Organic Chemistry 101: Analysis and Spectra

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.

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.

It 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.

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.

They are determined by the characteristics of a bond:

• number of electron pairs

• polarity of the bond

• masses of the atoms involved

• double and triple bonds

• polar bonds

• bonds that include lighter atoms

Remember, IR vibration frequencies are determined by the characteristics of the bond.

• single bonds

• nonpolar bonds

• bonds that include heavier atoms

Remember, IR vibration frequencies are determined by the characteristics of the bond.

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.

hydroxyl (-OH) functional group

Hydroxyl groups always give a broad absorption peak in the range of 3100-3500 cm-1.

carbonyl (C=O) functional group

Carbonyl groups always give a sharp absorption peak around 1700 cm-1.

amine (N-H) functional group

Amine groups always give a sharp absorption peak in the range of 3100-3500 cm-1.

carbonyl functional group

Carbonyl groups always give a sharp absorption peak around 1700 cm-1. The graph above is the spectrum of propanone.

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.

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.

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.

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.

The fingerprint region, which covers the range from 500 cm-1 to 1450 cm-1, can be disregarded.

The fingerprint region often does not contain distinct characteristic peaks.

It can identify a molecule’s electronic energy levels.

Electrons bound to different molecules will have different excited states. These molecules will absorb photons of light that correspond to the difference between the ground and excited energy levels, which exist in the visible and near-UV regions of the spectrum.

It must have nonbonding electrons that can be excited without breaking a bond.

The most common examples are:

• conjugated molecules

• molecules with double and triple bonds

• molecules with nonbonding pi electrons

• molecules that contain transition metals

Molecules are complementary in color to the light that they absorb.

For example, if a molecule is illuminated with white light and absorbs green light, it must reflect light from the remainder of the spectrum, namely red. The molecule will appear reddish in color.

The molecule must absorb green light.

More broadly, the absorbed light must have a frequency around the yellow/green/blue area of the spectrum.

A molecule that absorbs green light will reflect red, making it appear reddish in color.

The molecule must absorb red light.

A molecule that absorbs red light will reflect light in the blue/green/yellow area of the spectrum, causing it to appear green in color.

It measures the path curvature of molecular fragments from a sample. This is then related to a “mass-to-charge ratio” for each fragment.

The mass-to-charge ratio can then be used to calculate molecular mass.

It bombards a vapor sample with sufficiently high-energy electrons to both fragment and ionize the sample.

The charged ions and fragments are then accelerated using an electric field and bent through a curved detector using a magnetic field. The radius of curvature for the ions depends on their mass and charge, allowing total mass to be calculated.

Mass spectrometry is never a good separation technique. It is used for analysis, not separation / purification.

Be careful to distinguish separation techniques from analytic ones. Mass spectrometry, for example, destroys the molecule that is being analyzed, so it would not be used to separate components of a mixture for later use. Actual separation techniques include extraction and distillation.

Nuclear Magnetic Resonance

Although there are multiple types of this process, only proton (H+) NMR is tested.

NMR operates by exposing a molecule in the solvent phase to a large, permanent magnetic field. It then measures its resonant absorptions of electromagnetic radiation.

It measures the environment of the protons, or hydrogen atoms, in a molecule.

Hydrogens bound to different atoms, and in different configurations, will respond differently to magnetic fields. This concept forms the essence of H+ NMR spectroscopy.

The chemical shift of a proton depends on how deshielded it is by the other atoms in the molecule.

Deshielding generally refers to the proton's proximity to electronegative groups. The less electron density an atom retains, the more deshielded it is, and the further downfield its chemical shift will be. Resonance structures and highly electronegative atoms tend to deshield protons.

This peak would be considered to be downfield. NMR peaks are relative, but the more downfield a peak is shifted, the higher its value will be. 9.5 ppm is significantly higher than a characteristic alkane peak around 0-4 ppm.

Downfield shifts are the result of deshielding, usually due to proximity to electronegative groups. 9.5 ppm is near the characteristic shift for an aldehyde.

The molecule contains non-identical protons. Signals in an NMR spectrum represent different chemical shifts due to different proton environments.

Identical protons will display signals at the same point, such as the protons in methane. In contrast, a molecule with many different proton environments will have multiple peaks, such as butanoic acid (4 peaks).

The set of equivalent protons corresponding to the larger peak must contain three times as many protons as the other. The area of an NMR peak is proportional to the number of identical protons with the given chemical shift.

Identical protons will have signals at the same point, and their signals will add up to a single peak in the measured spectrum. Integrated area is often given if it is necessary, but peak height can also be used as a valid approximation.

between 0 and 4 ppm

For example, the protons on ethane have an NMR value of about 0.86.

between 4 and 7 ppm

For example, the protons on ethene have an NMR value of about 5.35.

between 6 and 8 ppm

For example, the protons on benzene have an NMR value of about 7.28.

a downfield shift of up to 3 ppm

For example, a proton in methane has a peak at about 0.25 ppm, while the chemical shift of a proton in chloromethane is around 3.0 ppm. The electronegative chlorine atom draws electron density towards itself, deshielding the other protons in the molecule by 2.75 ppm.

a downfield chemical shift between 8.5 and 11 ppm.

Simply remember this value as around 9.5 ppm.

For example, the protons on formaldehyde have an NMR value of about 9.61.

a downfield chemical shift between 10 and 12.5 ppm

For example, the carboxylic acid proton on acetic acid has an NMR value of about 11.8.

This molecule has only one peak in the range for aromatic protons, so all protons are likely part of an aromatic structure.

This is the NMR spectrum of benzene, a common organic molecule.

This molecule has two unique peaks in the sp3-hybridized range, probably corresponding to alkane protons. It also contains a single proton in the aldehydic range.

This is the NMR spectrum of propanal, an aldehyde.

It results in peaks that look jagged. A peak that displays splitting will look like multiple peaks packed extremely close together.

However, such a peak will still only refer to one proton or set of equivalent protons.

Each unique proton's peak is split into (n+1) subpeaks, where n is the number of protons bound to adjacent carbons.

"Adjacent" refers to carbons next to the carbon to which the proton in question is bound.

The blue peak is a doublet, meaning that the adjacent carbon is bound to one proton.

Since the peak is split into a doublet (2 subpeaks), the (n+1) rule states that a single adjacent proton must be present (n = 1). To recall the n+1 rule, remember that a peak corresponding to a proton with zero adjacent protons will be a singlet.

The spectrum shown above is of propene.