An aldehyde is a carbonyl compound in which the C=O group is at the end of the carbon chain, formed by oxidising a primary alcohol.

What colour change occurs when acidified KMnO 4 oxidises an alcohol?

The solution changes from purple to colourless . The MnO 4 - ions are reduced to colourless Mn 2+ ions.

True or False? To produce an aldehyde, the product must be distilled off as it forms to prevent further oxidation to a carboxylic acid.

True. Aldehydes have a lower boiling point than the alcohol they came from. Distilling the aldehyde off immediately prevents it from being further oxidised to a carboxylic acid under reflux.

Ketones are produced by oxidising .......... alcohols. Unlike aldehydes, they .......... undergo further oxidation.

Ketones are produced by oxidising secondary alcohols. Unlike aldehydes, they cannot undergo further oxidation.

What are the two common oxidising agents used to prepare aldehydes and ketones from alcohols?

Acidified potassium dichromate(VI) (K 2 Cr 2 O 7 ) and acidified potassium manganate(VII) (KMnO 4 ), both acidified with dilute sulfuric acid.

True or False? When K 2 Cr 2 O 7 oxidises a secondary alcohol, the dichromate ions change from orange to green.

True. The orange Cr 2 O 7 2- ions are reduced to green Cr 3+ ions as they oxidise the alcohol.

A ketone is a carbonyl compound in which the C=O group is within the carbon chain (not at the end), formed by oxidising a secondary alcohol.

To synthesise an aldehyde, warm primary alcohol is slowly added to the .......... and the aldehyde is .......... off immediately.

To synthesise an aldehyde, warm primary alcohol is slowly added to the oxidising agent and the aldehyde is distilled off immediately.

Why does a ketone not need to be distilled off immediately after it forms during oxidation of a secondary alcohol?

Ketones cannot be further oxidised , so there is no risk of the product being converted to another compound. Immediate distillation is therefore unnecessary.

Define hydroxynitrile .

A hydroxynitrile is a compound containing both a hydroxy (-OH) group and a nitrile (-CN) group, formed by nucleophilic addition of HCN to an aldehyde or ketone.

What type of alcohol is formed when an aldehyde is reduced, and what type when a ketone is reduced?

Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols . Both NaBH 4 and LiAlH 4 can act as reducing agents.

True or False? In the nucleophilic addition of HCN to a carbonyl compound, the CN - ion acts as the nucleophile and attacks the carbonyl carbon.

True. The cyanide ion (CN - ) has a lone pair on its carbon atom and attacks the slightly positive carbonyl carbon, forming a negatively charged intermediate that is then protonated to give the hydroxynitrile.

Aldehydes and ketones react with HCN using .......... as a catalyst to produce .......... .

Aldehydes and ketones react with HCN using KCN as a catalyst to produce hydroxynitriles .

What effect does nucleophilic addition of HCN have on the length of the carbon chain of an aldehyde or ketone?

The carbon chain increases by one carbon atom , as the CN - ion adds a carbon to the carbonyl compound.

True or False? NaBH 4 and LiAlH 4 are both suitable reducing agents for converting aldehydes and ketones to alcohols.

True. Both NaBH 4 and LiAlH 4 can reduce aldehydes and ketones to alcohols. NaBH 4 is a milder reducing agent than LiAlH 4 .

The nucleophilic addition of HCN to an aldehyde always produces a .......... hydroxynitrile compound because a new .......... centre is created.

The nucleophilic addition of HCN to an aldehyde always produces a chiral hydroxynitrile compound because a new chiral centre is created.

Define nucleophilic addition .

A nucleophilic addition is a reaction in which a nucleophile attacks and adds across a polar C=O double bond, forming a new single bond and breaking the pi bond.

What is the role of the cyanide ion in the nucleophilic addition of HCN to ethanal, and where does the negative charge reside on CN - ?

CN - acts as a nucleophile , donating its lone pair to the carbonyl carbon. The negative charge on the cyanide ion resides on the carbon atom , not the nitrogen atom.

Define carbonyl group .

A carbonyl group is a C=O double bond found in aldehydes and ketones. It is polarised because oxygen is more electronegative than carbon, leaving the carbon slightly positively charged.

Why is the carbonyl carbon of an aldehyde or ketone susceptible to nucleophilic attack?

The oxygen atom is more electronegative than carbon, drawing electron density towards itself. This leaves the carbonyl carbon with a slight positive charge (δ+) , making it susceptible to attack by a nucleophile .

True or False? The negative charge on the cyanide ion (CN - ) is located on the nitrogen atom.

False. The negative charge on CN - is on the carbon atom , not the nitrogen atom. It is the carbon end that acts as the nucleophile and attacks the carbonyl carbon.

In step 1 of the HCN addition mechanism, the .......... ion attacks the carbonyl carbon, forming a .......... charged intermediate.

In step 1 of the HCN addition mechanism, the cyanide ion attacks the carbonyl carbon, forming a negatively charged intermediate.

What happens in step 2 of the nucleophilic addition of HCN to an aldehyde?

The negatively charged oxygen in the intermediate reacts with an aqueous H + ion (from HCN, water or dilute acid) to form the 2-hydroxynitrile product.

True or False? The addition of HCN to a carbonyl compound is a two-step mechanism involving nucleophilic attack followed by protonation.

True. Step 1 is nucleophilic attack of CN - on the carbonyl carbon to form a negatively charged intermediate. Step 2 is protonation of the oxygen by H + to give the hydroxynitrile.

The carbonyl carbon is susceptible to nucleophilic attack because the oxygen draws .......... density towards itself, giving the carbon a slight .......... charge.

The carbonyl carbon is susceptible to nucleophilic attack because the oxygen draws electron density towards itself, giving the carbon a slight positive charge.

Define 2-hydroxynitrile .

A 2-hydroxynitrile is the product of nucleophilic addition of HCN to an aldehyde or ketone, containing an -OH group and a -CN group on the same carbon atom.

What source of H + ions can protonate the negatively charged intermediate in step 2 of the HCN addition mechanism?

H + ions can come from HCN itself, from water or from dilute acid present in solution.

A 2,4-DNPH (2,4-dinitrophenylhydrazine) is a reagent used to detect carbonyl compounds. It reacts with aldehydes and ketones in a condensation reaction to produce a deep-orange precipitate.

How can the identity of a specific aldehyde or ketone be confirmed after a positive 2,4-DNPH test?

The deep-orange precipitate is purified by recrystallisation and its melting point is measured. The value is compared with literature values to identify the specific carbonyl compound.

True or False? Carboxylic acids and esters give a positive result with 2,4-DNPH.

False. Only aldehydes and ketones give a positive result. Carboxylic acids and esters do not react with 2,4-DNPH to form a precipitate.

Tollens' reagent is an aqueous alkaline solution of .......... in excess .......... solution.

Tollens' reagent is an aqueous alkaline solution of silver nitrate in excess ammonia solution.

What observation is made when an aldehyde is warmed with Tollens' reagent, and what is reduced?

A silver mirror forms on the inside of the tube. The Ag + ions are reduced to silver atoms (Ag) as they oxidise the aldehyde to a carboxylate ion.

True or False? Ketones give a positive result with both Fehling's solution and Tollens' reagent.

False. Ketones cannot be oxidised and therefore give a negative result with both Fehling's solution and Tollens' reagent. Only aldehydes give a positive result.

Define Fehling's solution .

A Fehling's solution is an alkaline solution containing Cu 2+ ions used to test for aldehydes. A positive result produces a red precipitate of copper(I) oxide as the clear blue solution turns opaque.

When an aldehyde is warmed with Fehling's solution, the .......... Cu 2+ solution turns .......... due to the formation of copper(I) oxide.

When an aldehyde is warmed with Fehling's solution, the clear blue Cu 2+ solution turns opaque red due to the formation of copper(I) oxide.

What type of reaction occurs between 2,4-DNPH and a carbonyl compound, and what small molecule is eliminated?

A condensation reaction occurs. A water molecule is eliminated as the 2,4-DNPH and carbonyl compound join together to form the deep-orange precipitate.

Define tri-iodomethane (iodoform) .

A tri-iodomethane (iodoform, CHI 3 ) is a yellow precipitate formed in the iodoform reaction when methyl ketones or ethanal are heated with iodine in alkaline solution.

Which structural feature must a compound have to give a positive iodoform test?

The compound must contain a CH 3 CO- group (a methyl ketone). Ethanal also contains this group and gives a positive test.

True or False? Ethanal gives a positive iodoform test because it contains a CH 3 CO- group.

True. Ethanal (CH 3 CHO) contains the CH 3 CO- group, so it undergoes the iodoform reaction to produce a yellow CHI 3 precipitate.

In the iodoform reaction, the reagent is heated with an .......... solution of iodine. The yellow precipitate formed is .......... .

In the iodoform reaction, the reagent is heated with an alkaline solution of iodine. The yellow precipitate formed is CHI 3 .

Describe the two steps of the iodoform reaction.

Step 1 (halogenation): All three H atoms of the -CH 3 group are replaced by iodine atoms to form a -CI 3 group. Step 2 (hydrolysis): The intermediate is hydrolysed by alkaline solution to form a sodium carboxylate salt (RCO 2 - Na + ) and the yellow CHI 3 precipitate.

True or False? Propanone gives a positive iodoform test because it is a methyl ketone.

True. Propanone (CH 3 COCH 3 ) contains a CH 3 CO- group, so it undergoes the iodoform reaction with alkaline iodine to produce a yellow CHI 3 precipitate.

In the halogenation step of the iodoform reaction, .......... iodine atoms replace the hydrogen atoms of the .......... group in the methyl ketone.

In the halogenation step of the iodoform reaction, three iodine atoms replace the hydrogen atoms of the -CH 3 group in the methyl ketone.

Define methyl ketone .

A methyl ketone is a ketone containing a CH 3 CO- group, which reacts with iodine in alkaline solution to form the yellow precipitate CHI 3 (iodoform).

What are the two products formed in the hydrolysis step of the iodoform reaction?

The two products are a sodium carboxylate salt (RCO 2 - Na + ) and a yellow precipitate of CHI 3 (tri-iodomethane / iodoform).

A LevelChemistryCambridge (CIE)Flashcards17. Carbonyl CompoundsAldehydes & Ketones

Aldehydes & Ketones (Cambridge (CIE) A Level Chemistry): Flashcards

Exam code: 9701

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Define aldehyde.

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Cards in this collection (45)

Define aldehyde.
An aldehyde is a carbonyl compound in which the C=O group is at the end of the carbon chain, formed by oxidising a primary alcohol.
What colour change occurs when acidified KMnO₄ oxidises an alcohol?
The solution changes from purple to colourless. The MnO₄^- ions are reduced to colourless Mn²⁺ ions.
True or False?
To produce an aldehyde, the product must be distilled off as it forms to prevent further oxidation to a carboxylic acid.
True.
Aldehydes have a lower boiling point than the alcohol they came from. Distilling the aldehyde off immediately prevents it from being further oxidised to a carboxylic acid under reflux.
Ketones are produced by oxidising .......... alcohols. Unlike aldehydes, they .......... undergo further oxidation.
Ketones are produced by oxidising secondary alcohols. Unlike aldehydes, they cannot undergo further oxidation.
What are the two common oxidising agents used to prepare aldehydes and ketones from alcohols?
Acidified potassium dichromate(VI) (K₂Cr₂O₇) and acidified potassium manganate(VII) (KMnO₄), both acidified with dilute sulfuric acid.
True or False?
When K₂Cr₂O₇ oxidises a secondary alcohol, the dichromate ions change from orange to green.
True.
The orange Cr₂O₇^2- ions are reduced to green Cr³⁺ ions as they oxidise the alcohol.
Define ketone.
A ketone is a carbonyl compound in which the C=O group is within the carbon chain (not at the end), formed by oxidising a secondary alcohol.
To synthesise an aldehyde, warm primary alcohol is slowly added to the .......... and the aldehyde is .......... off immediately.
To synthesise an aldehyde, warm primary alcohol is slowly added to the oxidising agent and the aldehyde is distilled off immediately.
Why does a ketone not need to be distilled off immediately after it forms during oxidation of a secondary alcohol?
Ketones cannot be further oxidised, so there is no risk of the product being converted to another compound. Immediate distillation is therefore unnecessary.
Define hydroxynitrile.
A hydroxynitrile is a compound containing both a hydroxy (-OH) group and a nitrile (-CN) group, formed by nucleophilic addition of HCN to an aldehyde or ketone.
What type of alcohol is formed when an aldehyde is reduced, and what type when a ketone is reduced?
Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols. Both NaBH₄ and LiAlH₄ can act as reducing agents.
True or False?
In the nucleophilic addition of HCN to a carbonyl compound, the CN^- ion acts as the nucleophile and attacks the carbonyl carbon.
True.
The cyanide ion (CN^-) has a lone pair on its carbon atom and attacks the slightly positive carbonyl carbon, forming a negatively charged intermediate that is then protonated to give the hydroxynitrile.
Aldehydes and ketones react with HCN using .......... as a catalyst to produce .......... .
Aldehydes and ketones react with HCN using KCN as a catalyst to produce hydroxynitriles.
What effect does nucleophilic addition of HCN have on the length of the carbon chain of an aldehyde or ketone?
The carbon chain increases by one carbon atom, as the CN^- ion adds a carbon to the carbonyl compound.
True or False?
NaBH₄ and LiAlH₄ are both suitable reducing agents for converting aldehydes and ketones to alcohols.
True.
Both NaBH₄ and LiAlH₄ can reduce aldehydes and ketones to alcohols. NaBH₄ is a milder reducing agent than LiAlH₄.
The nucleophilic addition of HCN to an aldehyde always produces a .......... hydroxynitrile compound because a new .......... centre is created.
The nucleophilic addition of HCN to an aldehyde always produces a chiral hydroxynitrile compound because a new chiral centre is created.
Define nucleophilic addition.
A nucleophilic addition is a reaction in which a nucleophile attacks and adds across a polar C=O double bond, forming a new single bond and breaking the pi bond.
What is the role of the cyanide ion in the nucleophilic addition of HCN to ethanal, and where does the negative charge reside on CN^-?
CN^- acts as a nucleophile, donating its lone pair to the carbonyl carbon. The negative charge on the cyanide ion resides on the carbon atom, not the nitrogen atom.
Define carbonyl group.
A carbonyl group is a C=O double bond found in aldehydes and ketones. It is polarised because oxygen is more electronegative than carbon, leaving the carbon slightly positively charged.
Why is the carbonyl carbon of an aldehyde or ketone susceptible to nucleophilic attack?
The oxygen atom is more electronegative than carbon, drawing electron density towards itself. This leaves the carbonyl carbon with a slight positive charge (δ+), making it susceptible to attack by a nucleophile.
True or False?
The negative charge on the cyanide ion (CN^-) is located on the nitrogen atom.
False.
The negative charge on CN^- is on the carbon atom, not the nitrogen atom. It is the carbon end that acts as the nucleophile and attacks the carbonyl carbon.
In step 1 of the HCN addition mechanism, the .......... ion attacks the carbonyl carbon, forming a .......... charged intermediate.
In step 1 of the HCN addition mechanism, the cyanide ion attacks the carbonyl carbon, forming a negatively charged intermediate.
What happens in step 2 of the nucleophilic addition of HCN to an aldehyde?
The negatively charged oxygen in the intermediate reacts with an aqueous H⁺ ion (from HCN, water or dilute acid) to form the 2-hydroxynitrile product.
True or False?
The addition of HCN to a carbonyl compound is a two-step mechanism involving nucleophilic attack followed by protonation.
True.
Step 1 is nucleophilic attack of CN^- on the carbonyl carbon to form a negatively charged intermediate. Step 2 is protonation of the oxygen by H⁺ to give the hydroxynitrile.
The carbonyl carbon is susceptible to nucleophilic attack because the oxygen draws .......... density towards itself, giving the carbon a slight .......... charge.
The carbonyl carbon is susceptible to nucleophilic attack because the oxygen draws electron density towards itself, giving the carbon a slight positive charge.
Define 2-hydroxynitrile.
A 2-hydroxynitrile is the product of nucleophilic addition of HCN to an aldehyde or ketone, containing an -OH group and a -CN group on the same carbon atom.
What source of H⁺ ions can protonate the negatively charged intermediate in step 2 of the HCN addition mechanism?
H⁺ ions can come from HCN itself, from water or from dilute acid present in solution.
Define 2,4-DNPH.
A 2,4-DNPH (2,4-dinitrophenylhydrazine) is a reagent used to detect carbonyl compounds. It reacts with aldehydes and ketones in a condensation reaction to produce a deep-orange precipitate.
How can the identity of a specific aldehyde or ketone be confirmed after a positive 2,4-DNPH test?
The deep-orange precipitate is purified by recrystallisation and its melting point is measured. The value is compared with literature values to identify the specific carbonyl compound.
True or False?
Carboxylic acids and esters give a positive result with 2,4-DNPH.
False.
Only aldehydes and ketones give a positive result. Carboxylic acids and esters do not react with 2,4-DNPH to form a precipitate.
Tollens' reagent is an aqueous alkaline solution of .......... in excess .......... solution.
Tollens' reagent is an aqueous alkaline solution of silver nitrate in excess ammonia solution.
What observation is made when an aldehyde is warmed with Tollens' reagent, and what is reduced?
A silver mirror forms on the inside of the tube. The Ag⁺ ions are reduced to silver atoms (Ag) as they oxidise the aldehyde to a carboxylate ion.
True or False?
Ketones give a positive result with both Fehling's solution and Tollens' reagent.
False.
Ketones cannot be oxidised and therefore give a negative result with both Fehling's solution and Tollens' reagent. Only aldehydes give a positive result.
Define Fehling's solution.
A Fehling's solution is an alkaline solution containing Cu²⁺ ions used to test for aldehydes. A positive result produces a red precipitate of copper(I) oxide as the clear blue solution turns opaque.
When an aldehyde is warmed with Fehling's solution, the .......... Cu²⁺ solution turns .......... due to the formation of copper(I) oxide.
When an aldehyde is warmed with Fehling's solution, the clear blue Cu²⁺ solution turns opaque red due to the formation of copper(I) oxide.
What type of reaction occurs between 2,4-DNPH and a carbonyl compound, and what small molecule is eliminated?
A condensation reaction occurs. A water molecule is eliminated as the 2,4-DNPH and carbonyl compound join together to form the deep-orange precipitate.
Define tri-iodomethane (iodoform).
A tri-iodomethane (iodoform, CHI₃) is a yellow precipitate formed in the iodoform reaction when methyl ketones or ethanal are heated with iodine in alkaline solution.
Which structural feature must a compound have to give a positive iodoform test?
The compound must contain a CH₃CO- group (a methyl ketone). Ethanal also contains this group and gives a positive test.
True or False?
Ethanal gives a positive iodoform test because it contains a CH₃CO- group.
True.
Ethanal (CH₃CHO) contains the CH₃CO- group, so it undergoes the iodoform reaction to produce a yellow CHI₃ precipitate.
In the iodoform reaction, the reagent is heated with an .......... solution of iodine. The yellow precipitate formed is ...........
In the iodoform reaction, the reagent is heated with an alkaline solution of iodine. The yellow precipitate formed is CHI₃.
Describe the two steps of the iodoform reaction.
Step 1 (halogenation): All three H atoms of the -CH₃ group are replaced by iodine atoms to form a -CI₃ group.
Step 2 (hydrolysis): The intermediate is hydrolysed by alkaline solution to form a sodium carboxylate salt (RCO₂^- Na⁺) and the yellow CHI₃ precipitate.
True or False?
Propanone gives a positive iodoform test because it is a methyl ketone.
True.
Propanone (CH₃COCH₃) contains a CH₃CO- group, so it undergoes the iodoform reaction with alkaline iodine to produce a yellow CHI₃ precipitate.
In the halogenation step of the iodoform reaction, .......... iodine atoms replace the hydrogen atoms of the .......... group in the methyl ketone.
In the halogenation step of the iodoform reaction, three iodine atoms replace the hydrogen atoms of the -CH₃ group in the methyl ketone.
Define methyl ketone.
A methyl ketone is a ketone containing a CH₃CO- group, which reacts with iodine in alkaline solution to form the yellow precipitate CHI₃ (iodoform).
What are the two products formed in the hydrolysis step of the iodoform reaction?
The two products are a sodium carboxylate salt (RCO₂^- Na⁺) and a yellow precipitate of CHI₃ (tri-iodomethane / iodoform).

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