Part 5. The chemistry of ALDEHYDES and KETONES

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 5.3 Methods of preparing aldehydes and ketones

INDEX of ALDEHYDES and KETONES revision notes

All Advanced A Level Organic Chemistry Notes

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Aliphatic alcohols

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

Structure reminder

and the relative structures of aldehydes and ketones.

Note: An aryl group means a benzene ring C₆H₅- or a substituted benzene ring e.g. CH₃C₆H₅-.

The oxidation of alcohols

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

When oxidised they form aldehydes e.g.

1. ==> , ethanol ==> ethanal

2. ==> , 2-methylpropan-1-ol ==> 2-methylpropanal

3.  ==> ,butan-1-ol ==> butanal

4. ==> , pentan-1-ol ==> pentanal

 

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

When oxidised they form relatively stable ketones e.g.

1.  ==> , propan-2-ol ==> propanone

2.  ==> , butan-2-ol ==> butanone (butan-2-one)

3.  ==> , pentan-3-ol ==> pentan-3-one

Oxidation of aliphatic primary alcohols to aliphatic aldehydes

It is possible using the same reagent of aqueous sodium/potassium dichromate(VI)–sulphuric acid to oxidise a primary alcohol to an aldehyde.

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).

The 25% sulphuric acid is placed in the flask and gently simmered (just heating up to ~60^oC with an electric heater is ok.

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.

After the initial heating, the exothermic reaction can maintain the required temperature.

Note the oxidation state colour change of the orange dichromate(VI) ion to the green chromium(III) ion.

Ideally the temperature should be higher than the boiling point of the aldehyde and lower than the boiling point of the alcohol - so the product, not the reactant, will preferentially distil over into the condenser and collection flask AND before it can be further oxidised to the corresponding carboxylic acid.

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=O^δ–, but no H bonding) is lower than the original more polar alcohol (^δ–O–H^δ+, 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), is not oxidised further.

For ethanal, the boiling point is so low, you need to cool the collection flask with ice or very cold water.

primary alcohol ==> aldehyde (e.g. preparation of ethanal and propanal)

Cr₂O₇^2–_(aq) + 3RCH₂OH_(aq) + 8H⁺_(aq) ===> 3RCHO_(aq) + 2Cr³⁺_(aq) + 7H₂O_(l)

reduction half reaction: Cr₂O₇^2–_(aq) + 14H⁺_(aq) + 6e^– ===> 2Cr³⁺_(aq) + 7H₂O_(l)

oxidation half reaction: 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 ==> propanal:  CH₃CH₂CH₂OH  +  [O]  ===>  CH₃CH₂CHO  +  H₂O

The oxidation of secondary alcohols to ketones

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 can reflux first the alcohol/K₂Cr₂O₇/H₂SO_4(aq) for a short time (see diagrams at the end), but not too long, in case the ketone is oxidised to a 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.

(Its the same intermolecular forces as for the aldehyde)

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).

primary alcohol ==> aldehyde (e.g. preparation of propanone and butanone)

reduction half reaction: Cr₂O₇^2–_(aq) + 14H⁺_(aq) + 6e^– ===> 2Cr³⁺_(aq) + 7H₂O_(l)

Cr₂O₇^2–_(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)

Examples using simplified symbol equations e.g.

propan-2-ol ==> propanone:  CH₃CH(OH)CH₃  +  [O]  ===>  CH₃COCH₃  +  H₂O

butan–2–ol ==> butanone:  CH₃CH(OH)CH₂CH₃  +  [O]  ===>  CH₃COCH₂CH₃  +  H₂O

 

A refluxing system (left diagram) can be used if the reaction is slow.

After completion, fit the still head and thermometer, rearrange the condenser and fractionally distil off the ketone (right diagram).

The aldehyde can also be fractionally distilled to get a more purer sample.

For ethanal, the boiling point is so low, you need to cool the collection flask with ice or very cold water.

 

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