17. Carbonyl Compounds Flashcards
(9 cards)
What are the two reducing agents commonly used to reduce aldehydes and ketones?
NaBH₄ and LiAlH₄
They are both strong enough to reduce both ketones and aldehydes
What are aldehydes and ketones reduced to respectively?
- Primary alcohols and secondary alcohols
- For example, ethanal + [H] forms ethanol
What is the reaction of HCN with aldehydes and ketones?
Excluding the mechanism
- Aqueous HCN, with KCN as a catalyst, is heated with an aldehyde or a ketone
- The CN⁻ acts as a nucleophile and is added across the double bond
- A hydroxynitrile (nitrile with hydroxyl group) is formed
- This is a nucleophilic addition reaction
- The idea is the same for ketones as it is with aldehydes, though aldehydes more commonly form chiral hydroxynitriles
Like the nucleophilic substitution of CN, this reaction adds a carbon
What is the mechanism for the nucleophilic addition reaction of HCN with propanone?
- The C=O bond is polar, giving the carbon a partial positive charge
- The CN⁻ nucleophile attacks the carbon, which forces the pi bond to break heterolytically, sending both atoms to the oxygen and forming a negatively charged intermediate with an O⁻ and the CN both attached to the carbon
- Next, an aqueous proton (H⁺) either from HCN or water bonds to the oxygen anion, neutralising the charge of the compound
- 2-hydroxy-2-methylpropanenitrile forms
Which compounds give a positive test result in the 2,4-dinitrophenylhydrazine (2,4,-DNPH) test?
- Ketones and aldehydes only (it tests for the presence of a -C=O group)
- No other class of compound gives a positive test result; carboxylic acids and esters will not because of their differing levels of reactivity
What happens in the case of a positive 2,4-dinitrophenylhydrazine (2,4-DNPH) test result?
- An aldehyde or ketone is added to 2,4-DNPH
- The carbonyl group on the carbonyl and an NH₂ on the 2,4-dinitrophenylhydrazine react in a condensation reaction, producing water
- The carbon that had the aldehyde group forms a double bond with the nitrogen
- The resultant compound gives a deep-orange precipitate, the melting point of which can be tested to identify the specific aldehyde
How can Fehling’s test be used to test for the presence of carbonyls and differentiate between them and what is the chemistry behind it?
- It only gives a positive result for aldehydes
- Fehling’s solution, which is an alkaline solution with Cu²⁺ ions, is warmed with the compound
- If it is an aldehyde, the aldehyde is oxidised to a carboxylic acid, which is neutralised to form a carboxylate ion that forms a carboxylate salt with a cation like Na⁺
- Cu²⁺ ions are reduced to Cu⁺ ions, which form copper (I) oxide with oxygen from OH ions
- The solution goes from blue (due to the Cu²⁺ ions) to having a red Cu₂O precipitate (this is the positive test result)
The result is only positive for aldehydes as ketones (and carboyxlic acids) cannot be easily oxidised, so they cannot reduce the copper ions to cause the colour change
How is Tollens’ reagent used to test for carbonyls and differentiate between them and what is the chemistry behind it?
- Similar to Fehling’s test, only aldehydes give a positive result when reacted with Tollens’ as ketones cannot be oxidised further
- Tollens’ reagent, an aqueous alkaline silver nitrate solution, is warmed with a compound
- If this compound is an aldehyde, it will form a carboxylic acid then a carboxylate ion, which can form a carboxylate salt with a cation from the alkaline environment
- Ag⁺ ions in the silver nitrate solution are reduced to Ag atoms, which precipitate out and form a layer of silver
- This layer of silver, which is commonly called a silver mirror, is the positive test result
Which compounds will the iodoform test give a positive result for?
The test itself is detailed in the alcohols deck
- Methyl ketones (ketones with a CH₃CO-R group)
- Ethanal
- Secondary alcohols in which the hydroxyl-bearing carbon is adjacent to a methyl group
- Ethanol
- This is because the test requires a methyl group next to a carbonyl carbon (or next to a carbon that can be oxidised to a carbonyl carbon)
- Carboxylic acids are too unreactive and resistant to oxidation for this to work
