Infrared Spectroscopy
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What is infrared spectroscopy used for in organic chemistry?
To help identify organic compounds by analysing the bonds present in a molecule
What causes bonds in molecules to vibrate?
Atoms in a bond vibrate naturally, similar to two balls joined by a spring
How does the mass of atoms affect IR absorption frequency?
Bonds between lighter atoms vibrate faster and absorb IR radiation at a higher frequency, while bonds with heavier atoms absorb at lower frequencies
What is the effect of bond strength on IR absorption frequency?
Stronger bonds vibrate at a higher frequency, so they absorb IR radiation at higher wave numbers
How does IR radiation interact with chemical bonds?
Bonds absorb IR radiation only if the radiation matches their natural frequency of vibration
What happens to IR radiation as it passes through a sample?
Specific frequencies are absorbed by bonds, so they are missing from the transmitted radiation
Why are the absorbed frequencies important in IR spectroscopy?
The missing frequencies reveal the types of bonds present in the compound
What does an infrared spectrometer do?
It passes a beam of infrared radiation with a range of frequencies through a sample and detects which frequencies are absorbed by the bonds in the sample
What does the infrared spectrometer plot?
It plots a graph of transmittance (intensity of radiation that passes through) against frequency
What do the peaks in an IR spectrum represent?
The absorption of infrared radiation by specific bonds in the sample - each peak corresponds to a particular type of bond
What is the wavenumber range of the O-H bond in alcohols?
Broad peak, between 3230 - 3550 cm-1 indicates an O-H bond in alcohols
What is the wavenumber range of the O-H bond in carboxylic acids?
Very broad peek, between 2500 - 3000 cm-1, indicates an O-H in carboxylic acids
What is the wavenumber range for the C=O bond, what functional groups have this wavenumber range?
Peak between 1680 and 1750 cm⁻¹, found in aldehydes, ketones and carboxylic acids
What is the fingerprint region in an infrared spectrum?
The part of the spectrum below about 1500 cm-1, containing many peaks from complex vibrations of the whole molecule
Why is it called fingerprint region?
The pattern of the peaks is unique to each compound
At what wavelength does the fingerprint region begin?
Just below 1500 cm -1
What causes the greenhouse effect?
Gases in the atmosphere absorb infrared radiation emitted from the Earth’s surface, preventing from escaping into space
How does infrared radiation interact with greenhouse gases?
It is absorbed by the bonds in greenhouse gas molecules, causing them to vibrate
How is the absorption of infrared radiation by greenhouse gases similar to IR spectroscopy?
In both, specific bonds absorb IR radiation at characteristic frequencies, causing molecular vibrations
What role does carbon dioxide play in the greenhouse effect?
Carbon dioxide has two C=O bonds that absorb infrared radiation, contributing to global warming
Why is infrared radiation important in the context of global warming?
It is the type of energy emitted by the Earth’s surface; if absorbed by gases, it leads to increased atmospheric temperatures