2025 NYJC H2 Chem Hydroxy Compounds Lecture Notes 1

1 Nanyang Junior College Chemistry (9729) Lecture Notes 14 Hydroxy Compounds Lecturer: Mrs Judy Tan JC2/2025 Content  Alcohols (exemplified by ethanol) (i) formation of halogenoalkanes (ii) reaction with sodium; oxidation; dehydration (iii) the tri-iodomethane test  Phenol (i) its acidity; reaction with bases and sodium (ii) nitration of, and bromination of, the aromatic ring Learning Outcomes Candidates should be able to: (a) recall the chemistry of alcohols, exemplified by ethanol: (i) combustion (ii) nucleophilic substitution to give halogenoalkanes (iii) reaction with sodium (iv) oxidation to carbonyl compounds and carboxylic acids (v) dehydration to alkenes (b) suggest characteristic distinguishing reactions for the different classes of alcohols (primary, secondary and tertiary alcohols), e.g. mild oxidation (c) deduce the presence of a CH 3CH(OH)– group in an alcohol from its reaction with alkaline aqueous iodine to form tri-iodomethane (d) recall the chemistry of phenol, as exemplified by the following reactions: (i) with bases (ii) with sodium (iii) nitration of, and bromination of, the benzene ring (e) explain the relative acidities of water, phenol and ethanol in aqueous medium (interpret as Brønsted-Lowry acids) Textbooks and References 1. Chemistry for advanced level; Peter Cann, Peter Hughes, John Murray (Publishers) Ltd Page 477- 493 2. Chemistry in Focus; John Andrew, Paul Rispoli; 2 nd edition Hodder & Stoughton Page 276-299 3. Organic Chemistry; John McMurry; 5th edition Brooks/Cole

2 1. INTRODUCTION  Hydroxy compounds contain the –OH functional group.  They can be classified as aliphatic compounds called alcohols or aromatic compounds, phenols. Tertiary alcohols 3 R groups Primary alcohols 1 R group Alcohols –OH group is bonded to an alkyl group General formula is C H OH Phenols –OH group is directly bonded to an aryl group OH OH CH3 Secondary alcohols 2 R groups R C H R OH H C H R OH R C R R OH Hydroxy compounds Note: “R” represents alkyl or aryl groups.

3 2. ALCOHOLS  General formula for alcohols: CnH2n+1OH or CnH2n+2O where n = 1, 2, 3, …  All alcohols (except methanol) are constitutional isomers (functional group isomers) of ethers. Ethers (C–O–C), have the same general formula as alcohols but its reactions are not required in the H2 syllabus. 2.1 Nomenclature and Classification of Alcohols  Alcohols have names ending with –ol e.g. Methanol, Ethanol, Propanol  Examples: IUPAC Name Molecular Formula Structural Formula Displayed formula (Full Structural Formula) Classification propan-1-ol C3H7OH CH3CH2CH2OH C CC H H H H O H H H H primary propan-2-ol C3H7OH CH3CH(OH)CH3 C CH H H H O C H H H H secondary 2-methylpropan-2-ol C4H9OH (CH3)3COH C C C C O H H H H H H H H H H tertiary cyclopentanol C5H9OH OH C C C C C O H H H H H H H H H H secondary phenylmethanol C7H7OH CH2OH CH H O H primary

4 3 Physical Properties  Alcohols have the same geometry as water. There are 2 lone pairs and 2 bond pairs of electrons with respect to the O atom. O CH3 H O H HCH3 3.1 Boiling Point 3.1.1 Alcohols vs Alkanes Alcohols have much higher boiling points compared to alkanes of similar Mr. Reason: Both alcohols and alkanes have simple molecular structure. Alcohols are able to form stronger intermolecular hydrogen bonds , which require more energy to overcome than the weaker intermolecular instantaneous dipole-induced dipole interactions found between alkanes of similar Mr. CH3 C O H H H CH3 C O H H H CH3 C O H H H 0 50 100 150 200 250 300 350 400 450 1 2 3 4 5 alkanes alcohols : : δ - δ - δ - δ + δ + δ + Bent wrt O atom hydrogen bonding No of C atoms Temperature in K

5 3.1.2 Length of Alkyl Chain Boiling point increases with increasing molecular mass in the alcohol homologous series. compound Mr b.p. / oC CH3OH 32.0 65 CH3CH2OH 46.0 78 CH3CH2CH2OH 60.0 97 Reason: Alcohols have simple molecular structure. As Mr increases with increasing length of alkyl chain, the size of electron cloud increases and becomes more polarisable. Hence more energy is required to overcome the stronger instantaneous dipole-induced dipole interactions between the alcohol molecules. 3.1.3 Straight-chain vs Branched-chain Branched alcohols have lower boiling points than their straight-chain isomers. compound Mr b.p. / oC (CH3)3COH 74.0 82 CH3CH2CH2CH2OH 74.0 117 Reason: Alcohols have simple molecular structure. The branched isomers are more spherical in shape, with smaller surface area of contact between molecules. Hence, the strength of intermolecular instantaneous dipole-induced dipole interactions is weaker, and less energy is required to overcome them. 3.2 Solubility  All alcohols are soluble in organic solvents.  The first three alcohols (C1 to C3) are completely miscible with water. Reason: This is due to the hydrogen bonding between the alcohol and the water molecules. CH3 C O H H H O H H O H H hydrogen bonding : : δ- δ+ δ+ δ+ δ+ δ+ δ- δ-

6  Alcohols with more than 4 carbons are partially miscible or immiscible with water. Reason: As the length of the carbon chain increases, the instantaneous dipole-induced dipole (id-id) interactions between the hydrocarbon chain s in the a lcohol become more significant. Amount of energy evolved during formation of id -id interactions between water and alcohol molecules is insufficient to overcome the id -id interactions between alcohol molecules and hydrogen bonding between water molecules. CH3 CH2 CH2 CH2 CH2 OH 3.2.1 Solubility of alcohol and its applications Alcohols are good solvents and are used in perfumes and flavourings to dissolve fats and oils. Heavier alcohols with long chains of hydrocarbons act as emulsifiers and surfactants, bringing oil and water together. Applications:  Ethanol is used as a fast-drying (due to its low b.p.) solvent in cosmetics and hairsprays.  Stearyl alcohol (1-octadecanol) is a non-ionic surfactant used as a hair coating in shampoos and conditioners. As water and oil do not dissolve in each other, a surfactant has to be added to the mixture to keep it from separating into layers. Stearyl alcohol can be used as a surfactant as it has both hydrophobic and hydrophilic ends. CH3 OH Stearyl alcohol C18H38O (1-octadecanol) Hydrocarbon chain forms instantaneous dipole-induced dipole interactions, making this part hydrophobic. Alcohols forms hydrogen bonding, making this part hydrophilic.

7 4 Chemical Reactions of Alcohols 4.1 Combustion Alcohols burn to give carbon dioxide and water. e.g. ethanol burns with a pale-blue flame CH3CH2OH + 3 O2  2 CO2 + 3 H2O 4.2 Other reactions of alcohols  Other reactions of alcohols can be divided into two groups: those that occur at the C–O bond and those that occur at the O–H bond. C R R R O H  Overview of the reactions of alcohols Type of reaction Conversion Reaction at O-H bond Reaction with Na reduction ROH + Na  RO−Na+ + ½ H2 Esterification condensation (nucleophilic acyl substitution) ROH + R’COOH ⇌ R’COOR + H2O ROH + R’COX  R’COOR + HX Oxidation oxidation ROH  aldehyde / carboxylic acid / ketone (depending on the classification of alcohol) Reaction with very strong bases (e.g. NaH, NaNH2, CH3MgBr) **Refer to Appendix Reaction at C-O bond Dehydration elimination ROH  alkene + H2O Halogenation nucleophilic substitution ROH  RX + HX Reactions that occur at the C— O bond Reactions that occur at the O— H bond

8 4.2.1 Reaction with sodium Type of Reaction Reduction Re