What is the structural difference between an aldehyde and a ketone?

In an aldehyde , the carbonyl group (C=O) is at the end of the carbon chain (bonded to at least one H). In a ketone , the carbonyl group is within the chain, bonded to two alkyl groups .

True or False? Ketones can be oxidised by Tollens' reagent to give a silver mirror.

False. Ketones cannot be oxidised by mild oxidising agents such as Tollens' reagent. Only aldehydes give a positive silver mirror test.

When Fehling's solution is warmed with an aldehyde, the initially .......... solution changes to a .......... precipitate of .......... . With a ketone, no change is observed.

When Fehling's solution is warmed with an aldehyde, the initially blue solution changes to a brick-red precipitate of Cu 2 O (copper(I) oxide) . With a ketone, no change is observed.

What colour change is observed when acidified K 2 Cr 2 O 7 oxidises an aldehyde?

The solution changes from orange to green as the Cr 2 O 7 2- is reduced to Cr 3+ . No colour change occurs with a ketone.

To oxidise a primary alcohol to an aldehyde, the aldehyde must be .......... as soon as it forms to prevent .......... oxidation to a .......... .

To oxidise a primary alcohol to an aldehyde, the aldehyde must be distilled off as soon as it forms to prevent further oxidation to a carboxylic acid .

What is Tollens' reagent and what does a positive result look like?

Tollens' reagent is ammoniacal silver nitrate, containing [Ag(NH 3 ) 2 ] + . A positive result with an aldehyde is a silver mirror deposited on the inside of the test tube. No mirror forms with a ketone.

True or False? Secondary alcohols can be directly oxidised to carboxylic acids using acidified K 2 Cr 2 O 7 .

False. Secondary alcohols are oxidised to ketones , which resist further oxidation. Carboxylic acids are produced only from primary alcohols (via the aldehyde stage).

The carbonyl group is a carbon atom double-bonded to an oxygen atom (C=O). It is the functional group of aldehydes (where the carbonyl carbon is bonded to at least one H, at the end of the chain) and ketones (where the carbonyl carbon is bonded to two alkyl groups, within the chain).

What reagent is commonly used to reduce carbonyl compounds, and what nucleophile does it generate?

NaBH 4 (sodium tetrahydridoborate / sodium borohydride) in aqueous solution generates the hydride ion (:H - ) nucleophile, which attacks the carbonyl carbon.

Reduction of an aldehyde with NaBH 4 gives a .......... alcohol. Reduction of a ketone with NaBH 4 gives a .......... alcohol.

Reduction of an aldehyde with NaBH 4 gives a primary alcohol. Reduction of a ketone with NaBH 4 gives a secondary alcohol.

True or False? NaBH 4 can reduce both C=O double bonds and C=C double bonds.

False. NaBH 4 can reduce C=O bonds but is not strong enough to reduce C=C bonds. The hydride ion is attracted to the electrophilic C in C=O but repelled by the high electron density of C=C.

What type of reaction mechanism is the reduction of a carbonyl by NaBH 4 ?

Nucleophilic addition. The hydride ion (:H - ) acts as the nucleophile and attacks the electrophilic carbonyl carbon.

LiAlH 4 can also reduce carbonyl compounds. Unlike NaBH 4 , it must be used in a .......... solvent. It is also capable of reducing .......... which NaBH 4 cannot.

LiAlH 4 can also reduce carbonyl compounds. Unlike NaBH 4 , it must be used in a non-aqueous solvent. It is also capable of reducing carboxylic acids which NaBH 4 cannot.

True or False? Reducing a carboxylic acid with NaBH 4 gives an aldehyde as the sole product.

False. In practice, a carboxylic acid is reduced directly to a primary alcohol — the aldehyde intermediate does not accumulate. However, NaBH 4 is too mild to reduce carboxylic acids; LiAlH 4 is needed.

Nucleophilic addition

A polymer is a long-chain molecule made up of many small repeating units called monomers, joined together by covalent bonds in an addition or condensation reaction.

Why is the carbonyl carbon susceptible to nucleophilic attack?

Oxygen is more electronegative than carbon, so the C=O bond is polarised . The carbon atom carries a δ+ charge, making it susceptible to attack by nucleophiles.

When HCN reacts with an aldehyde, the .......... ion acts as the nucleophile. In step 1, it attacks the .......... carbon. In step 2, the negatively charged oxygen reacts with .......... to form a hydroxynitrile.

When HCN reacts with an aldehyde, the CN - ion acts as the nucleophile. In step 1, it attacks the carbonyl carbon. In step 2, the negatively charged oxygen reacts with H + to form a hydroxynitrile.

True or False? The addition of HCN to ethanal is important in organic synthesis because it increases the length of the carbon chain.

True. The cyanide ion adds a carbon to the chain. The resulting hydroxynitrile can be hydrolysed (with dilute acid) to an α-hydroxy acid, making it a useful synthetic step.

What is a hydroxynitrile ?

The product of nucleophilic addition of HCN to a carbonyl compound. It contains both an –OH group and a –CN (nitrile) group. The nitrile is the highest-priority group and is named as carbon 1.

Why does nucleophilic addition of HCN to an aldehyde or ketone produce a racemic mixture?

The carbonyl group is planar , so CN - has an equal probability of attacking from either face . Attack from each side gives one enantiomer, producing a 50:50 racemic mixture .

A racemic mixture is formed in nucleophilic addition to a carbonyl because the CN - can attack from .......... side of the planar C=O with .......... probability, giving both .......... in equal amounts.

A racemic mixture is formed in nucleophilic addition to a carbonyl because the CN - can attack from either side of the planar C=O with equal probability, giving both enantiomers in equal amounts.

True or False? A racemic mixture rotates plane-polarised light because it contains chiral molecules.

False. Although a racemic mixture contains chiral molecules, the rotations of the two enantiomers cancel out, giving a net rotation of zero . The mixture is optically inactive.

A LevelChemistryAQAFlashcardsAdvanced Organic ChemistryAldehydes & Ketones

Aldehydes & Ketones (AQA A Level Chemistry): Flashcards

Exam code: 7405

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What is the structural difference between an aldehyde and a ketone?

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What is the structural difference between an aldehyde and a ketone?
In an aldehyde, the carbonyl group (C=O) is at the end of the carbon chain (bonded to at least one H). In a ketone, the carbonyl group is within the chain, bonded to two alkyl groups.
True or False?
Ketones can be oxidised by Tollens' reagent to give a silver mirror.
False.
Ketones cannot be oxidised by mild oxidising agents such as Tollens' reagent. Only aldehydes give a positive silver mirror test.
When Fehling's solution is warmed with an aldehyde, the initially .......... solution changes to a .......... precipitate of .......... . With a ketone, no change is observed.
When Fehling's solution is warmed with an aldehyde, the initially blue solution changes to a brick-red precipitate of Cu₂O (copper(I) oxide). With a ketone, no change is observed.
What colour change is observed when acidified K₂Cr₂O₇ oxidises an aldehyde?
The solution changes from orange to green as the Cr₂O₇^2- is reduced to Cr³⁺. No colour change occurs with a ketone.
To oxidise a primary alcohol to an aldehyde, the aldehyde must be .......... as soon as it forms to prevent .......... oxidation to a .......... .
To oxidise a primary alcohol to an aldehyde, the aldehyde must be distilled off as soon as it forms to prevent further oxidation to a carboxylic acid.
What is Tollens' reagent and what does a positive result look like?
Tollens' reagent is ammoniacal silver nitrate, containing [Ag(NH₃)₂]⁺. A positive result with an aldehyde is a silver mirror deposited on the inside of the test tube. No mirror forms with a ketone.
True or False?
Secondary alcohols can be directly oxidised to carboxylic acids using acidified K₂Cr₂O₇.
False.
Secondary alcohols are oxidised to ketones, which resist further oxidation. Carboxylic acids are produced only from primary alcohols (via the aldehyde stage).
Carbonyl group
The carbonyl group is a carbon atom double-bonded to an oxygen atom (C=O). It is the functional group of aldehydes (where the carbonyl carbon is bonded to at least one H, at the end of the chain) and ketones (where the carbonyl carbon is bonded to two alkyl groups, within the chain).
What reagent is commonly used to reduce carbonyl compounds, and what nucleophile does it generate?
NaBH₄ (sodium tetrahydridoborate / sodium borohydride) in aqueous solution generates the hydride ion (:H^-) nucleophile, which attacks the carbonyl carbon.
Reduction of an aldehyde with NaBH₄ gives a .......... alcohol. Reduction of a ketone with NaBH₄ gives a .......... alcohol.
Reduction of an aldehyde with NaBH₄ gives a primary alcohol. Reduction of a ketone with NaBH₄ gives a secondary alcohol.
True or False?
NaBH₄ can reduce both C=O double bonds and C=C double bonds.
False.
NaBH₄ can reduce C=O bonds but is not strong enough to reduce C=C bonds. The hydride ion is attracted to the electrophilic C in C=O but repelled by the high electron density of C=C.
What type of reaction mechanism is the reduction of a carbonyl by NaBH₄?
Nucleophilic addition. The hydride ion (:H^-) acts as the nucleophile and attacks the electrophilic carbonyl carbon.
LiAlH₄ can also reduce carbonyl compounds. Unlike NaBH₄, it must be used in a .......... solvent. It is also capable of reducing .......... which NaBH₄ cannot.
LiAlH₄ can also reduce carbonyl compounds. Unlike NaBH₄, it must be used in a non-aqueous solvent. It is also capable of reducing carboxylic acids which NaBH₄ cannot.
True or False?
Reducing a carboxylic acid with NaBH₄ gives an aldehyde as the sole product.
False.
In practice, a carboxylic acid is reduced directly to a primary alcohol — the aldehyde intermediate does not accumulate. However, NaBH₄ is too mild to reduce carboxylic acids; LiAlH₄ is needed.
Nucleophilic addition
A polymer is a long-chain molecule made up of many small repeating units called monomers, joined together by covalent bonds in an addition or condensation reaction.
Why is the carbonyl carbon susceptible to nucleophilic attack?
Oxygen is more electronegative than carbon, so the C=O bond is polarised. The carbon atom carries a δ+ charge, making it susceptible to attack by nucleophiles.
When HCN reacts with an aldehyde, the .......... ion acts as the nucleophile. In step 1, it attacks the .......... carbon. In step 2, the negatively charged oxygen reacts with .......... to form a hydroxynitrile.
When HCN reacts with an aldehyde, the CN^- ion acts as the nucleophile. In step 1, it attacks the carbonyl carbon. In step 2, the negatively charged oxygen reacts with H⁺ to form a hydroxynitrile.
True or False?
The addition of HCN to ethanal is important in organic synthesis because it increases the length of the carbon chain.
True.
The cyanide ion adds a carbon to the chain. The resulting hydroxynitrile can be hydrolysed (with dilute acid) to an α-hydroxy acid, making it a useful synthetic step.
What is a hydroxynitrile?
The product of nucleophilic addition of HCN to a carbonyl compound. It contains both an –OH group and a –CN (nitrile) group. The nitrile is the highest-priority group and is named as carbon 1.
Why does nucleophilic addition of HCN to an aldehyde or ketone produce a racemic mixture?
The carbonyl group is planar, so CN^- has an equal probability of attacking from either face. Attack from each side gives one enantiomer, producing a 50:50 racemic mixture.
A racemic mixture is formed in nucleophilic addition to a carbonyl because the CN^- can attack from .......... side of the planar C=O with .......... probability, giving both .......... in equal amounts.
A racemic mixture is formed in nucleophilic addition to a carbonyl because the CN^- can attack from either side of the planar C=O with equal probability, giving both enantiomers in equal amounts.
True or False?
A racemic mixture rotates plane-polarised light because it contains chiral molecules.
False.
Although a racemic mixture contains chiral molecules, the rotations of the two enantiomers cancel out, giving a net rotation of zero. The mixture is optically inactive.

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