Biological Chemistry FIRST YEAR ORGANIC CHEMISTRY Lecture Six Alcohols and Ethers Convenor : Dr. Fawaz Aldabbagh

Alcohols and Ethers Alcohols and Ethers can be regarded as derivatives of water in which one or two of the H atoms has been replaced by an alkyl group Electronegativity of oxygen causes an unsymmetrical distribution of charge Saturated molecules are sp 3 hybridized

Alcohols are found to have much higher bpt than those of alkanes or haloalkanes of comparable size, e.g. Methanol (65 o C), Chloromethane and Methane are gases ; Ethanol (78.5 o C), Chloroethane (12 o C) and Ethane is a gas Methanol and Ethanol are classed as Polar Molecules (Hydrophilic) – They are Infinitely Soluble in Water Why? Answer – Hydrogen Bonding H-bonds weaker than covalent bonds, although these bonds can be continually broken and reformed – a highly ordered structure results – H-Bonding to water can also occur Water (mw = 18) is a liquid, bpt 100 o C – otherwise a gas

As R-group increases in size, so does the solubility in non- polar solvents As the number –OHs increases so does solubility in water Bpt increase with chain length and number of –OHs Methanol, CH 3 OH - Solvent in varnishes, paint - Racing Car Fuel (easy to put out flames) - Highly Toxic – “Blindness” - Formaldehyde Ethanol, CH 3 OH -Drinking Alcohol - 50% Ethanol is flammable

Preparation of Ethanol - Fermentation of Sugar – Break down of sugar to CO 2 and Ethanol by Yeast Enzymes - Industrial Process – Hydration of Ethene Ethanol content; Beer, 3-9% ; Wine, 11-13% ; Whisky, 40-45% ; Vanilla Extracts, 35% ; Night Nurse, 25% ; Listerine, 25% Naming Alcohols Methyl alcohol (methanol) Ethyl alcohol (ethanol)Propyl alcohol (propanol) Isopropyl alcohol2-Ethyl-1-butanol

Naming Alcohols Polyhydroxy alcohols are alcohols that possess more than one hydroxyl group 1,2-Ethanediol (ethylene glycol) 1,2-Propanediol (propylene glycol) 1,2,3-Propanetriol (glycerol) Extremely Toxic Calcium Oxalate crystallises in the kidney leading to renal problems Harmless

Alcohols are weak Acids The Conjugate base of an alcohol is an alkoxide ion Because most alcohols are weaker acids than water ; they’re respective alkoxide is a stronger base than hydroxide ion OH - pK a CH 3 OH (15.5), H 2 O (15.74), CH 3 CH 2 OH (15.9), (CH 3 ) 3 COH (18.0)

Weak acids make Poor Leaving Groups in Organic Reactions Strong Acids have Conjugate Bases that are Good Leaving Groups in Organic Reactions Sodium Ethoxide -OH is a Poor Leaving Group We have to convert OH into a Good Leaving Group

OMs and OTs are excellent leaving groups in Nucleophilic Substitution Reactions- Derivatives of Sulfuric Acid

Other Good Leaving Groups ; R-I > R-Br > R-Cl > R-F It follows Acid Strength HI > HBr > HCl > HF S N 2 – Substitution, Nucleophilic, Bimolecular Backside Nucleophilic Attack – Inversion in Configuration Concerted Mechanism Optically Active Enantiomericaly Pure

S N 1 – Substitution, Nucleophilic, Unimolecular Professor George Olah Nobel Prize 1994 Carbocation is sp 2 -planar

Evidence for S N 1 is Racemization of an optically active compound The Carbocation intermediate is attacked by water from either side by the same rate Phenols Phenol 4-Methylphenol 1-Naphthol2-Naphthol Much high bpt, because of H-Bonding (e.g. Phenol, 182C, toluene, 111C

Phenols are much stronger acids than alcohols

ETHERS, RO-OR 1-Propoxypropane Methoxybenzene Methoxycyclohexane Bpt are similar to alkanes – No H-bonding to one another But are soluble in water- H-bonding to water - Polar Flammable – Ether can cause flash fires Low Reactivity – Make Good Reaction Solvents Cyclic Ethers Non-Flammable Anaesthetics