Aldehydes and Ketones Flashcards

Active recall practice for carbon-oxygen double bonds, Fehling/Tollens tests, NaBH4 reductions, and nucleophilic addition mechanisms.

Structures & Distinction: read all 5 questions and answers

What is the general structure of an aldehyde?
R-CHO (where the carbonyl C=O group is at the end of the carbon chain).
What is the general structure of a ketone?
R-CO-R' (where the carbonyl C=O group is in the body of the carbon chain).
Why can aldehydes be oxidized to carboxylic acids, but ketones cannot?
Aldehydes have a hydrogen atom attached directly to the carbonyl carbon, which can be easily oxidized. Ketones do not have this C-H bond, requiring breaking a strong C-C bond to oxidize.
State the reagent and observations for Tollens' test.
Reagent: Ammoniacal silver nitrate [Ag(NH₃)₂]⁺. Observation: Aldehydes form a silver mirror; ketones show no reaction.
State the reagent and observations for Fehling's test.
Reagent: Fehling's solution (contains blue Cu²⁺ complexes). Observation: Aldehydes form a brick-red precipitate of copper(I) oxide (Cu₂O); ketones show no reaction.

Reduction Reactions: read all 6 questions and answers

What reagent is commonly used to reduce aldehydes and ketones to alcohols?
Sodium tetrahydridoborate(III), NaBH₄, in aqueous or alcoholic solution.
What nucleophile is provided by NaBH4 in reduction reactions?
The hydride ion, H⁻ (a lone pair on hydrogen).
What type of alcohol is formed when an aldehyde is reduced?
A primary alcohol.
What type of alcohol is formed when a ketone is reduced?
A secondary alcohol.
Write the simplified equation for the reduction of propanal using [H].
CH₃CH₂CHO + 2[H] → CH₃CH₂CH₂OH
Write the simplified equation for the reduction of propanone using [H].
CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃

Nucleophilic Addition Mechanisms: read all 5 questions and answers

Why do aldehydes and ketones undergo nucleophilic addition rather than substitution?
The C=O bond is polar (carbon is delta+, oxygen is delta-). The unsaturated double bond allows addition to occur, forming a stable saturated tetrahedral product.
Describe the first step in the mechanism of nucleophilic addition to a carbonyl group.
The nucleophile attacks the delta+ carbonyl carbon atom, and the pair of electrons in the pi bond moves to the carbonyl oxygen atom, forming an intermediate with a negative charge on oxygen.
Explain the role of HCN in nucleophilic addition reactions.
It acts as a reagent to add a nitrile group (-CN) and a hydroxyl group (-OH) to form a hydroxynitrile, increasing the carbon chain length by one.
State a hazard associated with using HCN or KCN in the laboratory.
HCN is a highly toxic, volatile gas; KCN is also highly toxic if ingested or absorbed, and can release HCN gas when in contact with acid.
What is the IUPAC name of the product formed from the reaction of propanone with KCN followed by dilute acid?
2-hydroxy-2-methylpropanenitrile