Aldehydes and Ketones Flashcards
What is a carbonyl group
C=O group
In terms of the position of the carbonyl group, what is the difference between aldehydes and ketones
Aldehydes: Carbonyl group on an end of carbon of the chain chain
Ketones: Carbonyl group on an inner carbon of the chain
What is the suffix for ketones
-one
What is the suffix for aldehydes
-al
What are aldehydes oxidised into and what are the conditions and reagent for this reaction
Carboxylic acids
Reagent: potassium dichromate (VI) solution and
dilute sulphuric acid.
Conditions: heat under reflux
What are ketones oxidised into
Ketones cant be oxidised easily
Write an equation showing the oxidation of propanal using [O] as oxidising agent
How do we make tollens
Add a few drops of NaOH to a solution of silver nitrate (forms a pale brown precipitate)
Add a few drops of dilute ammonia until the precipitate dissolves
This is tollens reagent
What are the practical steps in using tollens to distinguish between an aldehyde and ketone
- Add aldehyde/ketone to tollens in a test tube
- Place in a hot water bath
If an aldehyde is present, silver mirror appears
If ketone is present no silver mirror appears
Why do we not heat the test tube with a Bunsen burner when testing for aldehydes and ketones
Aldehydes and ketones are flammable
Write an equation for the reaction between aqeuous tollens and ethanal
State what occurs
CH3CHO + 2Ag+ + H2O —> CH3COOH + 2Ag + 2H+
Aldehydes only are oxidised by
Tollen’s reagent into a carboxylic acid and
the silver(I) ions are reduced to silver atoms
What is the observation when aldehydes are oxidised into a carboxylic acid using dichromate
Orange dichromate ion (Cr2O72-) is reduced to the green Cr3+ ion
Write the full equation for the oxidation of ethanal (using dichromate) into ethanoic acid
3CH3CHO + Cr2O72- + 8H+ —> 3 CH3COOH + 4H2O + 2Cr3+
What are the conditions and positive test result for distinguishing between aldehydes and ketones using Fehling’s
Conditions: heat gently
Observation: Aldehydes :Blue Cu 2+ ions in solution
change to a red precipitate of Cu2O. Ketones do
not react.
Write an equation for the reaction between Fehling’s solution and ethanal
State what occurs
CH3CHO + 2Cu2+ + 2H2O —> CH3COOH + Cu2O + 4H+
Aldehydes :Blue Cu 2+ ions in solution
change to a red precipitate of Cu2O. Ketones do
not react.
What do we use as a reducing agent when reducing aldehydes and ketones
NaBH4
Represented as [H]
Source of hydride (H-) ions
What are aldehydes reduced to
Primary alcohols
How many [H]’s are required for the reduction aldehydes and ketones and where do these [H]’s come from
2 are required
H- from reducing agent
H+ from water
What are ketones reduced to
Secondary alcohols
What is the name of the mechanism by which aldehydes and ketones are reduced using NaBH4
Nucleophilic addition
Outline the general mechanism for the reduction of aldehydes and ketones, by representing R as either H for an aldehyde or an alkyl group for a ketone
State the name of this mechanism
Nucleophilic addition
What is an alkyl group
e.g. CH3
What are carbonyl compounds reduced to by CN- ions
Hydroxynitriles
What is the name of the mechanism by which carbonyl compounds are reduced to hydroxynitriles
Nucleophilic addition
What is a hydroxynitrile
A molecule which contains an OH and CN group
Outline the general mechanism by which CN- ions reduce carbonyl compounds to a hydroxynitrile
What are the 2 methods of reducing a carbonyl compound to a hydroxynitrile. State the reagent and conditions for both
Method 1:
Reagent: HCN
Conditions: None
Method 2:
Reagent KCN
Conditions: Dilute H2SO4(aq) (Source of H+)
What is the general equation for the reduction of carbonyl compounds to hydroxynitriles using CN- ions
Evaluate the different safety risks of using KCN or HCN in the reduction of carbonyl compounds to form hydroxynitriles
Both release toxic CN-
But KCN is a solid so is only toxic if ingested
HCN is a gas so easier to inhale especially is there’s a leak
Therefore for safety reasons KCN and dilute H2SO4 is often used over HCN
Evaluate the different rates of reaction when using KCN or HCN in the reduction of carbonyl compounds to form hydroxynitriles
HCN: SLOW RATE. HCN is a weak acid and only partially dissociates and so lower [CN-]
KCN: FASTER RATE. KCN completely dissociates therefore a higher [CN-]
What is a hazard of using KCN
FASTER RATE. KCN completely dissociates therefore a higher [CN-]
Toxic if ingested
When do we form a mix of enantiomers in the nucleophilic addition reactions of KCN, followed by dilute acid
If we use an unsymmetric ketone or aldehyde (apart from methanal)
