Question 1

How do I identify an aldehyde vs a ketone using chemical tests?

Accepted Answer

Aldehydes can be distinguished from ketones using mild oxidising agents like Tollens' reagent or Fehling's solution. Tollens' reagent gives a silver mirror with aldehydes (they are oxidised to carboxylic acids) but no reaction with ketones. Fehling's solution gives a red precipitate of copper(I) oxide with aldehydes. Ketones do not react with either reagent under these conditions. Also, the iodoform test is positive for methyl ketones (CH₃COR) and ethanol, but not for most aldehydes.

Question 2

What does the molecular ion peak tell you in mass spectrometry?

Accepted Answer

The molecular ion peak (M⁺) corresponds to the intact molecule with one electron removed. Its m/z value gives the relative molecular mass (Mr) of the compound. For example, if the M⁺ peak is at m/z 58, the Mr is 58. This helps determine the molecular formula. However, be aware that some compounds may have a very weak or absent M⁺ peak if the molecule fragments easily. Also, isotope peaks (e.g., M+1 for ¹³C) can provide additional information.

Question 3

How do I interpret splitting patterns in ¹H NMR?

Accepted Answer

Splitting patterns arise from spin-spin coupling between non-equivalent protons on adjacent carbons. The n+1 rule states that a proton with n neighbouring non-equivalent protons will be split into n+1 peaks. For example, a CH₃ group next to a CH₂ group gives a triplet (n=2, so 3 peaks). The CH₂ group next to the CH₃ gives a quartet (n=3, so 4 peaks). The relative intensities follow Pascal's triangle (1:1 for doublet, 1:2:1 for triplet, 1:3:3:1 for quartet). Protons on heteroatoms (like –OH) often appear as broad singlets and do not cause splitting.

Question 4

What are the key IR absorptions I need to know for AQA A-Level?

Accepted Answer

Key IR absorptions include: O–H (alcohol) broad 3200–3600 cm⁻¹; O–H (carboxylic acid) very broad 2500–3300 cm⁻¹; N–H (amine) two peaks 3300–3500 cm⁻¹ (primary) or one peak (secondary); C–H 2850–3100 cm⁻¹; C≡N 2200–2260 cm⁻¹; C=O 1700–1750 cm⁻¹ (aldehyde, ketone, carboxylic acid, ester, amide); C=C 1620–1680 cm⁻¹; C–O 1000–1300 cm⁻¹. You should be able to identify functional groups from these absorptions.

Question 5

How do I distinguish between a primary, secondary, and tertiary alcohol using chemical tests?

Accepted Answer

Primary alcohols can be oxidised to aldehydes and then to carboxylic acids; secondary alcohols oxidise to ketones; tertiary alcohols do not oxidise under mild conditions. Use acidified potassium dichromate(VI): primary alcohols turn the orange solution green (Cr³⁺) and produce a carboxylic acid (or aldehyde if distilled); secondary alcohols also turn it green but produce a ketone; tertiary alcohols do not change colour (remain orange). Alternatively, the iodoform test is positive for ethanol (a primary alcohol) and for secondary alcohols with a CH₃CH(OH)– group (e.g., propan-2-ol).

Question 6

What is the difference between ¹H NMR and ¹³C NMR?

Accepted Answer

¹H NMR (proton NMR) detects hydrogen nuclei and gives information about the number of proton environments, their chemical shifts, integration (relative number of protons), and splitting patterns (due to coupling with neighbouring protons). ¹³C NMR detects carbon-13 nuclei and gives information about the number of carbon environments and their chemical shifts. ¹³C NMR spectra are simpler because carbon-carbon coupling is usually not observed (due to low natural abundance of ¹³C), so each carbon environment gives a single peak. Both techniques are used together to deduce structure.

Organic analysis

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Related Topics in AQA A-Level Chemistry

Acids and bases (A-level only)

Alcohols

Aldehydes and ketones (A-level only)

Alkanes