Chapter 26: Carbonyl Compounds Flashcards
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 are unreactive (they don’t have the H group to be removed, and oxidation can also be defined as the loss of hydrogen)
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)
Describe 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
Describe 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, you have to follow the method for purifying an organic solid
- You then compare the melting point value against the values in a data base
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
