Topic 22: Analytical Techniques

Question 1

Definitions I:
(a) Infrared spectroscopy
(b) Mass spectrum
(c) Molecular ion

Answer 1

Definitions I:
(a) Technique identifying functional groups by IR absorption, e.g., C=O.
(b) Plot of ion abundance vs. mass-to-charge ratio (m/e).
(c) Ion of intact molecule, gives molecular mass in mass spectrum.

Question 2

In a mass spectrum, M⁺ = 58 m/e. Suggest the molecular formula of this compound. [2 Marks]

Answer 2

M⁺ = 58, likely CH₃COCH₃ (C₃H₆O) (1 mark)
Matches ketone, 12×3 + 6 + 16 = 58 (1 mark)

Question 2

Identify the functional groups in a molecule with IR peaks at 3300 cm⁻¹ and 1710 cm⁻¹. [2 Marks]

Answer 2

3300 cm⁻¹: O-H (alcohol/acid), 1710 cm⁻¹: C=O (ketone/acid) (1 mark)
Possible -COOH or -OH with C=O (1 mark)

Question 3

Calculate Aᵣ of Cl from isotopic peaks: 75 m/e (75%) and 77 m/e (25%). [2 Marks]

Answer 3

Aᵣ = (75 × 75 + 77 × 25) / 100 (1 mark)
Aᵣ = (5625 + 1925) / 100 = 75.5 (1 mark)

Question 4

In a lab, a compound’s IR shows 1680 cm⁻¹. What group is present, and how do you confirm? [2 Marks]

Answer 4

1680 cm⁻¹: C=O (aldehyde/ketone) (1 mark)
Confirm with 2,4-DNPH (orange ppt) (1 mark)

Question 5

A mass spectrum has M⁺ at 72 m/e and [M+1]⁺ at 2.2% of M⁺. Calculate the number of carbons. [2 Marks]

Answer 5

n = (100 × 2.2) / (1.1 × 100) = 2 (1 mark)
2 carbons, e.g., C₂H₅CHO (72 m/e) (1 mark)

Question 5

An IR spectrum has a broad peak at 3000-3500 cm⁻¹. What group and reaction confirm it? [2 Marks]

Answer 5

3000-3500 cm⁻¹: O-H (alcohol/acid) (1 mark)
Na → H₂ (alcohol) or NaOH → salt (acid) (1 mark)

Question 5

Analyse the mass spectrum of CH₃CH₂Cl: predict M⁺ and [M+2]⁺ peaks. [3 Marks]

Answer 5

M⁺ = 64 (CH₃CH₂³⁵Cl), [M+2]⁺ = 66 (CH₃CH₂³⁷Cl) (1 mark)
Ratio ~3:1 (Cl isotopes 75%:25%) (1 mark)
Confirms chlorine presence (1 mark)

Question 6

In a sample, a fragment at 15 m/e appears. Suggest its identity from CH₃CH₂OH. [2 Marks]

Answer 6

15 m/e = CH₃⁺ (1 mark)
Fragment from CH₃CH₂OH losing CH₂OH (1 mark)

Question 7

Deduce the molecular mass of a compound with M⁺ at 86 m/e in its mass spectrum. [2 Marks]

Answer 7

Molecular mass = 86 (1 mark)
From M⁺ peak at 86 m/e (1 mark)

Question 8

A mass spectrum shows peaks at 78, 80, and 82 m/e (9:6:1). What element is present? [2 Marks]

Answer 8

Peaks 78, 80, 82 (9:6:1) = Br (⁷⁹Br:⁸¹Br ~1:1) (1 mark)
[M+2]⁺ pattern confirms bromine (1 mark)

Question 9

Analyse the IR and mass spectrum of CH₃CHO: list key peaks and deduce its formula. [3 Marks]

Answer 9

IR: 1720 cm⁻¹ (C=O), 2800 cm⁻¹ (C-H aldehyde) (1 mark)
Mass: M⁺ = 44 (CH₃CHO), fragment 29 (CHO⁺) (1 mark)
Formula: C₂H₄O, aldehyde (1 mark)