Organic Analysis Flashcards

Active recall practice for qualitative test tube reactions, mass spectrometry, and IR spectroscopy.

Chemical Test Tube Reactions: read all 6 questions and answers

State the test and positive observation for an alkene.
Add bromine water. The orange solution decolourises (turns colourless).
State the test and positive observation for a halogenoalkane.
Warm with sodium hydroxide solution, neutralise with nitric acid, then add silver nitrate solution. Precipitates: white (AgCl), cream (AgBr), or yellow (AgI).
State the test and positive observation for a primary/secondary alcohol.
Add acidified potassium dichromate(VI) and warm. Orange solution turns green.
State the test and positive observation for an aldehyde.
Warm with Fehling's solution (blue solution to brick-red precipitate) or Tollens' reagent (silver mirror forms).
State the test and positive observation for a carboxylic acid.
Add sodium hydrogencarbonate (NaHCO₃) or sodium carbonate (Na₂CO₃). Effervescence (bubbling) is observed due to the release of carbon dioxide gas.
How do you test for the presence of carbon dioxide gas?
Bubble the gas through limewater. Limewater turns cloudy/milky.

Mass Spectrometry: read all 3 questions and answers

What does the molecular ion peak (\(\text{M}^{+\bullet}\) peak) in a mass spectrum represent?
The relative molecular mass (\(M_r\)) of the whole molecule.
What is high-resolution mass spectrometry?
A type of mass spectrometry that measures \(m/z\) values to four decimal places, allowing the exact molecular formula to be determined.
Explain why high-resolution mass spectrometry can distinguish between propane (C₃H₈) and acetaldehyde (C₂H₄O).
Although both have an approximate \(M_r\) of 44, their exact relative isotopic masses are slightly different (C₃H₈ = 44.0626, C₂H₄O = 44.0262) due to the precise mass values of C, H, and O.

Infrared (IR) Spectroscopy: read all 7 questions and answers

How does infrared spectroscopy work?
Infrared radiation is passed through a sample. Chemical bonds absorb specific frequencies of IR radiation, causing them to vibrate (stretch or bend).
What is the fingerprint region on an IR spectrum?
The region below 1500 cm⁻¹. It is unique to every organic compound and can be compared with a database of known spectra to identify a compound.
Identify the characteristic wave number range for a C=O bond absorption.
1680 to 1750 cm⁻¹ (sharp, strong peak).
Identify the characteristic range for an O-H bond absorption in an alcohol.
3230 to 3550 cm⁻¹ (broad, strong peak).
Identify the characteristic range for an O-H bond absorption in a carboxylic acid.
2500 to 3000 cm⁻¹ (very broad peak, often overlapping the C-H peak).
Identify the characteristic range for a C-H bond absorption.
2850 to 3300 cm⁻¹.
How can you use IR spectroscopy to show that a primary alcohol has been fully oxidized to a carboxylic acid?
Check that the alcohol O-H peak (3230-3550 cm⁻¹) is replaced by a carboxylic acid O-H peak (2500-3000 cm⁻¹), and a strong C=O peak (1680-1750 cm⁻¹) has appeared.