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Pre-university Advanced A Level Organic Chemistry: Oxidation of alcohols to aldehydes, ketones & carboxylic acids

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Part 4. The chemistry of ALCOHOLS - OXIDATION

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

Part 4.5 The controlled oxidation of alcohols with selected oxidising agents ==> aldehydes, ketones or carboxylic acids depending on structure of alcohol and reaction conditions

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Sub-index for this alcohol chemistry page 4.5

4.5.1 Reminders of the structure of the sub-classes of aliphatic alcohols

4.5.2 Possible oxidation sequences starting with an alcohol and testing for the products

4.5.3 Summary of oxidising agents and products from oxidised alcohols

4.5.4 Oxidation of primary alcohols to aldehydes

4.5.5 Oxidation of primary alcohols to carboxylic acids

4.5.6 Oxidation of secondary alcohols to ketones

4.5.7 Oxidation of tertiary alcohols

4.5.8 Oxidations of alcohols with potassium manganate(VII)

4.5.9 Industrial oxidation of alcohol processes

4.5.1 Reminders of the structure of the sub-classes of aliphatic alcohols

diagram structure of alcohol functional group general structural formula advanced level organic chemistry Aliphatic alcohols

You need to know the structures of the sub-classes of alcohols - primary, secondary and tertiary.

 

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4.5.2 Possible oxidation sequences starting with an alcohol

Most alcohols can be readily oxidised to aldehydes and ketones.

 Aldehydes are easily oxidised further to carboxylic acids.

The reagent is often potassium dichromate(VI) K2Cr2O7 , acidified with diluted sulphuric acid H2SO4(aq) (colour change is orange to green).

However the oxidation products depend on the original structure of the alcohol.

The alcohol functional group -OH in aliphatic alcohols is classified into primary, secondary and tertiary types.

(When the -OH is attached directly to a benzene ring the molecule is called a phenol and their oxidation is not included in this section).

 

Primary aliphatic alcohols R-OH, R is H or alkyl:

When oxidised they form aldehydes and further oxidation gives a relatively stable carboxylic acid e.g.

  1. aldehydes and ketones nomenclature (c) doc b ===> aldehydes and ketones nomenclature (c) doc b ===> aldehydes and ketones nomenclature (c) doc b

    • ethanol ===> ethanal ===> ethanoic acid

  2. aldehydes and ketones nomenclature (c) doc b ===> aldehydes and ketones nomenclature (c) doc b ===> (c) doc b 

    • 2-methylpropan-1-ol ==> 2-methylpropanal ==> 2-methylpropanoic acid

  3. aldehydes and ketones nomenclature (c) doc b ==> aldehydes and ketones nomenclature (c) doc b==>aldehydes and ketones nomenclature (c) doc b

    • butan-1-ol ==> butanal ==> butanoic acid

  4. aldehydes and ketones nomenclature (c) doc b ===> aldehydes and ketones nomenclature (c) doc b ===> aldehydes and ketones nomenclature (c) doc b 

    • pentan-1-ol ==> pentanal ==> pentanoic acid

 

Secondary aliphatic alcohols R-CH(OH)-R', R or R' are both alkyl (can be aryl):

When oxidised they form relatively stable ketones (see NOTE below) e.g.

  1. aldehydes and ketones nomenclature (c) doc b  ===> aldehydes and ketones nomenclature (c) doc b , propan-2-ol ==> propanone

  2. aldehydes and ketones nomenclature (c) doc b ===> aldehydes and ketones nomenclature (c) doc b , butan-2-ol ==> butanone (butan-2-one)

  3. aldehydes and ketones nomenclature (c) doc b  ===>  aldehydes and ketones nomenclature (c) doc b , pentan-3-ol ===> pentan-3-one

 

Tertiary aliphatic alcohols RR'R"C-OH, where R,R' or R" are all alkyl (or aryl):

These are relatively stable to oxidation (see NOTE 1. further down) e.g.

  1. 2-methylpropan-2-ol , aldehydes and ketones nomenclature (c) doc b or aldehydes and ketones nomenclature (c) doc b

  2. 2-methylbutan-2-o l, aldehydes and ketones nomenclature (c) doc b or aldehydes and ketones nomenclature (c) doc b

  3. 3-methylpentan-3-o l, aldehydes and ketones nomenclature (c) doc b or aldehydes and ketones nomenclature (c) doc b

 

Footnotes:

  1. Ketones and carboxylic acids are relatively stable to further oxidation because a strong C-C bond must be broken in the process. Prolonged oxidation with H2SO4(aq)/K2Cr2O7 or using a more powerful oxidising agent, results in the formation of carbon dioxide, water and carboxylic acids of shorter carbon chain length than the original alcohol or ketone.

  2. When a primary alcohol is oxidised to an aldehyde, the oxidation to the carboxylic acid is rapid. If the aldehyde formed first is the desired product, it must be immediately distilled off to prevent further oxidation.

  3. Important examination note:

    • Unless hydrogen gas or oxygen gas are used directly in the redox synthesis reaction [O] and [H] should be used in simplified equations and examples will be quoted in each section and some syllabuses specifically state so.

 

Aldehydes chemical test (R–CHO, R = H, alkyl or aryl) to distinguish from ketones (R2C=O, R = alkyl or aryl) and also reducing sugars.

Note:

(1) Test (b)(i) and (ii) can be used to distinguish aldehydes (reaction) and ketones (no reaction).

Aldehydes are stronger reducing agents than ketones and reduce the metal ion and are oxidised in the process

RCHO + [O] ==> RCOOH

Simple qualitative test for aldehydes and ketones as the products of oxidising alcohols
(a) Add a few drops of the suspected carbonyl compound to Brady's reagent (2,4–dinitrophenylhydrazine solution) (a) A yellow–orange precipitate forms with both types of carbonyl compound. The aldehyde or ketone 2,4–dinitrophenylhydrazone is formed

R2C=O + (NO2)2C6H3NHNH2 ==>

(NO2)2C6H3NHN=CR2 + H2O

(R = H, alkyl or aryl)

This tells you its an aldehyde or ketone, but can't distinguish them, read on below!
(b)(i) warm a few drops of the compound with Tollens' reagent [ammoniacal silver nitrate]

(b)(ii) simmer with Fehling's or Benedict's solution [a blue complex of Cu2+(aq)]

 (b) Only the aldehyde produces (i) A silver mirror on the side of the test tube.

(ii) Blue solution to a brown or brick red  ppt.

 (i) reduction of silver(I) ion to silver metal

RCHO + 2Ag+ + H2O ==> RCOOH + 2Ag + 2H+

(ii) reduction of copper(II) to copper(I) i.e. the blue solution of the Cu2+ complex changes to the brown/brick red colour of insoluble copper(I) oxide Cu2O.

RCHO + 2Cu2+ + 2H2O ==> RCOOH + Cu2O + 4H+

With tests (b)(i)/(ii) no reactions with ketones.

 

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4.5.3 Summary of some oxidising agents and the products from oxidised alcohols

homologous series change on oxidation molecular structure change

Apart from CH3OH

R = alkyl or aryl

 (a) heat with mod conc. H2SO4 and K2Cr2O7(aq) (lab method) (b) reflux with KMnO4/NaOH(aq) (lab method) (c) oxygen + catalyst or thermal decomposition (industrial methods)
primary alcohol ==> aldehyde ==> carboxylic acid RCH2OH ==> RCHO ==> RCOOH YES, can separate intermediate aldehyde, or allow complete oxidation to carboxylic acid YES but only get RCOOH and of little synthetic use e.g. CH3CH2OH ==> CH3CHO (Cu/500oC)
secondary alcohol ==> ketone R2CHOH ==> R2C=O YES YES but of little synthetic use (CH3)2CHOH ==> CH3COCH3 (Cu/500oC)
tertiary alcohol ==> ? R3C–OH fairly stable (if oxidised C–C bonds broken ==> lower RCOOH, CO2, H2O products) not readily oxidised – no synthetic use not readily oxidised – no synthetic use not readily oxidised – no synthetic use

I've included other oxidising agents and industrial processes, but the potassium dichromate(VI)/sulfuric acid reagent is the most important to know for school chemistry and their application is described in the experiments in the next few sections.

Remember for redox reactions in this context.

(i) Oxidation is electron loss and reduction is electron gain.

(ii) Not as fundamental as (i), but oxidation of organic compounds often involves hydrogen loss.

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4.5.4 Oxidation of primary alcohols to aldehydes

It is possible using the same reagent of aqueous sodium/potassium dichromate(VI)–sulphuric acid to oxidise a primary alcohol to either the aldehyde, or the carboxylic acid, depending on the reaction conditions.

In order to selectively isolate the aldehyde this initial oxidation products must be removed from the reaction mixture as quickly as possible, otherwise oxidation proceeds to the carboxylic acid (diagram PD1 below).

By this method you can distil off the aldehyde before it is oxidised to a carboxylic acid.

diagram oxidation of primary alcohols ethanol to aldehydes ethanal potassium dichromate(VI) sulfuric acid reaction conditions equations advanced organic chemistry revision notes doc brown

The 25% sulphuric acid is placed in the flask and gently simmered. The alcohol and aqueous sodium/potassium dichromate(VI) solution is dripped onto the hot acid. Immediately, the orange dichromate(VI) is reduced by the alcohol to the green chromium(III) ion and the alcohol is oxidised to the aldehyde or ketone.

The diagram shows a bunsen burner being used to supply the heat ('my days'), these days its more likely, and safer, to use an electrical heater that the round bottomed flask fits in snugly.

A spot of theory to explain the separation of the aldehyde/ketone from the reaction mixture.

For the same carbon number, the boiling point of the polar aldehyde/ketone (δ+C=Oδ–, but no H bonding) is lower than the original more polar alcohol (δ–O–Hδ+, hydrogen bonding in the alcohol) whose bpt. is higher.  Therefore, as long as the bpt. of the aldehyde/ketone is not too high, in the set–up shown above, the aldehyde rapidly distils over and condenses in the collection tube/flask with some water.

This rapid in situ extraction ensures that most of the aldehyde (or ketone), 9.1(a) is not oxidised further.

If the carboxylic acid of the same carbon number is required from a primary alcohol, the mixture is refluxed using the set–up illustrated in diagram PD5.

 

(i) primary alcohol ==> aldehyde

Cr2O72–(aq) + 3RCH2OH(aq) + 8H+(aq) ===> 3RCHO(aq) + 2Cr3+(aq) + 7H2O(l)

reduction half-equation: Cr2O72–(aq) + 14H+(aq) + 6e– ===> 2Cr3+(aq) + 7H2O(l)

oxidation half-equation: RCH2OH(aq) ===> RCHO(aq) + 2H+(aq) + 2e–(aq) (R = alkyl or aryl)

Examples using simplified symbol equations e.g.

ethanol ==> ethanal: CH3CH2OH + [O] ==> CH3CHO + H2O

propan–1–ol (1–propanol, n–propyl alcohol, n–propanol) ==> propanal

CH3CH2CH2OH + [O] ==> CH3CH2CHO + H2O

Test observations: The distillate will give a yellow precipitate with Brady's reagent, a silver mirror with Tollen's reagent and a red-brown precipitate with either Benedict's or Fehlings reagent - showing the presence of an aldehyde.

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reflux diagram oxidation of primary alcohols ethanol to carboxylic acid ethanoic acid potassium dichromate(VI) sulfuric acid reaction conditions equations advanced organic chemistry revision notes doc brown4.5.5 Oxidation of primary alcohols to carboxylic acids

The technique illustrated on the right (diagram PD5) is called heating under reflux, a method which enables a reaction to be carried out at a higher temperature than room temperature to speed up the reaction AND retain the solvent (reaction medium e.g. water) and any volatile reactant or product (e.g. an alcohol/aldehyde/ketone).

As the mixture boils, the vapours of the solvent or volatile reactant/product are condensed back into the flask in the vertical condenser, so any volatile reactant is used up and no volatile product lost (at least at this stage in a preparation!).

Following on from 4.5.4 where the primary alcohol is oxidised to the aldehyde, the procedure is repeated under reflux conditions with excess of the potassium dichromate(VI)/sulfuric acid mixture.

 The further oxidation is ...

(ii) aldehyde ==> carboxylic acid

Cr2O72–(aq) + 3RCHO(aq) + 8H+(aq) ==> 3RCOOH(aq) + 2Cr3+(aq) + 4H2O(l) (R = alkyl or aryl)

oxidation half–reaction: RCHO(aq) + H2O(l) ==> RCOOH(aq) + 2H+(aq) + 2e–(aq)

Examples using simplified symbol equations:

ethanal ==> ethanoic acid,  CH3CHO + [O] ==> CH3COOH

The overall oxidation starting with the ethanol is: CH3CH2OH  +  2[O]  ===>  CH3COOH  +  H2O

propanal (propionaldehyde) ==> propanoic acid (propionic acid)

CH3CH2CHO + [O] ==> CH3CH2COOH

The overall oxidation starting with propan-1-ol is: CH3CH2CH2OH  +  2[O]  ===>  CH3CH2COOH  +  H2O

butanal (butyraldehyde) ==> butanoic acid (butyric acid)

CH3CH2CH2CHO + [O] ==> CH3CH2CH2COOH

Overall oxidation starting with butan-1-ol : CH3CH2CH2CH2OH  +  2[O]  ===>  CH3CH2CH2COOH  +  H2O

so the redox equations overall for reflux conditions gives

(iii) primary alcohol ==> carboxylic acid

2Cr2O72–(aq) + 3RCH2OH(aq) + 16H+(aq) ==> 3RCOOH(aq) + 4Cr3+(aq) + 11H2O(l)

oxi'n half–reaction: RCH2OH(aq) + H2O(l) ==> RCOOH(aq) + 4H+(aq) + 4e–(aq)  (R = alkyl or aryl)

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4.5.6 Oxidation of secondary alcohols to ketones

diagram oxidation of secondary alcohols propan-2-ol to ketones propanone potassium dichromate(VI) sulfuric acid reaction conditions equations advanced organic chemistry revision notes doc brown

In the case of secondary alcohols you only get the ketone if you distil the product off immediately as shown in diagram PD1 above.

You do NOT reflux the alcohol/K2Cr2O7/H2SO4(aq) for a long time, in case the ketone is oxidised to lower carbon number carboxylic acids, carbon dioxide and water etc. if the carbon chain is broken

However, ketones are quite stable to further oxidation due to the strong carbon–carbon (C–C) bonds that have to be broken.

So, as with the aldehyde, you can distil off the ketone, having a lower boiling point than the parent secondary alcohol.

To be on the safe side it is better to make the ketone under the same restricted reaction conditions used to produce the aldehyde (details above with diagram PD1).

Cr2O72–(aq) + 3R2CHOH(aq) + 8H+(aq) ==> 3R2C=O(aq) + 2Cr3+(aq) + 7H2O(l)

oxidation half–reaction: R2CHOH(aq) ==> R2C=O(aq) + 2H+(aq) + 2e–(aq) (R = alkyl or aryl)

Simplified equation: R2CHOH(aq) +  [O]  ===> R2C=O(aq) +   H2O(l)   (R = alkyl or aryl)

Test observations: The distillate will give a yellow precipitate with Brady's reagent, BUT NO silver mirror with Tollen's reagent and NO red-brown precipitate with either Benedict's or Fehlings reagent - showing the presence of a ketone.

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4.5.7 Oxidation of tertiary alcohols

Tertiary alcohols, R3COH (R = alkyl or aryl), are not readily oxidised because strong carbon–carbon bonds have to be broken.

If a tertiary alcohol is refluxed with a powerful oxidising agent, they will break down to give lower chain carboxylic acids, carbon dioxide and water and therefore of no synthetic use.

 

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reflux diagram oxidation of primary alcohols to carboxylic acid potassium manganate(VII)sodium hydroxide reaction conditions equations advanced organic chemistry revision notes doc brown4.5.8 Oxidations of alcohols with potassium manganate(VII) and industrial processes

School laboratory preparation

Heating a primary alcohol with a aqueous sodium hydroxide and potassium manganate(VII) mixture under reflux (diagram PD2) will give the sodium salt of the carboxylic acid and it is not possible to isolate the intermediate aldehyde.

However, the acid/dichromate(VI) method 9.1(a) under reflux is better, and the carboxylic acid is less liable to further degradative oxidation. The complex reaction can be summarised as:

RCH2OH(aq) + NaOH(aq) + 2[O] ==> RCOO–Na+(aq) + 2H2O(l)

(R = alkyl or aryl)

After removing the excess KMnO4/MnO2 the weak acid is freed from its sodium salt  by adding strong dilute hydrochloric acid.

RCOO–(aq) + H+(aq) ==> RCOOH(aq/s)

Oxidation of secondary alcohols

Ketones are produced by refluxing secondary alcohols with NaOH/KMnO4(aq), but further oxidation is likely to take place because this reagent is a stronger oxidising agent than acidified potassium dichromate(VI).

(CH3)2CHOH + [O] ==> (CH3)2C=O + H2O

(propan–2–ol ==> propanone)

 

4.5.9 Industrial oxidation of alcohols processes

Many chemical feedstocks are oxidised directly with molecular oxygen/transition metal catalyst to produce useful products in industry.

e.g. oxidation of primary alcohols

2CH3OH + O2 ==> 2HCHO + 2H2O (Ag/500oC, methanol ==> methanal)

or  2CH3CH2OH + O2 ==> 2CH3CHO + 2H2O (Ag/500oC, ethanol ==> ethanal)

and the latter reaction can also be achieved via a thermal decomposition using a different catalyst,

e.g. CH3CH2OH ==> CH3CHO + H2 (Cu/500oC),

which is still an oxidation, right carbon (–1) to (+1) and 2 x hydrogen (+1) to (0).

 

e.g. oxidation of secondary alcohols

2(CH3)2CHOH(g) + O2(g) ==> 2(CH3)2C=O(g) + 2H2O(g) (Ag/500oC)

and this reaction can also be achieved via a thermal decomposition using a different catalyst.

(CH3)2CHOH(g) ==> (CH3)2C=O(g) + H2(g) (Cu/500oC)

which is still an oxidation, right carbon (–1) to (+1) and 2 x hydrogen (+1) to (0).

 

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