Chapter 18 Carbonyl compounds Flashcards
Aldehydes and ketones are
carbonyl compounds containing a C=O group
-They can be prepared from the oxidation of primary and secondary alcohols respectively
Production of Aldehydes & Ketones: Oxidising agents
- The oxidising agents used to prepare aldehydes and ketones from alcohols include acidified potassium dichromate (K2Cr2O7) and acidified potassium manganate (KMnO4)
- Acidified with dilute sulfuric acid, potassium dichromate(VI), K2Cr2O7, is an orange oxidising agent
- When the alcohols are oxidised the orange dichromate ions (Cr2O72-) are reduced to green Cr3+ ions
- Acidified with dilute sulfuric acid, potassium manganate(VII), KMnO4 is a purple oxidising agent
- When the alcohols are oxidised the purple manganate ions (MnO4–) are reduced to colourless Mn2+ ions
The oxidising agents change colour when they oxidise an alcohol and get reduced themselves
Production of Aldehydes & Ketones: Synthesis of aldehydes
- To make an aldehyde, warm primary alcohol is slowly added to the oxidising agent
- The formed aldehyde has a lower boiling point than the alcohol and can therefore be distilled off as soon as it forms
- The aldehyde is then condensed into a liquid and collected
Production of Aldehydes & Ketones: Synthesis of ketones
- To make a ketone, warm secondary alcohol is slowly added to the oxidising agent
- Since the formed ketone cannot be further oxidised it does not need to be distilled off straightaway after it has been formed
Reactions of Aldehydes & Ketones: Reduction of aldehyde & ketones
- Aldehydes and ketones can be reduced by reducing agents such as NaBH4 or LiAlH4
- Aldehydes are reduced to primary alcohols
- Ketones are reduced to secondary alcohols
Reactions of Aldehydes & Ketones: Nucleophilic addition with HCN
- Aldehydes and ketones can undergo nucleophilic addition with hydrogen cyanide, HCN
- The cyanide ion, CN–, acts as a nucleophile and adds across the C-O bond
- Aldehydes and ketones react with HCN, KCN as catalyst and heat to produce hydroxynitriles
- Hydroxynitriles are nitriles containing a hydroxy, -OH, group
Reaction of Hydrogen Cyanide with Aldehydes & Ketones
- The carbonyl group -C=O, in aldehydes and ketones is polarised
- The oxygen atom is more electronegative than carbon drawing electron density towards itself
- This leaves the carbon atom slightly positively charged and the oxygen atom slightly negatively charged
- The carbonyl carbon is therefore susceptible to attack by a nucleophile, such as the cyanide ion
Reaction of Hydrogen Cyanide with Aldehydes & Ketones: Nucleophilic addition
- The nucleophilic addition of hydrogen cyanide to carbonyl compounds is a two-step process
- In step 1 the cyanide ion attacks the carbonyl carbon to form a negatively charged intermediate
- In step 2 the negatively charged oxygen atom in the reactive intermediate quickly reacts with aqueous H+ (either from HCN, water or dilute acid) to form 2-hydroxynitrile
Testing for Carbonyls: what is 2,4-DNPH
2,4-dinitrophenylhydrazine (also known as 2,4-DNPH) is a reagent which detects the presence of carbonyl compounds (compounds with -C=O group)
- The carbonyl group of aldehydes and ketones undergoes a condensation reaction with 2,4-dinitrophenylhydrazine
- A condensation reaction is a reaction in which two molecules join together and a small molecule (such as H2O or HCl) is eliminated
- The product formed when 2,4-DNPH is added to a solution that contains an aldehyde or ketone is a deep-orange precipitate which can be purified by recrystallisation
- The melting point of the formed precipitate can then be measured and compared to literature values to find out which specific aldehyde or ketone had reacted with 2,4-DNPH
The test tube on the left shows a negative 2,4-DNPH test and the tube on the right shows a positive test
Fehling’s solution
Fehling’s solution is an alkaline solution containing copper(II) ions which act as the oxidising agent
- When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Cu2+ ions are reduced to Cu+ ions
- In the alkaline conditions, the carboxylic acid formed will be neutralised to a carboxylate ion (the -COOH will lose a proton to become -COO– )
- The carboxylate ion (-COO–) will form a salt with a positively charged metal ion such as sodium (-COO–Na+)
- The clear blue colour of the solution turns opaque red due to the formation of a copper(I) oxide precipitate
- Ketones cannot be oxidised and therefore give a negative test when warmed with Fehling’s solution
The copper(II) ions in Fehling’s solution are oxidising agents, oxidising the aldehyde to a carboxylic acid and getting reduced themselves to copper(I) ions in the Cu2O precipitate
Tollens’ reagent
- Tollen’s reagent is an aqueous alkaline solution of silver nitrate in excess ammonia solution
- Tollen’s reagent is also called ammoniacal silver nitrate solution
- When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Ag+ ions are reduced to Ag atoms
- In the alkaline conditions, the carboxylic acid will become a carboxylate ion and form a salt
- The Ag atoms form a silver ‘mirror’ on the inside of the tube
- Ketones cannot be oxidised and therefore give a negative test when warmed with Tollens’ reagent
The Ag+ ions in Tollens’ reagent are oxidising agents, oxidising the aldehyde to a carboxylic acid and getting reduced themselves to silver atoms
The reaction of ethanal with iodoform results in the formation of a yellow CHI3 precipitate
