Unit 2-Experimental Determination Of Structure Flashcards
What is meant by the empirical formula of a molecule?
Simplest ratio of the elements in a molecule
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?
Elemental microanalysis
Elemental microanalysis can be determined from?
Combustion product masses or
percentage product by mass
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?
- Mass of nitrogen = total mass - mass of carbon - mass of hydrogen.
Mass of nitrogen = 0.00250-0.00133-0.00039 = 0.00078g.
- 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.
- 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
- Empirical formula is C2H7N
What information can be determined by carrying out mass spectroscopy?
The accurate gram formula mass (GFM) and structural features of an organic compound.
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
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
Explain the effect infrared radiation has on the bonds within molecules and how this allows different functional groups to be identified.
Bond vibrate/bend/stretch.
Different bonds/functional groups absorb at different wavelengths.
How does infrared spectroscopy work?
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.
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?
C-O (stretch)
What information can be determined by Proton NMR spectroscopy?
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
What is the standard reference substance used in 1H NMR spectroscopy?
What chemical shift value is it assigned?
Tetramethylsilane (TMS) which has a chemical shift value equal to zero.
How many different proton environments would be found in the NMR spectra of propan-1-ol and propan-2-ol respectively?
Propan-1-ol has 4 proton environments and propan-2-ol has 3 proton environments.