Alcohols and Ethers
Senior 5 · Chemistry
Alcohols and Ethers
Lesson 4 of 5
25 min
In 1954 the American police officer and scientist Robert Borkenstein patented the Breathalyzer. The suspect blew into a tube containing acidified potassium dichromate: any ethanol in the breath was oxidised, and the orange dichromate turned green in proportion to the amount of alcohol. Modern devices use fuel cells or infrared light instead, but the chemistry of that first instrument, the oxidation of an alcohol, is the most important reaction in this unit. It is also the basis of the standard test that tells primary, secondary and tertiary alcohols apart.
Definition
Oxidation (in organic chemistry): the addition of oxygen or the removal of hydrogen; written with [O] for oxygen supplied by an oxidising agent.
Heating under reflux: heating a mixture with a vertical condenser fitted, so that vapours condense and run back into the flask; volatile substances are not lost.
Iodoform test: warming a compound with iodine and sodium hydroxide solution; a pale yellow precipitate of triiodomethane (iodoform), , shows a or group.
Acidified potassium dichromate(VI) is an oxidising agent: its orange ion is reduced to green :
The alcohol loses the H from its O–H group and the H from the carbon carrying –OH, so that carbon must have at least one hydrogen.
| Class | Product | Equation | Colour |
|---|---|---|---|
| primary | aldehyde, then carboxylic acid | ; | orange → green |
| secondary | ketone | orange → green | |
| tertiary | no reaction | no H on the C–OH carbon | stays orange |
Figure 4.1: Reflux (left) and distillation (right). The set-up decides whether a primary alcohol gives a carboxylic acid or an aldehyde.
To stop at the aldehyde, use excess alcohol and distil the product off as it forms: ethanal boils at 21 °C, far below ethanol (78 °C), so it escapes before it can be oxidised further. For the carboxylic acid, use excess oxidising agent and heat under reflux. The full ionic equation for ethanol to ethanal is:
Acidified potassium manganate(VII) does the same job (purple → colourless). Passing the alcohol vapour over hot copper at about 300 °C removes hydrogen instead: primary alcohols give aldehydes and secondary alcohols give ketones, while tertiary alcohols are dehydrated to alkenes.
Warm a few drops of the compound with iodine solution, then add sodium hydroxide solution until the brown colour of iodine just disappears. A pale yellow precipitate with an antiseptic smell is triiodomethane. Alcohols containing the group give it, because they are first oxidised to the group, whose three hydrogens are replaced by iodine before the C–C bond breaks:
Positive: ethanol (the only primary alcohol to give it), propan-2-ol, butan-2-ol. Negative: methanol, propan-1-ol, 2-methylpropan-2-ol.
| Alcohol | Uses |
|---|---|
| ethanol | alcoholic drinks; fuel blended with petrol; solvent for medicines, perfumes and inks; hand rub; making ethanoic acid and esters |
| methanol | making methanal and plastics; fuel; solvent; screen-wash antifreeze |
| propan-2-ol | disinfectant swabs and hand rub; cleaning electronics |
| ethane-1,2-diol | antifreeze in car radiators; making polyester (PET) |
| propane-1,2,3-triol (glycerol) | moisturiser in cosmetics and hand rub; food additive; making explosives |
Ethanol is a drug that depresses the nervous system. The World Health Organization estimates that alcohol consumption caused about 2.6 million deaths worldwide in 2019, through liver disease, cancers, heart disease, road crashes and violence. Methanol is far more toxic: the body converts it to methanal and then methanoic (formic) acid, which, as CDC guidance explains, cause acid build-up in the blood, blindness and death. Illegally distilled spirits contaminated with methanol cause outbreaks of poisoning in many countries.
Worked example
Problem. Give the organic product, if any, when each is heated with acidified potassium dichromate: (a) propan-1-ol, distilling off the product as it forms; (b) propan-1-ol, heated under reflux with excess oxidant; (c) propan-2-ol; (d) 2-methylpropan-2-ol. Write a balanced ionic equation for (c).
Solution. (a) propanal, . (b) propanoic acid, . (c) propanone. (d) no reaction; the solution stays orange.
(c): each propan-2-ol loses 2 H (2 electrons); each gains 6 electrons, so the ratio is 3 : 1: .
Worked example
Problem. X, Y and Z are three of the alcohols . X gives a yellow precipitate with iodine and alkali, and is oxidised by dichromate to a ketone. Y does not turn acidified dichromate green. Z is oxidised to an aldehyde, does not give the iodoform test, and has a branched chain. Identify X, Y and Z.
Solution. X is secondary and contains : butan-2-ol. Y is tertiary: 2-methylpropan-2-ol. Z is primary and branched: 2-methylpropan-1-ol, .
Warning
Safety. Potassium dichromate is toxic and a carcinogen, and must be handled with gloves and collected for disposal; it is no longer allowed in many schools, where acidified manganate(VII) is used instead. Never taste any alcohol in the laboratory: laboratory ethanol may be denatured with methanol. Anyone who may have swallowed methanol needs emergency medical treatment at once.
Tip
Count the H atoms on the C–OH carbon: 2 → aldehyde, then acid; 1 → ketone; 0 → no oxidation. For the iodoform test, look for a next to the C–OH carbon.
Chemistry Student: Oxidation of Alcohols, A-level Chemistry (14 min). Watch how the class of alcohol decides the product, and how distillation or reflux decides whether a primary alcohol stops at the aldehyde.
Practice
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