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Organic Synthesis and Analysis

Combining spectra to elucidate a structure

  • 3 things to know
  • Component 2

Overview

Recall questions on identifying compounds from a combination of mass, infrared and NMR evidence, which technique gives the molecular formula, which the functional groups and which the skeleton, worked identifications, and why one technique alone is rarely enough.

What you need to know

Answer each one before you reveal it.

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Key term

How does the proton NMR spectrum of ethanoic acid confirm its identity?

Key term

A compound of formula C₃H₆O gives one singlet in its proton NMR. What is it?

Key term

How would propanal be distinguished from propanone by proton NMR?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    A compound of relative molecular mass 60 shows a broad infrared absorption from 2500 to 3200 cm⁻¹ and a strong peak at 1710 cm⁻¹. What is it?

    Model answer

    Ethanoic acid, since that pair of absorptions is the carboxylic acid O—H and C=O and the formula CH₃COOH has a relative molecular mass of 60.

  2. 2

    Confirm that identification from the proton NMR.

    Model answer

    Ethanoic acid gives two singlets in the ratio 3 : 1, the methyl near 2.1 and the acid hydrogen far downfield beyond 10 parts per million.

  3. 3

    A compound of formula C₃H₆O gives one singlet in its proton NMR. What is it?

    Model answer

    Propanone, whose six hydrogen atoms are all equivalent.

  4. 4

    How would propanal be distinguished from propanone by proton NMR?

    Model answer

    Propanal gives three peaks including one near 9.7 parts per million from the aldehyde hydrogen, while propanone gives a single peak.

  5. 5

    How would propanal be distinguished from propanone by infrared alone?

    Model answer

    Propanal shows a weak C—H absorption near 2750 to 2850 cm⁻¹ from the aldehyde group, which propanone does not.

  6. 6

    How would propanal be distinguished from propanone by a chemical test?

    Model answer

    Propanal reduces Tollens' reagent to give a silver mirror and reduces Fehling's solution to a red precipitate, while propanone does neither.

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