oxidation of alcohols

A small amount of potassium dichromate(VI) solution is acidified with dilute sulphuric acid and a few drops of the aldehyde or ketone are added. If nothing happens in the cold, the mixture is warmed gently for a couple of minutes – for example, in a beaker of hot water. You need to produce enough of the aldehyde (from oxidation of a primary alcohol) or ketone (from a secondary alcohol) to be able to test them. There are various things which aldehydes do which ketones don’t. These include the reactions with Tollens’ reagent, Fehling’s solution and Benedict’s solution, and are covered on a separate page. You would then add a few drops of the alcohol to a test tube containing potassium dichromate(VI) solution acidified with dilute sulphuric acid.

Rate Equations

• You get an aldehyde if you use an excess of the alcohol, and distil off the aldehyde as soon as it forms.
• Again, complexing the copper(II) ions prevents the formation of a precipitate – this time of copper(II) carbonate.
• You need to produce enough of the aldehyde (from oxidation of a primary alcohol) or ketone (from a secondary alcohol) to be able to test them.
• The alcohol is heated under reflux with an excess of the oxidising agent.
• The support material then goes on to say “The equations for their formation are not too difficult.” Does that mean that you have to know them?

You need to use an excess of the oxidising agent and make sure that the aldehyde formed as the half-way product stays in the mixture. The full equation for this reaction is fairly complicated, and you need to understand about electron-half-equations in order to work it out. If you used ethanol as a typical primary alcohol, you would produce the aldehyde ethanal, CH3CHO. At the time of writing, no question involving these equations had been asked. Whether or not you find the equations difficult depends on how much time you have spent learning how to write equations from electron-half-equations. If you look at the equations on the page you have read, and find them scary, ignore them!

The half-equation for the oxidation of the aldehyde obviously varies depending on whether you are doing the reaction under acidic or alkaline conditions. The problem is that what is important in using these reactions as tests is the colour change in the oxidising agent. In this particular reaction, you have to explain, for example, why the solution turns green. Any equation that you write has got to show the production of the chromium(III) ions. Tertiary alcohols don’t have a hydrogen atom attached to that carbon.

Using Fehling’s solution or Benedict’s solution

If you are studying a UK-based syllabus and haven’t got any of these things, follow this link to find out how to get them. You can draw simple structures to show the relationship between the primary alcohol and the aldehyde formed. Potassium manganate(VII) is a powerful enough oxidising agent to break carbon-carbon bonds in ketones, and so you wouldn’t get a reliable result. You will remember that the difference between an aldehyde and a ketone is the presence of a hydrogen atom attached to the carbon-oxygen double bond in the aldehyde. These half-equations are then combined with the half-equations from whatever oxidising agent you are pin up casino using.

Methanal is such a powerful reducing agent that the copper(II) ions may be reduced to metallic copper – often seen as a very nice copper mirror on the tube. Because I am going to be using electron-half-equations quite a lot on the rest of this page, it would definitely be worth following this link if you aren’t happy about them. You will find details of these reactions further down the page. You are expected to know all of the above methods which can be used to distinguish between an aldehyde and a ketone! However, Tollens’ reagent is the most commonly used method, if trying to identify an unknown sample for example. That is particularly important with the ketone phenylethanone, or any other ketone where the carbonyl group is on a side chain attached to a benzene ring.

• A small amount of potassium dichromate(VI) solution is acidified with dilute sulphuric acid and a few drops of the aldehyde or ketone are added.
• When the reaction is complete, the carboxylic acid is distilled off.
• Follow this link if you haven’t come across these compounds before.

Acidified potassium manganate oxidises any carbon-containing side chain in these circumstances to a -COOH group attached to the ring. Support material for teachers says that you should know the identities of the inorganic products of the Fehling’s and Tollens’ test (copper(I) oxide and silver respectively). If this is the first set of questions you have done, please read the introductory page before you start. You will need to use the BACK BUTTON on your browser to come back here afterwards. You get an aldehyde if you use an excess of the alcohol, and distil off the aldehyde as soon as it forms. Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level.

If you get a positive result for any of these reactions, you have an aldehyde. You will find all of this on the page oxidation of aldehydes and ketones. While you are warming the reaction mixture in the hot water bath, you can pass any vapours produced through some Schiff’s reagent. Benedict’s solution contains copper(II) ions complexed with citrate ions in sodium carbonate solution. Again, complexing the copper(II) ions prevents the formation of a precipitate – this time of copper(II) carbonate.

Follow this link if you haven’t come across these compounds before. Fehling’s solution and Benedict’s solution are variants of essentially the same thing. Both contain complexed copper(II) ions in an alkaline solution. The presence of that hydrogen atom makes aldehydes very easy to oxidise. You need to be able to remove those two particular hydrogen atoms in order to set up the carbon-oxygen double bond.

Oxidation of Alcohols

Fehling’s solution contains copper(II) ions complexed with tartrate ions in sodium hydroxide solution. Complexing the copper(II) ions with tartrate ions prevents precipitation of copper(II) hydroxide. Alternatively, you could write separate equations for the two stages of the reaction – the formation of ethanal and then its subsequent oxidation. If you are in the UK A level system (or its equivalent), it is highly likely that your examiners will accept equations involving O. To be sure, consult your syllabus, past papers and mark schemes.

As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

Building equations for the oxidation reactions

Under alkaline conditions, this couldn’t form because it would react with the alkali. Because ketones don’t have that particular hydrogen atom, they are resistant to oxidation. Only very strong oxidising agents like potassium manganate(VII) solution (potassium permanganate solution) oxidise ketones – and they do it in a destructive way, breaking carbon-carbon bonds. Primary alcohols can be oxidised to either aldehydes or carboxylic acids depending on the reaction conditions. In the case of the formation of carboxylic acids, the alcohol is first oxidised to an aldehyde which is then oxidised further to the acid.

Carboxylic Acids & Derivatives

Assuming that you know it has to be one or the other, in each case, a ketone does nothing. It must, however, be used absolutely cold, because ketones react with it very slowly to give the same colour. If you heat it, obviously the change is faster – and potentially confusing. Schiff’s reagent is a fuchsin dye decolourised by passing sulphur dioxide through it. In the presence of even small amounts of an aldehyde, it turns bright magenta.

Author: Stewart Hird

Even if you don’t find them scary, I wouldn’t spend too much time on them. The support material then goes on to say “The equations for their formation are not too difficult.” Does that mean that you have to know them? In each of the following examples, we are assuming that you know that you have either an aldehyde or a ketone. There are lots of other things which could also give positive results.

For example, if you heat the secondary alcohol propan-2-ol with sodium or potassium dichromate(VI) solution acidified with dilute sulphuric acid, you get propanone formed. Tollens’ reagent contains the diamminesilver(I) ion, Ag(NH3)2+. You add a drop of sodium hydroxide solution to give a precipitate of silver(I) oxide, and then add just enough dilute ammonia solution to redissolve the precipitate. To carry out the test, you add a few drops of the aldehyde or ketone to the freshly prepared reagent, and warm gently in a hot water bath for a few minutes.