Analysis and Spectra Flashcards
Molecular spectra, whether NMR, IR, or mass spec, can be intimidating when they appear on the MCAT. Use these cards to master equivalent protons, memorize important shifts, and learn how these complex methods relate.
Two common analytic techniques in organic chemistry are infrared (IR) spectroscopy and ultraviolet (UV) spectroscopy. What do these procedures have in common?
Both techniques involve absorption of electromagnetic radiation by the molecule that is being analyzed.
Radiation with a specific frequency will be absorbed by the electrons in a molecule, exciting them to higher energy states.
What happens if a molecule is simultaneously exposed to several frequencies of light?
The molecule will only absorb radiation from the particular frequencies at which it has matching energy states. This concept is known as resonance.
Radiation with other frequencies will pass through or reflect. Spectroscopy, or the process of measuring the absorbed radiation, can yield information about the composition of the molecule.
What can infrared (IR) spectroscopy identify about an organic molecule?
IR spectroscopy can identify the functional groups present on the molecule.
Different covalent bonds vibrate at different, characteristic frequencies when light is absorbed by the molecule. These frequencies correspond to certain common groups.
In infrared (IR) spectroscopy, how can molecules with exactly the same atoms absorb light differently?
Different covalent bonds vibrate at different, characteristic frequencies. These vibrations can exist in different forms, such as stretching, bending, and rocking.
Molecules that contain the same atoms but different functional groups will absorb light at their specific characteristic frequencies. Their spectra will not necessarily overlap.
What determines the frequency of an IR vibration?
IR vibration frequencies are determined by the characteristics of a bond: the number of electron pairs, the polarity of the bond, and the masses of the atoms involved.
What qualities characterize bonds with high IR vibration frequencies?
Double and triple bonds, polar bonds, and bonds that include lighter atoms will have high vibration frequencies.
Remember, IR vibration frequencies are determined by the characteristics of the bond.
What qualities characterize bonds with low IR vibration frequencies?
Single bonds, nonpolar bonds, and bonds that include heavier atoms will have low frequencies.
Remember, IR vibration frequencies are determined by the characteristics of the bond.
How can a molecule be identified using an IR spectrum?
A molecule is identified via characteristic peaks in its spectrum.
Though functional groups may have many absorption peaks across the full spectrum, certain ranges are important to know.
Which functional group yields the IR peak shown below?
This IR peak corresponds to a hydroxyl (-OH) functional group.
Hydroxyl groups always give a broad absorption peak in the range of 3100-3500 cm-1.
Which functional group yields the IR peak shown below?
This IR peak corresponds to a carbonyl (C=O) functional group.
Carbonyl groups always give a sharp absorption peak around 1700 cm-1.
Which functional group yields the IR peak shown below?
This IR peak corresponds to a amine (N-H) functional group.
Amine groups always give a sharp absorption peak in the range of 3100-3500 cm-1.
What functional group must be present in a molecule with the IR spectrum shown below?
The molecule must contain a carbonyl functional group.
Carbonyl groups always give a sharp absorption peak around 1700 cm-1. The graph above is the spectrum of propanone.
What functional group must be present in a molecule with the IR spectrum shown below?
The molecule must contain a hydroxyl functional group.
Hydroxyl groups always give a broad absorption peak in the range of 3100-3500 cm-1. The graph above is the spectrum of propan-1-ol.
What functional group must be present in a molecule with the IR spectrum shown below?
The molecule must contain an amine functional group.
Amine groups always give a sharp absorption peak in the range of 3100-3500 cm-1. The graph above is the spectrum of 1-aminobutane.
What functional group must be present in a molecule with the IR spectrum shown below?
The molecule must contain a hydroxyl functional group.
Hydroxyl groups always give a broad absorption peak in the range of 3100-3500 cm-1. The graph above is the spectrum of ethanol.
What is the fingerprint region of an IR spectrum?
The fingerprint region is the area of the spectrum between 500 and 1450 cm-1.
Every molecule’s fingerprint region is unique, but the signal in this area tends to be complex. For the MCAT, don’t worry about identifying peaks in this area.