Organic Analysis

Question 1

What is the test for alkenes?

Answer 1

shake with bromine water

Question 2

What is the test for haloalkanes?

Answer 2

1. Add NaOH (aq) and warm
2. Acidify with NHO3
3. Add AgNO3

Question 3

What is the test for primary or secondary alcohols?

Answer 3

1. Add acidified K2Cr2O7
2. Warm

Question 4

What is the test for aldehydes?

Answer 4

Warm with Fehling’s Solution OR Warm with Tollen’s Solution

Question 5

What is the test for carboxylic acids?

Answer 5

Add Na2CO3 (either solution or solid)

Question 6

What is the result of the alkenes test?

Answer 6

Bromine water turns from Orange to Clear and Colourless if alkene present

Question 7

What is the result for the haloalkanes test?

Answer 7

A white, cream or yellow ppt will form of AgCl, AgBr or AgI if a haloalkane was present

Question 8

What is the result for the primary or secondary alcohols test?

Answer 8

Solution will turn from Orange to Green if 1⁰ or 2⁰ Alcohol is present. (Also for aldehyde too)

Question 9

What is the result for the aldehydes test?

Answer 9

Blue solution turns to Brick Red ppt OR Silver mirror forms if aldehyde present

Question 10

What is the result for the carboxylic acids test?

Answer 10

Effervescence (as CO2 is given off

Question 11

What is the name of the peak furthest to the right on the mass spectrum?

Answer 11

the molecular ion peak

Question 12

What does the molecular ion peak represent?

Answer 12

the molecules which have made it all the way through the spectrometer to the detector and stayed in tact

Question 13

What is the m/z ratio of the molecular ion peak equal to?

Answer 13

the Mr of the molecule

Question 14

What is the difference between a mass spectra and high resolution mass spectrometry?

Answer 14

Mass spectra often show masses and m/z ratios to the nearest whole number whereas high resolution mass spectrometry can measure the mass of an ion to up to 4 decimal places

Question 15

What is infrared spectroscopy used for?

Answer 15

used to identify the functional groups present in an organic molecule

Question 16

How does infrared spectroscopy work?

Answer 16

Every different type of bond has its own ‘natural frequency’ which falls within the infrared region of the electromagnetic spectrum. When you shine infrared radiation through a solution containing a molecule, if the natural frequency of any of the bonds in the molecule match the frequency of the IR light being shone through the sample, that light will be absorbed by the molecule. This means the IR radiation which passes through the solution will be missing some frequencies, and these can be used to indicate which bonds, and therefore which functional groups, are present in the molecule.

Question 17

What is the process which occurs inside an IR spectrometer?

Answer 17

1, A beam of IR light containing a wide range of frequencies is passed through a sample.
2, Some of the frequencies of IR light are absorbed by the molecule. Many of the frequencies are not absorbed, and so pass straight through the sample. The light that emerges through the other side of the sample is missing the frequencies which are absorbed.
3, Once detected, a computer plots a graph of the intensity of the light which emerges from the sample. This is called transmittance.
4, The frequency is plotted on the x-axis with the units ‘wavenumbers’ which are cm-1.

Question 18

How should you answer an IR question?

Answer 18

Always quote the absorption range for the peak, and the functional group it represents

Question 19

What is the fingerprint region?

Answer 19

• the area of the spectrum 1500 cm-1 and below
• a complicated combination of peaks formed by different twisting and bending vibrations of the molecule as a whole - individual peaks cannot be interpreted as coming from specific bonds

Question 20

How can fingerprint regions be used?

Answer 20

each molecule has its own unique fingerprint region, so a computer can compare the fingerprint region to a database of samples to confirm the identity of the molecule

Question 21

How can IR spectra be used to identify impurities?

Answer 21

The IR spectra of a huge range of pure samples have been collected and are saved in databases. If another sample of a compound has been obtained, its IR spectrum can be compared to that of the pure spectrum to indicate if impurities are present.

Question 22

Define functional group

Answer 22

the functional group is the part of the molecule which determines how it reacts

Question 23

Define homologous series

Answer 23

a homologous series of organic molecules have the same functional group but each successive member has an additional -CH2