26 - Carbonyls and carboxylic acids Flashcards
F: RCHO
G: RCHOR C=C
Describe suitable chemical tests, with observations, that would confirm the presence of the
functional groups in F and G.
F/aldehyde
Tollens’ (reagent) -> Silver (mirror/precipitate/ppt/solid)
G/alkene/C=C
Bromine/Br2 -> goes colourless/decolourised
G/ketone -> 2,4-dinitrophenylhydrazine
orange/yellow/red precipitate
G/ketone
Tollens’ (reagent) ->no silver mirror/no change/no reaction
Explain why the mechanism in (i) involves heterolytic fission
Heterolytic
One (bonded) atom/O receives both/2 electrons ✓
Fission
Breaking of a covalent bond ✓
Suggest a suitable chemical test to confirm that both compounds contain an unsaturated
carbon chain.
Your answer should include the reagent and observations.
Bromine/ Br2
goes colourless/decolourise
Describe a chemical test to confirm that both compounds contain an aldehyde functional
group.
Your answer should include the reagent and observations
Tollens’ (reagent)
Silver (mirror/precipitate/ppt/solid)
Describe a chemical test to confirm that cinnamaldehyde and methylcinnamaldehyde contain a carbonyl group.
How could the products of this test be used to distinguish between the two compounds?
Add 2,4-dinitrophenylhydrazine
orange/yellow/red precipitate forms
Take melting point (of crystals)
Compare to known values/database
Test for aldehyde.
Reagent(s)
Observation(s)
Equation
Tollens’ (reagent)
Silver (mirror) ✓
RCHO + [O] → RCOOH
Test for carboxylic acid
Reagent(s)
Observation(s)
Equation
sodium carbonate
Fizzing/effervescence/bubbling ✓
2RCOOH + Na2CO3 → 2RCOONa + CO2 +
H2O
Suggest a chemical test to distinguish the ketone from the two esters.
Reagent(s)
Observation(s)
2,4−dinitrophenylhydrazine
AND
Orange/yellow/red precipitate for the ketone
Suggest how the hydroxide ion can act as a nucleophile
It is an electron pair donor OR donates a lone pair
