TJC 2025 Alcohols Notes (Student)

1 ©TJC IP Chemistry Temasek Junior College Integrated Programme Year 4 Chemistry Topic 10.4: Alcohol Notes Name: Class: Date: Learning objectives At the end of this notes, you must be able to: (a) describe alcohols as a homologous series containing the –OH group (b) draw the structures of branched and unbranched alcohols C1–C4 and name the unbranched alcohols methanol to butanol (c) describe the reactions of alcohols in terms of combustion , acid -metal displacement and oxidation to carboxylic acids (d) describe the formation of ethanol by the catalysed addition of steam to ethene and by fermentation of glucose Topic Outline 1 Introduction 1.1 Nomenclature 2 Physical Properties of Alcohols 2.1 Melting and Boiling points 3 Isomerism in Alcohols 4 Chemical Properties of Alcohols: Reaction of alcohols 4.1 Combustion 4.2 Acid-Metal displacement 4.3 Oxidation 4.4 Condensation 5 Manufacturing Ethanol from Ethene 5.1 Addition of Steam (Hydration) 5.2 Fermentation of Yeast 6 Use of Ethanol as a Biofuel

2 ©TJC IP Chemistry 1 Introduction • Alcohols are organic compounds made up of carbon, hydrogen and oxygen. • General Formula : CnH2n+1 OH or ROH • Functional Group : –OH 1.1 Nomenclature • Alcohol is named with a suffix ‘–ol’ to indicate presence of the alcohol group. • The following table shows the first four members of the alcohol homologous series. Structural formula Displayed / Full structural formula and Name General formula of alcohols, in terms of n, where n is an integer starting with n = 1 CH3OH methanol CH3CH2OH ethanol CH3CH2CH2OH propan-1-ol CH3CH2CH2CH2OH butan-1-ol ____________ functional group H H H C H C H H H C H H C O H H H H C H H C H H C O H H H H C H H C O H H H H C O H

3 ©TJC IP Chemistry 2 Physical Properties of Alcohols • Alcohol molecules have an alkyl group as well as a –OH group. • In addition to intermolecular forces of attraction, hydrogen bonds between the molecules are present as well. • The hydrogen bond between alcohol molecules refers to the electrostatic forces of attraction between a hydrogen atom that is covalently bonded to the oxygen atom (an electronegative atom) and a lone pair of electrons on an oxygen atom from another alcohol molecule. Example: Ethanol CH3 – CH2 – OH 2.1 Boiling Point and Melting Point (i) Boiling points of alcohols are higher than hydrocarbons of similar Mr. Boiling points of alcohols are much higher than hydrocarbons of similar Mr. Reason: This is due to the –OH group which allows for hydrogen bonding between alcohol molecules. Hydrogen bonding between alcohol molecules: Compound Mr Boiling Point / °C CH3CH2OH 46.0 78 CH3CH2CH3 44.0 –42 The –OH group forms hydrogen bonds with other molecules

4 ©TJC IP Chemistry (ii) Boiling point increases with increasing total number of electrons present. Reason: As the size of the electron cloud increases, this leads to stronger intermolecular forces of attraction between molecules. Hence, greater amount of energy is required to overcome these forces leading to higher boiling points. Straight–chain isomers have higher boiling points than their branched isomers. Example: Butanol has a higher boiling point than 2–methylpropan–1–ol. Isomer Type of isomer Boiling Point / °C butanol Straight chain isomer 118 2-methylpropan-1-ol Branched chain isomer 108 Reason: • Butanol has a straight chain while 2–methylpropan–1–ol has a branched chain. • Extent of surface area of contact : butanol > 2-methylpropan-1-ol • Extent of intermolecular forces of attractions : butanol > 2-methylpropan-1-ol • Energy requirement: butanol > 2-methylpropan-1-ol Alcohol Boiling Point / °C methanol 65 ethanol 78 propanol 97

5 ©TJC IP Chemistry 3 Isomerism Alcohols exhibit constitutional isomerism. Molecular formula of alcohol Isomer of alcohol Type of isomerism and reasoning CH4O Not applicable. Only CH3OH C2H6O Not applicable. Only CH3CH2OH C3H8O CH3CH2CH2OH propan–1–ol CH3CH(OH)CH3 propan–2–ol propan-1-ol and propan-2-ol exhibit _________________ isomerism due to ___________________________ C4H10O CH3CH2CH2CH2OH butan–1–ol CH3CH2CH(OH)CH3 butan–2–ol (CH3)2CHCH2OH 2–methylpropan–1–ol (CH3)3COH 2–methylpropan–2–ol 1. Constitutional (Positional) isomerism Reason: 2. Constitutional (Chain) isomerism Reason:

6 ©TJC IP Chemistry 4 Chemical Properties of Alcohols: Reaction of alcohols 4.1 Combustion (Self-directed learning) Similar to hydrocarbon, alcohols can undergo combustion. 4.1.1 Complete Combustion Complete combustion of alcohol will yield CO2 and H2O as products. Example complete combustion of ethanol: CH3CH2OH + 3O2 → 2CO2 + 3H2O 4.1.2 Incomplete Combustion Incomplete combustion of alcohol will yield CO and H2O as products. Example incomplete combustion of ethanol: CH3CH2OH + 2O2 → 2CO + 3H2O 4.2 Acid−Metal displacement reaction • Alcohols are very weak acids. • Hence, alcohols can undergo acid-metal displacement reaction with reactive metals to form a salt and hydrogen gas. General equation: ROH + Na ⎯→ RO–Na+ + 1 2 H2 Reagents and Conditions: ……………….………………………………………. Type of reaction: ……………………………………….………………………… Observation: Effervescence of H 2 observed that extinguishes a lighted splint with a pop sound e.g. Reaction of ethanol with sodium Note: Alcohols do not react with bases and carbonates.

7 ©TJC IP Chemistry 4.3 Oxidation • Primary alcohols are alcohols where the carbon atom bonded to the –OH group is bonded to two or three hydrogen atoms. Examples of primary alcohol: • Primary alcohols can undergo oxidation to form _______________________________. General equation: RCH2OH + 2[O] → RCOOH + H2O Reagents & Conditions: KMnO4(aq), dilute H2SO4, heat under reflux OR K2Cr2O7(aq), dilute H2SO4, heat under reflux Type of Reaction: Oxidation Observation: Purple KMnO4 solution is decolourised OR Orange K2Cr2O7 solution turns green Example: Oxidation of ethanol by heating under reflux with KMnO4 or K2Cr2O7 acidified with H2SO4 Chemical equation: CH3CH2OH + 2[O] → CH3COOH + H2O ethanol ethanoic acid Quick-check: 1. Write balanced chemical equation for the oxidation of propan–1–ol when heated under reflux with acidified potassium dichromate(VI). Chemical equation: ……………………………………………………………………………… 2. Write balanced chemical equation for the oxidation of butan–1–ol when heated under reflux with acidified potassium dichromate(VI). State the observation expected. Chemical equation: ……………………………………………………………………………… Observation: ……………………………………………………………………………………… C H H R O H O C R O H H2O + 2[O] + primary alcohol carboxylic acid

8 ©TJC IP Chemistry 4.4 Condensation Alcohols can react with carboxylic acids to form esters through condensation reactions, and it is named esterification. Details of this will be covered in Carboxylic acids notes. 5 Manufacturing Ethanol from Ethene Ethanol can be manufactured through two methods: addition and fermentation. 5.1 Addition of Steam (Hydration) Ethanol is synthesised through the addition of steam to ethene. Reagent: