CHAPTER 17 - SPECTROSCOPY

Question 1

What happens when an organic compound is placed in a mass spectrometer

Answer 1

Loses electron and forms a positive ion - the molecular ion

(pg 252)

Question 2

What does a mass spectrometer do?

Answer 2

Detect the mass-to-charge ratio (m/z) of the molecular ion which gives the molecular mass of the compound

Question 3

How do you find the molecular ion peak (M+ peak) on a mass spectrum

Answer 3

The clearest peak at the highest m/z value on the right hand side

Question 4

What is the M+1 peak and why does it happen

Answer 4

A small unit after the M+ peak

Due to 1.1% of carbon present as Carbon-13 isotope

Question 5

What is fragmentation

Answer 5

The breakdown of molecular ions down into smaller pieces known a fragments or fragment ions

Question 6

What are the other peaks on the mass spectrum caused by, not M+ or M+1 peaks

Answer 6

Fragment ions

Question 7

What does a simple fragmentation of a molecular ion form

Answer 7

Forms a fragment ion (charged and detected by spectrum)

Radical (uncharged and not detected by spectrum)

Question 8

Explain why there sometimes is a peak in the spectrum after the molecular ion peak

Answer 8

The M+1 peak arises because there is a small percentage of Carbon-13 in the sample and this adds one to the molecular mass of the molecular ion

Question 9

What are the two types of vibration

Answer 9

Stretch and Bend

Question 10

What is a stretch

Answer 10

A rhythmic movement along the line between atoms so that the distance between the atomic centres increase and decrease

Question 11

What is a bend

Answer 11

Change in bond angle

Question 12

What does the amount of bond stretch or bend depend on

Answer 12

The mass of the atom - Heavier atoms vibrate slower than lighter atoms

The strength of the bond - stronger bonds vibrate faster than weaker bonds

Question 13

What impact does Infra-red (IR) have upon bonds

Answer 13

Absorbed by bonds

makes them bend or stretch more

Question 14

What is wavenumber

Answer 14

The scale used in IR spectroscopy, used for convenient scale

Question 15

How are greenhouse gases affected by Infra-red radiation

Answer 15

Absorb longer-wavelength IR radiation, as it has the same frequency as the natural frequency of their bonds

Vibrating bonds in these molecules re-emit this energy as radiation that increases the temperature of atmosphere close to earth’s surface, leading to global warming

Question 16

What is IR spectroscopy used for

Answer 16

Identification of functional groups present in organic molecules

Question 17

What is the method for Infra-red Spectroscopy

Answer 17

Sample under investigation is placed inside an IR spectrometer

A beam if IR radiation in the range 200-4000 cm-1 is passed through the sample

The molecule absorbs some of the IR frequencies and the emerging bean of radiation is analysed to identify the frequencies absorbed by the sample

The IR spectrometer is usually connected to a computer that plots a graph of transmittance against wavenumber

(pg 258, 259 and 260)

Question 18

What is the fingerprint region

Answer 18

Below 1500cm-1, which contains unique peaks to identify the particular molecule under investigation

Question 19

What uses are there for IR spectroscopy

Answer 19

Identification of Pollutants

Remote sensors for vehicle emissions

Breathalysers

Question 20

What are the 3 techniques used for an identification sequence and what do each do

Answer 20

Elemental analysis -
use of percentage composition data to determine the empirical formula

Mass Spectrometry -
Use of the molecular ion peak from a mass spectrum to determine the molecular mass; use of fragment ions to identify sections of a molecule

Infrared Spectroscopy -
Use of absorption peaks from an infrared spectrum to identify bonds and functional groups present on a molecule