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AN INTRODUCTION TO THE CHEMISTRY OF ALCOHOLS THE CHEMISTRY OF ALCOHOLS CONTENTS • Structure of alcohols • Nomenclature • Isomerism • Physical properties • Chemical properties of alcohols • Revision check list CLASSIFICATION OF ALCOHOLS Aliphatic • general formula CnH2n+1OH - provided there are no rings • the OH replaces an H in a basic hydrocarbon skeleton CLASSIFICATION OF ALCOHOLS Aliphatic • general formula CnH2n+1OH - provided there are no rings • the OH replaces an H in a basic hydrocarbon skeleton Aromatic (Not required for NCEA Level 3) • in aromatic alcohols (or phenols) the OH is attached directly to the ring • an OH on a side chain of a ring behaves as a typical aliphatic alcohol The first two compounds are classified as aromatic alcohols (phenols) because the OH group is attached directly to the ring. CLASSIFICATION OF ALCOHOLS Aliphatic • general formula CnH2n+1OH - provided there are no rings • the OH replaces an H in a basic hydrocarbon skeleton Aromatic (Not required for NCEA Level 3) • in aromatic alcohols (or phenols) the OH is attached directly to the ring • an OH on a side chain of a ring behaves as a typical aliphatic alcohol The first two compounds are classified as aromatic alcohols (phenols) because the OH group is attached directly to the ring. Structural differences • alcohols are classified according to the environment of the OH group • chemical behaviour, eg oxidation, often depends on the structural type PRIMARY 1° SECONDARY 2° TERTIARY 3° NAMING ALCOHOLS Alcohols are named according to standard IUPAC rules • select the longest chain of C atoms containing the O-H group; • remove the e and add ol after the basic name • number the chain starting from the end nearer the O-H group • the number is placed after the an and before the ol ... e.g butan-2-ol • as in alkanes, prefix with alkyl substituents • side chain positions are based on the number allocated to the O-H group e.g. CH3 - CH(CH3) - CH2 - CH2 - CH(OH) - CH3 is called 5-methylhexan-2-ol CONSTITUTIONAL ISOMERISM IN ALCOHOLS Different structures are possible due to... A Different positions for the OH group and B Branching of the carbon chain butan-1-ol 2-methylpropan-2-ol butan-2-ol 2-methylpropan-1-ol BOILING POINTS OF ALCOHOLS Increases with molecular size due to increased intermolecular forces. Alcohols have higher boiling points than similar molecular mass alkanes This is due to the added presence of inter-molecular hydrogen bonding. More energy is required to separate the molecules. propane C3H8 ethanol C2H5OH Mr 44 46 bp / °C -42 just instantaneous dipole-dipole interactions +78 instantaneous dipole-dipole interactions + hydrogen bonding BOILING POINTS OF ALCOHOLS Increases with molecular size due to increased van der Waals’ forces. Alcohols have higher boiling points than similar molecular mass alkanes This is due to the added presence of inter-molecular hydrogen bonding. More energy is required to separate the molecules. propane C3H8 ethanol C2H5OH Mr 44 46 bp / °C -42 just instantaneous dipole-dipole interactions +78 instantaneous dipole-dipole interactions + hydrogen bonding Boiling point is higher for “straight” chain isomers. butan-1-ol CH3CH2CH2CH2OH butan-2-ol CH3CH2CH(OH)CH3 2-methylpropan-2-ol (CH3)3COH bp / °C 118 Greater branching = 100 lower inter-molecular forces 83 SOLVENT PROPERTIES OF ALCOHOLS Solubility Low molecular mass alcohols are miscible with water Due to hydrogen bonding between the two molecules Heavier alcohols are less miscible Solvent properties Alcohols are themselves very good solvents They dissolve a large number of organic molecules REACTIONS OF ALCOHOLS ELIMINATION OF WATER (DEHYDRATION) Reagent/catalyst conc. sulfuric acid (H2SO4) Conditions reflux at 180°C Product alkene Equation e.g. C2H5OH(l) ——> CH2 = CH2(g) + H2O(l) Note 1 There must be an H on a carbon atom adjacent the carbon with the OH Note 2 Alcohols with the OH in the middle of a chain can have two ways of losing water. This gives a Mixture of alkenes from unsymmetrical alcohols... OXIDATION OF ALCOHOLS All alcohols can be oxidised depending on the conditions Oxidation is used to differentiate between primary, secondary and tertiary alcohols The usual reagent is acidified potassium dichromate(VI) Primary Easily oxidised to aldehydes and then to carboxylic acids. Secondary Easily oxidised to ketones Tertiary Not oxidised under normal conditions. They do break down with very vigorous oxidation PRIMARY 1° SECONDARY 2° TERTIARY 3° OXIDATION OF PRIMARY ALCOHOLS Primary alcohols are easily oxidised to aldehydes e.g. CH3CH2OH(l) + [O] ethanol ——> CH3CHO(l) + H2O(l) ethanal it is essential to distil off the aldehyde before it gets oxidised to the acid CH3CHO(l) + [O] ethanal ——> CH3COOH(l) ethanoic acid OXIDATION OF PRIMARY ALCOHOLS Primary alcohols are easily oxidised to aldehydes e.g. CH3CH2OH(l) + [O] ethanol ——> CH3CHO(l) + H2O(l) ethanal it is essential to distil off the aldehyde before it gets oxidised to the acid CH3CHO(l) + [O] ethanal ——> CH3COOH(l) ethanoic acid Practical details • • • • the alcohol is dripped into a warm solution of acidified K2Cr2O7 aldehydes have low boiling points - no hydrogen bonding - they distil off immediately if it didn’t distil off it would be oxidised to the equivalent carboxylic acid to oxidise an alcohol straight to the acid, reflux the mixture compound formula intermolecular bonding boiling point ETHANOL C2H5OH HYDROGEN BONDING 78°C ETHANAL CH3CHO DIPOLE-DIPOLE 23°C ETHANOIC ACID CH3COOH HYDROGEN BONDING 118°C OXIDATION OF PRIMARY ALCOHOLS Controlling the products e.g. CH3CH2OH(l) + [O] ——> CH3CHO(l) + H2O(l) then CH3CHO(l) + [O] ——> CH3COOH(l) OXIDATION TO ALDEHYDES DISTILLATION OXIDATION TO CARBOXYLIC ACIDS REFLUX Aldehyde has a lower boiling point so distils off before being oxidised further Aldehyde condenses back into the mixture and gets oxidised to the acid OXIDATION OF SECONDARY ALCOHOLS Secondary alcohols are easily oxidised to ketones e.g. CH3CHOHCH3(l) + [O] propan-2-ol ——> CH3COCH3(l) + H2O(l) propanone The alcohol is refluxed with acidified K2Cr2O7. However, on prolonged treatment with a powerful oxidising agent they can be further oxidised to a mixture of acids with fewer carbon atoms than the original alcohol. OXIDATION OF TERTIARY ALCOHOLS Tertiary alcohols are resistant to normal oxidation ESTERIFICATION OF ALCOHOLS Reagent(s) carboxylic acid + strong acid catalyst (e.g conc. H2SO4 ) Conditions reflux Product ester Equation Notes e.g. CH3CH2OH(l) + CH3COOH(l) ethanol ethanoic acid CH3COOC2H5(l) + H2O(l) ethyl ethanoate Concentrated H2SO4 is a dehydrating agent - it removes water causing the equilibrium to move to the right and increases the yield ESTERIFICATION OF ALCOHOLS Reagent(s) carboxylic acid + strong acid catalyst (e.g conc. H2SO4 ) Conditions reflux Product ester Equation e.g. CH3CH2OH(l) + CH3COOH(l) ethanol ethanoic acid CH3COOC2H5(l) + H2O(l) ethyl ethanoate Notes Concentrated H2SO4 is a dehydrating agent - it removes water causing the equilibrium to move to the right and increases the yield Uses of esters Esters are fairly unreactive but that doesn’t make them useless Used as flavourings Naming esters Named from the alcohol and carboxylic acid which made them... CH3OH + CH3COOH from ethanoic acid CH3COOCH3 + H2O CH3COOCH3 METHYL ETHANOATE from methanol OTHER REACTIONS OF ALCOHOLS OXYGEN Alcohols make useful fuels C2H5OH(l) + 3O2(g) ———> 2CO2(g) + 3H2O(l) Advantages have high enthalpies of combustion do not contain sulfur so there is less pollution can be obtained from renewable resources CHLORINATION OF ALCOHOLS Reagent(s) conc. hydrochloric acid HCl(aq) with ZnCl2 (Lucas Reagent) Product haloalkane Equation C2H5OH(l) + conc. HCl(aq) Reagent(s) thionyl chloride SOCl2 Product haloalkane Equation ROH (l) + SOCl2 (l) Reagent(s) phosphorus pentachloride PCl5 Product haloalkane Equation ROH (l) + PCl5 (l) ———> C2H5Cl(l) + H2O(l) RCl (l) + SO2(g) + HCl (g) RCl (l) + POCl3(l) + HCl (g) LABORATORY PREPARATION OF ALCOHOLS from haloalkanes - reflux with aqueous sodium or potassium hydroxide from aldehydes - reduction with sodium tetrahydridoborate(III) - NaBH4 from alkenes - acid catalysed hydration using concentrated sulfuric acid Details of the reactions may be found in other sections. REVISION CHECK What should you be able to do? Recall and explain the physical properties of alcohols Recall the different structural types of alcohols Recall and explain the chemical reactions of alcohols Write balanced equations representing any reactions in the section Understand how oxidation is affected by structure Recall how conditions and apparatus influence the products of oxidation CAN YOU DO ALL OF THESE? YES NO You need to go over the relevant topic(s) again Click on the button to return to the menu WELL DONE! Try some past paper questions AN INTRODUCTION TO THE CHEMISTRY OF ALCOHOLS THE END