Chapter 26: Carbonyl Compounds
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What functional group do aldehydes and ketones have?
• C=O, known as the carbonyl functional group
Can aldehydes and ketones be oxidised, and why?
• Only aldehydes can be oxidised • Ketones can’t (oxidation can also be defined as the loss of hydrogen, and they don’t have the H group to be removed)
Give the equation and the conditions for the oxidation of an aldehyde. How is this reaction different to the oxidation of alcohols, and why?
• Aldehyde + [O] -> carboxylic acid • Conditions: reflux, acidified potassium dichromate (VI) • Water is not formed as it was already formed in the step going from an alcohol to an aldehyde
What observation can be made during the oxidation of aldehydes?
• The dichromate ions change colour from orange to green
What influences the reactivity of aldehydes and ketones, and in what ways? Therefore, what mechanism do aldehydes and ketones undergo?
• The C=O double bond • It is polar, so the positive dipole on the carbon atom attracts nucleophiles • It is a double bond, so it undergoes addition reactions • Nucleophilic addition
What does [H] represent?
• Reducing agent
What is another way of describing reduction?
• The gain of hydrogen (formation of more C-H bonds)
What are aldehydes and ketones reduced to form, and how is this done?
• Aldehydes are reduced to form 1° alcohols, and ketones are reduced to form 2° alcohols • Aqueous NaBH4 acts as the reducing agent
Give the equation for the reduction of an aldehyde.
• Aldehyde + 2[H] • Arrow: NaBH4/ H2O • Primary alcohol
Give the equation for the reduction of a ketone.
• Ketone + 2[H] • Arrow: NaBH4/ H2O • Secondary alcohol
What acts as the nucleophile in the reduction of aldehydes and ketones with NaBH4?
• The hydride ion (H- with a lone pair)
Draw the mechanism for the reduction of aldehydes and ketones.
• Check the mechanism
Name another compound carbonyl compounds undergo nucleophilic addition reactions with. What part exactly is the nucleophile?
• HCN (hydrogen cyanide) • The cynanide ion- C≡N, and C has a lone pair and negative charge
Why is the reaction of HCN with aldehydes/ ketones important?
• It increases the length of the carbon chain
Give the equation of the reaction of aldehyes/ ketones with hydrogen cyanide.
• Aldehyde/ ketone + HCN • Arrow: NaCN/ H2SO4 • hydroxynitrile
Explain the reaction conditions for the reaction of aldehydes and ketones with HCN.
• HCN is too dangerous, so NaCN and H2SO4 are used to produce it in the reaction
Draw the mechanism for the reaction of aldehydes and ketones with hydrogen cyanide.
• Check the mechanism
How can carbonyl compounds be tested for?
• Add 2,4-DNP • If an aldehyde or ketone is present, a yellow-orange precipitate will form
After proving the presence of a carbonyl compound, how can you identify the compound?
• From the melting point of its DNP derivative; the compound it forms with 2,4-DNP • To find the melting point, recrystallise • You then compare the melting point value of the crystals against the values in a database
How can you distinguish between aldehydes and ketones?
• Add Tollens’ reagent and warm gently in a water bath • Aldehydes will form a silver mirror precipitate as they are oxidised • Ketones cannot be oxidised, so they don’t react and will stay colourless
Give equations and an explanation for the reaction of aldehydes with Tollens’ reagent.
• Ag+(aq) + e- -> Ag(s) • Aldehyde + [O] -> carboxylic acid • The silver ion acts as the oxidising agent, and is reduced