IR spectroscopy and mass spectrometry.

Question 1

How is a molecular ion formed?

Answer 1

When an organic compound is placed in a mass spectrometer, loses an electron and forms a positive ion.

Question 2

What does a mass spectrometer do?

Answer 2

Detects the mass to charge ratio (m/z) of the molecular ion(M+), which then gives the molecular mass of the compound.

Question 3

How to find the molecular mass from a mass spectrum.

Answer 3

Locate the M+ peak.
This is the highest m/z value on the right side of the mass spectrum.

Question 4

What is an M+1 peak?

Answer 4

A very small peak 1 unit after the M+ peak because 1.1% of carbon is present as C-13.

Question 5

What is fragmentation?

Answer 5

When some molecular ions break down into smaller pieces(fragments).
This causes other peaks in a mass spectrum from these fragment ions.
Breaks molecular ion into 2 species-Positive fragment ion and a radical.

Question 6

How to calculate a compounds formula from the mass spectra.

Answer 6

PRACTICE QUESTIONS.

Question 7

How to calculate the number of carbon atoms from a mass spectra.

Answer 7

Height of M+1 peak/Height of M peak x 100.

Question 8

How does Infrared radiation cause covalent bonds to vibrate more and absorb energy?

Answer 8

Bonds possess energy and vibrate around a central point.
Increased temperature=increased vibration.
Absorb more IR=bonds can bend/stretch more.

Question 9

Stretch definition.

Answer 9

A rhythmic movement along the line between atoms.
Distance between 2 atomic centres decreases and increases.

Question 10

Bend definition.

Answer 10

Results in a change in bond angle.

Question 11

Factors affecting bend/stretch amounts of a bond.

Answer 11

Heavier atoms vibrate more slowly than lighter ones.
Stronger bonds vibrate faster than weaker ones.
Any bond can only absorb radiation that has the same frequency as the natural frequency of the bond.

Question 12

Absorption of IR by atmospheric gases.

Answer 12

Water vapour, CO2 and methane absorb longer wave IR radiation(same frequency as natural frequency of their bonds)(C=O, O-H, C-H).
Vibrating bonds in these molecules re-emit this energy as radiation.
Increases temperature of atmosphere close to earth’s surface=global warming.
More emphasis on renewable energy that does not emit CO2.

Question 13

What is infrared spectroscopy used for?

Answer 13

To identify functional groups in organic molecules.

Question 14

Where are the peaks in an IR spectrum?

Answer 14

The dips in the graphs.

Question 15

What is the fingerprint region?

Answer 15

Below 1500cm^-1.
Used to identify the particular molecule under investigation.

Question 16

Infrared spectra of common functional groups.

Answer 16

Outside fingerprint region:
Alcohols(O-H)=3200-3600.
Aldehydes/ketones(C=O)=1630-1820.
Carboxylic acids(COOH)=1630-1820(C=O) and 2500-3300(O-H,broad peak).
ALL organic compounds have peak at 2850-3100 because of C-H.

Question 17

Uses of infrared spectroscopy.

Answer 17

Detecting pollutants in city centres, IR spectra detect vehicle emissions and measure CO, CO2 and other hydrocarbons to monitor pollution.
IR-based breathalysers, bonds present in ethanol are detected, more IR absorbed=more ethanol in breath.

Question 18

Sequence to interpret and predict/identify substance.

Answer 18

Elemental analysis(% composition data to find empirical formula)
Mass spectrometry(M+ peak for molecular mass , fragment ions for sections of a molecule)
Infrared spectroscopy(absorption peaks to identify bonds and functional groups present).
PRACTICE QUESTIONS.