Spectra

Question 1

How do molecules absorb electromagnetic radiation of a specific frequency?

Answer 1

Electromagnetic radiation of specific frequencies will be absorbed by the electron(s) in a molecule, by exciting them to higher energy states.

Energy levels, electron states, and frequencies must be matched exactly (resonance), or radiation will not be absorbed.

Question 2

What happens if a molecule is exposed to many frequencies of light simultaneously?

Answer 2

When the molecule is exposed to many frequencies of light simultaneously, it will only absorb radiation from those particular frequencies at which it has matching energy states (resonance).

Other wavelengths will pass through or reflect unaffected. The process of measuring these absorbtions is known as spectroscopy. For example, a molecule with carbons, hydrogens, and oxygens could be formed into an alcohol or aldehyde, which each have very different absorbtion values.

Question 3

Why do molecules, which can have the same exact atoms, absorb light in Infrared (IR) spectroscopy differently?

Answer 3

Different covalent bonds vibrate at different, characteristic frequencies. These vibrations can come in different forms, such as stretching, bending, and rocking.

Molecules containing the same atoms, but different functional groups will absorb light at their specific characteristic frequencies and not necessarily overlap.

Question 4

What does Infrared (IR) spectroscopy identify about an organic molecule?

Answer 4

IR spectroscopy identifies the functional groups present on an organic molecule.

Different covalent bonds vibrate at different, characteristic frequencies, and those vibrations are due to light being absorbed into the molecule.

Question 5

What determines the frequency of an IR vibration?

Answer 5

IR vibration frequencies are determined by the characteristics of a bond: number of electron pairs, the polarity of the bond, and the masses of the atoms in the bond.

Question 6

What determines a high frequency of an IR vibration?

Answer 6

Double and triple bonds, polar bonds, and bonds including lighter atoms will have higher frequencies.

IR vibration frequencies are determined by the characteristics of a bond.

Question 7

What determines a low frequency of an IR vibration?

Answer 7

Single bonds, nonpolar bonds, and bonds including heavier atoms will have lower frequencies.

IR vibration frequencies are determined by the characteristics of a bond.

Question 8

How is a molecule identified using an IR spectroscopy spectrum?

Answer 8

A molecule is identified via characteristic peaks in its IR spectrum.

Though functional groups may have many absorbtion peaks across the full spectrum, certain ranges of peaks will be unique to that molecule.

Question 9

What functional group yields the IR peak shown below?

Answer 9

This IR peak corresponds to a hydroxyl (O-H) functional group.

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

Question 10

What functional group yields the IR peak shown below?

Answer 10

This IR peak corresponds to a carbonyl (C=O) functional group.

Carbonyl groups always give a sharp absorption peak in the range of 1700-1800 cm^-1.

Question 11

What functional group yields the IR peak shown below?

Answer 11

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.

Question 12

What functional group is present in the molecule whose IR spectrum is shown below?

Answer 12

The molecule must contain a carbonyl functional group.

Carbonyl groups always give a sharp absorption peak in the range of 1700-1800 cm^-1. The graph below is for propanone.

Question 13

What functional group is present in the molecule whose IR spectrum is shown below?

Answer 13

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 below is for propan-1-ol.

Question 14

What functional group is present in the molecule whose IR spectrum is shown below?

Answer 14

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 below is for 1-aminobutane.

Question 15

What functional group is present in the molecule whose IR spectrum is shown below?

Answer 15

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 below is for ethanol.

Question 16

What is the fingerprint region of an IR spectrum?

Answer 16

The fingerprint region of an IR spectrum is the area between 500 and 1450 cm^-1.

Every molecule’s fingerprint region is unique, but the signal in this area tends to be complex. Assigning peaks in the fingerprint region is not necessary on the MCAT.

Question 17

What region of the IR spectroscopy spectrum can generally be disregarded?

Answer 17

The “fingerprint region”, in the range 500cm^-1 to 1450cm^-1, is diregarded.

The fingerprint region often has no one distinct characteristic peak.

Question 18

What does UV-Visible spectroscopy identify about an organic molecule?

Answer 18

UV-Visible spectroscopy identifies the electronic energy levels of an organic molecule.

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

Question 19

What are the common features of organic molecules which show signals in UV-Vis Spectroscopy?

Answer 19

UV-Vis active molecules have nonbonding electrons which 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 containing transition metals

Question 20

What does the color of an organic molecule indicate about the frequency of light that molecule absorbs?

Answer 20

The color of the molecule is the complimentary color of the frequency it absorbs.

Ex: if a molecule is illuminated with white light, and absorbs green light (the yellow/green/blue range), it will reflect the red photons (the remaining spectrum), and appear reddish in color.

Question 21

What color photons are absorbed by a molecule which appears reddish in color?

Answer 21

The molecule absorbs yellow/green/blue photons.

A molecule which absorbs green light will reflect red photons, and appear reddish in color.

Question 22

What color photons are absorbed by a molecule which appears greenish in color?

Answer 22

The molecule absorbs mostly red photons.

A molecule which absorbs red light will reflect blue, green and yellow photons, and appear greenish in color.

Question 23

What color photons are absorbed by a molecule which appears orangeish in color?

Answer 23

The molecule absorbs blue and green photons.

A molecule which absorbs blue and green light will reflect red and yellow photons, and appear orangeish in color.

Question 24

What color photons are absorbed by a molecule which appears blue in color?

Answer 24

The molecule absorbs mostly red and yellow photons.

A molecule which absorbs red and yellow light will reflect blue photons, and appear blue in color.

Question 25

What does a mass spectrometer measure?

Answer 25

Mass spectrometers measure the path curvature for molecular fragments from a sample, and relates that to a “mass-to-charge” ratio for each fragment.

The mass-to-charge ratio of fragments from the substance is characteristic based on the radius of curvature, and total molecular mass can then be calculated.

Question 26

How does a mass spectrometer work?

Answer 26

A mass spectrometer 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 is a function of their mass and charge, allowing total mass to be calculated.

Question 27

What is the quantity reported by a mass spectrometer?

Answer 27

The Mass-Charge Ratio of various ions and fragments is the quantity actually reported in mass spectrometry.

By looking at all the various fragments that appear after electron bombardment, the mass of the original molecule is determined.

Question 28

What is NMR spectroscopy, and what is the general process?

Answer 28

NMR spectroscopy is (proton) Nuclear Magnetic Resonance.

It operates by exposing a molecule in solvent phase to a large permanent magnetic field, then measuring its resonant absorption of smaller RF-frequency electromagnetic radiation.

Although there are several kinds of NMR spectroscopy, only proton(H⁺) NMR is tested explicitly on the MCAT.

Question 29

What does H⁺NMR spectroscopy measure?

Answer 29

H⁺NMR spectroscopy measures the environment of the bare protons (hydrogen atoms) in a molecule.

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

Question 30

What is measured by the chemical shift of various protons in NMR spectroscopy?

Answer 30

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

The less electron density a proton retains, the more deshielded it is, and the further downfield (large ppm) its chemical shift will be. Resonance structures and highly electronegative atoms tend to deshield protons more thoroughly.

Question 31

What does an NMR spectrum with multiple peaks show about the molecule?

Answer 31

The peaks (or signals) in an NMR spectrum are different chemical shifts due to different proton environments.

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

Question 32

What is being measured by the integrated area of a peak in NMR spectroscopy?

Answer 32

The area of an NMR spectrum 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. On the MCAT peak height can be used as a valid approximation of integrated area.

Question 33

What is the range for chemical shift of a proton bound to an alkane (sp³-hybridized) carbon?

Answer 33

A proton bound to an sp³-hybridized carbon will have a downfield chemical shift between 0 and 4 ppm.

Ex: ethane has an NMR value of about 0.86.

Question 34

What is the range for chemical shift of a proton bound to an alkene (sp²-hybridized) carbon?

Answer 34

A proton bound to an sp²-hybridized carbon will have a downfield chemical shift between 4 and 7 ppm.

Ex: ethene has an NMR value of about 5.35.

Question 35

What is the range for chemical shift of a proton bound to an alkyne (sp-hybridized) carbon?

Answer 35

A proton bound to an sp-hybridized carbon will have a downfield chemical shift between 2 and 4 ppm.

Ex: ethyne has an NMR value of about 2.05.

Question 36

What is the range for chemical shift of a proton bound to an aromatic carbon?

Answer 36

A proton bound to an aromatic carbon will have a downfield chemical shift between 6 and 8 ppm.

Ex: benzene has an NMR value of about 7.28.

Question 37

What is the NMR shift for a proton due to an adjacent electronegative atom or functional group?

Answer 37

Nearby electronegative atoms and functional groups result in a downfield shift of 0 to 3 ppm.

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

Question 38

What is the range for chemical shift of a proton bound to an aldehydic (CHO) carbon?

Answer 38

A proton bound to an aldehydic carbon will have a downfield chemical shift between 8.5 and 11 ppm.

Ex: formaldehyde has an NMR value of about 9.61.

Question 39

What is the range for chemical shift of a carboxylic acid (COOH) proton?

Answer 39

A carboxylic acid proton will have a downfield chemical shift between 10 and 12.5 ppm.

Ex: acetic acid has an NMR value of about 11.8.

Question 40

What types of protons are present in the molecule whose NMR spectrum appears below?

Answer 40

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

This is the NMR spectrum of benzene, a common example molecule for the MCAT.

Question 41

What types of protons are present in the molecule whose NMR spectrum appears below?

Answer 41

This molecule has protons with two unique peaks in the sp³-hybridized range, and a single proton in the aldehydic range.

This is the NMR spectrum of propanal, a common example molecule for the MCAT.

Question 42

Define:

spin-spin splitting in an NMR spectrum

Answer 42

Spin-spin splitting is the phenomena which explains the fine “jagged” structure of the peaks in an NMR spectrum.

Each unique proton’s peak is split into a series of subpeaks packed extremely close together.

Question 43

What is the rule for number of subpeaks in a proton’s NMR?

Answer 43

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

Question 44

How many protons are present on the adjacent carbon to the proton represented by the blue peak (far right) in the NMR spectrum below?

Answer 44

The adjacent carbon has one proton bound to it.

Since the peak is split into a double (2), the (n+1) rule states that 2 = n + 1, so n = 1 proton, hence the adjacent carbon has one proton bound to it.

The spectrum shown is of propene.