Aldehydes and Ketones Flashcards by Saleh Taha

What is a carbonyl group

C=O group

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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

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What is the suffix for ketones

-one

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What is the suffix for aldehydes

-al

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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

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What are ketones oxidised into

Ketones cant be oxidised easily

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Write an equation showing the oxidation of propanal using [O] as oxidising agent

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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

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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

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Why do we not heat the test tube with a Bunsen burner when testing for aldehydes and ketones

Aldehydes and ketones are flammable

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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

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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

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Write the full equation for the oxidation of ethanal (using dichromate) into ethanoic acid

3CH3CHO + Cr2O72- + 8H+ —> 3 CH3COOH + 4H2O + 2Cr3+

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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.

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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.

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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)

Outline general 3 stages in how we can measure the melting point of a sample to test its purity

Put sample in a MP capillary tube Place in an oil bath or melting point apparatus Heat slowly to establish melting point range Broad range of melting point indicates presence of impurities OR melting point agrees with/close to data book value

Outline stages in how we can measure the melting point of a sample to test its purity using Thiele tube

Put sample in a Durham tube and place a MP capillary tube inside with closed end at the top and attach to a thermometer using rubber band Place oil in Thiele tube so that its above the triangle Submerge in the sample with thermometer in oil in the Thiele Tube Ensure bung is on tight Heat slowly by applying heat to the arm of the Thiele tube (RHS) to establish melting point range Broad range of melting point indicates presence of impurities OR melting point agrees with/close to data book value https://youtu.be/74vCYTIHo5Q?si=v5tQR330uADpl1K_