17 Spectroscopy

Question 1

What is a molecular ion? (1 mark)

Answer 1

A molecule that is ionised in a mass spectrometer.

Question 2

What does a molecular ion fragment into? (2 marks)

Answer 2

Another positive fragment ion (which is detected)
And a neutral radical (which is not detected)

Question 3

Why does the fragmentation happen in the spectrometer? (1 mark)

Answer 3

The covalent bonds have been weakened by the loss of the electron from the molecule.

Question 4

In a mass spectra for an alkane, how can you tell the stability of the carbocation? (1 mark)

Answer 4

Higher peak on mass spectra

Question 5

What do carbonyls typically fragment into? (2 marks)

Answer 5

Acylium ion
Alkyl radical

Question 6

What affect does the C-13 isotope have on the mass spectra? (1 mark)

Answer 6

Creates an ‘M+1’ peak ( a tiny peak after the molecular ion peak)

Question 7

What happens when an organic compound is placed in a mass spectrometer? (1 mark)

Answer 7

It loses an electron and forms THE MOLECULAR ION (M+).

Question 8

How can the number of carbon atoms be found? (Hint: formular involving M+1 height) (2 marks)

Answer 8

(Height of M+1 peak/ height of M+ peak) x 100

Question 9

What does infrared radiation do to covalent bonds? (1 mark)

Answer 9

Make them bend or stretch more

Question 10

If covalent bonds posses energy, what does this mean they do? (2 marks)

Answer 10

Vibrate naturally about a central point

Question 11

What is a stretch? (2 marks)

Answer 11

Rhythmic movement along the line between atoms
The distance between the two atoms centres increases and decreases.

Question 12

What does a bend result in? (1 mark)

Answer 12

A change in bond angle.

Question 13

What does how much a bond stretches/bends rely on? (2 marks)

Answer 13

Mass of the atoms in the bond (heavier = vibrate slower).

Strength of the bond (stronger = vibrate faster)

Question 14

Any bond can only absorb radiation that…
(2 marks)

Answer 14

…has the same frequency as the natural frequency as the natural frequency pf the bond.

Question 15

Describe and explain how global warming occurs.
(4 marks)

Answer 15

Some visible and IR radiation is re-emitted from the Earth’s surface,
in the form of longer-wavelength IR.
Greenhouse gases absorb these because it has the same frequency as their natural bonds.
Eventually, the vibrating bonds re-emit this energy as radiation,
that increases the temperature of the atmosphere close to the earth’s surface.

Question 16

IR spectroscopy can be used to identify the functional groups present in organic molecules. Describe how an IR spectroscope works.
(5 marks)

Answer 16

1. Place sample inside
2. Beam of IR radiation (200-4000cm-1) is passed through sample
3. Molecule absorbs some of the IR frequencies
4. The emerging beam is analysed to find out which frequencies were absorbed
5. Usually connected to a computer which plots a graph of transmittance against wavenumber

Question 17

What is the typical sequence of techniques used for identification of an organic sample?
(3 marks)

Answer 17

Elemental analysis (% composition)
Mass spectrometry
IR spectroscopy