Experimental Determination of Structure

Question 1

What is elemental Microanalysis used to determine

Answer 1

The masses of C,H,O,S and N in a sample of and organic compound, to find the empirical formula

Question 2

What is an Empirical Formula

Answer 2

It shows the simplest ratio of the elements in a molecule

Question 3

Elemental Microanalysis can be determined from….

Answer 3

• Combustion Product Masses
• Percentage Product by Mass

Question 4

What is Mass Spectroscopy used for?

Answer 4

To determine the accurate GFM and structural features of an organic compound

Question 5

Describe the Process of Mass Spectroscopy

Answer 5

• Small sample of organic compound id 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

Question 6

How is the Mass spectrum obtained

Answer 6

A mass spectrum is obtained showing a plot of the relative abundance of the ions detected against the mass-to-charge (m/z) ratio

Question 7

The mass-to-charge ratio of a parent ion can be used to determine…..

Answer 7

The GFM of the molecular ion and so a molecular formula can be determined using the empirical formula

Question 8

What is Infrared Spectroscopy is used to identify?

Answer 8

Certain functional groups in an organic compound

Question 9

What happens when infrared radiation is absorbed by organic compounds

Answer 9

Bonds within the molecule vibrate (strech and bend).

Question 10

The wavelengths of infrared 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 os the absorbance of infrared radiation measured in?

Answer 11

Wavenumbers (reciprocal of wavelength) in units of cm^-1

Question 12

What happens in infrared Spectroscopy

Answer 12

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 13

What does proton NMR stand for

Answer 13

Proton nuclear magnetic resonance spectroscopy

Question 14

What does Proton NMR tell you

Answer 14

NMR Gives information about the different Chemical Environments of hydrogen atoms in an organic molecule, and about how many hydrogens there are in each of the environments

Question 15

How does NMR Work

Answer 15

• H nuclei behave like tiny magnets and in a strong magnetic field some (lower energy nuclei) align with the field , whilst the rest align against it (higher energy nuclei).
• Absorption of radiation in the
  radio frequency region of the electromagnetic spectrum causes the H 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 16

What does the Chemical Shift in NMR tell you

Answer 16

Is related to the environment of the H atom and is measured in parts per million

Question 17

What does the area under the paek of an NMR tell you

Answer 17

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

Question 18

What is Tetramethylsilane’s (TMS) chemical shift value?

Answer 18

0

Question 19

H NMR spectra can be obtaoned using what type of resolution NMR

Answer 19

High and Low

Question 20

Benefits of high resolution NMR

Answer 20

High-resolution H NMR uses higher radio frequencies than those used in low-resolution H NMR and provides more detailed spectra.

Question 21

Describe high resolution NMR in terms of peaks

Answer 21

In a high-resolution H NMR an interaction with H atoms on neighbouring carbon atoms can result in the splitting of peaks into multiplets.

The number of H 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 H atoms on the neighbouring carbon atom.

Question 22

What is a benefit of low-resolution NMR

Answer 22

Spectra can be sketched for any given compound