3.6 Organic Analysis

Question 1

How do you test for alkenes? What is the result?

Answer 1

Shake with bromine water, result is bromine water is decolourised (orange to colourless)

Question 2

How do you test for haloalkanes? What is the result?

Answer 2

Add NaOH (aq) and warm, acidify with HNO3, add AgNO3 (aq)
Result: precipitate of AgX (for Cl=white, for Br=cream, for I=yellow)

Question 3

How do you test for alcohols? What is the result?

Answer 3

Add acidified K2Cr2O7 (potassium dichromate(VI)) and heat.

Result: colour change from orange to green for primary and secondary alcohols (note: no change for tertiary alcohols)

Question 4

How do you test for aldehydes? What is the result? (2 ways)

Answer 4

1. Warm with Fehling’s solution, result: brick red ppt forms (from blue solution)
2. Warm with Tollens’ reagent, result: “silver mirror” (Ag(s) ppt) forms

Question 5

How do you test for carboxylic acids? What is the result?

Answer 5

Add Na2CO3 (aq), result: CO2 (g) given off- effervescence

Question 6

What is mass spectrometry? How does it work?

Answer 6

Used to find the relative molecular masses of organic
compounds.
Compound is dissolved in solution, ionised by a high voltage supply (to mostly 1+ ions), accelerated by a negatively charged plate, becomes a beam of ionised molecules, reach detector and cause a current to flow. Time of flight used to work out m/z value and plot graph.

Question 7

What does the x axis show on a mass spectrum?

What does this effectively show and why?

Answer 7

Shows m/z value (mass divided by ionic charge).

Since most ions are 1+, this effectively shows Mr

Question 8

Why are there multiple peaks when molecules are put into a mass spectrometer?

Answer 8

The molecular ion is shown as a peak, but it will also FRAGMENT into smaller molecules, so these peaks are shown as well.

Also, due to isotopes of atoms, different peaks may be seen

Question 9

How does gas chromatography work and what does it tell you?

Answer 9

A stream of gas carries a mixture of vapours through a column packed with solids, different compounds move through at different speeds, so they are separated. The amount of each compound can then be measured.

Question 10

What does GCMS stand for?

Answer 10

Gas chromatography linked to mass spectrometry

Question 11

What might GCMS be used for?

Answer 11

Powerful chemical analysis - forensic work, measuring water pollution, drug testing on athletes, racehorses

Question 12

What is high resolution mass spectrometry?

Answer 12

Mass spectrometers which give Mr to 3d.p. or 4d.p. are called high resolution (low resolution is to nearest whole number)

Question 13

What does high resolution mass spec allow you to do?

Answer 13

Distinguish between compounds that have the
same Mr to the nearest whole number, but are
made up of different atoms and therefore have
different values of Mr to 3.d.p

Question 14

Why do atoms and chemical bonds absorb infrared

radiation?

Answer 14

They are constantly vibrating - they can absorb infrared radiation that is the same frequency as their frequency of vibration

Question 15

What effect does a stronger bond have on the frequency of vibration?

Answer 15

Vibrate faster (with higher frequency)

Question 16

What effect do heavier atoms have on the frequency

of vibration?

Answer 16

Vibrate slower (with lower frequency)

Question 17

How does infrared spectroscopy work?

Answer 17

Every bond has a unique vibration frequency in the infrared region of the EM spectrum
Bonds absorb radiation that has the same frequency as their frequency of vibration
Infrared radiation emerged from a sample is missing the frequencies that have been absorbed → this information can be used to identify the compound’s
functional group

Question 18

What happens inside an infrared spectrometer?

Answer 18

Beam of infrared radiation with a range of frequencies is passed through the sample
Radiation that emerges is missing frequencies that have been absorbed by the bonds in the sample
Graph is plotted of intensity against frequency of radiation

Question 19

What do the troughs on an infrared spectrum show?

Answer 19

The frequencies where radiation has been absorbed - match to table to find out which bonds they represent

Question 20

What is the fingerprint region?

Answer 20

Area of the infrared spectrum below wavenumber
of 1500cm-1

Many peaks, caused by complex vibrations of the
whole molecule.

Unique to every compound, so
can be used to identify compounds

Question 21

How is the fingerprint region used to identify compounds?

Answer 21

The fingerprint region’s pattern is matched to a database on a computer to identify the compound accurately