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MCAT Org. Chemistry > Analysis and Spectra > Flashcards

Flashcards in Analysis and Spectra Deck (42):
1

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.

2

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.

3

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.

4

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.

5

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.

6

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.

7

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.

8

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. 

9

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.

10

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

11

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

12

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 below is the spectrum of propanone.

13

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 below is the spectrum of propan-1-ol.

14

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 below is the spectrum of 1-aminobutane.

15

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 below is the spectrum of ethanol.

16

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. 

17

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

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. 

18

What can UV-visible spectroscopy identify about an organic molecule?

UV-visible spectroscopy 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.

19

What features characterize the organic molecules that display signals in UV-vis spectroscopy?

Molecules that appear active in UV-vis must have nonbonding electrons that can be excited without breaking a bond.

The most common examples are:

  • conjugated molecules (most important for the MCAT)
  • molecules with double and triple bonds
  • molecules with nonbonding pi electrons
  • molecules that contain transition metals

 

20

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

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.

21

What color of light must be absorbed by a molecule that appears 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.

22

What color of light must be absorbed by a molecule that appears greenish in color?

The molecule must absorb mostly 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.

23

What does a mass spectrometer measure?

Mass spectrometers measure 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.

24

Briefly describe how a mass spectrometer works.

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 depends on their mass and charge, allowing total mass to be calculated.

25

A student is looking for a lab technique that will help him separate two components of a sample. What conditions must be satisfied for mass spectrometry to be a good choice?

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

On the MCAT, 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.

26

What does the "NMR" stand for in NMR spectroscopy?

NMR stands for nuclear magnetic resonance. Although there are multiple types of this process, only proton (H+) NMR is tested explicitly on the MCAT.

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.

27

At the molecular level, what does H+ NMR spectroscopy measure?

H+ NMR spectroscopy 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.

28

What determines the chemical shift of a proton in 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.

29

A proton near a certain functional group displays an NMR peak of 9.5 ppm. Would this peak be considered to be upfield or downfield?

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.

30

A certain NMR spectrum displays multiple distinct peaks. What does this show about the molecule?

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).

31

A certain NMR signal has an integrated area three times that of another peak. What does this show about the protons involved?

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. On the MCAT, integrated area is often given if it is necessary, but peak height can also be used as a valid approximation.

32

In what range is the NMR chemical shift of a proton bound to an alkane (sp3-hybridized)?

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

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

33

In what range is the NMR chemical shift of a proton bound to an alkene (sp2-hybridized) carbon?

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

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

34

In what range is the NMR chemical shift of a proton bound to an aromatic carbon?

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

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

35

By what amount does an adjacent electronegative atom usually affect the chemical NMR shift of a proton?

Nearby electronegative atoms and functional groups cause 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.

36

In what range is the NMR chemical shift of a proton bound to an aldehydic (CHO) carbon?

A proton bound to an aldehydic carbon will have a downfield chemical shift between 8.5 and 11 ppm. For the MCAT, simply remember this value as around 9.5 ppm.

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

37

In what range is the NMR chemical shift of a carboxylic acid (COOH) proton?

A carboxylic acid proton will have 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.

38

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

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 and one often mentioned on the MCAT.

39

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

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.

40

What does spin-spin splitting look like in an NMR spectrum?

Spin-spin splitting 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.

41

In proton NMR, how can one determine the amount of splitting of a single peak?

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.

42

With regard to splitting, what name is given to the blue peak (far right) in the NMR spectrum below, and how many adjacent protons does it represent?

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, simply remember that a peak with zero adjacent protons must still be a singlet.

The spectrum shown below is of propene.