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Distinguishing between organic compounds

What these questions look like

You are given several compounds and a set of observations, and must match them up.
Or you are given two or three compounds and asked to devise tests that tell them apart.
Either way the marks are for the observation, not just the name of the reagent.

The complete test table

Reagent	Positive observation	Reacts with	Does not react with
$\text{Br}_2(aq)$	orange → colourless	alkene	alkane, alcohol, carbonyl
$\text{Na}_2\text{CO}_3(aq)$	bubbles of $\text{CO}_2$	carboxylic acid	alcohol, ester, aldehyde
water	steamy fumes of HCl	acyl chloride	ester, amide
damp red litmus	turns blue	amine	amide, alcohol
Tollens'	silver mirror	aldehyde	ketone, alcohol
Fehling's / Benedict's	orange-red precipitate	aldehyde	ketone, alcohol
$\text{Cr}_2\text{O}_7^{2-}/\text{H}^+$	orange → green	1°/2° alcohol, aldehyde	3° alcohol, ketone
$\text{MnO}_4^-/\text{H}^+$	purple → colourless	1°/2° alcohol, aldehyde	3° alcohol, ketone

Choosing the right test

Some tests are specific — only one family responds. Use these first:
- carbonate → acid, water → acyl chloride, litmus → amine, bromine water → alkene
Others respond to more than one family, so they need a follow-up:
- dichromate goes green with a primary alcohol, a secondary alcohol and an aldehyde
- Tollens' then separates the aldehyde from the alcohols
So the standard strategy is: use a specific test if one exists; otherwise use two tests in sequence.

Distinguishing a primary alcohol from an aldehyde

This pair comes up constantly, because both turn dichromate green.

Tollens' reagent separates them: the aldehyde gives a silver mirror, the alcohol gives no change.
The reason is that Tollens' is a mild oxidising agent — strong enough to oxidise an aldehyde but not an alcohol.

Distinguishing a carboxylic acid from an acyl chloride

Both eventually fizz with carbonate, because the acyl chloride hydrolyses in the aqueous solution to the acid first.
So test with water alone: the acyl chloride gives steamy fumes, the acid does not.
Do the water test first, then the carbonate test.

Writing a good answer

Name the reagent precisely — "acidified potassium dichromate", not "dichromate".
Give the observation for the positive case and for the negative one.
If a colour change, give both colours.
If a gas, say what it is and how you would identify it.

Tips and Tricks

"Add Tollens' reagent" is not an answer. "Warm with Tollens' reagent — a silver mirror forms with the aldehyde and there is no change with the ketone" is. The observations carry the marks.

Give both colours in a colour change. "Decolourises" is weak; "orange to colourless" is precise. Never write "goes clear" — a solution can be clear and coloured at the same time.

Watch for compounds with two functional groups. A molecule containing both a C=C and a $-\text{COOH}$ will decolourise bromine water and fizz with carbonate, and a matching question may rely on that pair of observations together.

Beware the acyl chloride when using carbonate — it hydrolyses to the acid and fizzes too. Test with water first.

If a compound gives a negative result with everything, that is itself informative. A ketone is the usual answer: it does not oxidise, does not fizz, is not basic and has no C=C.

Worked ExampleMatching compounds to observations

Six compounds are listed below.

	Compound
1	$\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$
2	$\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO}$
3	$\text{CH}_3\text{COCH}_2\text{CH}_3$
4	$\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$
5	$\text{CH}_3\text{CH}_2\text{COCl}$
6	$\text{CH}_3\text{CH}_2\text{CH}{=}\text{CH}_2$

Match each set of observations to the correct compound number, and justify each choice.

(a) Adding a few drops of water produces steamy fumes.
(b) Adding sodium carbonate solution produces bubbles of gas.
(c) Warming with Tollens' reagent gives a silver mirror.
(d) Bromine water changes from orange to colourless; acidified dichromate stays orange.
(e) Acidified dichromate turns from orange to green; Tollens' gives no change.
(f) No reaction with any of the above reagents.

Step 1 — Identify the functional group in each compound

	Compound	Functional group
1	$\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$	primary alcohol (butan-1-ol)
2	$\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO}$	aldehyde (butanal)
3	$\text{CH}_3\text{COCH}_2\text{CH}_3$	ketone (butanone)
4	$\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$	carboxylic acid (butanoic acid)
5	$\text{CH}_3\text{CH}_2\text{COCl}$	acyl chloride (propanoyl chloride)
6	$\text{CH}_3\text{CH}_2\text{CH}{=}\text{CH}_2$	alkene (but-1-ene)

Step 2 — Work through the specific tests first

(a) Steamy fumes with water → Compound 5. Only an acyl chloride is hydrolysed by water at room temperature, releasing hydrogen chloride gas:

$\text{CH}_3\text{CH}_2\text{COCl} + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{CH}_2\text{COOH} + \text{HCl}$

(b) Bubbles with sodium carbonate → Compound 4. Only a carboxylic acid is acidic enough to release carbon dioxide from a carbonate:

$2\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{CH}_3\text{CH}_2\text{CH}_2\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$

(Compound 5 would also fizz once hydrolysed, which is why test (a) is done separately and first.)

(c) Silver mirror with Tollens' → Compound 2. Only an aldehyde has a hydrogen on its carbonyl carbon, allowing the mild oxidant to oxidise it while the silver(I) ions are reduced to silver metal.

(d) Bromine water decolourised, dichromate unchanged → Compound 6. Decolourising bromine water indicates a C=C double bond. The dichromate staying orange rules out an alcohol or aldehyde, confirming the compound is a hydrocarbon alkene with no other oxidisable group.

Step 3 — Use the two-test sequence for the remaining pair

Compounds 1 and 3 are left, matched to observations (e) and (f).

(e) Dichromate orange → green, but no silver mirror → Compound 1. Butan-1-ol is a primary alcohol, so it is oxidised by acidified dichromate and the solution turns green.

But Tollens' is a much milder oxidising agent and is not strong enough to oxidise an alcohol, so there is no silver mirror.

That combination — green with dichromate, nothing with Tollens' — is what distinguishes a primary alcohol from an aldehyde, since an aldehyde would give both.

(f) No reaction with anything → Compound 3. Butanone is a ketone:

not oxidised by dichromate, because its carbonyl carbon has no hydrogen → stays orange
no silver mirror with Tollens', for the same reason
not acidic, so no fizzing with carbonate
no C=C, so bromine water stays orange
no acyl chloride, so no steamy fumes

Distinguishing between organic compounds

Covers AS91391

What these questions look like

You are given several compounds and a set of observations, and must match them up.
Or you are given two or three compounds and asked to devise tests that tell them apart.
Either way the marks are for the observation, not just the name of the reagent.

The complete test table

Reagent	Positive observation	Reacts with	Does not react with
$\text{Br}_2(aq)$	orange → colourless	alkene	alkane, alcohol, carbonyl
$\text{Na}_2\text{CO}_3(aq)$	bubbles of $\text{CO}_2$	carboxylic acid	alcohol, ester, aldehyde
water	steamy fumes of HCl	acyl chloride	ester, amide
damp red litmus	turns blue	amine	amide, alcohol
Tollens'	silver mirror	aldehyde	ketone, alcohol
Fehling's / Benedict's	orange-red precipitate	aldehyde	ketone, alcohol
$\text{Cr}_2\text{O}_7^{2-}/\text{H}^+$	orange → green	1°/2° alcohol, aldehyde	3° alcohol, ketone
$\text{MnO}_4^-/\text{H}^+$	purple → colourless	1°/2° alcohol, aldehyde	3° alcohol, ketone

Choosing the right test

Some tests are specific — only one family responds. Use these first:
- carbonate → acid, water → acyl chloride, litmus → amine, bromine water → alkene
Others respond to more than one family, so they need a follow-up:
- dichromate goes green with a primary alcohol, a secondary alcohol and an aldehyde
- Tollens' then separates the aldehyde from the alcohols
So the standard strategy is: use a specific test if one exists; otherwise use two tests in sequence.

Distinguishing a primary alcohol from an aldehyde

This pair comes up constantly, because both turn dichromate green.

Tollens' reagent separates them: the aldehyde gives a silver mirror, the alcohol gives no change.
The reason is that Tollens' is a mild oxidising agent — strong enough to oxidise an aldehyde but not an alcohol.

Distinguishing a carboxylic acid from an acyl chloride

Both eventually fizz with carbonate, because the acyl chloride hydrolyses in the aqueous solution to the acid first.
So test with water alone: the acyl chloride gives steamy fumes, the acid does not.
Do the water test first, then the carbonate test.

Writing a good answer

Name the reagent precisely — "acidified potassium dichromate", not "dichromate".
Give the observation for the positive case and for the negative one.
If a colour change, give both colours.
If a gas, say what it is and how you would identify it.

Tips and Tricks

Give both colours in a colour change. "Decolourises" is weak; "orange to colourless" is precise. Never write "goes clear" — a solution can be clear and coloured at the same time.

Beware the acyl chloride when using carbonate — it hydrolyses to the acid and fizzes too. Test with water first.

If a compound gives a negative result with everything, that is itself informative. A ketone is the usual answer: it does not oxidise, does not fizz, is not basic and has no C=C.

Worked ExampleMatching compounds to observations

Six compounds are listed below.

	Compound
1	$\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$
2	$\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO}$
3	$\text{CH}_3\text{COCH}_2\text{CH}_3$
4	$\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$
5	$\text{CH}_3\text{CH}_2\text{COCl}$
6	$\text{CH}_3\text{CH}_2\text{CH}{=}\text{CH}_2$

Match each set of observations to the correct compound number, and justify each choice.

(a) Adding a few drops of water produces steamy fumes.
(b) Adding sodium carbonate solution produces bubbles of gas.
(c) Warming with Tollens' reagent gives a silver mirror.
(d) Bromine water changes from orange to colourless; acidified dichromate stays orange.
(e) Acidified dichromate turns from orange to green; Tollens' gives no change.
(f) No reaction with any of the above reagents.

Step 1 — Identify the functional group in each compound

	Compound	Functional group
1	$\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$	primary alcohol (butan-1-ol)
2	$\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO}$	aldehyde (butanal)
3	$\text{CH}_3\text{COCH}_2\text{CH}_3$	ketone (butanone)
4	$\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$	carboxylic acid (butanoic acid)
5	$\text{CH}_3\text{CH}_2\text{COCl}$	acyl chloride (propanoyl chloride)
6	$\text{CH}_3\text{CH}_2\text{CH}{=}\text{CH}_2$	alkene (but-1-ene)

Step 2 — Work through the specific tests first

(a) Steamy fumes with water → Compound 5. Only an acyl chloride is hydrolysed by water at room temperature, releasing hydrogen chloride gas:

$\text{CH}_3\text{CH}_2\text{COCl} + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{CH}_2\text{COOH} + \text{HCl}$

(b) Bubbles with sodium carbonate → Compound 4. Only a carboxylic acid is acidic enough to release carbon dioxide from a carbonate:

$2\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{CH}_3\text{CH}_2\text{CH}_2\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$

(Compound 5 would also fizz once hydrolysed, which is why test (a) is done separately and first.)

Step 3 — Use the two-test sequence for the remaining pair

Compounds 1 and 3 are left, matched to observations (e) and (f).

(e) Dichromate orange → green, but no silver mirror → Compound 1. Butan-1-ol is a primary alcohol, so it is oxidised by acidified dichromate and the solution turns green.

But Tollens' is a much milder oxidising agent and is not strong enough to oxidise an alcohol, so there is no silver mirror.

That combination — green with dichromate, nothing with Tollens' — is what distinguishes a primary alcohol from an aldehyde, since an aldehyde would give both.

(f) No reaction with anything → Compound 3. Butanone is a ketone:

not oxidised by dichromate, because its carbonyl carbon has no hydrogen → stays orange
no silver mirror with Tollens', for the same reason
not acidic, so no fizzing with carbonate
no C=C, so bromine water stays orange
no acyl chloride, so no steamy fumes