§-Quick questions
WAChemistryUnit 4: Organic Chemistry and Chemical Synthesis
Quick questions on Infrared spectroscopy: WACE Year 12 Chemistry
3short 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 are diagnostic absorption bands?Show answer
The bands you must recognise (values are approximate ranges from the data table) include:
What is the fingerprint region?Show answer
The region below about 1500 cm is the fingerprint region, a complex pattern unique to each compound. It is too complex to assign band by band, but it can confirm identity by matching against a reference spectrum of a known compound.
What is reading a spectrum in practice?Show answer
A reliable routine for an examination spectrum is to scan from high wavenumber down. First look above : a broad band there signals O-H (alcohol or, if very broad and extending lower, a carboxylic acid) or N-H (amine or amide). Next look around : a strong sharp band there signals a C=O carbonyl, present in aldehydes, ketones, carboxylic acids, esters and amides. Then combine the evidence: a C=O on its own (no O-H) points to an aldehyde or ketone; a C=O together with a very broad O-H points to a carboxylic acid; a C=O together with N-H points to an amide.
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