Unit 2-Experimental Determination Of Structure

Question 1

Which analysis is used to determine the masses of C, H, O, S and N in a sample
of an organic compound in order to determine its empirical formula?

Answer 1

Elemental microanalysis

Question 2

What is meant by the empirical formula of a molecule?

Answer 2

Simplest ratio of the elements in a molecule

Question 3

Elemental microanalysis can be determined from?

Answer 3

Combustion product masses or
Percentage product by mass

Question 4

0.00250 g of a compound containing carbon, hydrogen and nitrogen was subjected to elemental microanalysis.

It was found to contain 0.00133 g of carbon and 0.00039 g of hydrogen.

What is the empirical formula?

Answer 4

1. Mass of nitrogen = total mass - mass of carbon - mass of hydrogen.

Mass of nitrogen = 0.00250-0.00133-0.00039 = 0.00078g.

2. Calculate number of moles of each.

Carbon = 0.00133/12 = 0.00011 moles

Hydrogen = 0.00039/1 = 0.00039 moles

Nitrogen = 0.00078/14 = 0.000056 moles.

3. Divide each number of moles by the smallest value.

C : H : O

0.00011 : 0.00039 : 0.000056

Divide by 0.000056 then round

2 : 7 : 1

4. Empirical formula is C₂H₇N

Question 5

Which peak is most important in a mass spectrum and what does it show?

Answer 5

The peak farthest right is the molecular ion, M+, peak. It shows the accurate gram formula mass (GFM).

Other peaks show structural features of an organic compound.

e.g. a peak at 15 could be a CH_3⁺

Question 6

A simplified mass spectrum of an organic compound is shown below.

Which of the following compounds produces this spectrum?

A Propane

B Propan-1-ol

C Propan-2-ol

D Propanone

Answer 6

Largest m/z value indicates the molecular ion at 58.

GFM of propane = 44g

GFM of propan-1-ol = 60g

GFM of propan-2-ol = 60g

GFM of propanone = 58g

Question 7

What can IR spectroscopy be used to identify?

Answer 7

Functional groups

Question 8

What happens to the molecule when it absorbed by organic molecules?

Answer 8

The bonds bend and stretch.

Question 9

Identify the groups responsible for the main peaks between 1500 and 4000 wavenumbers

Answer 9

C=O at 1700

O-H at 3000

Question 10

What does proton NMR identify?

Answer 10

hydrogen atoms in different chemical environments

Question 11

How many different environments are present in the attached spectrum?

Identify each peak.

Answer 11

3 environements:

1. 3 - CH3
2. 6 - CH2
3. 4 - OH

Question 12

What is TMS?

Answer 12

This is a standard reference which has a chemical shift value of zero

Question 13

What does a singlet mean?

What does a triplet mean?

What does a quartet mean?

Answer 13

The H is isolated

The hydrogens are next to a CH2 group

The hydrogens are next to a CH3 group