organic analysis (3.3.6)
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what can mass spectrometry be used to do?
determine the molecular formula of a compound
what is the difference between the Mr values derived from low resolution mass spectrometry and high resolution mass spectrometry?
the value from HRMS is given to 5 decimal places while the value from LRMS is given to the nearest whole number
what do bonds in a molecule do?
absorb infrared radiation at characteristic wavenumbers (1/wavelength of light in cm)
why do the bonds in a molecule absorb infrared radiation?
• the bonds are constantly vibrating (stretching and bending) • the bonds absorb radiation with the same frequency as the bond vibration • when a bond absorbs radiation the degree of stretching or bending increases
what do the bonds in CO2, methane and water vapour do?
absorb infrared radiation which leads to global warming
how does the greenhouse effect occur?
• short wavelength UV radiation passes through the atmosphere to the Earth’s surface and heats it up • the Earth’s surface radiates out long wavelength infrared radiation • the C=O bonds in CO2 absorb infrared radiation so it doesn’t escape from the atmosphere • this energy is transferred to other molecules in the atmosphere by collisions so the atmosphere is warmed
how can you use infrared data to identify where a certain functional group is present on a spectra?
• use the IR absorption data on the data sheet to check the wavenumber range for that functional group • identify where on the spectra that wavenumber range is and which peak corresponds to this (remember may be more than one peak eg. for an OH alcohol)
what is the fingerprint region on a spectra?
a complex series of peaks specific to the molecule being studied
what does fingerprinting allow?
identification of a molecule by comparison of spectra
what should you do when using infrared spectra to distinguish between two molecules using their fingerprint regions?
• identify the fingerprint region • compare the fingerprint region to a database and look for an exact match
what is the functional group for an aldehyde?
H-C=O
what is the functional group for a ketone?
C=O where carbon atom is directly bonded to two other cabon atoms and not any hydrogen atoms
how do you test for an alkene?
shake with bromine water which will be decolourised
how do you test for an alkane?
shake with bromine water and there will be no visible change
how do you test for an aldehyde?
warm with Fehling’s solution and the blue solution will become a red precipitate
what is an alternative method of testing for an aldehyde?
warm with Tollen’s reagent and a silver mirror will form (coats inside of test tube)
how do you test for a ketone?
warm with Tollen’s reagent or Fehling’s solution and there will be no visible change
how do you test for a carboxylic acid?
• add sodium carbonate and effervescence will be observed due to the formation of CO2 • if the gas produced is bubbled through limewater, the limewater will turn cloudy
how do you test for a primary or secondary alcohol?
warm with acidified potassium dichromate (VI) and there will be a colour change from orange to green
how do you test for a tertiary alcohol?
warm with acidified potassium dichromate (VI) and there will be no visible change
how do you test for a chloroalkane?
warm with silver nitrate and there will be a slow formation of a white precipitate of AgCl
what happens if you add acidified potassium dichromate (VI) to an aldehyde?
there will also be a colour change from orange to green (if you are testing compounds and have an aldehyde and a primary/secondary alcohol, test with Tollen’s reagent first to identify the aldehyde)