3.3.6.3 infra-red spectroscopy

why is infra-red spectrometry used by organic chemists?

its often used to help them identify the type of bonds in a compound

describe how infra-red spectroscopy works

• each bonds vibrates at the same frequency as IR

• the frequency depends on the mass of the atoms in the bond, the bond strength, + the type of vibration

• observing which frequencies of radiation are absorbed allows bonds to be identified

describe what happens in an IR spectrometer

• a beam of IR radiation containing a spread of frequencies is passed through a sample

• bonds in a molecule absorb infrared radiation at characteristics wavenumber

• the radiation that emerges is missing the frequencies that are absorbed by the bonds found in the sample

• the instruments plots a graph of the intensity of the radiation emerging from the sample, called the transmittance, against the frequency of radiation

how is the frequency of radiation measured?

as a wavenumber, measured in cm⁻¹

for the spectrum of methanol, label the bonds that are responsible for each stretch + give the wavenumber for each one

for the spectrum of methanoic acid, label the bonds that are responsible for each stretch + give the wavenumber for each one

for the spectrum of propanal, label the bonds that are responsible for each stretch + give the wavenumber for each one

what is the fingerprint region?

the area of an infra-red spectrum below 1500cm⁻¹

how does the fingerprint region differ for different substances?

its shape is unique for any particular substance

what can the fingerprint region be used to do?

to identify the compound

how can the fingerprint region be used to identify compounds?

we can use a computer to match the fingerprint region of a sample with those on a database of known compounds → an exact match confirms identification of the sample

for example, spectra of propan-1-ol and propane-2-ol are both similar, why is that? what makes them identifiable then?

• both spectra have absorption in the 3230-3350cm⁻¹ region which is due to the O-H bond in alcohols

• but they have different pattern of peaks in the fingerprint region

what can IR spectra also be used to show?

the presence of impurities

how may impurities be revealed?

these may be revealed in absorptions that should not be there in the pure compound

eg the oxidation of an alcohol to a carboxylic acid:

if an IR spectrum is recorded during the reaction + a broad stretch is found at 3400cm⁻¹, which impurity is present?

• O-H (alcohol) peak between 3230-3550cm⁻¹

• unreacted alcohol purity

eg the oxidation of an alcohol to a carboxylic acid

why might it be difficult to see if an aldehyde impurity is present in the final product?

• C=O peak between 1680-1750cm⁻¹

• both aldehydes + carboxylic acids have C=O in this range so peaks may overlap

give 3 examples of greenhouse gases

• water vapour

• carbon dioxide

• methane

how do greenhouse gases lead to global warming?

they absorb radiation from the sun + eventually re-emit it as long-wavelength radiation that increases the temperature of the earth → the trap heat in the earth’s atmosphere

what is the link between infrared radiation + global warming? how is this found?

• the infrared spectra of water vapour, methane + of carbon dioxide shows this link

• all three bonds, C=O, C-H + O-H vibrate in the same frequency as infrared light (although slightly different frequencies within IR region)

what is the criteria for the ‘best greenhouse gas’?

the best greenhouse gas absorbs the most infrared radiation + traps the heat in the earth’s atmosphere

the more ____ the bond, the more IR radiation the bonds absorb

polar

the bigger the area of a peak, the more…

IR radiation absorbed by that bond