Alcohols: Classification, Production & Combustion (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about classifying alcohols, how alcohols are produced, and the combustion of alcohols.

All of it turns on the carbon that carries the hydroxy group.

Every alcohol contains a hydroxy group, and the carbon carrying it is what everything turns on. How many other carbons are attached to that carbon classifies the alcohol as primary, secondary or tertiary, and that classification decides whether it can be oxidised. The route used to make an alcohol decides which class you end up with. Combustion, on the other hand, happens whatever the class.

We'll take the classification first, then what it decides about oxidation, then briefly how alcohols are made, and finally burning them and their use as biofuels.

Primary alcohols are alcohols in which the carbon atom bonded to the hydroxy group is attached to one other carbon atom, or alkyl group. Secondary alcohols are those in which that carbon is attached to two other carbon atoms. Tertiary alcohols are those in which it is attached to three.

That is the whole classification: count the carbons attached to the carbon that carries the OH group. It is that count which decides how the alcohol behaves.

Only primary and secondary alcohols can be oxidised and the oxidising agent acidified potassium dichromate can be used to do this. Primary alcohols are mildly oxidised to aldehydes, and secondary alcohols to ketones. Tertiary alcohols do not undergo oxidation with acidified potassium dichromate at all.

Because of that, only the oxidation of primary and secondary alcohols changes the colour of the acidified potassium dichromate solution, as the orange dichromate ions are reduced to green chromium ions. With a tertiary alcohol, no colour change is observed. The classification is what this test is reading.

Alcohols are compounds that contain at least one hydroxy group, and they can be prepared by a wide range of chemical reactions.

Fermentation is a batch process: sugar or starch is dissolved in water, yeast is added, and the mixture is kept between 15 and 35 degrees Celsius in the absence of oxygen. Enzymes within the yeast break the sugar down into ethanol and carbon dioxide. If the temperature is too low the rate is low and if it is too high the enzymes are denatured. The enzymes are killed off anyway once the alcohol concentration reaches around 15 percent.

The other industrial route is hot steam reacted with an alkene, using concentrated phosphoric acid or sulfuric acid as a catalyst. This is electrophilic addition that forms the alcohol. Which route is used determines which class of alcohol you get.

Alcohols react with oxygen in the air when ignited and undergo complete combustion to form carbon dioxide and water, and that happens whatever the class of alcohol. Lower alcohols burn with an almost invisible flame and make good fuels.

A biofuel is a fuel produced from biological material, and ethanol produced by fermentation can be used as one. A carbon-neutral fuel produces no net emissions of carbon dioxide to the atmosphere. Photosynthesis absorbs six moles of carbon dioxide, while fermentation and combustion together release six, so the net is zero. The claim does not hold in practice, because fossil fuels are burned to transport the crop and the ethanol, to power the farming machinery, and to run the fractional distillation that purifies it.

When defining carbon-neutral, you must refer to carbon dioxide, not just carbon, and state that the emissions are to the atmosphere. The mark scheme also requires the idea that the carbon dioxide taken in equals the carbon dioxide released, not just that no carbon dioxide is produced.

Be careful when balancing equations for the combustion of alcohols, as students often forget to count the oxygen already in the alcohol.

The carbon that carries the hydroxy group is what classifies an alcohol: one other carbon attached makes it primary, two makes it secondary and three makes it tertiary. That class decides the oxidation, with primary alcohols going to aldehydes, secondary to ketones, and tertiary not oxidising at all. Which is why acidified potassium dichromate turns green for the first two and stays orange for the third.

Fermentation and the hydration of alkenes are the industrial routes, and which one is used decides the class. Combustion, though, happens whatever the class.

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