Oxidation & Elimination Reactions of Alcohols (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about the oxidation of alcohols and their elimination reactions.

Both attack the same carbon, the one carrying the hydroxy group, but they take it in opposite directions.

Both of these reactions start at the same place, the carbon carrying the hydroxy group. Oxidation keeps the oxygen and builds it into a carbon-oxygen double bond, giving an aldehyde, a carboxylic acid or a ketone. Elimination removes the oxygen altogether as water, leaving a carbon-carbon double bond and an alkene.

We'll take the oxidation products first, then the oxidising agents and their colour changes, then how the conditions decide whether you stop at the aldehyde or go on to the carboxylic acid, then the tests that distinguish aldehydes from ketones, and finally the dehydration that gives an alkene.

Primary alcohols can be oxidised to form aldehydes, which can undergo further oxidation to form carboxylic acids. Secondary alcohols can be oxidised to form ketones only. Tertiary alcohols do not undergo oxidation.

Aldehydes and ketones are carbonyl compounds that contain a carbon-oxygen double bond, and they are prepared by oxidising primary and secondary alcohols respectively. That carbon-oxygen double bond is the oxygen being kept and built up, rather than removed.

Common oxidising agents for alcohols are acidified potassium dichromate(VI) and acidified potassium manganate(VII). Acidified means the reagent is dissolved in a dilute acid such as dilute sulfuric acid. For the oxidising agent to act as an oxidising agent it must itself be reduced, and that reduction requires hydrogen ions, which are supplied by the acidic medium.

Potassium dichromate is orange, and when alcohols are oxidised the orange dichromate ions are reduced to green chromium ions. Potassium manganate is purple, and it is reduced to colourless manganese ions. The colour change is the visible sign that the alcohol has been oxidised.

The method used to oxidise a primary alcohol decides which product you get. The alcohol is added to a limited amount of oxidising agent and gently warmed, and because the aldehyde has a lower boiling point than the alcohol it came from, it can be distilled off as soon as it forms.

If the aldehyde is not removed, further heating under reflux with excess oxidising agent oxidises it to a carboxylic acid. Reflux holds the reaction at its boiling point for extended periods without losing volatile solvent or reactants, with a vertical condenser cooling the vapours and returning them to the flask. The top of the condenser is never sealed, to avoid pressure building up. Ketones cannot be further oxidised, so a ketone does not need to be distilled off immediately.

The presence of an aldehyde group in an unknown compound can be determined by two oxidising agents, Fehling's solution and Tollens' reagent.

Fehling's solution is an alkaline solution containing copper(II) ions. Warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the copper ions are reduced. The clear blue solution turns brick red as a copper(I) oxide precipitate forms. Tollens' reagent is an aqueous alkaline solution of silver nitrate in excess ammonia, also known as ammoniacal silver nitrate. Warmed with an aldehyde in a water bath, the silver ions are reduced to silver atoms, which deposit on the inside of the test tube as a characteristic silver mirror. Ketones cannot be oxidised, so they give a negative result in both tests.

Alcohols can also undergo dehydration to form alkenes. Alcohol vapour is passed over a hot catalyst of aluminium oxide powder or pieces of porous pot at around 600 degrees Celsius. Alternatively, excess hot concentrated sulfuric acid or phosphoric acid is used as the catalyst.

This is an elimination reaction, in which a small molecule is formed as a by-product. The water molecule is formed from the hydroxy group of the alcohol and the hydrogen ion from the acid catalyst. The neighbouring hydrogen is then lost as a hydrogen ion, which regenerates the acid catalyst. Here the oxygen leaves altogether, and what is left behind is a carbon-carbon double bond. Alkenes produced by this method can be used to produce addition polymers without using monomers derived from crude oil.

Students often confuse this acid-catalysed elimination of water from alcohols with the base-induced elimination of hydrogen halides from halogenoalkanes, drawing a hydroxide ion attacking a hydrogen. These mechanisms often carry several marks, so it is worth investing the time to practise them.

In this mechanism, any carbon-hydrogen bond that breaks must have the hydrogen drawn explicitly. The usual stick shorthand is penalised here, even though it is often accepted elsewhere.

Both reactions start at the carbon carrying the hydroxy group. Oxidation keeps the oxygen and builds a carbon-oxygen double bond. Primary alcohols give aldehydes and then carboxylic acids, secondary alcohols give ketones, and tertiary alcohols do not oxidise at all. The conditions decide which product you get, distilling the aldehyde off as it forms or refluxing with excess oxidising agent to reach the carboxylic acid. Fehling's and Tollens' distinguish the aldehyde from the ketone.

Elimination goes the other way: the oxygen leaves as water, and a carbon-carbon double bond is left behind.