4.2.4 Analytical Techniques

Question 1

What happens when a molecule absorbs infrared radiation?

Answer 1

• It makes the covalent bond vibrate more in a stretching or bending motion

Question 2

What factors affect the amount of vibration of a bond?

Answer 2

• Bond strength
• Bond length
• Mass of each atom in the bond

Question 3

How does infrared spectroscopy work?

Answer 3

• Every bond has a unique vibration frequency in the infrared region of the EM spectrum
• Bonds absorb radiation with the same frequency as their vibration
• Infrared radiation emerging from the sample is missing the frequencies absorbed, which can identify the functional group

Question 4

What do the peaks on an infrared spectrum represent?

Answer 4

• Absorbance of energy from the infrared radiation

Question 5

What are the two peaks that must be present to identify a substance as a carboxylic acid?

Answer 5

• Very broad peak at 2500-3300 cm⁻¹ (O-H group)
• Sharp peak at 1680-1750 cm⁻¹ (C=O group)

Question 6

What happens inside a mass spectrometer?

Answer 6

• Organic compound is vaporized and passed through the spectrometer
• Some molecules lose an electron, forming molecular ions
• Excess energy from ionization makes bonds vibrate more
• Vibration causes bond weakening
• Molecular ion splits by fragmentation

Question 7

What is the symbol of a molecular ion?

Answer 7

• M⁺

Question 8

Is fragmentation predictable? Why?

Answer 8

• No, because it can happen anywhere in the molecule

Question 9

In mass spectrometry, how is a molecular ion represented?

Answer 9

• It is the peak with the highest mass/charge ratio (m/z)

Question 10

The molecular mass of the molecular ion is equal to what?

Answer 10

• Relative molecular mass of the compound

Question 11

Will the molecular ion peaks of two isomers of the same compound be the same or different?

Answer 11

• Same

Question 12

What is the m/z value of CH₂⁺?

Answer 12

• 14

Question 13

What is the m/z value of CH₃⁺?

Answer 13

• 15

Question 14

What is the m/z value of OH⁺ from alcohol?

Answer 14

• 17

Question 15

What is the m/z value of CH₂CH⁺?

Answer 15

• 29

Question 16

What is the m/z value of C₃H₇⁺?

Answer 16

• 43

Question 17

What is the m/z value of C₄H₉⁺?

Answer 17

• 57

Question 18

What are the advantages of using mass spectrometry?

Answer 18

• Cheap
• Only small quantities of sample are required

Question 19

What is the main disadvantage of using mass spectrometry?

Answer 19

• The sample is completely destroyed