Aldehydes and Ketones

Aldehyde functional group

-CHO (C=O)

How are aldehydes produced?

From oxidation of primary alcohols

Can aldehydes oxidise further?

Yes, into carboxylic acids

Ketone functional group

C=O

How are ketones produced?

From oxidation of secondary alcohols

What type of mechanism is used to reduce carbonyls to alcohols?

Nucleophilic addition

Give the reagents required for nucleophilic addition of carbonyls

Reagent sodium borohydride (NaBH4), aqueous solution

Give the conditions required for reduction of carbonyls

Room temperature and pressure

Equation for reduction of aldehydes by nucleophilic addition (ethanal as example)

Ethanal + 2[H] —> ethanol

Where are the first and second [H] in reduction of carbonyls produced from?

H- from NaBH4, H+ from aqueous solution

Equation for reduction of ketones by nucleophilic addition

Propanone + 2[H] —> propan-2-ol

What product is formed when aldehydes undergo nucleophilic addition by HCN?

Hydroxy nitriles

What can hydroxy nitriles form using H+/H2O?

Carboxylic acids

What is the basis for the tests for aldehydes and ketones?

Aldehydes can be further oxidised to carboxylic acids whereas ketones cannot

What is Tollen’s Reagent made up of?

A mixture of aqueous ammonia and silver nitrate [Ag(NH3)2]+

Observations when aldehydes and ketones are tested using Tollen’s Reagent

Aldehydes - a silver mirror forms coating the inside of test tube

Ketones - no visible change

Why does a silver precipitate form when aldehydes are tested with Tollen’s?

Silver atoms are reduced to silver atoms

Test for presence of carboxylic acid

Sodium carbonate - will fizz/produce CO2

What ions does Fehling’s solution contain?

Blue Cu2+ ions

Observations when aldehydes and ketones are tested with Fehling’s solution

Aldehydes - red precipitate (of Cu2O)

Ketones - no reaction

Describe the solubility of carbonyls in water

Smaller carbonyls are soluble in water because they can form hydrogen bonds with water

Give the reagents for the oxidation of aldehydes

Potassium dichromate solution and dilute sulfuric acid

Give the conditions for oxidation of aldehydes

Heat under reflux

Give the equation for the test for aldehydes using Tollen’s reagent (using ethanal)

CH3CHO + 2Ag+ + H2O —> CH3COOH + Ag + 2H+

Give the equation for testing for aldehydes with Fehling’s solution (using ethanal)

CH3CHO + 2Cu2+ + 2H2O —> CH3COOH + Cu2O + 4H+

What conditions are needed to test for aldehydes using Fehling’s and Tollens?

Heat gently

Give the reagent required for addition of hydrogen cyanide to carbonyls to form hydroxynitriles

Potassium cyanide (KCN) and dilute sulfuric acid

Give the reason each reagent is needed for addition of hydrogen cyanide to carbonyls

KCN supplies the CN- ions and the sulfuric acid supplies the H+ needed for second step of mechanism

What are the advantages of using KCN or NaCN over HCN for nucleophilic addition of carbonyls?

There will be a high concentration of CN- ions as these compounds will completely ionise - HCN is a weak acid and will only partially ionise