A GUIDE TO OXIDATION REACTIONS OF ALCOHOLS Compounds containing the alcohol functional group (–OH) can be oxidised to produce carbonyl compounds. Here’s how it happens.

THE REAGENTS THE APPARATUS TESTING FOR REACTION PRODUCTS

DISTILLATION H CH CH3 3 1˚ alcohol aldehyde Oxidising agents can be represented simply in chemical WATER OUT (excess alcohol used) equations as [O]. An example reaction is shown below. H3C C OH H3C C OH H3C C OH H H CH3 O O WATER IN [O] [O] PRIMARY (1˚) ALCOHOL SECONDARY (2˚) ALCOHOL TERTIARY (3˚) ALCOHOL OH Carbon attached to –OH has one Carbon attached to –OH has two Carbon attached to –OH has three other carbon directly attached. other carbons directly attached. other carbons directly attached. H OH

Alcohols can be oxidised to carbonyl compounds ETHANOL ETHANAL ETHANOIC ACID (containing a C=O bond) using an oxidising agent. ALCOHOL & ACIDIFIED DICHROMATE Note: In step 1, water (H O) is lost as a side product of the reaction Acidified dichromate (VI) salts can be used, though due 2

to their toxicity alternative reagents can also be utilised, DISTILLED PRODUCT 2– + 3+ 3 C2H5OH + 2 Cr2O7 + 16 H 3 CH3COOH + 4 Cr + 11 H2O such as pyridinium chlorochromate (PCC). HEAT DICHROMATE (ORANGE) CHROMIUM ION (GREEN) An aldehyde can be obtained from Na2Cr2O7 K2Cr2O7 primary alcohols using distillation (above). There are two different chemical reactions that can be Otherwise, heating under reflux (below) used to identify the products of oxidation reactions. SODIUM DICHROMATE POTASSIUM DICHROMATE is used to make sure the alcohol is fully oxidised before distilling off the product. FEHLING’S SOLUTION TOLLEN’S REAGENT The type of compound obtained from the reaction HEATING UNDER depends on the starting alcohol (shown below). When an WATER OUT 2+ REFLUX Contains complexed Cu ions. Contains the diamine silver ion, + oxidation reaction is carried out with a dichromate salt, 1˚ alcohol carboxylic acid Aldehydes reduce these ions to red [Ag(NH3)2] . Aldehydes reduce this 2– 3+ (excess oxidising agent used) copper (I) oxide. Ketones don’t react to metallic silver, forming a silver the dichromate ion (Cr2O7 ) is reduced to the Cr ion, giving a colour change from orange to dark green. 2˚ alcohol ketone with Fehling’s solution. mirror on the glass surface.

WATER IN DISTIL REFLUX REACTION PRODUCT WARM ALDEHYDE WARM ALDEHYDE 1˚ ALCOHOL ALDEHYDE CARBOXYLIC ACID CONDENSES AND Blue Red Colourless Silver mirror DROPS BACK IN TO (or grey silver precipitate) PEAR-SHAPED FLASK REFLUX

2˚ ALCOHOL KETONE PRODUCTS ALCOHOL & ACIDIFIED WITH DIFFERENT DICHROMATE WARM KETONE WARM KETONE DISTIL ALCOHOLS Solution remains blue; Solution remains colourless; 3˚ ALCOHOL NO REACTION HEAT no reaction no reaction

C © COMPOUND INTEREST 2016 - WWW.COMPOUNDCHEM.COM | Twitter: @compoundchem | Facebook: www.facebook.com/compoundchem This graphic is shared under a Creative Commons Attribution-NonCommercial-NoDerivatives licence. BY NC ND