Chemistry · Organic chemistry · Grade 10

Alcohols and ethers

Why do ethanol and dimethyl ether have the same formula but boiling points differing by over 100 °C?

The situation

The disinfectant alcohol at the pharmacy, the antifreeze in a car radiator and the glycerol in hand cream are all alcohols: their molecules contain an –OH group. Ethanol forms when grape juice ferments; methanol, which looks and smells much the same, is a powerful poison: a few millilitres can blind a person. Dimethyl ether, which has the same formula as ethanol, is a gas at room temperature. How much does a single –OH group change the properties of a substance, and how can alcohols be told apart?

Topic

Alcohols and ethers. Disinfectant alcohol, antifreeze and glycerol cream: monohydric and polyhydric alcohols; blue colour with Cu(OH)₂

Key ideas

–OH group and hydrogen bonding
With Na, CuO, Cu(OH)₂
Ethers — isomers

Formulas

C_{n} H_{2 n + 1} OH

The homologous series of saturated monohydric alcohols: methanol $CH_{3} OH$ , ethanol $C_{2} H_{5} OH$ , propan-1-ol $C_{3} H_{7} OH$ … The general formula is $C_{n} H_{2 n + 2} O$ — the same as for ethers.

2 C_{2} H_{5} OH + 2 Na \overset{}{\to} 2 C_{2} H_{5} ONa + H_{2} ↑

Alcohols with active metals: the hydrogen of the O–H bond is replaced by the metal (practical 3.10). 1 mol of Na gives 0.5 mol of $H_{2}$ .

C_{2} H_{5} OH + CuO \overset{\overset{}{t}}{\to} CH_{3} CHO + Cu + H_{2} O

A primary alcohol is oxidised by CuO to an aldehyde: the black wire turns red copper again.

C_{6} H_{12} O_{6} \overset{\overset{}{yeast}}{\to} 2 C_{2} H_{5} OH + 2 CO_{2} ↑

Alcoholic fermentation: glucose is converted into ethanol and carbon dioxide by enzymes of yeast fungi.

2 C_{3} H_{5} (OH)_{3} + Cu (OH)_{2} \overset{}{\to} Cu (C_{3} H_{7} O_{3})_{2} + 2 H_{2} O

Qualitative test for polyhydric alcohols: a fresh $Cu (OH)_{2}$ precipitate turns into a deep blue solution (practical 3.5).

Theory

Monohydric alcohols: series, naming, isomerism (3.1)

Alcohols are compounds in which one or more hydrogen atoms of a hydrocarbon molecule have been replaced by a hydroxyl group –OH. The series of saturated monohydric alcohols: $CH_{3} OH$ methanol, $C_{2} H_{5} OH$ ethanol, $C_{3} H_{7} OH$ propanol… general formula $C_{n} H_{2 n + 1} OH$ . Depending on the carbon bearing the –OH, alcohols are primary (the C is bonded to one other C: propan-1-ol), secondary (to two C: propan-2-ol) and tertiary (to three C: 2-methylpropan-2-ol). Naming: choose the longest chain that carries the –OH, number it from the end nearer to the –OH, add “-ol” to the name of the alkane and give the position of the –OH: $CH_{3} - CH (OH) - CH_{3}$ is propan-2-ol. Isomerism: of the carbon chain (butan-1-ol and 2-methylpropan-1-ol), of the –OH position (butan-1-ol and butan-2-ol) and between classes — with ethers (ethanol and dimethyl ether are both $C_{2} H_{6} O$ ).

Hydrogen bonding and physical properties

The O–H bond is strongly polarised: the H atom is attracted to the oxygen of a neighbouring molecule — a hydrogen bond (O–H···O, ≈ 0.19 nm). Because the molecules “stick” to one another, alcohols boil at much higher temperatures than alkanes of similar mass: methanol 64.7 °C (ethane −88.6 °C), ethanol 78.3 °C (propane −42.1 °C). The boiling point rises as the chain gets longer, and branching lowers it: butan-1-ol 117.7 °C, 2-methylpropan-2-ol 82.4 °C. The lower alcohols mix with water in all proportions (they also form hydrogen bonds with water); from butanol onwards the solubility drops sharply. Methanol is a powerful poison: if drunk, it is converted in the body into formaldehyde and formic acid; there are recorded cases in which even 4–15 mL permanently damaged the optic nerve, and the lethal dose is a few tens of millilitres. It cannot be told from ethanol by smell or appearance. Ethanol, too, is a narcotic substance and poisons the liver and the brain.

Preparation and chemical properties (3.2; practical 3.10)

Preparation: ethanol — by fermenting sugary substances with yeast (grapes; the starch of grain is first broken down to sugar; yeast survives up to ≈ 15–18 % ethanol, and pure alcohol is obtained by distillation) and, in industry, by hydrating ethene ( $H_{3} PO_{4}$ , ≈ 300 °C, steam at high pressure). Methanol — from “synthesis gas”: $CO + 2 H_{2} \overset{}{\to} CH_{3} OH$ (Cu/ZnO catalyst, heating, pressure). Properties: 1) with active metals $H_{2}$ is released and an alkoxide forms; alcohols are even weaker acids than water (ethanol p $K_{a}$ ≈ 15.9), so the reaction is calmer than in water, and the alkoxide gives an alkaline solution in water; 2) the –OH group is replaced by a halogen: $C_{2} H_{5} OH + HBr \overset{}{\to} C_{2} H_{5} Br + H_{2} O$ (the oxygen picks up H⁺ — the basic property of alcohols); 3) dehydration: with $H_{2} SO_{4}$ at ≈ 170 °C ethene, at ≈ 140 °C diethyl ether; 4) oxidation: with CuO an aldehyde, dehydrogenation when passed over heated copper ( $C_{2} H_{5} OH \overset{}{\to} CH_{3} CHO + H_{2}$ ), combustion: $C_{2} H_{5} OH + 3 O_{2} \overset{}{\to} 2 CO_{2} + 3 H_{2} O$ . Uses: solvent, fuel additive; 70 % ethanol — disinfectant, medicinal tinctures; methanol — synthesis of formaldehyde and ethanoic acid. In Uzbekistan ethyl alcohol is produced, for example, at the Biokimyo plant in Yangiyul.

Polyhydric alcohols: ethylene glycol and glycerol (3.3–3.5)

Ethylene glycol $HO - CH_{2} - CH_{2} - OH$ (dihydric) and glycerol $HO - CH_{2} - CH (OH) - CH_{2} - OH$ (trihydric) are sweet-tasting, viscous liquids that mix with water; having many –OH groups, they have high boiling points: 197.3 °C and 290 °C. Ethylene glycol is the base of antifreeze (a 60 % mixture freezes at ≈ −45 °C) and a raw material for lavsan (polyester); its molecules join through ether bonds to give diethylene glycol and polyethylene glycols (medicines, cosmetics). Ethylene glycol is made by hydrating ethylene oxide. It is a powerful poison: because of its sweet taste it is especially dangerous for children and animals. Glycerol is used in cosmetics, medicine and the food industry, and nitroglycerine is made from it; it is part of fats (a fat is an ester of glycerol). Qualitative test (practical 3.5): 2 mL of NaOH solution and a few drops of $CuSO_{4}$ are added to a test tube — a blue $Cu (OH)_{2}$ precipitate forms; if glycerol (or ethylene glycol) is added and the tube is shaken, the precipitate dissolves and a deep blue solution forms — the copper(II) glycerate complex. Monohydric alcohols do not do this: the blue precipitate does not change.

Ethers (3.8)

Ethers $R - O - R^{'}$ have two hydrocarbon radicals joined through an oxygen “bridge”: dimethyl ether $CH_{3} - O - CH_{3}$ , ethyl methyl ether, diethyl ether $C_{2} H_{5} - O - C_{2} H_{5}$ . They are the isomers (between classes) of alcohols with the same formula: $C_{2} H_{6} O$ — ethanol and dimethyl ether, $C_{4} H_{10} O$ — butanol and diethyl ether. An ether molecule has no O–H, so the molecules do not form hydrogen bonds with one another: dimethyl ether boils at −24 °C and diethyl ether at 34.6 °C (butan-1-ol 117.7 °C). Preparation: heating an alcohol with conc. $H_{2} SO_{4}$ at ≈ 140 °C — intermolecular dehydration. It does not react with sodium. Diethyl ether is a good solvent, very volatile and extremely flammable; its vapour forms an explosive mixture with air, and explosive peroxides accumulate on long standing. In the nineteenth century it was one of the first anaesthetics. Dimethyl ether is used in aerosol cans and as a substitute for diesel fuel.

Safety and the limits of the model

Safety: never taste alcohols — methanol and ethylene glycol are poisons; work in safety goggles. Do not touch sodium with your hands (use tweezers), put only a small piece into anhydrous alcohol and keep the rest under kerosene; the hydrogen released forms an explosive mixture with air. Keep alcohol and ether away from open flames and work with ether only in a fume cupboard. If alkali gets into the eyes, rinse with plenty of water. Model: the water-bath temperature approaches 20 + 82·(power) °C as a first-order process, and the liquid in a test tube does not heat above its boiling point; the piece of sodium is a sphere that shrinks at a constant rate (the duration of the reaction is approximate); the gas in the syringe is measured at 20 °C and 101.3 kPa, and the expansion of alcohol vapour and air is not taken into account; the volume at STP uses $V_{m}$ = 22.4 L/mol. Fermentation time is sped up (1 s = 6 hours), the yield is theoretical; in limewater, ≈ 0.4 % of the $CO_{2}$ in the bubbles is assumed to be absorbed at first, and ≈ 0.1 % once the precipitate has formed. The molecules have real bond lengths and angles (1 Å ≈ 3 cm), and the reaction animations show only which bond breaks and which forms — the real mechanism has more steps.

Did you know?

On 16 October 1846, at the Massachusetts General Hospital in Boston, the dentist William Morton put a patient to sleep with diethyl ether vapour and publicly demonstrated a painless surgical operation.
70 % ethanol disinfects better than pure (almost anhydrous) alcohol: to destroy the protein of microbes water is needed too, while pure alcohol quickly “hardens” the surface of the cell and has no time to get inside.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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