Empirical and Molecular Formula:
• Do elemental analysis by using percentage composition data to determine empirical formula and
then molecular formula.
• Sometimes volumetric analysis given, use Mr Moles to work out Mr.
• If empirical formula and molecular formula are same, still need to prove and state empirical
formula is molecular formula.
• Mass Spec:
• Molecular ion peak m/z = 46 so Mr = 46.
• Fragment ions m/z = 45 to indicate CH+
.
• Choose fragment ion with tallest peak. Try and fit key molecules in like OH+
first, not CH.
• In question may be left with more than one compound, even if question doesn’t say more than
one compound- use more fragment ions.
• Always use fragment ions in mass spec to prove, even if doesn’t ask.
Infrared Spec:
• IR shows absorption between 1640 to 1820cm-1 due to C=O bond indicating ketone or aldehyde.
• Need ‘broad’ for OH in carboxylic.
• When given range of structures, say ‘no broad absorption between 2500 and 3300 so no O-H, so
can’t be carboxylic acid’ to rule something out.
• C must be primary because it oxidised to carboxylic acid. C cannot be tertiary because it doesn’t
react with acidified dichromate as lack of C=O and OH peak.
• May need to identify E or Z isomer, even not stated.
• ‘Two possible structures’- means need to find two isomers.