organic analysis Flashcards
(20 cards)
Test for C=C (alkene)?
test - bromine water
positive result - decolourise water
Test for O-H in 1°+ 2° alcohols?
test - acidified potassium dichromate
positive result - orange solution —> green solution
Test for -CHO (aldehyde)?
test - Fehling’s solution + water bath
positive result - blue solution —> brick-red ppt
test - Tollens’ reagent
positive result - colourless solution —> silver mirror
Test for -COOH (carboxylic acid)?
test - sodium carbonate
positive result - gentle fizzing
Test for halogenoalkane?
test - silver nitrate solution (warm with NaOH)
positive result - ppt forms
Test for acyl chlorines?
test - contact with moisture
positive result - steamy white fumes
Mass spectrometry.
Mass spectrometry is used to find the relative molecular mass (Mr) of compounds.
Compounds are converted into 1+ ions (often called the molecular ion).
Electron impact.
what it does - removes one electron to form M+ ion
M(g) —> M+(G) + e-
which compounds - compounds with low Mr
how is it done - high energy electrons from an electron gun are fired at the sample.
*electron impact often breaks the molecular ion into smaller fragments. In this work we are ignoring any fragments.
Electrospray ionisation.
what it does - adds one proton to form MH+ ion
M(g) + H+(g) —> MH+ (g)
which compounds - compounds with high Mr
how is it done - the compound is dissolved in a volatile solvent and sprayed out into a fine mist via a hypodermic needle whose tip is connected to the positive terminal of a high voltage power supply.
The impact of 13C and 2H isotopes on signals for the molecular ion.
Spectra measures the mass of individual ions that hit the detector. Therefore whether an ion contains a 13C rather than a 12C atom, or a 2H instead of 1H, affects the mass detected.
The more C and H atoms in the organic molecule, the more chance of there being one 13C (1.1% of C atoms) or 2H (0.015% of H atoms). There is often a small peak with a value that is 1 greater than that for the molecular ion.
Isotopes of Cl and Br.
Chlorine contains 75% 35Cl and 25% 37Cl (and so 3/4 of Cl atoms have mass 35 and 1/4 of Cl atoms have mass 37).
Bromine contains 50% 79Br and 50% 81Br (and so 1/2 of Br atoms have mass 79 and 1/2 of Br atoms have mass 81).
Molecular ions in compounds containing chlorine or bromine are impacted significantly by these isotopes.
High resolution mass spectrometry.
Most mass spectrometers record masses to the nearest 1 or 0.1, but high resolution mass spectrometers record masses to a much higher resolution (e.g. 0.0001).
The Mr given by a high resolution mass spectrometer allows the molecular formula of a compound to be determined.
High resolution mass spectrometry does not identify a compound, but it does give the molecular formula.
For example:
If a low resolution mass spectrometer gives Mr = 60, then there are many compounds it could be with several different molecular formulas.
If a high resolution spectrometer gives Mr = 60.0211 then it indicates that the molecular formula mass is C2H4O2, narrowing down what the compound could be.
Infrared spectroscopy.
Infrared spectroscopy is a very important technique that can be used to identify organic compounds.
All covalent bonds vibrate at a characteristic frequency (stretching and contacting as well as bending vibrations are the commonest types).
The frequency depends on the mass of the atoms in the bond, the bond strength, and the type of vibration.
The frequencies at which they vibrate are in the infrared region of the electromagnetic spectrum.
If infrared light is passed through the compound, it will absorb some or all of the light at the frequencies at which its bonds vibrate.
Rather than using the actual vases of the wavelength or frequency, the IR light is measured in wavenumbers (1/frequency in cm) because it gives convenient numbers in the range 4000-400 cm-1.
There are two main things you need to be able to do with infrared spectra:
1. identify functional group signals (above 1500 cm-1) to identify functional groups.
2. use the “fingerprint” region (below 1500 cm-1) to identify specific compounds.
How to identify O-H alcohol?
range = 3230 - 3550 cm-1
description = these are often very broad, obvious and smooth.
How to identify O-H acid?
range = 2500 - 3000 cm-1
description = these are often very broad and obvious, but with C-H signal overlapping as well so making them “bumpy”.
How to identify C=O aldehyde/ketone/esters/acids?
range = 1680 - 1750 cm-1
description = these are often very narrow but very strong.
How to identify C=C alkenes?
range = 1620 - 1680 cm-1
description = these are often very narrow but relatively weak absorption.
How to identify C≡C nitriles?
range = 2220 - 2260 cm-1
description = these are often narrow and strong.
How to identify N-H amines?
range = 3300 - 3500 cm-1
description = these are very obvious large peaks - for primary amines (with NH2 group) there are two peaks.
Using the fingerprint region (below 1500cm-1).
This part of the spectrum is more complicated and contains many signals which means that we o not use it to identify functional groups.
However, this part of the spectrum is unique for every compound, and so it can be used as a “fingerprint”. Comparison of the spectrum to that of known compounds can identify it.
This region can also be used to check if a compound is pure. If a comparison of the spectrum of a sample is made to the spectrum of the pure compound, they should be identical. If there are any extra peaks in the fingerprint region, they must be due to an impurity.
