Organic Chemistry: Carbonyls (Edexcel International A Level (IAL) Chemistry): Flashcards
Exam code: YCH11
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What is an aldehyde?
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What is an aldehyde?
An aldehyde is a carbonyl compound where the C=O group is at the end of the carbon chain. Aldehydes have the general formula RCHO and are named with the suffix -al.
What is a ketone?
A ketone is a carbonyl compound where the C=O group is in the middle of the carbon chain. Ketones have the general formula RCOR’ and are named with the suffix -one.
The simplest aldehyde is .......... and the simplest ketone is .......... because a ketone requires an alkyl group on .......... side of the carbonyl carbon.
The simplest aldehyde is methanal and the simplest ketone is propan-2-one because a ketone requires an alkyl group on each side of the carbonyl carbon.
Why do aldehydes and ketones have lower boiling points than their corresponding alcohols?
Aldehydes and ketones cannot form hydrogen bonds with each other. They only have permanent dipole-dipole interactions and London forces, which are weaker than the hydrogen bonds in alcohols.
Why can small aldehydes and ketones dissolve in water, but larger ones cannot?
Smaller carbonyls can form hydrogen bonds with water via the lone pairs on their oxygen atom. In larger carbonyls, the long hydrocarbon chain disrupts water's hydrogen bonding without forming new ones, so they do not dissolve.
True or False?
Aldehydes and ketones can hydrogen bond with each other.
False.
Aldehydes and ketones have no O-H or N-H bonds, so they cannot form hydrogen bonds with each other. They can, however, hydrogen bond with water molecules.
What intermolecular forces exist between carbonyl molecules?
Carbonyl molecules experience permanent dipole-dipole interactions and London dispersion forces. The electronegative oxygen of the C=O group creates a dipole.
True or False?
In naming an aldehyde, the carbonyl carbon is always carbon 1.
True.
The carbonyl group in an aldehyde is always at the end of the chain, so the carbonyl carbon is automatically carbon 1. The number is not included in the name.
How can acidified potassium dichromate(VI) distinguish between an aldehyde and a ketone?
An aldehyde is oxidised by acidified K2Cr2O7, producing an orange to green colour change. A ketone is not oxidised, so no colour change occurs.
What is Tollens' reagent and what is a positive result?
Tollens' reagent is ammoniacal silver nitrate, containing [Ag(NH3)2]+ ions. A positive result is a silver mirror: the aldehyde reduces Ag+ to solid silver. Ketones give no reaction.
What colour change indicates a positive result with Fehling's solution?
A positive result is a blue to brick red colour change. The aldehyde reduces Cu2+ ions to Cu+, forming a brick red precipitate of copper(I) oxide, Cu2O.
What type of reaction is the addition of HCN to a carbonyl compound, and what type of product is formed?
It is a nucleophilic addition reaction. The CN- ion attacks the carbonyl carbon, and the intermediate is then protonated to form a hydroxynitrile.
Reduction of an aldehyde with LiAlH4 gives a .......... alcohol, while reduction of a ketone gives a .......... alcohol.
Reduction of an aldehyde with LiAlH4 gives a primary alcohol, while reduction of a ketone gives a secondary alcohol.
What is 2,4-DNPH used for, and what is a positive result?
2,4-DNPH (2,4-dinitrophenylhydrazine) is used to identify carbonyl compounds. A positive result is a deep-orange precipitate, which can be recrystallised and its melting point compared to literature values.
What is the iodoform test and what does a positive result indicate?
The iodoform test uses iodine in alkaline solution. A positive result is a yellow precipitate of CHI3 (tri-iodomethane), indicating the presence of a CH3CO- group (methyl ketones or ethanal).
True or False?
Ethanal gives a positive iodoform test.
True.
Ethanal contains a CH3CO- group, so it produces a yellow precipitate of CHI3 in the iodoform test, just like methyl ketones.
Why is the C=O group in aldehydes and ketones susceptible to nucleophilic attack?
The electronegative oxygen draws electron density towards itself, leaving the carbonyl carbon slightly positive (δ+). This makes it susceptible to attack by nucleophiles such as CN-.
True or False?
Ketones give a positive result with Fehling's solution.
False.
Ketones are resistant to oxidation and do not react with Fehling's solution. The solution remains blue. Only aldehydes give a positive (brick red precipitate) result.
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