6.1.2 - carbonyl compounds

• The carbonyl group is C=O
• The general formula of an aldehyde is RCHO
• The general formula for a ketone is RCOR’
• To name aldehydes you add the -al suffix
• To name ketones add the -one suffix and add the position number for longer carbon chains
• The carbonyl group has permanent dipole dipole due to the polar C=O bond. O is §- and C is §+
• Carbonyls are soluble in water as they form hydrogen bonds between water and carbonyls. As chain length increases, solubility decreases.
• The C=O bond in carbonyls is involved in reactions due to the polarity of the bond because of the difference in electronegativity between C and O
• C=O is the strongest bond in a carbonyl
• this is the mechanism for nucleophilic addition

  
• The addition of HCN (cyanide) to carbonyls is important as it increases the length of the carbon chain so more useful molecules can be made.
• When HCN is added to carbonyl compounds it forms hydroxynitriles

  they have both OH and CN groups
• fehlings solution is copper complex ions, it is blue
• When an aldehyde is added to fehlings solution the copper ions are reduced to Cu+ ions and there is a colour change to a brick red precipitate.
• When a ketone is added to fehlings solution there is no visible change so solution stays blue.
• To test for a carbonyl compound you use 2,4-DNP (Brady’s reagent)

  if a carbonyl compound is present an orange precipitate is formed.
• Tollens reagent is silver complex ions in a colourless solution
• When an aldehyde is added to tollens reagent Ag+ is reduced to Ag (s) and a silver mirror forms
• When a ketone is added to tollens reagent there is no visible change
• An oxidising agent for alcohols and aldehydes is acidified potassium dichromate which changes colour froom orange to green
• A reducing agent for aldehydes and ketones is NaBH4 which releases a H- ion
• The reduction of an aldehyde is nucleophilic addition. This is the general mechanism.

  
• The reduction of pentan-2-one is:

  CH3COCH2CH2CH3 + 2[H] ——> CH3CH(OH)CH2CH2CH3
• To convert an aldehyde into a carboxylic acid you oxidise the aldehyde using acidified potassium dichromate
• This is the general mechanism for nucleophilic addition using NaBH4

  
• tollens reagent is made when sodium hydroxide is added to silver nitrate until a brown precipitate is formed. then dilute ammonia is added dropwise until the brown precipitate redissolves.