Aldehydes And Ketones Flashcards
What type of group do aldehydes and ketones contain?
A carbonyl group
What is the difference between the carbonyl groups in aldehydes and ketones
• Carbonyl group is located in different positions
• Aldehydes: carbonyl group is at the end of the carbon chain- al
• Ketones: Carbonyl group is in the middle of the carbon chain- one
What do aldehydes produce when oxidised?
Carboxylic acids
Why can’t ketones be oxidised
They do not have a hydrogen attached to their carbonyl
What happens to the other compound when an aldehyde is oxidised
The other compound is reduced
Give the two oxidising agents to test for aldehydes and ketones and the product of the reaction
- Tollens reagent: colourless solution of silver nitrate dissolved in aqueous ammonia
- Tollens reagent is heated in a test tube with an aldehyde, Ag+ ions are reduced to Ag atoms and a silver mirror forms
- Ketones can’t be oxidised so there’s no colour change
- Fehling’s solution/benedict’s solution: blue solution of copper (II) ions dissolved in sodium hydroxide
- Heated with an aldehyde , the copper (II) ions are reduced to a brick-red precipitate of copper (I) oxide
- With ketones, no precipitate is formed
What product is formed when aldehydes are reduced?
Alcohols
What product is formed when secondary alcohols are oxidised
They form ketones
What reducing agent is used to reduce aldehydes to alcohols
NaBH4 (sodium tetrahydridoborate (III) or sodium borohydride
What type of reaction is the reduction of aldehydes to produce alcohols
• Nucleophilic addition reactions
• H- ions acts as a nucleophile and adds on to the s+ carbon atom
Give the diagram for the reduction of aldehydes to form primary alcohols and ketones to secondary alcohols
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Give a detailed diagram of how aldehydes are reduced
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Where do the H- ions and H+ ions come from?
• H-: from the reducing agent
• H+: from water
How does potassium cyanide react with carbonyls?
By Nucleophilic addition reactions
What is the product of the reaction of potassium cyanide and carbonyls
Hydroxynitriles, molecules with a CN and an OH group
Give the equation for how potassium cyanide dissociates in water
KCN -> K+ + CN-
Describe the reaction between potassium cyanide and a carbonyl group (aldehyde). Give the overall equation for an aldehyde and ketone
- The CN- group attacks the partially positive carbon atom and donates a pair of electrons. Both electrons from the double bond transfer to the oxygen
- H+ ions add to the oxygen to form the hydroxyl group (OH). Acidified KCN is usually used so there’s a source of H+ ions
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Why is potassium cyanide a dangerous chemical to work with and what are some safety measure
• Potassium cyanide is an irritant
• Dangerous if ingested or inhaled
• Reacts with moisture to produce hydrogen cyanide- highly toxic gas
•Wear gloves, safety goggles, lab coat, experiment in a fume cupboard
Solution X reacts with liquid ketones to form a crystalline solid.
This reaction can be used to identify a ketone if the crystalline solid is separated, purified by recrystallisation, and the melting point is determined.
Describe how the crystalline solid separated and purified
- Filter/decant (pouring off the top content of a mixture/draining the bottom content)
- Dissolve in minimum volume of hot solvent
- Cool/leave (to crystallise) and filter (under reduced pressure)
- Wash with cold solvent/ water and dry with method
Why is KCN used instead of HCN in the reaction with propanone
HCN is weak
Draw the nucleophilic addition-elimination mechanism of a compound with a carbonyl group and halogen group that is being attacked by a alcohol to produce an ester and one to produce a halide
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- What are the oxidizing species in Tollens reagent
- What happens to these oxidizing species during the oxidation of carbonyls
- Write a half equation to represent this change
- Write an equation to show the reaction that takes place when Tollens reagent is warmed with pentanal
- Explain if Tollens reagent will produce a silver mirror when added to 3-methylbutanone
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