organic analysis [O1] PAPER 2

Question 1

why can the precise relative molecular masses of propanal and prop-2-en-1-ol obtained via mass spectrometry not be used to distinguish between the two molecules?

Answer 1

both have a molecular formula of of C3H6O, therefore have identical precise relative masses

Question 2

why is the precise relative atomic mass of the 12C isotope exactly 12?

Answer 2

by definition
OR
the standard/reference value/isotope

Question 3

Answer 3

Question 4

test for an aldehyde?

Answer 4

1)
• add Tollen’s reagent
• positive result = colour change from colourless to silver mirror
• negative result = remains colourless
2)
• add Fehling’s solution
• positive result = red precipitate formed
• negative result = remains blue

Question 5

test for unsaturated hydrocarbons?

Answer 5

• add bromine water
• positive result = colour change from orange to colourless
• negative result = remains orange

Question 6

test for carboxylic acid?

Answer 6

• react with sodium carbonate
• observation: effervescence
• bubble gas formed (carbon dioxide) through limewater
• positive result = limewater turns cloudy

Question 7

what is the fingerprint region on an IR spectrum?

Answer 7

region on the right-hand side of the spectrum which allows the molecule to be identified

Question 8

test for an alcohol?

Answer 8

• add acidified potassium dichromate (VI)
• positive result = colour change from orange to green - ONLY primary and secondary alcohols
• negative result = remains orange - tertiary alcohols would give a negative result

Question 9

Describe fully how infrared spectra can be used to distinguish between different molecules.

Answer 9

• [molecule] has a [OH? C=O? etc.] peak at [range for the bond] cm-1 → repeat for each molecule
• [if some molecules have the same bond thing] ____ have unique fingerprint regions (below 1500cm-1)
• Compare to known spectra/database and look for exact matches

Question 10

Suggest why mass spectrometry using electrospray ionisation isn’t a suitable method for distinguishing between isomers.

Answer 10

They all have the same Mr

Question 11

Explain how carbon dioxide causes global warming.

Answer 11

• C=O Bonds in CO2 absorb infrared radiation (of 2350 cm-1)
• IR radiation emitted by the earth does not escape (from the atmosphere)

Question 12

[dicarboxylic acid’s Mr is 118 to the nearest whole number]

Answer 12

precise relative molecular masses are different

Question 13

Answer 13