7 - Modern analytical techniques

Question 1

What does the largest m/z mean in a mass spectrum?

Answer 1

The peak would be caused by the complete molecule, and will be equal to the Molar mass of the molecule. This peak is called the molecular ion.

Question 2

What type of substance is read by the mass spectrometer?

Answer 2

positive ions.

Question 3

How does a molecular ion form and how do you draw it?

Answer 3

• bombarded with electrons.

- brackets with positive charge and dot

Question 4

What is fragmentation? What is formed by fragmentation?

Answer 4

• when the molecular ion is broken into fragments by breaking covalent bonds.
• positive ion (responsible for the peak).
• free radical

Question 5

what does a molecule become?

Answer 5

molecular ion e.g [CH3CH2CHO]+.

Question 6

what does molecular ion become

Answer 6

• positive ion (gets read by spectrometer).

- radical.

Question 7

What do the peaks indicate on the mass spectrum?

Answer 7

The mass/charge for every fragment.

Question 8

What sort of modern analytical technique is used in breath alcohol anaslyzers?

Answer 8

Infrared spectroscopy.

Question 9

What is infrared spectroscopy used for?

Answer 9

• predicting the spectrum of an organic compound
• deducting the functional groups from a list of wave numbers.
• deducing the structure from wavenumbers and molecular formula.

Question 10

What does the amount of infrared radiation energy absorbed by a molecule depend on (stretching vibrations).

Answer 10

• length of the bond
• strength of the bond
• mass of each atom involved in the bond.

Question 11

Why don’t simple non-polar molecules e.g H2 and Cl2 absorb infrared radiation?

Answer 11

They do not have a dipole. When the molecule stretches due to a vibration, there is no change in the dipole, so there is no absorption of IR.

Question 12

How does bond stretching occur?

Answer 12

Bond absorbs IR and uses it to alter the length of the bond.

Question 13

What is the fingerprint region?

Answer 13

• between 1500 and 500 cm^-1
• caused by the vibration of the molecule as a whole (and not the individual bonds).
• the fingerprint region is unique for every compound.

Question 14

What does the region above 1500^-1 show? (above fingerprint region)

Answer 14

the points of strong absorption (above 1500cm^-1) can show what bonds and functional groups are present in a compound.

Question 15

Do we use the fingerprint region (1500 - 500cm^-1) when identifying bonds from wave numbers?

Answer 15

• no

- use above 1500cm^-1.