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Topic 10.4 Alcohols Assessment Statements 10.4.1 Describe, using equations, the complete combustions of alcohols. 10.4.2 Describe, using equations, the oxidation reactions of alcohols. 10.4.3 Determine the products formed by the oxidation of primary and secondary alcohols. Alcohols An alcohol consists of a carbon chain with a hydroxy group (-OH) attached Methanol Ethanol Propanol Phenol 3 Alcohol Classification Alcohols may be classified as - Primary - Secondary - Tertiary Depending on whether the carbon atom that is attached to the –OH group is surrounded by one, two or three other carbon atoms 4 Primary Alcohols A primary alcohol has only one carbon atom attached 5 Secondary Alcohols A secondary alcohol has 2 carbon chains attached to the group on which the –OH resides. 6 Tertiary Alcohols 7 Polyhydroxy alcohols Alcohols that have more than one OH group are known as polyhydroxyl alcohols. Two examples are shown below 8 Name and classify the following alcohols. 2-butanol (Secondary) 2-propene-1-ol (Primary) 2-methyl-1-butanol (Primary) 2-methyl-2-butanol (Tertiary) 9 Properties of Alcohols The physical properties of alcohols are similar to those of both water and hydrocarbons The shorter chain alcohols such as methanol and ethanol are similar to water, in general they -- have higher boiling points than hydrocarbons but lower than water -- dissolve in water to some degree -- are more polar than hydrocarbons but less polar than water 10 Physical Properties of ALcohols State at r.t.p.* Name Molecular formula Solubility In Water Methanol CH3OH Very Soluble Liquid Ethanol C2H5OH Soluble Liquid Propanol C3H7OH Soluble Liquid Butanol C4H9OH Slightly soluble Liquid (solid/liquid/gas) Reactions of Alcohols Alcohols undergo several types of reactions including: Oxidation Dehydration Reactions with active metals Esterification 12 Assessment Statements 10.4.1 Describe, using equations, the complete combustions of alcohols. Combustion An alcohol burns in air to produce carbon dioxide and water vapour. ethanol + oxygen → carbon dioxide + water vapour C2H5OH + 3O2 → 2CO2 + 3H2O C2H5OH + 3O2 → 2CO2 + 3H2O Combustion Flame produced is clean Combustion of alcohols are useful because: 1. Methanol is less likely than conventional fuel to explode in an accident. It is a clean fuel. 2. When alcohol beverage is burnt with food, it gives a distinct flavour. Assessment Statements 10.4.2 Describe, using equations, the oxidation reactions of alcohols. 10.4.3 Determine the products formed by the oxidation of primary and secondary alcohols. Oxidation Alcohols are oxidized to alkanals (aldehydes) or alkanones (ketones) 19 Oxidation We can oxidise an alcohol (for example, ethanol) by warming it with acidified potassium dichromate(VI). Experimental setup: (ethanol) C2H5OH + acidified K2Cr2O7 cold water heat CH3COOH (ethanoic acid) Oxidation The colour of potassium dichromate(VI) changes from orange to green. Oxidation of alcohol is used in: Breathalyzer to test the amount of alcohol consumed by drivers. 1º, 2º, 3º Carbons Primary Alcohols are Oxidized to Alkanals The most common oxidizing agents are KMnO4 in basic solution, K2Cr2O7 in acidic solution, or oxygen from the air. The mechanisms of such reactions are usually complex 23 Oxidation Chemical equation: + 2 [O] Ethanol + Ethanoic acid H2 O Oxidation of 1° Alcohols 1° alcohol to aldehyde to carboxylic acid Difficult to stop at aldehyde Use pyridinium chlorochromate (PCC) to limit the oxidation. PCC can also be used to oxidize 2° alcohols to ketones. OH CH3CH2CH2CH2 N H CrO3Cl O CH3CH2CH2CH => Chapter 11 25 Stopping at Aldehydes For IB, distillation of aldehyde as it is produced increases its yield Improving Yield of Carboxylic Acid For IB, Heating under reflux Secondary Alcohols are oxidized to Alkanones Example 28 Oxidation of 2° Alcohols 2° alcohol becomes a ketone Reagent is Na2Cr2O7/H2SO4 Active reagent probably H2CrO4 Color change: orange to greenish-blue OH CH3CHCH2CH3 Na2Cr2O7 / H2SO4 O CH3CCH2CH3 => Chapter 11 29 Tertiary Alcohols are not easily oxidized There is no hydrogen attached to the tertiary carbon. It is not possible for a carbonyl group on the tertiary carbon. 30 Tertiary Alcohols are not easily Oxidised Furthermore the large R groups on the tertiary carbon, prevent reacting groups. Hence tertiary alcohols are rather unreactive. 31 3° Alcohols Don’t Oxidize Cannot lose 2 H’s Basis for chromic acid test => Chapter 11 32 Question Starting with prop- 1- ene, show a chemical pathway to produce propanoic acid. Dehydration Many alcohols undergo intramolecular dehydration, removing water to form alkenes. 34 Reactions with Metals • Alcohols are far less acidic than water but they react with highly reactive metals such as potassium and sodium. • The metal replaces the hydrogen atom in the alcohol group forming a salt and hydrogen gas 35 Esterification An alcohol reacts with an alkanoic acid to form an ester and water. 36 Thiols If the oxygen in the –OH group is replaced with a sulfur atom, a group with somewhat similar properties results. The –SH functional group is known as a thiol or mercaptan. 37 Some Thiols Thiols are well known for their intense, foul odors Responsible for skunk odor Added to natural gas to give it a rotten egg smell 38 Name and classify the following alcohols. 2-butanol (Secondary) 2-propene-1-ol (Primary) 2-methyl-1-butanol (Primary) 2-methyl-2-butanol (Tertiary) 39 Alcohols and Natural Products Urushiol is the active ingredient in poison ivy and poison oak.