Flashcards · 9701 · AS Topic 22

Analytical techniques (AS Level) flashcards

9 cards on AS Topic 22, subtopics 22.1 to 22.2. Pick a subtopic or study them all.

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What does infrared spectroscopy detect, and what decides the absorption frequency?: Bonds absorb IR radiation at frequencies matching their vibrations (stretching and bending). The frequency depends on the bond strength and the masses of the atoms.
IR: alcohol vs carboxylic acid vs aldehyde/ketone: Alcohol: broad O–H at 3200–3600 cm⁻¹, no C=O. Carboxylic acid: very broad O–H at 2500–3000 plus C=O. Aldehyde, ketone or ester: strong C=O (about 1670–1750), no O–H.
IR absorptions for C=O and C≡N: C=O: strong, sharp, about 1670–1740 cm⁻¹ (esters 1710–1750, amides 1640–1690). C≡N: sharp, 2200–2250 cm⁻¹.
A use of infrared spectroscopy outside the lab: Breathalysers (ethanol) and monitoring of exhaust gases, since CO and CO₂ absorb characteristically.

Molecular ion peak (M⁺): The peak at the highest m/e (ignoring isotope peaks); it gives the relative molecular mass.
M and M+2 peaks: what do the ratios 3 : 1 and 1 : 1 show?: 3 : 1: one chlorine atom (³⁵Cl and ³⁷Cl). 1 : 1: one bromine atom (⁷⁹Br and ⁸¹Br).
How does the M+1 peak indicate the number of carbon atoms?: It comes from ¹³C (about 1.1% per carbon). Number of carbons ≈ (height of M+1 as a % of M) ÷ 1.1. e.g. 6.6% → about 6 carbons.
Common fragment losses: 15, 17, 29 and 45: 15: CH₃. 17: OH. 29: CHO or C₂H₅. 45: COOH.
Ar from a mass spectrum of an element: Ar = Σ(isotopic mass × % abundance) ÷ 100.