Oxidation reactions of alcohols

Reactions with sodium dichromate. Sodium dichromate is an oxidising agent, ie it oxidises other chemicals, being reduced in the process. Orange chromium (vi) is reduced to green chromium (iii) when heated with acid. Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H20

Oxidation of primary alcohols. Primary alcohols are oxidised as orange chromium (vi) is reduced to green chromium (iii) when heated with acid. Two organic products are possible; Aldehydes or Carboxylic Acids.

Aldehydes vs Carboxylic Acids Aldehydes have a carbonyl group C=O, at one end. Eg Propanal Carboxylic acids have the functional group; -CO2H Eg; Ethanoic acid

The alcohol is oxidised; Dichromate is reduced; CH3CH2OH → CH3HC=O + 2H+ + 2e- Ethanol Ethanal Dichromate is reduced; Cr2O7 2- + 14H+ + 6e- → 2Cr3+ + 7H20 The aldehyde is then distilled as fast as it is formed so that it is unable to react further to form the carboxylic acid.

Oxidising alcohols to aldehydes using distillation. Aldehydes evaporate

Oxidation of aldehydes. Aldehydes are oxidised as orange chromium (vi) is reduced to green chromium (iii) when heated with acid. Only one organic product is possible; a Carboxylic Acids.

Reflux produces the carboxylic acid. Aldehyde condenses Reflux produces the carboxylic acid. So the aldehyde can be oxidised to a carboxylic acid. Aldehyde evaporates.

The carboxylic acid is then obtained by distillation. Carboxylic acid evaporates

The aldehyde is oxidised; CH3CHO + H2O→ CH3CO2H + 2H++ 2e- Ethanal Ethanoic acid Dichromate is reduced. Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H20 NB Instead of H+ [H] or ‘H’ can be used in writing half equations.

Distinguishing between aldehydes and carboxylic acids. Aldehydes are neutral. Aldehydes turn blue Cu(ii) orange on heating. Carboxylic acids are acidic. Carboxylic acids do not react.

Carboxylic acids do not. Aldehydes give a silver mirror when heated with ammonical silver nitrate. Carboxylic acids do not.

Effects of ethanol on the body. Ethanol depresses the activity of the central nervous system. Reducing stress, tension and anxiety and inducing a feeling of relaxation. Unfortunately it also reduces reaction times and impairs judgement. Blood Alcohol Concentration (BAC) is measure in mg/100ml of blood. The legal limit is 80mg/100ml.

The UK legal limit for drivers is 80 mg of alcohol per 100 ml of blood Detecting alcohol Alcohol is not digested on absorption nor chemically changed in the blood. The UK legal limit for drivers is 80 mg of alcohol per 100 ml of blood But it takes time to analyse blood…

Breathalysers It is much easier to analyse exhaled air. The oxidation reaction of ethanol is the basis of the breathalyser.

As the blood flows through the lungs some of the alcohol moves across the alveoli into the air, because it is volatile. The concentration of alcohol in the alveolar is directly proportional to the concentration of alcohol in the blood. As the alveolar air is exhaled the alcohol can be detected by a breathalyzer, giving an instant result.

Biological recognition layer How Breathalyzers Work Breath sample with alcohol Amplifier Biological recognition layer Photosystem cell The reacted mixture is compared to a vial of unreacted mixture in a photosystem cell that produces an electric current, displayed in a meter. Enzyme catalyses the reaction of the alcohol with potassium dichromate to chromium sulphate, potassium sulphate, acetic acid and water. The reddish-brown dichromate ion changes to a green chromium ion.

½O2 + 2H++ 2e- → H2O Modern breathalysers use a fuel cell. At one electrode ethanol is oxidised; CH3CH2OH+ H2O → CH3CO2H+ 2H++ 2e- At the other oxygen is reduced; ½O2 + 2H++ 2e- → H2O The voltage produced at varying concentrations of ethanol can then be measured to calibrate the machine. A green light will then indicate no ethanol, amber a little and red on or near the limit.

Oxidation of secondary alcohols. Secondary alcohols are also oxidised as orange chromium (vi) is reduced to green chromium (iii) when heated with acid. Only one organic product ispossible; a Ketone.

CH3C=OCH3 + 2H+ + 2e- Eg; CH3CH(OH)CH3 Propan 2ol ↓ Propanone Ketones cannot react further, so oxidation stops.

Aldehydes vs Ketones Aldehydes have a carbonyl group C=O, at one end. Ketones also have a carbonyl group, but in the interior of the molecule.

Ketones cannot be oxidised by acidified potassium dichromate. Oxidation of Ketones Ketones cannot be oxidised by acidified potassium dichromate. To oxidise them a much stronger oxidising agent is needed that can break C/C bonds.

Oxidation of tertiary alcohols Tertiary alcohols cannot be oxidised by acidified potassium dichromate. To oxidise them a much stronger oxidising agent is needed that can break C/C bonds.

Ethanol as a renewable fuel Ethene is derived from crude oil, a non-renewable resource. But sugars are renewable. Ethanol derived from fermentation can be burnt as a renewable fuel. In some countries ethanol, from sugar cane, is mixed with petrol to form gasohol.

Carbon footprints of biodiesel and ethanol vs diesel. The carbon in diesel was taken out of circulation hundreds of millions of years ago. When it is burnt it therefore results in a net increase of carbon dioxide in the atmosphere. But the carbon in biodiesel or ethanol from fermentation was only recently taken up, so its release does not result in an increase in atmospheric carbon dioxide, ie it is carbon neutral.

Carbon neutrality 6CO2 + 6H2O→C6H12O6 + 6O2 C6H12O6 → 2C2H5OH + 2CO2 Carbon dioxide is taken up during photosynthesis. 6CO2 + 6H2O→C6H12O6 + 6O2 But released upon fermentation. C6H12O6 → 2C2H5OH + 2CO2 And when ethanol is burnt; 2[C2H5OH → 2CO2 + 3H2O]

Carbon budget Carbon uptake = 6CO2 Carbon production = 2CO2 +2[ 2CO2] = 6CO2 So there is no net change, but there are hidden costs in ethanol production and distribution which mean that ethanol is in fact not completely carbon neutral.