Carbonyl Compounds Aldehydes and Ketones

Contents Aldehydes vs. Ketones Testing for Carbonyl Compounds Distinguishing Between Aldehydes and Ketones  Mild Oxidation Oxidation of Aldehydes (Primary Alcohols) Oxidation of Ketones (Secondary Alcohols) Neucleophilic Addition Reaction Definitions Summary

Aldehydes vs. Ketones

Testing for Carbonyl Compounds

Distinguishing Between Aldehydes and Ketones  Mild Oxidation

Oxidation of Aldehydes (Primary Alcohols)

Oxidation of Ketones (Secondary Alcohols)

Neucleophilic Addition Reaction

Definitions

Summary

Aldehydes vs. Ketones The functional group is: >C=O Polar bond The aldehyde has the functional group attached to one carbon Suffix = -al Oxidation of primary alcohols Has a low boiling point Soluble in water The functional group is: >C=O Polar bond The ketone has the functional group attached to two carbon Suffix = -one Oxidation of secondary alcohols Has a low boiling point Soluble in water Back to Contents

The functional group is: >C=O

Polar bond

The aldehyde has the functional group attached to one carbon

Suffix = -al

Oxidation of primary alcohols

Has a low boiling point

Soluble in water

The functional group is:

>C=O

Polar bond

The ketone has the functional group attached to two carbon

Suffix = -one

Oxidation of secondary alcohols

Has a low boiling point

Soluble in water

Testing for Carbonyl Compounds Brady’s reagent or a 2,4-DNPH solution When added to a carbonyl compound it produces a yellow-orange precipitate Works with ketones and aldehydes Does not work with carboxylic acids, esters, esters, acid clorides and amides which contains the >C=O group Find the boiling point of the precipitate and the identity of the carbonyl compound can be found Back to Contents

Brady’s reagent or a 2,4-DNPH solution

When added to a carbonyl compound it produces a yellow-orange precipitate

Works with ketones and aldehydes

Does not work with carboxylic acids, esters, esters, acid clorides and amides which contains the >C=O group

Find the boiling point of the precipitate and the identity of the carbonyl compound can be found

Distinguishing Between Aldehydes and Ketones  Mild Oxidation Tollen's Reagent Involves a ammoniacal silver nitrate solution A aldehyde or ketone is shaken with the ammoniacal silver nitrate solution If there is an aldehyde then the test tube will form a “silver mirror”  silver precipitate This is due to the silver ion being reduced The silver may be formed as a black precipitate This works with all aldehydes Fehling's Reagent Involving copper (II) ions When added to a aldehyde and placed into a water bath, it will form a red precipitate The red precipitate is Cu₂O But it only works with aliphatic aldehydes In both tests the aldehyde is oxidised to a carboxylate salt Tollen’s Reagent is better because it works with all aldehydes. Both don’t work with ketones but vigorous oxidation will work Back to Contents

Tollen's Reagent

Involves a ammoniacal silver nitrate solution

A aldehyde or ketone is shaken with the ammoniacal silver nitrate solution

If there is an aldehyde then the test tube will form a “silver mirror”  silver precipitate

This is due to the silver ion being reduced

The silver may be formed as a black precipitate

This works with all aldehydes

Fehling's Reagent

Involving copper (II) ions

When added to a aldehyde and placed into a water bath, it will form a red precipitate

The red precipitate is Cu₂O

But it only works with aliphatic aldehydes

Oxidation of Aldehydes (Primary Alcohols) Mild oxidation CH₃CH₂OH (l) [O]  CH₃CHO (l) + H₂O(l) This is done by distillation CH₃CHO (l) + [O]  CH₃COOH (l) This is done by refluxing Easily oxidised to acids unlike ketones Back to Contents

Mild oxidation

CH₃CH₂OH (l) [O]  CH₃CHO (l) + H₂O(l)

This is done by distillation

CH₃CHO (l) + [O]  CH₃COOH (l)

This is done by refluxing

Easily oxidised to acids unlike ketones

Oxidation of Ketones (Secondary Alcohols) CH₃CHOHCH₃ (l) + [O]  CH₃OCOCH₃ (l) + H₂O C₂HCOCH₂CH₃ (l) + 3[O]  C₂HCOOH (l) + CH₃COOH (l) Only oxidised under vigorous conditions to acids Back to Contents

CH₃CHOHCH₃ (l) + [O]  CH₃OCOCH₃ (l) + H₂O

C₂HCOCH₂CH₃ (l) + 3[O]  C₂HCOOH (l) + CH₃COOH (l)

Only oxidised under vigorous conditions to acids

Neucleophilic Addition Reaction Occurs with both aldehydes and ketones Reagent: NaBH₄ Conditions: Aqueous/ alcoholic solution Nucleophile: H⁻ CH₃CHO + 2[H]  CH₃CH₂OH CH₃COCH₃ + 2[H]  CH₃CHOHCH Mechanism: Primary alcohol Secondary alcohol Back to Contents

Occurs with both aldehydes and ketones

Reagent: NaBH₄

Conditions: Aqueous/ alcoholic solution

Nucleophile: H⁻

CH₃CHO + 2[H]  CH₃CH₂OH

CH₃COCH₃ + 2[H]  CH₃CHOHCH

Mechanism:

Definitions Redox reaction – both reduction and oxidation take place Reflux – continual boiling and condensing of a reaction mixture to ensure that the reaction takes place without the contents of the flask boiling dry Neucleophile – an atom or group of atoms attracted to an electron-deficient centre, where it donates a pair of electrons to form a new covalent bond Functional group – the part of an organic molecule responsible for its chemical reactions Electronegativity – a measure of the attraction of a bonded atom for the pair of electrons in a covalent bond Stem – the longest carbon chain present in an organic molecule Suffix – the part of the name added after the stem Back to Contents

Redox reaction – both reduction and oxidation take place

Reflux – continual boiling and condensing of a reaction mixture to ensure that the reaction takes place without the contents of the flask boiling dry

Neucleophile – an atom or group of atoms attracted to an electron-deficient centre, where it donates a pair of electrons to form a new covalent bond

Functional group – the part of an organic molecule responsible for its chemical reactions

Electronegativity – a measure of the attraction of a bonded atom for the pair of electrons in a covalent bond

Stem – the longest carbon chain present in an organic molecule

Suffix – the part of the name added after the stem

Summary Back to Contents Aldehyde Reacts with 2,4-DNP(H) Yellow/orange precipitate Reduced to Primary alcohol Oxidised to Carboxylic acid Ketones Reacts with 2,4-DNP(H) Yellow/orange precipitate Reduced to Secondary alcohol Not oxidised