Title: Lesson 5 Alcohols Learning Objectives: Explain the relative ease of combustion of the alcohols Describe the oxidation reactions of the alcohols Investigate the oxidation reactions of the alcohols

Refresh Which products can be potentially obtained from crude oil and are economically important? Plastics Margarine Motor fuel I and II only I and III only II and III only I, II and III Reviewing Your Notes You should spend 60 seconds reviewing your notes from last lesson before attempting this.

Alcohols General structure CnH2n+1OH -OH Functional group The –OH group is polar  increased solubility in water compared to alkanes of comparable molar mass Alcohols are used as fuels: As a fuel for cars – either pure or blended with petrol Methanol as fuel for competitive motorsports including dragsters and monster trucks TOK: A large amount of ethanol is fermented from crops…crops that could otherwise be eaten, forcing up food prices. Is this ok?

Alcohols as Fuels Alcohols combust more readily than equivalent alkanes but release less energy since they are already partially oxidised Alcohol + oxygen  carbon dioxide + water (+ lots of energy!) Amount of energy released per mole increases as we go up the homologous series, due to increasing numbers of carbon dioxide molecules produced As with hydrocarbons, in limited supply of oxygen, alcohols will produce carbon monoxide instead of carbon dioxide

Oxidation of alcohols It is possible for them to react with oxidising agents, which selectively oxidise the carbon atom attached to the –OH group, keeping the carbon skeleton intact A range of compounds will oxidise them so the oxidiser is often represented as [O] One oxidising agent you need to know is potassium dichromate, K2Cr2O7. When using this, orange Cr (VI) is reduced to green Cr (III) Reread the chapter 9 redox for a reminder of how this works The oxidation reactions of the different alcohols are as follows:

Primary alcohols Oxidised in a two step reaction If you leave a bottle of wine exposed to air, bacteria slowly oxidise the ethanol to ethanoic acid… Giving that smell of vinegar! Oxidised in a two step reaction First forming the aldehyde Oxidised further to the carboxylic acid It is possible to stop the reaction after the first step using distillation to remove the aldehyde (which has a lower boiling point than ethanol or ethanoic acid) before it oxidises further. Lower boiling point is because it has no hydrogen bonding. You can also ensure you get ethanoic acid by using reflux to prolong the contact of the aldehyde with the oxidising agent. Primary = Carbon attached to one functional group and at least two hydrogens!

Distillation v. Reflux heating Introduction to Reflux Video

Secondary alcohols Oxidised to ketones by a similar process: Secondary = Carbon attached to one functional group, one hydrogens and two alkyl groups! Secondary alcohols Oxidised to ketones by a similar process:

Tertiary alcohols Not readily oxidised under comparable conditions Tertiary = Carbon attached to one functional group and three alkyl groups! Tertiary alcohols Not readily oxidised under comparable conditions This would break the carbon skeleton This would require significantly more energy

Summary of oxidation of alcohols

Summary of Oxidation reaction scheme

Esterification of alcohols Alcohols react with carboxylic acids to form esters in a condensation reaction Water is also produced E.g. reaction between ethanoic acid and ethanol: The ester has the lowest boiling point so can be separated by distillation Ester have no free –OH groups so cannot form hydrogen bonds and are mostly insoluble in water Other examples:

Oxidising alcohols Complete the following investigation into the oxidation of alcohols Instructions here

Key Points Alcohols are highly combustible Primary alcohols oxidise to form aldehydes, which oxidise to form carboxylic acids Secondary alcohols oxidise to form ketones Tertiary do not oxidise due to the 3 strong C-C bonds surrounding the –OH carbon