11.4 Spectroscopic Identification Of Organic Compounds

Question 1

What does the index of hydrogen deficient (IHD) indicate?

Answer 1

The number of double bond equivalents (the number of double bonds or rings) in a compounds.

Question 2

What does a triple bond count as?

Answer 2

Two double bond equivalents.

Question 3

What is the IHD for such a molecule:

CcHhNnOoXx

Answer 3

IHD = 0.5 [2c+2-h-x+n]

Question 4

What is infrared spectroscopy?

Answer 4

Can be used to determine the bonds present in a molecules. The bonds responsible for absorption bands in the spectrum can be determined by comparison to the frequencies in the data booklet.

Question 5

What are the two main functional groups shown on an absorption spectrum?

Answer 5

OH hydrogen bonded in carboxylic acids

C=O in aldehydes, ketons, carboxylic acids and esters.

Question 6

Why do all organic molecules show an absorption in the region 2850-3090cm-1?

Answer 6

Due to the C-H bonds present.

Question 7

What is the region below 1500cm-1?

Answer 7

The fingerprint region and is characteristic of all molecules. The region contains many absorption’s and is difficult to interpret. We only look at the fingerprint region to confirm the presence of a particular vibration.

Question 8

What happens in a mass spectrometer?

Answer 8

An organic compound is bombarded with higher energy electrons to produce positive ions Positive ions pass through the mass spectrometer where they are separated according to mass and detected.

Question 9

What happens on a molecular level?

Answer 9

When a sample of an organic compound is introduced into a mass spectrometer some of the molecules are converted to the molecular ion M+. Other molecules will be split up into smaller fragments and any positive ions formed will produce a signal in the spectrum - this is the fragmentation pattern.

Question 10

What is important about the molecular ion peak?

Answer 10

Is at the highest mass value (not necessarily the highest peak) in the spectrum and indicates the relative molecular mass of the compound.

Question 11

How can the mass spectrum be interpreted?

Answer 11

In terms of the fragments lost from the whole molecule or in terms of the masses of the fragment ions formed.

Question 12

What is nuclear magnetic resonance spectroscopy?

Answer 12

This technique looks at the hydrogen atoms (specifically hydrogen nuclei protons) in a molecule and requires electromagnetic radiation in the radio frequency range.

Question 13

What does the number of peaks/signals in the spectrum correspond to?

Answer 13

The number of different chemical environments for H.

Question 14

What does the area under the peaks of each peak depend on?

Answer 14

The ratio between the numbers of H atoms in each environments. This is also represented by the vertical height of the steps in the integration trace.

Question 15

What is important to remember when determining the number of hydrogen atoms in different chemical environments?

Answer 15

H atoms attached to the same C are all in the same environments, so take care to notice whether the molecule is symmetrical.

Question 16

What happens in high resolution NMR?

Answer 16

In high resolution NMR spectra it can be seen that the signals from low resolution NMR spectra are slit into clusters of small peaks, the splitting is due to the presence of H atoms on adjacent atoms.

Question 17

What does a particular signal split into n smaller peaks where n is the multiplicity indicate?

Answer 17

That the number of hydrogen atoms on the adjacent atom is n-1.

Question 18

What is the multiplicity and number of H atoms on the adjacent C for a singlet?

Answer 18

Multiplicity 1

0 H atoms on adjacent carbon

Question 19

What is the multiplicity and number of H atoms on the adjacent C for a doublet?

Answer 19

Multiplicity 2

1 H atoms on adjacent carbon

Question 20

What is the multiplicity and number of H atoms on the adjacent C for a triplet?

Answer 20

Multiplicity 3

2 H atoms on adjacent carbon

Question 21

What is the multiplicity and number of H atoms on the adjacent C for a quartet?

Answer 21

Multiplicity 4

3 H atoms on adjacent carbon

Question 22

What does a chemical shift scale provide information on?

Answer 22

The environment that the H atoms are in - the chemical shifts in a spectrum can be matched up to the H atoms in a molecule by reference to the data booklet.

Question 23

What is TMS (tetramethylsilane)?

Answer 23

Used as a reference standard for the chemical shift scale - the proton in TMS are assigned a chemical shift of 0.00

Question 24

Why was TMS chosen as the standard?

Answer 24

It has 12 protons all in the same environment
The chemical shift of the protons in TMS is well away from (lower than) the chemical shifts of protons in most organic molecules
It is non toxic and inert.

Question 25

What is X ray crystallography?

Answer 25

Uses X rays to probe the structure of molecules. X ray crystallography provides a detailed 3D structure for a molecule including bond lengths and bond angles.