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VICChemistryQuick questions
Unit 4: How are organic compounds categorised, analysed and used?
Quick questions on Mass spectrometry and infrared spectroscopy: VCE Chemistry Unit 4
12short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is mass spectrometry (MS) of organic compounds?Show answer
A mass spectrometer ionises the molecule (usually with electron impact), accelerates the ions in a vacuum, separates them by mass-to-charge ratio (m/z), and detects them. The output is a stick spectrum of relative abundance against m/z. Three things to look at in any organic mass spectrum:
What is infrared spectroscopy (IR)?Show answer
IR radiation excites bond vibrations. Each bond has a characteristic stretching frequency (cm^-1) determined by bond strength and atomic masses. The spectrum plots % transmittance (y, with peaks pointing down) against wavenumber (x, cm^-1, conventionally running 4000 to 400 from left to right).
What is the IR-MS workflow?Show answer
Given an unknown spectrum and an MS:
What is carboxylic acid vs ester vs ketone?Show answer
A common VCE puzzle is to distinguish three carbonyl species:
What is 1. Molecular ion peak?Show answer
The peak at the highest m/z (ignoring the small isotope peaks above it) corresponds to the whole molecule with a single electron removed. Its m/z equals the molecular mass (in u). For an unknown C3H8O, M+ appears at m/z = 60.
What is 2. Isotope peaks?Show answer
A small peak at one mass unit above the molecular ion (the M+1 peak) is due to natural ^13C (about 1.1% abundance). Its relative intensity is roughly 1.1% per carbon atom, which can be used to estimate the number of carbons.
What is 3. Fragmentation pattern?Show answer
The high-energy collisions break the molecular ion into smaller cations. Common diagnostic losses:
What is reading the IR axis the wrong way?Show answer
Wavenumber on the x-axis runs right to left (high to low cm^-1). Transmittance on the y-axis dips downward for absorbance peaks.
What is missing the M+2 isotope clue?Show answer
A strong M+2 peak the same height as M+ means one bromine atom. A 3:1 ratio means one chlorine.
What is forgetting that the molecular ion can be weak or absent?Show answer
For some compounds, M+ is barely visible. Use the next highest peak and an even-electron loss to back-calculate.
What is calling every C=O a ketone?Show answer
A C=O alone could be an aldehyde, ketone, ester, amide or acid. Combine with the O-H or C-O or N-H to identify the family.
What is skipping the degree of unsaturation?Show answer
A DoU of 1 implies one C=C or one ring; 4 implies a benzene ring.