3.4 experimental determination of structure

Question 1

what can elemental microanalysis be used to determine the masses of

Answer 1

C H O S N in a sample of an organic compound in order to determine its empirical formula

Question 2

what does an empirical formula show

Answer 2

the simplest ratio of the elements in a molecule

Question 3

what can elemental microanalysis can be determined from

Answer 3

combustion product masses and percentage product by mass

Question 4

what can mass spectrometry be used to determine

Answer 4

the accurate gram formula mass and structural features of an organic compound

Question 5

describe the process of mass spectrometry

Answer 5

• a small sample of an organic compound is bombarded by high energy electrons
• this removes electrons from the organic molecule generating positively charged molecular ions known as parent ions
• these molecular ions then break into smaller positively charged ion fragments
• a mass spectrum is obtained showing a plot of the relative abundance of the ions detected against the mass to charge ratio

Question 6

what can the mass to charge ratio of the parent ion be used to determine

Answer 6

the gram formula mass of the molecular ion, and so a molecule formula can be determined using the empirical formula

Question 7

what data can be interpreted to gain structural information

Answer 7

fragmentation data

Question 8

what can be used to identify certain functional groups in an organic compound

Answer 8

infrared spectroscopy

Question 9

what happens when infrared radiation is absorbed by organic compounds

Answer 9

bonds within the molecule vibrate (stretch and bend).

Question 10

what do the wavelengths of IR radiation that are absorbed depend on

Answer 10

the type of atoms that make up the bond and the strength of the bond

Question 11

what happens in IR spectroscopy

Answer 11

IR 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 12

what is the absorbance of IR radiation measured in

Answer 12

wavenumbers, the reciprocal of wavelength

Question 13

what does proton NMR or 1H NMR stand for

Answer 13

proton nuclear magnetic resonance spectroscopy

Question 14

what can proton NMR give information about

Answer 14

• the different chemical environments of hydrogen atoms in an organic molecule
• how many hydrogen atoms there are in each of these environments

Question 15

what do proton nuclei behave like

Answer 15

tiny magnets

Question 16

where do the proton nuclei align in a strong magnetic field

Answer 16

some align with it, if they have lower energy, and some align against it, if they have higher energy

Question 17

what does the absorption of radiation in the radio frequency region of the EM spectrum cause

Answer 17

the proton nuclei to flip from the lower to the higher energy alignment. as they fall back from the higher to the lower energy alignment the emitted radiation is detected and plotted on a spectrum

Question 18

what is the chemical shift

Answer 18

the peak position

Question 19

what is the chemical shift related to in a proton NMR spectrum

Answer 19

the environment of the proton NMR

Question 20

what is the area under the peak on a proton NMR spectrum related to

Answer 20

the number of proton atoms in that environment

Question 21

what is the area under the peak related to the numbre of proton atoms often given by

Answer 21

an integration curve on a spectrum

Question 22

what is the height of an integration spectrum proportional to

Answer 22

the number of proton atoms in that environment, allowing a ratio of the proton atoms in each environment to be determined

Question 23

what is the standard reference substance using in proton NMR spectroscopy

Answer 23

tetramethylsilane (TMS)

Question 24

how can proton NMR spectra be obtained

Answer 24

using low resolution or high resolution NMR

Question 25

what does high resolution proton NMR use that allows it to produce more detailed spectra than in low resolution proton NMR

Answer 25

higher radio frequencies

Question 26

what occurs in high resolution proton NMR

Answer 26

an interaction with the proton atoms on neighbouring carbon atoms can result in the splitting of peaks into multiplets. the number of proton atoms on neighbouring carbon atoms will determine the number of peaks within a multiplet and can be determined using the n+1 rule, where n is the number of proton atoms on the neighbouring carbon atom

Question 27

which proton NMR spectra can be sketched for any given compound

Answer 27

low resolution