Molecular Structure and Spectra

Question 1

What three functional groups are found in IR at 3300 cm^-1

Answer 1

1. Vinyllic hydrogen to a triple bond
2. N-H bond
3. O-H bond

Question 2

What two functional groups are found in IR at 3100 cm^-1?

Answer 2

1. Vinyllic hydrogen to a double bond
2. Carboxylic Acid

Question 3

What functional group is found in IR at 2900 cm^-1?

Answer 3

Alkane

Question 4

What functional group is found in IR at 2800 cm^-1?

Answer 4

Aldehydes.

Question 5

What functional group is found in IR at 2200 cm^-1?

Answer 5

Alkyne

Question 6

What functional group is found in IR at 1700 cm^-1?

Answer 6

Carbonyl bond

Question 7

What functional group is found in IR at 1600 cm^-1?

Answer 7

Alkane

Question 8

Describe intramolecular vibrations and rotations and which is measured in IR?

Answer 8

• Vibrations: bonds can stretch, compress and bend like a spring. It is this vibration that is measured in IR-spec.
• Rotations: molecules can rotate. Rotations produce waves mainly in the microwave region. However, part of the rotation spectra does overlap with the vibration spectra.

Question 9

Is the frequency of vibration higher or lower in the left peaks of IR?

Answer 9

Peaks toward the left have higher frequency of vibration.

Question 10

Hydrogen bonds cause (broad/sharp) peaks?

Answer 10

Broad peaks are due to hydrogen bonding

Question 11

Anything above 3000 cm^-1 in IR involves what type of bond:

Answer 11

Anything around 3000 cm^-1 involves a hydrogen atom, be it O-H, N-H, or C-H.

Question 12

Anything around 2000 cm-1 and below does not involve what type of bond?

Answer 12

Anything around 2000 cm^-1 and below does not involve hydrogen, be it C=O, C=C, C-C, or C-O.

Question 13

Rate from least sharp to most sharp in IR: O-H, N-H, or alkyne C-H

Answer 13

OH is the broadest. N-H is slightly sharper. Alkyne C-H is very sharp.

Question 14

H-indicator <–> H+ + Indicator-

Describe what happens at high pH, low pH, and neutral pH

Answer 14

• At low pH, high [H+], H-indicator and its color will predominate.
• At high pH, low [H+], indicator- and its color will predominate.
• At neutral pH, both H-indicator and indicator- will co-exist in an equilibrium, so the color will be a mixture of the two.

Question 15

Colors of the universal indicator and acidity

Answer 15

• Red: very acidic
• Orange: acidic
• Yellow: weakly acidic
• Green: neutral
• Blue: basic
• Purple: very basic
  *

Question 16

Why do double bonds absorb UV light?

Answer 16

Double bonds absorb UV because the pi electrons transition from the bonding and non-bonding molecular orbitals to the anti-bonding orbitals

Question 17

What do conjugated systems do to to energy and wavelength?

Answer 17

• Conjugated systems decreases the energy of electromagnetic radiation that is absorbed.
• The more conjugated double bonds there are, the longer the wavelengths of absorbed radiation.
• If there are enough conjugated double bonds, the molecule will start to absorb in the visible region.

Question 18

What happens during mass spectrometry?

Answer 18

Mass spec is when you bombard a molecule with electrons.

Question 19

What if electrons miss the molecule in MS?

Answer 19

Your molecule is neither fragmented nor ionized. Uncharged molecules are not detected and are not included in the mass spectra.

Question 20

What if the electrons do not break apart your molecule, but merely ionizes it? (MS)

Answer 20

this “molecular ion” will be detected as the parent peak, also called the molecular ion peak.

Question 21

What if the electrons not only ionize but also fragment your molecule? (MS)

Answer 21

all the fragmented ions will be detected and plotted in the mass spectra.

Question 22

The more fragmentation, the (smaller/larger) the molecular ion peak?

Answer 22

The more fragmentation, the smaller the molecular ion peak.

Question 23

What do peaks clustered close together in a mass spectrometer indicate?

Answer 23

Peaks clustered really close to one another depicts isotopes.

Question 24

What does the base peak show?

Answer 24

The base peak is the tallest peak (most abundant species).

Question 25

What is mass spec. useful for?

Answer 25

• Measuring the molecular weight of a molecule.
• Identify the molecule by fragmentation patterns.
• Identity heteroatoms by their characteristic isotope ratios.

Question 26

What is the HNMR ppm value for a carboxylic acid?

Answer 26

10-12 ppm

Question 27

What is the HNMR ppm for an aldehyde?

Answer 27

10 ppm

Question 28

What is the HNMR ppm for a phenyl group or an amide?

Answer 28

7ish ppm.

Question 29

What is the HNMR ppm for an alkene?

Answer 29

5

Question 30

What is the HNMR ppm for an alkyl halide?

Answer 30

4

Question 31

What is the HNMR ppm for an alkyne, alcohol, or amine?

Answer 31

3

Question 32

What is the HNMR ppm for an alkane?

Answer 32

0-2

Question 33

When things are deshielded, what happens to the magnetic field, the chemical shift, and the field?

Answer 33

LARGER magnetic field, LARGER chemical shifts, and DOWNFIELD (to the left)

Question 34

What determines the number of peaks in a HNMR spectra?

Answer 34

It’s the number of neighboring hydrogens plus 1.