chemistry - 4.4 Flashcards
In organic chemistry, what are carried out to verify the chemical structure of a substance?
A number of experimental techniques.
What is elemental microanalysis used for?
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.
What does an empirical formula show?
The simplest ratio of the elements in a molecule.
What can elemental microanalysis be determined from?
- combustion product masses
- percentage product by mass
What can mass spectrometry be used for?
To determine the accurate gram formula mass and structural features of an organic compound.
In mass spectrometry, what happens to a small sample of an organic compound and what does this do?
It is bombarded by high-energy electrons. This removes electrons from the organic molecule generating positively charged molecular ions known as parent ions.
What do these parent ions break into?
Smaller positively charged ion fragments.
A mass spectrum is obtained, what does this show?
A plot of the relative abundance of the ions detected against the mass-to-charge (m/z) ratio.
What can the mass-to-charge ratio of the parent ion be used for?
To determine the GFM of the molecular ion, and so a molecular formula can be determined using the empirical formula.
What can the fragmentation data be interpreted to gain?
Structural information.
What is infrared spectroscopy used for?
To identify certain functional groups in an organic compound.
What happens when infrared radiation is absorbed by organic compounds?
Bonds within the molecule vibrate (stretch and bend).
What do the wavelengths of infrared radiation that are absorbed depend on?
The type of atoms that make up the bond and the strength of the bond.
In infrared spectroscopy, what happens?
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. .
What is the absorbance of infrared radiation measured in?
Wavenumbes, the reciprocal of wavelength, in units of cm-1.
Where are characteristic absorptions by particular vibrations given?
In the data booklet.
What can Proton nuclear magnetic resonance spectroscopy (proton NMR or 1H NMR) give information about?
The different chemical environments of hydrogen atoms (protons or 1H) in an organic molecule, and about how many hydrogen atoms there are in each of these environments.
What do 1H nuclei behave like?
Tiny magnets and in a strong magnetic field some align with the field (lower energy), whilst the rest align against it (higher energy).
What does absorption of radiation in the radio frequency region of the electromagnetic spectrum do?
It causes the 1H nuclei to ‘flip’ from the lower to the higher energy alignment.
As the 1H nuclei fall back from the higher to the lower energy alignment, what happens?
The emitted radiation is detected and plotted on a spectrum.
In a 1H NMR spectrum, what is the chemical shift (peak position) related to?
The environment of the 1H atom and is measured in parts per million (ppm).
Where are chemical shift values for 1H in different chemical environments given?
In the data booklet.
What is the area under the peak related to?
The number of 1H atoms in that environment and is often given by an integration curve on a spectrum.
What is the height of an integration curve proportional to?
The number of 1H atoms in that environment, and so a ratio of 1H atoms in each environment can be determined.
