Unit 3-Experimental Determination Of Structure

Question 1

What is meant by the empirical formula of a molecule?

Answer 1

Simplest ratio of the elements in a molecule

Question 2

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 2

Elemental microanalysis

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

What information can be determined by carrying out mass spectroscopy?

Answer 5

The accurate gram formula mass (GFM) and structural features of an organic compound.

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

Explain the effect infrared radiation has on the bonds within molecules and how this allows different functional groups to be identified.

Answer 7

Bond vibrate/bend/stretch.

Different bonds/functional groups absorb at different wavelengths.

Question 8

How does infrared spectroscopy work?

Answer 8

Infrared radiation is passed through a sample of the organic compound and then into a detector that measures the intensity of the transmitted radiation at different wavelengths.

Question 9

The infra-red spectrum of an organic compound is shown below.

When burned completely the only products were carbon dioxide and water.

Which bond is responsible for the peak at 1140 cm^-1?

Answer 9

C-O (stretch)

Question 10

What information can be determined by Proton NMR spectroscopy?

Answer 10

The different chemical environments of hydrogen atoms (protons or 1 H) in an organic molecule,

and

how many hydrogen atoms there are in each of these environments

Question 11

What is the standard reference substance used in ¹H NMR spectroscopy?

What chemical shift value is it assigned?

Answer 11

Tetramethylsilane (TMS) which has a chemical shift value equal to zero.

Question 12

How many different proton environments would be found in the NMR spectra of propan-1-ol and propan-2-ol respectively?

Answer 12

Propan-1-ol has 4 proton environments and propan-2-ol has 3 proton environments.

Question 13

The infra-red spectrum of an organic compound is shown below.

When burned completely the only products were carbon dioxide and water.

Which bond is responsible for the peak at 1140 cm^-1?

Answer 13

C-O (stretch)