Carbonyl Compounds (OCR A Level Chemistry A): Flashcards
Exam code: H432
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Define carbonyl group
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Define carbonyl group
A carbonyl group is the C=O functional group found in aldehydes and ketones, where the carbon is bonded to oxygen by a double bond.
Why can aldehydes be oxidised but ketones cannot, under mild conditions?
Aldehydes have a hydrogen atom on the carbonyl carbon that can be removed during oxidation, whereas ketones lack this and are resistant to oxidation by mild oxidising agents.
Aldehydes can be oxidised to carboxylic acids using .......... with sulfuric acid.
Aldehydes can be oxidised to carboxylic acids using acidified potassium dichromate with sulfuric acid.
True or False?
When oxidising a primary alcohol to an aldehyde, the reaction mixture is heated under reflux.
False.
To isolate an aldehyde, the apparatus is set up to distil it off as it forms; reflux is used when the aim is to oxidise the primary alcohol all the way to a carboxylic acid.
Why is the carbonyl carbon in aldehydes and ketones susceptible to nucleophilic attack?
Oxygen is more electronegative than carbon, creating a dipole that leaves the carbonyl carbon slightly positive and open to attack by a nucleophile.
Define nucleophilic addition
A nucleophilic addition is a reaction in which a nucleophile attacks an electron-deficient carbon, breaking the pi bond of C=O to form a new single bond and adding across the double bond.
In the reduction of carbonyl compounds, sodium tetrahydridoborate generates the .......... ion, which acts as a nucleophile.
In the reduction of carbonyl compounds, sodium tetrahydridoborate generates the hydride ion, which acts as a nucleophile.
What product is formed when HCN undergoes nucleophilic addition with a carbonyl compound, and why is this reaction synthetically useful?
The product is a hydroxynitrile, formed when the cyanide ion attacks the carbonyl carbon and the intermediate is then protonated. This reaction is useful because it increases the carbon chain length by one carbon.
True or False?
Ketones are reduced to primary alcohols by sodium tetrahydridoborate.
False.
Ketones are reduced to secondary alcohols because the hydride ion adds to a carbonyl carbon bonded to two alkyl groups, placing the OH group in the middle of the chain.
Define condensation reaction
A condensation reaction is a reaction in which two molecules join together and a small molecule (such as H2O or HCl) is eliminated.
What is the positive result when 2,4-DNPH is added to a carbonyl compound?
A deep-orange precipitate forms, which can be purified by recrystallisation and its melting point measured to identify the specific aldehyde or ketone.
The .......... of the orange precipitate formed with 2,4-DNPH is measured and compared to literature values to identify the carbonyl compound.
The melting point of the orange precipitate formed with 2,4-DNPH is measured and compared to literature values to identify the carbonyl compound.
True or False?
Carboxylic acids give a positive result with 2,4-DNPH.
False.
Only aldehydes and ketones give a positive result; carboxylic acids and esters do not have a reactive carbonyl group that reacts with 2,4-DNPH.
Why does Tollens' reagent produce a silver mirror with an aldehyde but not a ketone?
An aldehyde is oxidised by the silver(I) complex ions, reducing them to metallic silver that coats the glass; a ketone cannot be oxidised under these mild conditions, so no silver mirror forms.
Tollens' reagent is also known as .......... and contains the silver(I) complex ion [Ag(NH3)2]+.
Tollens' reagent is also known as ammoniacal silver nitrate and contains the silver(I) complex ion [Ag(NH3)2]+.
How do you distinguish an aldehyde from a ketone using two reagents in sequence?
First add 2,4-DNPH: a deep-orange precipitate confirms a carbonyl compound is present. Then add Tollens' reagent and warm gently: a silver mirror confirms an aldehyde, while no reaction indicates a ketone.
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