Solubility in water The alcohol groups form hydrogen bonding which makes the short chain molecules soluble in water. The solubility in water decreases as the chain length increases .
Low-mass alcohols are soluble in water (because they hydrogen bond with water). As the hydrocarbon chain lengthens, the solubility decreases . This photo shows ethanol, propan-1-ol and butan-1-ol in water. The first two are completely miscible in water, while butan-1-ol is not miscible in water. Ethanol Propan-1-ol Butan-1-ol
Boiling Points of Alcohols Increases with molecular size due to increased instantaneous dipoles 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 M r bp / °C propane C 3 H 8 44 -42 just instantaneous dipoles ethanol C 2 H 5 OH 46 +78 instantaneous dipoles + hydrogen bonding Boiling point is higher for “straight” chain isomers. bp / °C butan-1-ol CH 3 CH 2 CH 2 CH 2 OH 118 butan-2-ol CH 3 CH 2 CH(OH)CH 3 100 2-methylbutan-2-ol (CH 3 ) 3 COH 83
CLASSIFICATION OF ALCOHOLS Aliphatic • general formula C n H 2n+1 OH - provided there are no rings • the OH replaces an H in a basic hydrocarbon skeleton 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° NB. Aliphatic - straight chain molecule (not a ring / cyclic)
Distinguishing alcohols Lucas reagent can be used to distinguish between low mass primary, secondary and tertiary alcohols. Lucas reagent contains anhydrous zinc chloride dissolved in concentrated hydrochloric acid. It contains a very high concentration of chloride ions and the Zn 2+ ion acts as a catalyst. Take 1–2 mL of Lucas reagent in a dry test tube, add a few drops of the alcohol and shake. If there is no reaction, place the test tube in a beaker of boiling water for a few minutes.
Distinguishing alcohols - Lucas test Primary alcohol - remain unchanged tertiary alcohol - turns cloudy immediately Secondary alcohol - will turn cloudy but takes a bit of time Lucas reagent = conc. HCl and ZnCl 2
Tertiary alcohols turn cloudy immediately. Once heated, the secondary alcohol quickly turned cloudy. The primary alcohol tube is unchanged.
OXIDATION OF PRIMARY ALCOHOLS <ul><li>Primary alcohols are easily oxidised to aldehydes </li></ul><ul><li>e.g. CH 3 CH 2 OH(l) + [O] ——> CH 3 CHO(l) + H 2 O(l) </li></ul><ul><li> ethanol ethanal </li></ul><ul><li>it is essential to distil off the aldehyde before it gets oxidised to the acid </li></ul><ul><li> CH 3 CHO(l) + [O] ——> CH 3 COOH(l) </li></ul><ul><li> ethanal ethanoic acid </li></ul><ul><li>Practical details </li></ul><ul><li>the alcohol is dripped into a warm solution of acidified K 2 Cr 2 O 7 </li></ul><ul><li>aldehydes have low boiling points - no hydrogen bonding - they distil off immediately </li></ul><ul><li>if it didn’t distil off it would be oxidised to the equivalent carboxylic acid </li></ul><ul><li>to oxidise an alcohol straight to the acid, reflux the mixture </li></ul><ul><li>compound formula intermolecular bonding boiling point </li></ul><ul><li>ETHANOL C 2 H 5 OH HYDROGEN BONDING 78°C </li></ul><ul><li>ETHANAL CH 3 CHO DIPOLE-DIPOLE 23°C </li></ul><ul><li>ETHANOIC ACID CH 3 COOH HYDROGEN BONDING 118°C </li></ul>
Oxidising a primary alcohol to an aldehyde Full oxidation is not wanted: use dilute acid and less dichromate. The reaction mixture is heated gently, ethanal vapourises (21°C) as soon as it is formed and distils over. This stops it being oxidised further to ethanoic acid.
Apparatus for the oxidation of ethanol to ethanoic acid
Oxidising a primary alcohol to a carboxylic acid reflux Distil to separate
Formation of ethanol by fermentation Conditions yeast warm, but no higher than 37°C (optimum temp. for yeast) Advantages LOW ENERGY PROCESS USES RENEWABLE RESOURCES - PLANT MATERIAL SIMPLE EQUIPMENT Disadvantages SLOW PRODUCES IMPURE ETHANOL - will need distilling to purify BATCH PROCESS
Formation of haloalkane Ethanol and PCl 5 C 2 H 5 OH (l) + PCl 5(s) C 2 H 5 Cl (g) + POCl 3(l) + HCl (g) fumes solid Ethanol and SOCl 2 C 2 H 5 OH (l) + SOCl 2(l) C 2 H 5 Cl (g) + SO 2(g) + HCl (g)
Formation of ethanol from ethene Advantages FAST PURE ETHANOL PRODUCED CONTINUOUS PROCESS Disadvantages HIGH ENERGY PROCESS EXPENSIVE PLANT REQUIRED USES NON-RENEWABLE FOSSIL FUELS TO MAKE ETHENE Uses of ethanol ALCOHOLIC DRINKS SOLVENT - industrial alcohol / methylated spirits FUEL - petrol substitute in countries with limited oil reserves
Dehydration of alcohols Reagent: concentrated sulphuric acid or passing the alcohol over aluminium oxide
Reaction with sodium The reaction is similar to the reaction of alkali metals with water, but less vigorous .
Esterification Catalyst : concentrated H 2 SO 4 (dehydrating agent - it removes water causing the equilibrium to move to the right and increases the yield Conditions: reflux
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... CH 3 OH + CH 3 COOH CH 3 COO CH 3 + H 2 O from ethanoic acid CH 3 COO CH 3 from methanol METHYL ETHANOATE Esters Ethanoate Methyl Methyl Ethanoate