with alcohols to form sweet smelling compounds called esters.• Concentrated sulphuric acid is added as a catalyst. The process of forming esters is called esterification.The formation of the esters from different carboxylic acids andalcohols: CH3COOH + C2H5OH CH3COOC2H5 + H2O ethanoic acid ethanol ethyl ethanoate CH3COOH + CH3OH CH3COOCH3 + H2O ethanoic acid methanol methyl ethanoate C2H5COOH + C2H5OH C2H5COOC2H5 + H2O propanoic acid ethanol ethyl propanoate
Chapter 16 Alcohols and Carboxylic Acids Uses of Ethanoic Acid• Ethanoic acid is a very important chemical used in manufacturing a wide variety of things.• Annual production of ethanoic acid in the world is estimated to be more than 6 million tonnes.• There is a wide range of products made from ethanoic acid. This includes: – making various kinds of plastics such as acetates for making fabric, films, adhesives and paints. – making drugs like aspirin, – making bleaching agents in detergents; chemicals like herbicides and dyes, and – vinegar, which is used in cooking, as an additive to chilli and tomato sauces and for preserving vegetables.
Chapter 16 Alcohols and Carboxylic Acids Esters Esters are organic compounds with characteristic fruity smells.• They are used in food flavouring and in making perfumes.• Esters can be prepared by reacting an alcohol with a carboxylic acid using concentrated sulphuric acid as the catalyst or dehydrating agent. E.g. Ethyl ethanoate can be prepared by reacting ethanoic acid with ethanol. In this case, the hydrogen in the –COOH group has been replaced by an ethyl group. The formula of ethyl ethanoate is:
Chapter 16 Alcohols and Carboxylic Acids Physical properties of esters Boiling points• Short-chain esters are more volatile than carboxylic acids of the same number of carbons.• They are not able to form hydrogen bonds with themselves due to the absence of hydroxyl group.• Thus the boiling points of esters are usually lower than that of carboxylic acids of the same number of carbon atoms. Solubility of esters in water• Short-chain esters are fairly soluble in water but solubility decreases with increasing chain length.• Esters are soluble in water because they are able to form hydrogen bonds with water molecules.• The slightly positive end of a water molecule can form a hydrogen bond with the lone pair of electrons on one of the oxygen atoms in an ester.
Chapter 16 Alcohols and Carboxylic AcidsChemical properties of esters• The ester linkage can be broken by addition of water.• The process is therefore known as hydrolysis.• Hydrolysing esters using only water is very slow.• The reaction is thus catalysed by using dilute acid like dilute hydrochloric acid or dilute sulphuric acid.• The ester is heated under reflux with a dilute acid and the products are separated by fractional distillation. (a) Hydrolysis of ethyl ethanoate: (b) Hydrolysis of methyl propanoate:
Chapter 16 Alcohols and Carboxylic Acids Alternative method • Alkalis can also be used. When an alkali is used, the salt of the acid, instead of the acid itself, is formed. • Hydrolysis of esters using an alkali is sometimes known as saponification. (used in producing soap) • E.g. In the hydrolysis of ethyl ethanoate using sodium hydroxide solution, sodium ethanoate instead of ethanoic acid is formed.CH3COOCH2CH3(aq) + NaOH(aq) CH3COO-Na+(aq) + CH2CH3OH(aq)
Chapter 16Alcohols and Carboxylic Acids Quick Check 41. (i) What is the general formula of a carboxylic acid? (ii) State the formula of a carboxylic acid with 10 carbon atoms.2. State the name and write the structural formula of a carboxylic acid with: (a) 2 carbon atoms, (b) 3 carbon atoms and (c) 4 carbon atoms.3. (a) What organic compound is formed when ethanoic acid reacts with ethanol? (b) What name is given to this type of reaction and what is the catalyst required? (c) Construct an equation to show the reaction between ethanoic acid and ethanol. Solution