Organic chemistry: Reactions of alcohols Syllabus Statements Understandings » Alcohols undergo nucleophilic substitution reactions with acids (also called esterification or condensation) and some undergo oxidation reactions. Applications » Writing equations for the complete combustion of alcohols. » Writing equations for the oxidation reactions of primary and secondary alcohols (using acidified potassium dichromate(VI) or potassium manganate(VII) as oxidizing agents). » Explanation of distillation and reflux in the isolation of the aldehyde and carboxylic acid products. » Writing the equation for the condensation reaction of an alcohol with a carboxylic acid, in the presence of a catalyst (eg concentrated sulfuric acid) to form an ester. Combustion of alcohols https://www.youtube.com/watch?v=Ku7TdLeEGsQ Combustion of alcohols » Complete combustion produces CO2 and water CH3CH2OH (g) + 3 O2 CO2 (g) + 3 H2O (g) (g) 2 » As for alkanes and alkenes, the longer the carbon chain, the larger the enthalpy of reaction (more Oxidation of alcohols The combustion of alcohols will completely oxidise the alcohol molecule producing heat, CO2 and H2O. It is also possible to selectively oxidise alcohols using acidified reagents such as KMnO4 or K2Cr2O7. Oxidation of alcohols In chapter 9, Redox reactions, you studied this oxidation reaction from an inorganic chemistry perspective • Oxidation is loss of electrons +6 -2 -2 +1 -2 +1 -2 +1 +1 2Cr2O72−(aq)+3CH3CH2OH(l)+16H+(aq) -2 0 () Oxidation +6 +3 () Reduction +3 0 +1 0 -2 -2 +1 +1 -2 4Cr3+(aq)+3CH3COOH(aq)+11H2O(l) Oxidation of alcohols In chapter 10, you will look at the same reactions from an organic chemistry perspective • Oxidation is gain of oxygen CH3CH2OH +[O], heat (l) CH3COOH (l) 1,2 and 3 alcohols » Primary (1), secondary(2) and tertiary(3) alcohols has the placement of the hydroxyl group (OH) on a carbon that is bonded to one, two or three other carbons H Primary Alcohol (1) CH3 C OH H The carbon that holds the hydroxyl group has one other carbon attached to it. 1,2 and 3 alcohols » Primary (1), secondary(2) and tertiary(3) alcohols has the placement of the hydroxyl group (OH) on a carbon that is bonded to one, two or three other carbons H 3 CH Secondary Alcohol Primary Alcohol (1)(2) CH3 C OH H The carbon that holds the hydroxyl group now has one has two other carbons carbon attached attachedto toit. it. 1,2 and 3 alcohols » Primary (1), secondary(2) and tertiary(3) alcohols has the placement of the hydroxyl group (OH) on a carbon that is bonded to one, two or three other carbons CH3 Tertiary Alcohol (3)(2) Secondary Alcohol CH3 C OH H CH3 The carbon that holds the two hydroxyl group now has three other carbons attached to it. 1,2 and 3 alcohols » Although Methanol does not have an additional carbon attached to the main carbon, it is considered a Primary (1) as it reacts in the same way. Primary alcohols » Primary, secondary and tertiary alcohols have different reaction outcomes » Primary alcohols can be oxidised twice: H CH3 C OH H Primary alcohols » Primary, secondary and tertiary alcohols have different reaction outcomes. » Primary alcohols can be oxidised twice: H CH3 C O H First to an aldehyde Where a double bonded oxygen is formed. Primary alcohols » Primary, secondary and tertiary alcohols have different reaction outcomes. » Primary alcohols can be oxidised twice: H CH3 C O OH Then to an carboxylic acid Where a hydroxyl group is added. Primary alcohols » This “Gain of oxygen” results in: H CH3 C OH CH3 C O H H Alcohol – one bond from carbon to oxygen. Aldehyde – two bonds from carbon to oxygen. CH3 C O OH Carboxylic acid – three bonds from carbon to oxygen » A carbon-hydrogen bond is broken in each step • Carbon-carbon bonds are not broken during oxidation (contrast this with combustion) . Primary alcohols Oxidised first to aldehydes, then to carboxylic acids O OH O [O], heat H3C C H [O], heat H3C C H H3C C (reflux) H H+/Cr(VI) Can we isolate the aldehyde? How? Cr(III) OH Isolating the Aldehyde Stopper (or thermometer attachment) Still head Condenser » When the aldehyde is produced, distillation can be used to collect the aldehydes as they have lower boiling points than alcohols (and carboxylic acids.) Round-bottom flask Why do aldehydes have a lower boiling point than alcohols and carboxylic acids? Isolating the Aldehyde Stopper (or thermometer attachment) Still head Condenser Round-bottom flask » When the aldehyde is produced, distillation can be used to collect the aldehydes as they have lower boiling points than alcohols (and carboxylic acids.) Producing the Acid Condensor » If the process of oxidation is allowed to continue, the aldehyde will continue to oxidise to a carboxylic acid. » Refluxing will help speed this process up. Round-bottom flask Aldehyde vs Acid » If the process of oxidation is allowed to continue, the aldehyde will continue to oxidise to a carboxylic acid. Condensor » Refluxing will help speed this process up. Round-bottom flask Reagent Selectivity Depending on the extent of oxidation required will determine the selection of oxidising agent. » Strong oxidising agent: dichromate ion H+/Cr2O72O O Mn O‒ O » or the permanaganate ion − + H /MnO4 » Weak oxidising agent: PCC: pyradinium chlorochromate (does not need to be acidified) Secondary alcohols » Secondary alcohols oxidise to ketones. CH3 H C CH3 [O], heat CH3 OH » Can use either H+/Cr2O72- or PCC H C CH3 O Tertiary alcohols » Tertiary alcohols do not oxidise. CH3 CH3 C OH +[O], heat No reaction CH3 Too much energy to break the carbon skeleton. Summary of Oxidation of Alcohols Alcohol Oxidation Product Colour Change with acidified K2Cr2O7 (aq) Colour Change with acidified KMnO4 (aq) Primary aldehyde carboxylic acid orange green purple red-brown Secondary ketone orange green purple red-brown Tertiary cannot be oxidised (no C-H bonds) no colour change no colour change Colour of Oxidised alcohols H+/Cr2O72- H+/MnO4- Esterification Esterification » Alcohols react with carboxylic acids to form an ESTER - ROOR’ » Condensation reaction: removal of water » Equilibrium reaction » Catalysed by concentrated H2SO4 Esterification carboxylic acid + alcohol RCOOH + conc. H2SO4 conc. H2SO4 R’OH ester + water RCOOR’ + ( & H+ ) H H O H C C H O H H O H O C H H Ethanoic acid Methanol Water HOH Esterification carboxylic acid + alcohol RCOOH + R’OH conc. H2SO4 conc. H2SO4 ester + water RCOOR’ + HOH ( & H+ ) H H O H C C H O C H H Ethanoicmethyl acid ethanoate Methanol H O H Water Esterification