Mass spectrometry and infrared spectrometry

Question 1

What can mass spectrometry be used to provide?

Answer 1

information about the structure of a molecule, it’s molecular mass, and its molecular formula

Question 2

What happens during a mass spectra of ethanol vapour?

Answer 2

-electrons from an electron gun knock electrons off the molecules of ethanol
-molecules usually contain pairs of electrons so when an electron is removed, an unpaired electron remains leaving the ion with a positive charge (called a radical ion)

-ethanol molecules lose an ion:
C2H5OH —-> [C2H5OH]+*
-then the positively charged ethanol ions are accelerated by an electric field, pass through a drift area, and then they are detected

Question 3

What symbol does the molecular ion have?

Answer 3

M+*

Question 4

How do you find out the molecular mass (Mr)

Answer 4

the m/z value for the molecular ion peak

Question 5

What can happen to molecular ions when they undergo fragmentation?

Answer 5

they form smaller ions and radicals

Question 6

What is a problem with only having a particular mass for a sample?

Answer 6

there are frequently several possible nuclear formulae for a particular mass, and so we need to know the mass of the particular ion more accurately

Question 7

How do we get a more accurate mass?

Answer 7

by using a high resolution mass spectrometer

Question 8

How many decimal places can be accurate?

Answer 8

4 decimal places

Question 9

What is infrared spectroscopy used for?

Answer 9

to identify the functional groups present in organic compounds

Question 10

How does infrared spectroscopy identify functional groups?

Answer 10

-chemical bonds absorb infrared radiation of specific frequencies so a particular bond can be matched to a particular absorption range on the infrared spectrum

Question 11

What happens when the molecules are exposed to infrared radiation?

Answer 11

it is absorbed by the bonds which makes them vibrate even more

Question 12

What are the three ways bonds can move?

Answer 12

-symmetric stretching
-bending
-asymmetric stretching

Question 13

What does the frequency of absorbed infrared radiation by different bonds depend on?`

Answer 13

-the bond strength
-the masses of the atoms

strong bonds between light atoms are high energy with large wavenumbers

weak bonds between heavy atoms are low energy with small wavenumbers

Question 14

Which bonds stretch?

Answer 14

O-H/ C-H/ N-H

Question 15

What does the energy corresponding to a bond stretch or bend correspond to?

Answer 15

a particular wavelength of electromagnetic radiation

Question 16

How do you calculate the frequency/wavelength/wavenumber?

Answer 16

E = hv
v = c/λ

E-energy
h-planck constant
v-frequency (nu)
c-velocity of light (s-1)
λ-wavelength (m)

Question 17

What is the unit of wavelength?

Answer 17

(cm-1)

Question 18

Why are wavenumbers used?

Answer 18

they are numerically more convenient than frequency or wavelength

Question 19

What are wavenumbers?

Answer 19

reciprocal wavelengths

1/λ cm-1

Question 20

What happens in the infrared spectrometer?

Answer 20

-the organic sample is subject to a continuous range of infrared frequencies
-this sample will absorb infrared radiation corresponding to the energies of the bond stretches and bends in it’s molecules
-by recording the intensity of the remaining infrared radiation a spectrum can be plotted
-the infrared spectrum can be used to identify the functional groups and the compound

Question 21

How do peaks/troughs appear on the spectrum?

Answer 21

when infrared radiation is absorbed by the bonds the transmission is reduced

Question 22

What shape do C-H bonds make?

Answer 22

typically sharp, longer than C≡N

Question 23

What shape do C≡N bonds make? (nitriles)

Answer 23

nitriles are very sharp, shorter than C-H

Question 24

What shape do O-H bonds make? (alcohols/carboxylic acids)

Answer 24

broad (due to hydrogen bonding)

Question 25

What shape do C=O bonds make? (aldehydes/ketones/carboxylic acids/esters)

Answer 25

sharp and very long

Question 26

What is the fingerprint region?

Answer 26

-1500-400 cm-1

-a complex region of the spectrum that contains that contains many peaks/a wide variety of bond vibrations

Question 27

What does the entire infrared spectrum of an organic compound provide?

Answer 27

a unique molecular fingerprint

Question 28

How can scientists identify a compound in the fingerprint region?

Answer 28

-compare the unknown spectrum with spectra from an infrared data-base
-an exact match means that the scientist has identified the compound since the fingerprint region is unique to each compound

Question 29

What does no match mean when comparing the fingerprint region?

Answer 29

no match means a unique fingerprint so the scientist may have made a new compound or it could be impure

Question 30

Name the approximate wavelengths for-
C=O
O-H
carboxylic acids

Answer 30

C=O - 1700cm-1
O-H - 3300cm-1
carboxylic acids- 3000cm-1