Identification of functional groups: tests, spectroscopy

What are Functional Groups?

Think of a functional group as the “personality” of a molecule – the part that decides how it behaves in a reaction. Just like a superhero’s special power, each group gives a compound unique properties.

Alcohols – The “Scent Test”

1. Take a small sample of the unknown liquid.
2. Drop a few drops of hydrogen bromide (HBr) onto it.
3. If a pleasant, fruity smell appears, you likely have an alcohol.

🔹 Exam Tip: Remember to write the reaction: $$\ce{ROH + HBr -> RBr + H2O}$$

Aldehydes – The “Benedict’s Test”

Use a copper(II) sulfate solution. If the mixture turns orange-red, the aldehyde is present.

🔹 Analogy: It’s like a red‑hot match that lights up when it meets the aldehyde.

🔹 Exam Tip: Write the reduction reaction: $$\ce{RCHO + 2Cu^{2+} + 5OH^- -> RCOO^- + 2Cu + 4H2O}$$

Ketones – The “Fehling’s Test”

Unlike aldehydes, ketones usually give a blue‑black precipitate.

🔹 Exam Tip: Note that ketones are “silent” in Fehling’s test unless they’re α‑hydroxy ketones.

Carboxylic Acids – The “Sodium Bicarbonate Test”

Adding NaHCO₃ to the sample produces fizzing (CO₂ gas).

🔹 Analogy: It’s like blowing up a balloon – the fizz shows the acid is there.

🔹 Exam Tip: Write the neutralisation: $$\ce{RCOOH + NaHCO3 -> RCOONa + CO2 + H2O}$$

Infrared (IR) Spectroscopy

• O–H stretch: sharp peak around 3400 cm⁻¹.
• C=O stretch: strong peak near 1700 cm⁻¹.
• Alkyne C≡C stretch: ~2100–2260 cm⁻¹.

🔹 Exam Tip: Remember the “fingerprint region” (400–1500 cm⁻¹) – it’s like a unique barcode for each functional group.

¹H NMR Spectroscopy

Protons in different environments give distinct chemical shifts.

• Alkanes: 0.8–2.5 ppm.
• Alkenes: 4.5–6.5 ppm.
• Aromatic: 6.0–8.5 ppm.

🔹 Exam Tip: Use splitting patterns (singlet, doublet, triplet) to deduce neighbouring protons.

Mass Spectrometry (MS)

Look for characteristic fragment ions.

• Loss of 15 Da → CH₃ group.
• Loss of 28 Da → CO group.

🔹 Exam Tip: Write the molecular ion as the starting point: $$\ce{M^+}$$ and trace fragmentation pathways.

Safety First!

• Wear goggles and gloves at all times.
• Handle acids and bases in a fume hood.
• Label all reagents clearly.

Record Keeping

Use a neat lab notebook. Write the date, reagent details, observations, and any anomalies.

🔹 Exam Tip: In the exam, you’ll be asked to interpret a lab report – clarity matters.

Multiple Choice Questions (MCQs)

• Look for keywords like “fizzing” (acid) or “orange‑red” (reducing agent).

• Eliminate obviously wrong answers first.

Short Answer Questions

• Show the reaction mechanism clearly.

• Use proper chemical notation and LaTeX for equations.

Data Interpretation

• Compare experimental values with literature values.

• Discuss possible sources of error.