chemistry - 4.4

Question 1

In organic chemistry, what are carried out to verify the chemical structure of a substance?

Answer 1

A number of experimental techniques.

Question 2

What is elemental microanalysis used for?

Answer 2

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.

Question 3

What does an empirical formula show?

Answer 3

The simplest ratio of the elements in a molecule.

Question 4

What can elemental microanalysis be determined from?

Answer 4

• combustion product masses
• percentage product by mass

Question 5

What can mass spectrometry be used for?

Answer 5

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

Question 6

In mass spectrometry, what happens to a small sample of an organic compound and what does this do?

Answer 6

It is bombarded by high-energy electrons. This removes electrons from the organic molecule generating positively charged molecular ions known as parent ions.

Question 7

What do these parent ions break into?

Answer 7

Smaller positively charged ion fragments.

Question 8

A mass spectrum is obtained, what does this show?

Answer 8

A plot of the relative abundance of the ions detected against the mass-to-charge (m/z) ratio.

Question 9

What can the mass-to-charge ratio of the parent ion be used for?

Answer 9

To determine the GFM of the molecular ion, and so a molecular formula can be determined using the empirical formula.

Question 10

What can the fragmentation data be interpreted to gain?

Answer 10

Structural information.

Question 11

What is infrared spectroscopy used for?

Answer 11

To identify certain functional groups in an organic compound.

Question 12

What happens when infrared radiation is absorbed by organic compounds?

Answer 12

Bonds within the molecule vibrate (stretch and bend).

Question 13

What do the wavelengths of infrared radiation that are absorbed depend on?

Answer 13

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

Question 14

In infrared spectroscopy, what happens?

Answer 14

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 15

What is the absorbance of infrared radiation measured in?

Answer 15

Wavenumbes, the reciprocal of wavelength, in units of cm-1.

Question 16

Where are characteristic absorptions by particular vibrations given?

Answer 16

In the data booklet.

Question 17

What can Proton nuclear magnetic resonance spectroscopy (proton NMR or 1H NMR) give information about?

Answer 17

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.

Question 18

What do 1H nuclei behave like?

Answer 18

Tiny magnets and in a strong magnetic field some align with the field (lower energy), whilst the rest align against it (higher energy).

Question 19

What does absorption of radiation in the radio frequency region of the electromagnetic spectrum do?

Answer 19

It causes the 1H nuclei to ‘flip’ from the lower to the higher energy alignment.

Question 20

As the 1H nuclei fall back from the higher to the lower energy alignment, what happens?

Answer 20

The emitted radiation is detected and plotted on a spectrum.

Question 21

In a 1H NMR spectrum, what is the chemical shift (peak position) related to?

Answer 21

The environment of the 1H atom and is measured in parts per million (ppm).

Question 22

Where are chemical shift values for 1H in different chemical environments given?

Answer 22

In the data booklet.

Question 23

What is the area under the peak related to?

Answer 23

The number of 1H atoms in that environment and is often given by an integration curve on a spectrum.

Question 24

What is the height of an integration curve proportional to?

Answer 24

The number of 1H atoms in that environment, and so a ratio of 1H atoms in each environment can be determined.

Question 25

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

Answer 25

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

Question 26

How can 1H NMR spectra be obtained?

Answer 26

Using low-resolution or high- resolution NMR.

Question 27

What does high-resolution 1H NMR use?

Answer 27

Hgher radio frequencies than those used in low-resolution 1H NMR and provides more detailed spectra.

Question 28

In a high-resolution 1H NMR what can an interaction with 1H atoms on neighbouring carbon atoms result in?

Answer 28

The splitting of peaks into multiplets.

Question 29

What will the number of 1H atoms on neighbouring carbon atoms determine?

Answer 29

The number of peaks within a multiplet and can be determined using the n+1 rule, where n is the number of 1H atoms on the neighbouring carbon atom.

Question 30

What can low- and high-resolution 1H NMR spectra be?

Answer 30

Analysed, and low- resolution 1H NMR spectra can be sketched for any given compound.