Aldehydes and Ketones Flashcards
Aldehydes and ketones are carbonyls. What is a carbonyl group?
C=O A carbon with a double bond to an oxygen
If the C=O is on the end of the chain with an H attached, what homologous series is present?
aldehyde
If a carbonyl is present in the middle of a carbon chain and has alkyl groups to both sides, which homologous series is present?
Ketone
What are the strongest kind of intermolecular forces present in aldehydes and ketones?
permanent dipole interactions
Why is the carbonyl group polarised?
The C=O bond is polarised because oxygen is more electronegative than carbon
Why does the carbonyl group in aldehydes and ketone react?
The positive carbon atom attracts nucleophiles.
What is a nucleophile?
Electron pair donor
Which oxidising agent is used to oxidise aldehydes and ketones?
acidified potassium dichromate solution OR (H+(aq) / K2Cr2O7(aq))
What can primary alcohols be oxidised to?
Primary alcohol → aldehydes → carboxylic acid
What are the conditions to oxidise a primary alcohol to an aldehyde?
excess alcohol; acidified potassium dichromate; distillation of product
What do you see when acidified potassium dichromate succesfully oxidises a carbonyl compound?
Colour change of orange to green
What are the conditions to oxidise an alcohol or aldehyde to a carboxylic acid?
heat under reflux; acidified potassium dichromate
Which acid can be used to acidify potassium dichromate?
dilute suphuric acid
What product is formed when you oxidise a secondary alcohol with acidified potassium dichromate?
ketone
What product is formed when you oxidise a tertiary alcohol with acidified potassium dichromate?
no reaction
What is Tollens Reagent?
Tollen’s reagent is formed by mixing aqueous ammonia and silver nitrate.
What is the active part of Tollens’ Reagent?
The active substance is the complex ion of [Ag(NH3)2]+ .
What do you see if Tollens Reagent acts as an oxidising agent?
a silver precipitate is formed
Give an equation for Tollens reagent acting as an oxidising agent
Ag+(aq) + e- → Ag (s)
Do aldehydes and/or ketones react with Tollens?
Just aldehydes (they are oxidised by the Tollens to form carboxylic acids - ketones can’t be oxidised)
What ions are present in Fehlings Reagent?
blue Cu 2+ ions.
Which substances are oxidised by Fehlings?
Just aldehydes (they are oxidised to form carboxylic acids - ketones can’t be oxidised)
What do you observe when Fehlings oxidises an aldehyde?
blue solution forms a red/brown precipitate of Cu+ oxide (Cu2O)
Name a reducing agent that can reduce an aldehyde to a primary alcohol?
NaBH4 In aqueous ethanol. Room temp and pressure.
Name a reducing agent that can reduce a carboxylic acid to a primary alcohol?
NaBH4 In aqueous ethanol. Room temp and pressure.
Name a reducing agent that can reduce a ketone to a secondary alcohol?
NaBH4 In aqueous ethanol. Room temp and pressure.
Give a balanced equation for the reduction of propanal. What are the conditions?
Conditions: NaBH4 in aqueous ethanol
Give a balanced equation for the reduction of propanone. What are the conditions?
Conditions: NaBH4 in aqueous ethanol.
Give the mechanism for the reduction of propanone.
Nae the mechanism
Nucleophilic substitution.
What are the conditions for catalytic hydrogenation?
Reagent: hydrogen (H2(g)) and nickel catalyst (Ni)
Conditions: high pressure
Give the mechanism for the addition of cyanide ions to carbonyls
Name the mechanism for the addition of cyanide ions to carbonyls
Nucleophilic substitution
What are the conditions for nucleophilic additions of CN- to carbonyls?
Reagent: sodium or potassium cyanide (NaCN or KCN) and dilute sulfuric acid.
Conditions: Room temperature and pressure
Why do chemists use H+/KCN and not HCN to make hydroxynitriles?
HCN is a toxic gas that is difficult to contain. The KCN is still toxic but it is a solution.
Why does nucleophilic addition of HCN to unsymmetrical carbonyls results in the formation of a racemate?
The carbonyl bond is planar.
it can be attacked from above or below by CN- with an equal chance.
This means that both enantiomers are formed in equal amounts.
