Spectroscopy

UV and visible, Infrared, Mass Spec and NMR

what does NMR spectroscopy stand for?

Nuclear magnetic resonance spectroscopy

What are the two types of NMR spectroscopy?

carbon 13 NMR spectroscopy

proton NMR spectroscopy

Scientists (dating back to the 1500s) identified elements by using the flame test. Now scientists use spectroscopy. give 3 advantages of spectroscopy than flame testing

1. sped up identification

2. only small amounts of the sample is needed for analysis

3. results are more accurate

Why cant NMR spectroscopy be used for all compounds/atoms?

only compounds with an nuclei with an odd mass number/odd atomic number will posses spin.

a nucleus without spin cannot be detected by NMR spectroscopy

Q1.Which amine has only three peaks in its proton NMR spectrum? Methylamine, Trimethylamine , Diethylamine , Propylamine

Diethylamine

The non-toxic, inert substance TMS is used as a standard in n.m.r. spectra. (i) Give two other reasons why TMS is used as a standard in recording n.m.r. spectra.(2 marks)

low boiling point-can be distilled off and used again

wont interfere with main signals

can easily be removed

produce a single peak

draw the displayed formula of Ethanol, how many peaks will Ethanol have in a carbon 13 NMR spectroscopy

2 peaks

draw the displayed structure of bromopropane and how many peaks will it have in a carbon 13 NMR spectroscopy

2 peaks

Top Tips! when working out how many peaks a compound has in carbon NMR

draw it displayed and follow each carbon atom through

draw the structural formula of TMS. (1 mark)

give the molecular formula of TMS

C₄H₁₂Si

Give the name of the compound used as a reference used in carbon 13 MNR spectroscopy

Tertramethylsilane

give the number of peaks you would expect for the carbon 13 NMR spectra of pentane

3

give the number of peaks you would expect for the carbon 13 NMR spectra of dimethylpropane

2

give the number of peaks you would expect for the carbon 13 NMR spectra of butanone

4

give the number of peaks you would expect for the carbon 13 NMR spectra of cyclohexane

1

which type of NMR spectroscopy does peak heigh matter?

proton NMR spectroscopy

why would CCl₄ and CDCL₃ good solvents for NMR?

• Inert

• They both have no protons so they will not produce peaks in a proton spectra

• produce one singlet peak in a carbon spectra that doesn’t interfere with other peaks

CCl₄ and CDCL₃ are used in NMR spectroscopy.

In terms of structure and polarity why is CCl₄ and CDCL₃ good solvents

CCl₄ has a tetrahedral shape and is symmetrical. therefore is non-polar and can dissolve non-polar substances

CDCL₃ has a tetrahedral shape and is non-symmetrical. therefore is polar and can dissolve polar substances

In proton NMR spectroscopy the number of signals/peaks tells you…

the number of different proton environments

In proton NMR spectroscopy the chemical shift tells you…

the general environment of the protons

In proton NMR spectroscopy the peak area/height tell you…

how many protons are in each environment

In proton NMR spectroscopy multiplicity tells you…

how many protons are on neighbouring carbon atoms.

In low resolution proton NMR spectra what is shown?

give the number of protons for each environmental group

represented as 1 peak

In high resolution proton NMR spectra what is shown?

multiplicity/splitting can happen for each peak.

each peak indicated the number of protons on adjacent carbon atoms

1 peaks is

• a singlet

• so zero neighbouring H

2 peaks is

• a doublet

• So 1 neighbouring H

3 peaks is

• a triplet

• so 2 neighbouring H

4 peaks is

• a quartet

• so 3 neighbouring H

5 peaks is

• a quintet

• 4 neighbouring H

6 peaks

• a sextet

• 5 neighbouring H

7 peaks

• a septet

• 6 neighbouring H

What do you do if an exam question asks you to work out the number of hydrogen atoms for each peak in a proton NMR spectra?

measure the distance between the to and bottom lines for each peak.

then compare the heights and make them into the simple ratio

Work out the ratio of hydrogen atoms present in each peak

5:10:15

so 1:2:3

how many peaks will Cl-CH₂-CH₂-Cl produce?

because the protons are in chemically equivalent environments, so no splitting occurs and a

1 singlet peak will form