ORGANIC CHEMISTRY -organic Analysis

Question 1

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

Answer 1

Add bromine water
Result - goes from orange to colourless

Question 2

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

Answer 2

Add (aq) silver nitrate (AgNO3)
Result: precipitate produced
Cl= white
Br= cream
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 1° and 2° alcohols. There is no change for 3° alcohols

Question 4

How do you test for aldehydes?

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
Result: CO2 (g) given off -effervescence

Question 6

How do you identify carbonyl compounds? (C=O)

Answer 6

Add 2,4-DNP
If carbonyl is present - orange ppt
Re-crystallise the product
Measure the melting point of the product
Compare to a data base of known carbonyls

Question 7

what is mass spectrometry? how does it work?

Answer 7

• used to find relative molecular masses of organic compounds
• compound is dissolved in solution, ionised by a high voltage supply, accelerated by a negatively charged plate. It becomes a beam of ionised molecules, reaching the detector causing a current to flow. TOF is used to work out the m/z value and plot graph.

Question 8

what does the x axis show on the mass spectrum? what does this effectively show and why?

Answer 8

m/z value ( mass dividede by ionic charge)
since most ions are 1+ this effectively shows Mr

Question 9

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

Answer 9

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 10

what is a molecular ion? where is it found?

Answer 10

the positive ion formed in mass spectrometry, as an electron has been knocked off it
found on the right end of the spectrum - tells us mass

Question 11

what occurs during fragmentation?

Answer 11

a bond breaks and a ion + radical is formed

Question 12

why aren’t all the fragments the same?

Answer 12

some fragments are more stable than each other so each fragment will not have the same abundance.

Question 13

why do atoms and chemical bonds absorb infrared radiation?

Answer 13

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

Question 14

what affect does a stronger bond have on the frequency of vibration?

Answer 14

They vibrate faster (with higher frequency)

Question 15

what affect does heavier atoms have on the frequency of vibration?

Answer 15

They vibrate slower (with lower frequency)

Question 16

How does IR spectroscopy work?

Answer 16

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 17

what happens inside an Infrared spectrometer?

Answer 17

beam of infrared radiation with different frequencies is passed through a sample
the radiation that emerges is missing the frequencies that correspond to the types of bonds found in the sample.
A graph of the intensity of the radiation from the sample,transmittance, against the frequency of the radiation is plotted.
The frequency is expressed as wave number.

Question 18

what do troughs on the IR spectrum show?

Answer 18

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

Question 19

what is the fingerprint region?

Answer 19

area of IR spectrum below wavenumber of 1500

Many peaks caused by complex vibrations of the whole molecule.

Question 20

how is the fingerprint region used to identify compounds?

Answer 20

The fingerprints regions pattern is matched to the database on a computer to identify the compound accurately.

Question 21

what are some practical applications of infrared spectroscopy?

Answer 21

• monitor air pollution
• forensics
• drug analysis
  -used in breathalysers to check level of alcohol in breath.

Question 22

summary of where OH, C=O and COOH can be found on the graph?

Answer 22

• one broad peak above 3000 (LEFT OF THE GRAPH) is for OH
  -one narrow peak about 1700 (MIDDLE OF THE GRAPH) for aldehyde and ketones
• two peaks for carboxylic acid = 1. at 1700 for (C=O) and a very broad one 2500-3300 (LEFT OF THE GRAPH) for (O-H)