Topic 37: Analytical Techniques Flashcards
Definitions I:
(a) Stationary phase
(b) Mobile phase
(c) Rf value
Definitions I:
(a) Fixed material, e.g., Al₂O₃ in TLC, interacts with compounds.
(b) Moving solvent/gas, e.g., ethanol in TLC, carries compounds.
(c) Ratio of spot distance to solvent front distance in TLC.
Definitions II:
(a) Retention time
(b) Chemical shift
(c) Splitting pattern
Definitions II:
(a) Time for compound to pass through gas chromatography column.
(b) Position of NMR peak relative to TMS, in ppm.
(c) Peak split by adjacent protons, e.g., triplet in NMR.
In a TLC lab, a spot moves 3 cm, solvent front 6 cm. Calculate Rf and interpret it. [2 Marks]
Rf = 3 / 6 = 0.5 (1 mark)
Moderately polar, balanced interaction/solubility (1 mark)
A gas chromatogram shows peaks at 2 min (40%) and 4 min (60%). What’s the composition? [2 Marks]
40% first compound, 60% second compound (1 mark)
Peak areas give percentage composition (1 mark)
Predict the number of peaks in the ¹³C NMR of CH₃CH₂OH and assign environments. [2 Marks]
2 peaks: CH₃ (17 ppm), CH₂OH (58 ppm) (1 mark)
Different C environments (1 mark)
In a proton NMR, CH₃CH₂Cl shows a triplet at 3.5 ppm. Explain the splitting. [2 Marks]
Triplet: 2 H on adjacent CH₂ (n+1 = 3) (1 mark)
CH₂Cl protons split by CH₃ (1 mark)
Why does TMS act as the standard in NMR spectroscopy? [2 Marks]
TMS: 0 ppm standard, 12 equivalent H, shielded (1 mark)
Non-reactive, distinct signal (1 mark)
Analyse the ¹H NMR of CH₃CH₂OH: peaks at 1.2 (t), 3.6 (q), 4.8 (s). Deduce structure. [3 Marks]
1.2 ppm (t, 3H, CH₃), 3.6 ppm (q, 2H, CH₂), 4.8 ppm (s, 1H, OH) (1 mark)
Ethanol: CH₃CH₂OH matches shifts (1 mark)
Splitting: CH₃ by CH₂ (t), CH₂ by CH₃ (q) (1 mark)
In a TLC with polar solvent, why does CH₃OH have a higher Rf than CH₃CH₂CH₃? [2 Marks]
CH₃OH: polar, soluble in polar solvent, less held by Al₂O₃ (1 mark)
CH₃CH₂CH₃: non-polar, lower Rf (1 mark)
Predict the ¹H NMR chemical shifts and splitting for CH₃CH₂CHO. [2 Marks]
CH₃: ~1 ppm (t), CH₂: ~2.5 ppm (q), CHO: ~9.5 ppm (t) (1 mark)
Splitting by adjacent H (n+1 rule) (1 mark)
Explain why CH₃COOH’s retention time is longer than CH₃OH in gas chromatography. [2 Marks]
CH₃COOH: polar, stronger interaction with stationary phase (1 mark)
Longer retention than less polar CH₃OH (1 mark)
Analyse the ¹³C NMR of CH₃COOCH₃ (peaks at 21, 51, 170 ppm) and confirm structure. [3 Marks]
21 ppm (CH₃), 51 ppm (CH₃O), 170 ppm (C=O) (1 mark)
3 C environments match CH₃COOCH₃ (1 mark)
Ester confirmed by shifts (1 mark)
