WM4 Instrumental analysis

The 3 key instrumental techniquesHow do we know that salicylic acid contains OH and COOH groups?

Mass spectroscopy (m.s.).Infrared (i.r.) spectroscopy.Nuclear magnetic resonance (n.m.r.) spectroscopy.

Making use of infrared spectroscopyAny unidentified, new substance has its i.r. spectrum recorded.

6.4 Infrared spectroscopyEnergy in molecules is quantised.In i.r., molecules are exposed to radiation between 1014Hz 1013Hz (wavelengths 2.5m -15 m). Remember: c = vBonds vibrate and stretch (pull apart then push together again) as they absorb energy.

Vibrational changes of CO2Asymmetric stretchSymmetric stretchbending

Infrared spectra: signals = stretchesAn IR spectrum hangs down from a baseline (100% transmittance = no absorbtion).The signals (look like icicles) on an IR spectra correspond to bonds absorbing a packet of energy and vibrating more.The OH and CO bonds in salicylic acid absorb energy at specific wavelengths ()/m and so wavenumbers (1/ )/ cm-1.

Generally, particular bonds can be matched to a particular absorption region.

Interpreting spectra: 2-ethylbut-1-ene?

Interpreting spectra: propanone

Regions in the IR spectrum where typical absorptions occurLabel these regions on the previous two examples; sketch the structures and link them to the main signals. Do this for the following examples, too.

Examples of infrared spectraButaneStrong absorption at 2970 cm-1 characteristic of C-H stretching in aliphatic compounds.No indication of any functional groups.

Examples of infrared spectraMethylbenzene2 types of C-H absorption ~3000 cm-1 (above = C-H on benzene; below = C-H on methyl group).No indication of any functional groups.Absorption pattern ~700 cm-1 is typical of a benzene ring with a substituted group.

Examples of infrared spectraBenzoic acid A sharp absorption at 3580 cm-1 is due to O-H bond.A strong absorption at 1760 cm-1 is the C=O group.Position of C-H absorption suggests an aromatic compound.

Summary of IR spectroscopyAn IR spectrum measures the extent to which electromagnetic radiation is transmitted through a sample of substance.Frequency ranges absorbed give clues about functional groups which are present.IR spectrum of salicylic acid gives evidence of C=O and OH groups.

Evidence from nuclear magnetic resonance (n.m.r.) spectroscopy.This technique helps to determine structure, as it investigates the different environments in which (hydrogen) nuclei are situated. The n.m.r. spectrum for salicylic acid shows signals for the different environments of the 6 protons: One proton in a COOH environment.One proton in a phenolic OH environment.Four protons attached to a benzene ring.

n.m.r. spectrum for salicylic acid12 11Absorption

The evidence so far.A combination of i.r. and n.m.r. spectroscopy shows that salicylic acid has an OH group and COOH group both attached to a benzene ring; we can now rename it HYDROXYBENZOIC ACID.However, it could be one of 3 possible isomers: 2-hydroxybenzoic acid, 3-hydroxybenzoic acid and 4-hydroxybenzoic acid.Mass spectroscopy can determine which isomer we have.

The mass spectrum of salicylic acidSignals correspond to positively charged ions formed from the parent compound, and fragment ions.Parent or molecular ionThe fragmentation pattern is characteristic of a particular compoundthe fragment at 120 can only come from 2-hydroxybenzoic acidcan you see why?mass

FragmentationPositive ions in a mass spectrometer can break down into building blocks. Example: 2-ethoxybutane.CH3-CH2-CH-O-CH2CH3 CH3 (M=102)+CH3-CH2-CH-O-CH2CH3(M=87)?+HO-CH-CH3 (M=45)Loss and rearrangement of CH3CH=CHCH3+CH3-CH2-CH(M=87) CH3?Loss of CH3-CH2???+Loss of CH3CH3-CH2 (M=29)+

Positively charged fragments form.Mass difference suggests functional groups.For each fragmentation, one product has a positive charge:

M+ A+ + B A + B+

The most stable ion usually forms.

Isotope peaks: heights are in the same ratio of abundance for particular elements.Pairs of peaks correspond to isotopes of 35Cl and 37Cl in the ratio of 75%:25% ie. 3:1. Highlight these.mass

Now its over to you!Do activity WM4: use accurate Mr values, isotope peaks and a database to lead you to the formula of salicylic acid.It shows you how chemists use fragmentation patterns to deduce or confirm a molecular structure.Do assignments 1 and 2 C.S. p110-111Do Problems for 6.5 on mass spectrometry, C.I., p145-146.