3.4 Experimental Determination of Structure

Question 1

how to verify the chemical structure of a substance

Answer 1

a number if experimental techniques

Question 2

elemental microanalysis use

Answer 2

• determine the masses of C, H, O, S, and N in a sample to determine its empirical formula
• can be determined from combustion product masses or percentage product by mass

Question 3

empirical formula

Answer 3

simplest ratio of elements in a molecule

Question 4

• determine the masses of C, H, O, S, and N in a sample to determine its empirical formula
• can be determined from combustion product masses or percentage product by mass

Answer 4

elemental microanalysis

Question 5

simplest ratio of elements in a molecule

Answer 5

empirical formula

Question 6

empirical formula

Answer 6

simplest ratio of elements in a molecule

Question 7

mass spectrometry

Answer 7

• used to determine the accurate gram formula mass and structural features of an organic compounds
• 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
• the mass-to-charge ratio of the parent ion can be used to determine the GFM of the molecular ions known, and so a molecular formula can be determined using the empirical formula
• the fragmentation data can be interpreted to gain structural information

Question 8

• used to determine the accurate gram formula mass and structural features of an organic compounds
• 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
• the mass-to-charge ratio of the parent ion can be used to determine the GFM of the molecular ions known, and so a molecular formula can be determined using the empirical formula
• the fragmentation data can be interpreted to gain structural information

Answer 8

mass spectrometry

Question 9

• used to identify certain functional groups in an organic compounds
• when infrared radiation is absorbed by organic compounds, bonds within the molecule vibrate (stretch and bend). The wavelengths of infrared radiation that are absorbed depends on the type of atoms that make up the bond and the strength of the bond

Answer 9

Infrared spectroscopy

Question 10

infrared spectroscopy

Answer 10

• used to identify certain functional groups in an organic compound

Question 11

Proton NMR

Answer 11

• gives information about the different chemical environments of hydrogen atoms (protons or 1H) in an organic molecule, and about how many H atoms there are in each of these environments

Question 12

• gives information about the different chemical environments of hydrogen atoms (protons or 1H) in an organic molecule, and about how many H atoms there are in each of these environments

Answer 12

Proton NMR

Question 13

1H nuclei in proton NMR

Answer 13

• 1H nuclei behave like tiny magnets and in a strong magnetic field done align with the field (lower energy), whilst the rest align against it (higher energy)
• absorption of radiation in the radio frequency region of the EM spectrum causes the 1H 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 14

1H NMR spectrum

Answer 14

• in a 1H NMR spectrum, the chemical shift, delta, (peak position) is related to the environment of the 1H atom and is measured in ppm
• chemical shift values for 1H in different chemical environments are in the data booklet

Question 15

area under peak in NMR

Answer 15

• the area under the peak is related to the number of 1H atoms in that that environment and is often given by an integration curve on a spectrum
• the height if the integration curve is proportional to the number of 1H atoms in the environment so a ratio of 1H atoms in each environment can be determined

Question 16

• the area under the peak is related to the number of 1H atoms in that that environment and is often given by an integration curve on a spectrum
• the height if the integration curve is proportional to the number of 1H atoms in the environment so a ratio of 1H atoms in each environment can be determined

Answer 16

area under curve in NMR

Question 17

reference substance in NMR

Answer 17

terramethylsilane (TMS), which is assigned a chemical shift value equal to zero

Question 18

what can H NMR be obtained from

Answer 18

• low resolution or high resolution NMR
• high resolution uses higher radio frequencies than those in low resolution and provide more detailed spectra
• in high resolution, 1H NMR interactions with 1H atoms on neighbouring C atoms can result in splitting peaks into multiplets. the number of 1H atoms on neighbouring C atoms will determine the number of peaks within a multiplet and can be determined using n+1 rule. n is the number of 1H atoms on the neighbouring carbon