carbonyls

Question 1

The following three carbonyl compounds are structural isomers of C5H10O2.
Describe chemical tests that you could carry out in test-tubes to distinguish between compounds C, D and E.Include appropriate reagents and any relevant observations.
Also include equations showing structures for the organic compounds involved.

Answer 1

FIRST react all with
Tollens’ reagent AND silver mirror/ppt/solid (formed) with
compound D
OR with Fehling’s/Benedict’s solutions AND
(brick-red/orange) solid/precipitate (formed) with
compound D 
NOTE: eliminates D

THEN react C and E with
H2SO4/H+ AND K2Cr2O7/ Cr2O7
2–
/Na2Cr2O7
AND colour change OR green colour with compound C
OR no change OR no reaction OR no green colour with
compound E 

Question 2

Describe a chemical test that you could use to detect the presence of a carbonyl group in an organic compound.

Answer 2

2,4-DNP(H)

orange precipitate 

Question 3

Describe a series of chemical tests that the technician could use to confirm that the compound in the bottle is a ketone.
Include appropriate reagents and any relevant observations.

Answer 3

One mark is for positive carbonyl test
 (Add) 2,4-dinitrophenylhydrazine
 AND
 orange/yellow/red precipitate 
One mark is for negative aldehyde test
EITHER
 (Add) Tollens’ reagent/Tollens’ test
 AND
 no change OR no reaction OR no silver (mirror)
OR
 (Add) H2SO4 AND K2Cr2O7
 AND
 no change OR no reaction OR no green colour 

Question 4

Describe how the technician could use the product of one of the tests in (i) to show whether the bottle contains pentan-2-one or pentan-3-one. The method used should not involve spectroscopy.

Answer 4

1st mark
Take melting point of orange crystals/derivative/product
from 2,4-DNP 
2nd mark
Compare melting point with known values
OR
compare melting point with value in database/reference
book 

Question 5

‘Methylglyoxal’, CH3COCHO, is formed in the body during metabolism.Describe one reduction reaction and one oxidation reaction of methylglyoxal that could be carried out in the laboratory.Your answer should include reagents, equations and observations, if any.

Answer 5

Equations
CH3COCHO + 4[H]  CH3CHOHCH2OH 
CH3COCHO + [O]  CH3COCOOH 
Reduction reagents and observation
Methylglyoxal is reduced by NaBH4 
Oxidation reagents and observation
Methylglyoxal is oxidised by H2SO4 AND K2Cr2O7 
Observation: turns green OR blue 
OR
Methylglyoxal is oxidised by Tollens’ reagent 
Observation: Silver (mirror) 

Question 6

A student was given three compounds, an aldehyde, a ketone, and a carboxylic acid.
The student carried out the same two chemical tests on each compound. This allowed her to distinguish between all three compounds.
• Describe two suitable tests that the student could have used.
• Show how the observations would allow her to distinguish between the compounds.

Answer 6

Tollens’ test AND ‘silver
precipitate/mirror’

is the aldehyde

react with 2,4-DNP(H)
and ‘orange precipitate’

must be the ketone

Tollens’ test AND ‘silver
precipitate/mirror’

is the aldehyde

react with carbonate/
hydrogencarbonate/
Na/Mg and ‘fizzes/
bubbles/ effervesces/ gas
evolved

must be the (carboxylic)
acid


2,4-DNP(H) AND orange
precipitate

is either aldehyde OR
ketone ALLOW carbonyl
OR C=O

Tollens’ test & ‘silver
ppt/mirror’

is the aldehyde

2,4-DNP(H) and no orange
precipitate

is the (carboxylic) acid

Tollens’ test & ‘silver
ppt/mirror’

is the aldehyde


Question 7

State what the biochemist would see when hydroxyethanal reacts with Tollens’ reagent.

Answer 7

silver mirror