1.1 10 Oxidation of alcohols and aldehydes
By the end of this spread, you should be able to …
1 Describe the oxidation of primary alcohols to form aldehydes and carboxylic acids. 1 Describe the oxidation of secondary alcohols to form ketones. 1 Describe the oxidation of aldehydes to form carboxylic acids.
Key definition Oxidation of alcohols You will recall from your AS chemistry studies that primary and secondary alcohols can A redox reaction is one in which both reduction and oxidation take place. be oxidised using an oxidising agent. s ! SUITABLE OXIDISING AGENT IS A SOLUTION CONTAINING ACIDIlED DICHROMATE IONS + 2− H /Cr2O7 .
s 4HE OXIDISING MIXTURE CAN BE MADE FROM POTASSIUM DICHROMATE +2Cr2O7, and sulfuric
acid, H2SO. During the reaction, the acidified potassium dichromate changes from orange to green. Remember that tertiary alcohols are not oxidised by acidified dichromate ions.
Primary alcohols Primary alcohols can be oxidised to aldehydes and to carboxylic acids (see AS Chemistry spread 2.2.3).
H H H O H O Oxidation Oxidation H CCOH H CC H CC
H H H H H OH Primary alcohol Aldehyde Carboxylic acid Figure 1 Ethanol oxidised to ethanal, and finally to ethanoic acid
The equation for the oxidation of ethanol to the aldehyde ethanal is shown below. Note that the oxidising agent is shown as [O] – this simplifies the equation.
CH3CH2OH + [O] }m CH3CHO + H2O When preparing aldehydes in the laboratory, you will need to distil the aldehyde from the reaction mixture as it is formed. This prevents the aldehyde from being oxidised further to a carboxylic acid.
Key definition When making the carboxylic acid, the reaction mixture is usually heated under reflux before distilling the product off. Reflux is the continual boiling and condensing of a reaction mixture to CH3CH2OH + 2[O] }m CH3COOH + H2O ensure that the reaction takes place without the contents of the flask boiling Secondary alcohols dry. Secondary alcohols can be oxidised to ketones (see AS Chemistry spread 2.2.3). Unlike aldehydes, ketones are not oxidised further.
H OH H H O H
Oxidation H CCC H H CCC H
H H H H H Secondary alcohol Ketone Figure 2 Propan-2-ol can be oxidised to propanone
The equation for the oxidation of propan-2-ol to the ketone propanone is:
CH3CHOHCH3 + [O] }m CH3COCH3 + H2O
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179 A2 chemistry.U1 M1.indd 22 29/10/08 13:17:09 Module 1 Rings, acids and amines Oxidising aldehydes Oxidation of alcohols and aldehydes You learnt in AS chemistry that aldehydes can be oxidised further to carboxylic acids. Acidified potassium dichromate(VI) is a suitable oxidising agent, and the reaction is normally carried out under reflux. The orange potassium dichromate(VI) changes colour, becoming green.
H CH3 O H CH3 O Oxidised H CCC + [O] H CCC
H H H H H OH AldehydeSignifies use of an Carboxylic acid oxidising agent Figure 3 Oxidation of an aldehyde to a carboxylic acid
STRETCH and CHALLENGE
Acidified potassium dichromate(VI) is an oxidising agent that oxidises primary alcohols, secondary alcohols and aldehydes. During oxidation, dichromate(VI) ions are reduced and the colour changes from orange to green. You can write a fully balanced equation for the reaction by combining the oxidation half-equation for the organic compound with the reduction half equation for the dichromate(IV). The example below shows oxidation of the aldehyde ethanal. + – oxidation CH3CHO(l) + H2O(l) }m CH3COOH(aq) + 2H (aq) + 2e 2– + – 3+ reduction Cr2O7 (aq) + 14H (aq) + 6e }m 2Cr (aq) + 7H2O(l) The two equations can be combined to give an overall equation. To do this, the first equation is multiplied by 3 to make the number of electrons in each equation the same, and then the two equations are added together. The overall equation becomes Figure 4 Potassium dichromate(VI) is reduced in its reactions with aldehydes 3CH CHO(l) + Cr O 2–(aq) + 8H+(aq) }m 3CH COOH(aq) + 2Cr3+(aq) + 4H O(l) 3 2 7 3 2 and changes colour from orange to green You will learn more about writing equations for redox reactions in spread 2.2.9.
‘Waiter, my wine is sour’
When a bottle of wine is opened, oxygen enters the bottle and an oxidation process Questions begins. Over time, the taste and colour of the wine change, and often a brownish 1 (a) Draw the displayed formulae tint can be seen when the glass containing the wine is held against a white background. of a primary alcohol, a This occurs because compounds in the wine called secondary alcohol and a tannins are oxidised when exposed to air. This oxidation tertiary alcohol that would be produces small quantities of hydrogen peroxide, which isomeric with pentan-1-ol. then oxidises ethanol to ethanal. Ethanal produces the (b) What products, if any, would oxidised odour and masks the be formed if the three natural fragrance of a wine. alcohols in part (a) were After a bottle of wine has oxidised? Name each been opened, ethanal can organic product. form within as little as two (c) Name a suitable oxidising hours. White wines and older agent and the reaction reds are particularly conditions for the oxidation. susceptible to oxidation. 2 What precautions would you need to take when oxidising a Figure 5 Wine tasting Figure 6 Wine tasters smell solution of propan-1-ol if you the wine before tasting in were hoping to make propanal? order to sample the bouquet 3 In an oxidation equation, what does the symbol [O] represent?
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