TOPIC 7 - MODERN ANALYTICSL TECHNIQUES I + II

Question 1

molecular ion peak

Answer 1

. Species formed from the molecule by the loss of one electron.
e.g. C4H10 + e- —-> C4H10^+ + 2e-

When an organic molecule loses an electron in the mass spectrometer- this can occur when an electron collides with the molecule, knocking out an electron so forming a positive ion.

Question 2

M+1 peak

Answer 2

A very small peak just to the right of the molecular ion peak. The naturally occurring isotope of carbon (carbon-13).

Question 3

Fragmentation

Answer 3

Molecular ion are energetically unstable breaks into smaller pieces:

1. Another +ve ion
2. A radical (neutral species) - does not show on mass spec as no charge and removed by vaccum pump.

So all peaks caused by +ve ions

• identical products of ethane = ch3+ + ch3 have same m/z = 15

Question 4

m/z ratio

Answer 4

mass/charge ratio (no. of e- lost)
- indicates which ions are present,  but height of peak indicates abundance.
m/1 (normal)
m/2 (halved)
m/3 (third)

• C=0 BONDS NEVER ALONE DO TO MASS SPEC NOT HAVING POLAR BONDS
• CH2 NEVER ON ITS OWN
• IF HAVE CARBOXYILIC ACID C=O-OH WRITE LIKE CHO+
• ALWAYS DRAW MOLECULE OUT TO HELP THEN SPLIT.

Question 5

Base peak

Answer 5

peak with the greatest abundance.
= tallest peak
most stable, common fragment

Question 6

What is mass spectrometry used to differentiate between?

Answer 6

Similar molecules

- used for carbon dating/half life as well

Question 7

mass spec of an element

Answer 7

means the no.of peaks means the no of isotopes of that element.

Determine peaks correspond to Br having 2 isotopes:

[Br79 - Br79] +
[Br79 - Br81]+ [Br81-Br79]+
[Br81 - Br81]+

Question 8

Infrared spectroscopy

Answer 8

Bonds between different atoms absorb different frequencies of IR radiation. POLAR BONDS SHOWN IN IR spec. That’s why polar bonds CO2, H2O and NO absorb IR - greenhouse gases - global warming. Non-polar molecules are NOT shown. A beam of IR radiation is passed through a sample of a chemical
It is absorbed by covalent bonds in the molecules, increasing their vibrational energy causes effect on bonds like Stretching and bending.

changes in vibration depend on frequency. High peak = high freq
EM wave and radiates sample detects the intensity of wavelength of IR
stretching - bond length increases and decreases.
bending- bond angle increases and decreases. CAUSES DIPS IN THE GRAPH

In the frequency absorbed, so OH groups on alcohols and carboxylic acids have different frequencies of absorption - SO LOCATION IS IMPORTANT.

IR graph:
y axis - Transmittance - value in an infrared spectrum that represents the amount of infrared radiation not absorbed (transmitted) at a particular wave number.
100% = NO ABSORPTION

x axis - Wavenumber - 1/wavelength in cm - represents the frequency of the infrared radiation that was absorbed by a particular bond in a molecule

H-BONDING CAUSES BROADNESS.
HAVE NO H ATOMS = PEAK SHARPER.

Question 9

Uses of IR

Answer 9

• predict spectrum of organic compounds
• deducing functional groups (polar bonds)
• deducing structure from wave no. and molecular formula.

Question 10

fingerprint region

Answer 10

unique for each compound caused by absorption from bending.

use to compare compounds and identify them