Organic analysis revision notes

Organic Analysis – Test‑tube Reactions, Mass Spectrometry and Infrared Spectroscopy

1. Test‑tube Reactions

Test‑tube reactions are qualitative‑analysis methods that give a visual change when a functional group is present. They are quick, inexpensive and give a clear *yes/no* result.

1.1 Alcohols – Lucas Test

• Reagents: Lucas reagent (concentrated HCl + ZnCl₂).
• Observation: Formation of a cloudy emulsion or a clear solution.
• Interpretation: Primary alcohols give a clear solution (slow reaction), secondary alcohols give a cloudy emulsion (fast reaction) and tertiary alcohols give a clear solution (very fast). The rate difference is due to the steric hindrance around the hydroxyl group.

1.2 Aldehydes – Tollen’s Test

• Reagents: Tollen’s reagent (Ag⁺ in NH₃, NaOH).
• Observation: Formation of a silver mirror on the inner wall of the test tube.
• Interpretation: Aldehydes are oxidised to carboxylic acids, reducing Ag⁺ to metallic Ag. Ketones do not give a silver mirror.

1.3 Alkenes – Bromine Water Test

• Reagents: Bromine water (Br₂ in H₂O).
• Observation: Decolourisation of the reddish‑brown bromine solution.
• Interpretation: Alkenes undergo electrophilic addition of Br₂, forming a dibromide and removing the colour. Alkanes do not react.

1.4 Carboxylic Acids – Bromine Water Test (Alternative)

• Observation: Decolourisation of bromine water.
• Interpretation: Carboxylic acids also react with bromine water, but the reaction is slower than for alkenes. The test is useful when the compound is a mixture of an alkene and a carboxylic acid.

2. Mass Spectrometry

Mass spectrometry (MS) provides the mass of the molecular ion and fragmentation pattern, which are key to structural identification.

2.1 Molecular Ion Peak

• Definition: The peak corresponding to the intact molecule (M⁺). Its m/z value equals the molecular mass.
• Use: Determines the empirical formula and, with the isotopic pattern, confirms the presence of halogens or other heteroatoms.

2.2 Fragmentation Evidence

• Common Fragmentations:
• α‑Cleavage: Loss of a neutral fragment adjacent to a heteroatom.
• McLafferty Rearrangement: Transfer of a γ‑hydrogen to a heteroatom, producing a characteristic ion.
• Cleavage of C–C bonds: Produces ions that help locate functional groups.
• Interpretation: By comparing the observed fragments with expected patterns, you can deduce the position of functional groups and the backbone of the molecule.

2.3 Practical Example – Ethanal (CH₃CHO)

• Molecular ion: m/z 44.
• Key fragments: m/z 29 (CH₃⁺) and m/z 30 (CH₃CO⁺). The presence of a fragment at m/z 30 indicates a carbonyl group.

3. Infrared Spectroscopy

Infrared (IR) spectroscopy measures the absorption of IR radiation by molecular bonds, producing a spectrum that is a fingerprint of the functional groups present.

3.1 Functional‑Group Absorptions

| Functional Group | Typical wavenumber (cm⁻¹) | Shape | Example |-------------------|--------------------------|-------|-------- | O–H (alcohol) | 3200–3600 | Broad, strong | Ethanol | C=O (aldehyde/carboxylic acid) | 1720–1740 | Sharp, strong | Acetaldehyde | C=C (alkene) | 1600–1680 | Sharp | Ethene | C–H (alkane) | 2850–2960 | Medium | Methane | N–H (amine) | 3300–3500 | Medium | Ethylamine | C≡C / C≡N | 2100–2260 | Sharp | Acetylene |

3.2 Fingerprint Region (600–1500 cm⁻¹)

• Definition: A complex pattern of peaks unique to each molecule.
• Use: Compare with reference spectra to confirm the identity of a compound when functional‑group peaks are ambiguous.

3.3 Monitoring Gases – CO₂, CH₄, H₂O

• CO₂: Strong absorption at 2349 cm⁻¹ (asymmetric stretch) and 667 cm⁻¹ (out‑of‑plane bend).
• CH₄: Absorption at 3015 cm⁻¹ (C–H stretch) and 1306 cm⁻¹ (C–H bend).
• H₂O: Broad absorption around 3400 cm⁻¹ (O–H stretch) and 1640 cm⁻¹ (H–O–H bend).
• Application: IR can detect trace amounts of these gases in a sample, useful in environmental monitoring and combustion analysis.

4. Integrating Techniques

• Sequential Approach: Use test‑tube reactions first to narrow down the functional group class. Then apply MS to confirm the molecular ion and deduce the empirical formula. Finally, use IR to pinpoint the exact functional groups and confirm the structure.
• Cross‑Validation: If MS suggests a molecular ion of 44 but IR shows a C=O stretch at 1725 cm⁻¹, the compound is likely an aldehyde (e.g., ethanal). If the IR shows no C=O but a broad O–H, the compound is an alcohol.

5. Practical Tips for the Exam

• Write down the reagent and observation for each test‑tube reaction; the examiner checks for accuracy.
• Label the peaks in a mass spectrum: molecular ion, base peak, and key fragments.
• Identify the most intense peaks in the IR spectrum and match them to functional‑group absorptions.
• Use the fingerprint region only when the functional‑group peaks are insufficient to distinguish between isomers.
• Remember the order of operations: test‑tube → MS → IR.

6. Common Mistakes

1. Confusing the rate of the Lucas test – mislabeling a clear solution as a positive result for a primary alcohol.
2. Forgetting the silver mirror in Tollen’s test – writing a negative result for an aldehyde.
3. Misidentifying the molecular ion – confusing a fragment peak with the molecular ion.
4. Overlooking the fingerprint region – assuming functional‑group peaks alone are sufficient.
5. Misreading IR peaks – assigning a C–H stretch to a C=O absorption.

7. Key Terms

• Lucas reagent
• Tollen’s reagent
• Bromine water
• Molecular ion (M⁺)
• Fragmentation
• McLafferty rearrangement
• Fingerprint region
• Functional‑group absorption
• Silver mirror
• α‑Cleavage
• Infrared (IR) spectrum

8. Exam Tips

• Practice writing clear, concise observations for each test‑tube reaction.
• Memorise the key functional‑group wavenumbers for quick identification.
• Use a systematic approach when interpreting mass spectra: start with the molecular ion, then look for the base peak and other significant fragments.
• Cross‑check your conclusions with at least two techniques to avoid misidentification.
• Keep a quick reference sheet of common IR peaks and MS fragmentation patterns for revision.

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