Qualitative Testing of Organic Compounds (VCE SSCE Chemistry): Revision Notes

Qualitative Testing of Organic Compounds

Introduction

When synthesising organic compounds, chemists often produce a mixture containing the desired product along with unwanted substances. These unwanted substances include:

• Intermediates: Compounds formed during the reaction pathway
• By-products: Substances produced alongside the intended product (such as isomers)

After synthesis, chemists must:

1. Separate the desired product from by-products
2. Purify the product
3. Confirm the identity and purity of the final product

This note covers laboratory techniques for isolating products and qualitative tests for identifying organic compounds.

Purification and separation techniques

Distillation

Distillation separates liquids that have different boiling points. This technique works best when the liquids have boiling points that differ by at least 50°C.

How distillation works:

1. The mixture is placed in a round-bottom flask and heated to the boiling point of the liquid you want to collect
2. Vapour rises from the flask and passes into a water-cooled condenser
3. The condenser cools the vapour below its boiling point, causing it to condense
4. The condensed liquid (called the distillate) is collected in a beaker
5. Liquids with higher boiling points remain in the flask

Fractional distillation

When liquids have boiling points that are close together (less than 50°C apart), fractional distillation provides better separation. This technique is particularly useful for separating volatile liquids - liquids that evaporate easily at low temperatures.

Key features of fractional distillation apparatus:

• A fractionating column packed with glass beads or shelves provides a large surface area for vapour to condense on
• Anti-bumping granules are added to the flask to prevent violent boiling
• A temperature gradient exists up the column (cooler at the top, hotter at the bottom)

How fractional distillation works:

The process can be thought of as multiple simple distillations happening simultaneously:

1. The mixture is heated, and vapours rise from the flask
2. These vapours contain a higher concentration of the lower boiling point components
3. Vapours rise up the column until they reach a height where the temperature is low enough for condensation
4. The condensed liquid trickles back down and is reheated by rising vapours, causing it to evaporate again
5. Each evaporation-condensation cycle enriches the vapour in the most volatile component
6. When vapour reaches the top of the column, it ideally consists of only the most volatile component
7. The temperature at the top remains relatively stable
8. The vapour enters the condenser, cools, and the distillate drips into the collection vessel

Industrial application: Crude oil fractional distillation

Crude oil is a mixture of hydrocarbons formed from plant and animal remains over millions of years. At oil refineries, crude oil undergoes fractional distillation in a fractionating tower.

Melting point determination

Melting point determination provides information about both the identity and purity of solid organic compounds.

Understanding melting points:

• A sharp melting point means the difference between the temperature at which melting begins and when it's complete is small (typically 0.5-2°C)
• Pure substances have sharp melting points because all molecules are identical and pack together in an orderly arrangement, maximising intermolecular forces
• Impure substances have lower melting points and broader melting point ranges (more than 2°C) because different molecules disrupt the crystal structure

Method:

1. Add the sample to a small glass capillary tube (closed at one end)
2. Attach the capillary tube to a thermometer and insert into a Thiele tube containing oil
3. Slowly heat the oil
4. Record two temperatures:
   • When the sample starts to melt
   • When the sample is completely liquified
5. The melting point range is the difference between these temperatures

Using melting points for identification:

To identify an unknown solid:

• Compare the experimentally determined melting point with known values
• If the melting point is higher than the expected value, the sample is not the desired product
• If the melting point is lower, the sample is either impure or not the desired product

Mixed melting point determination:

To confirm identity, mix the unknown substance with a pure sample of the suspected compound and measure the melting point:

• If the mixture has a wide melting point range below the expected value, the compounds are different
• If the mixture melts at the expected temperature, the unknown is confirmed as that compound

Qualitative tests for functional groups

After separating a desired product, chemists use simple chemical tests to confirm the presence of specific functional groups.

Testing for carbon-carbon double bonds

The bromine test is a quick and easy way to detect carbon-carbon double bonds (C=C).

Principle:

Alkenes are unsaturated hydrocarbons that readily undergo addition reactions. In these reactions, small molecules add across the carbon-carbon double bond.

When ethene reacts with bromine:

$\text{H}_2\text{C=CH}_2 + \text{Br}_2 \rightarrow \text{H}_2\text{BrC-CBrH}_2$

Observations:

Testing for hydroxyl groups

Two types of tests can identify the hydroxyl functional group (-OH) in alcohols:

1. Esterification test

Method:

1. Gently heat a mixture of the organic compound with ethanoic acid and a few drops of sulfuric acid (catalyst)
2. Pour the reaction mixture into cold water
3. Smell the surface of the water

Observations:

• If a hydroxyl group is present, you will detect the characteristic smell of an ester

Explanation:

Alcohols react with carboxylic acids to form esters:

$\text{ROH}(l) + \text{CH}_3\text{COOH}(l) \rightarrow \text{CH}_3\text{COOR}(l) + \text{H}_2\text{O}(l)$

(R represents an alkyl group)

If no ester smell is detected, the hydroxyl group is not present.

2. Oxidation tests

After confirming the presence of a hydroxyl group, oxidation tests can determine the type of alcohol:

Test A: Acidified potassium dichromate

Observations:

• Dichromate ion (Cr₂O₇²⁻) solution is orange
• When added to a primary or secondary alcohol, the dichromate is reduced to chromium(III) ions (Cr³⁺)
• The colour changes from orange to green
• With tertiary alcohols, the orange colour remains unchanged

Test B: Acidified potassium permanganate

Observations:

• Permanganate ion (MnO₄⁻) solution is deep purple
• When added to a primary or secondary alcohol, the permanganate is reduced to manganese(II) ions (Mn²⁺)
• The colour changes from purple to colourless
• With tertiary alcohols, the purple colour remains unchanged

Testing for carboxyl groups

The carboxyl functional group (-COOH) can be detected using sodium hydrogen carbonate.

Method:

1. Add a small amount of solid sodium hydrogen carbonate (NaHCO₃) to the organic compound

Observations:

• Effervescence (fizzing) occurs if the carboxyl group is present
• Carbon dioxide gas is produced

Confirmation test:

Bubble the gas through limewater:

• The limewater turns 'milky' or 'cloudy' due to precipitation of calcium carbonate

Explanation:

When metal hydrogen carbonates react with acids, the products are a salt, water, and carbon dioxide:

$\text{Ca(OH)}_2(aq) + \text{CO}_2(g) \rightarrow \text{CaCO}_3(s) + \text{H}_2\text{O}(l)$

Summary of qualitative tests

Chemical test	Method	Observations	Explanation
Carbon-carbon double bond	Add bromine (Br₂) solution to the unknown compound	Red-orange colour rapidly decolourises	Addition reaction occurs. As bromine adds across the C=C bond, the solution loses colour: R₂C=CR₂ + Br₂ → R₂BrC-CBrR₂
Carboxyl group	Add solid sodium hydrogen carbonate (NaHCO₃) to the unknown compound	Effervescence occurs as colourless gas is produced. When bubbled through limewater, the solution turns milky/cloudy	NaHCO₃(s) + RCOOH(aq) → RCOONa(aq) + CO₂(g) + H₂O(l) Limewater test: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
Hydroxyl group	Gently heat mixture of organic compound with ethanoic acid and sulfuric acid. Pour into cold water	Characteristic smell of ester on water surface	Condensation reaction between hydroxyl and carboxyl groups: ROH(l) + CH₃COOH(l) → CH₃COOR(l) + H₂O(l)

Identifying unknown compounds - worked example

Iodine number

Iodine number (also called iodine value or iodine index) measures the degree of unsaturation in fats and oils.

Definition: The iodine number is the mass of iodine (in grams) that reacts with 100 g of a fat or oil.

Background

Fats and oils contain triglycerides - large molecules made from glycerol and three fatty acids. Understanding their structure helps explain their chemical behavior:

• Fatty acids have a carboxyl group attached to a long hydrocarbon chain
• Fatty acids are classified as saturated (no C=C bonds) or unsaturated (contain C=C bonds)
• Halogens like iodine undergo addition reactions with unsaturated fatty acids
• Iodine is safer to use than bromine (which releases toxic fumes)
• Iodine solutions are stable and can be standardised

Calculating the number of double bonds

Calculating iodine number

Typical iodine numbers:

Fat or oil	Iodine number (g I₂ per 100 g)
Beef fat	42-48
Butter	25-42
Canola oil	110-126
Coconut oil	6-11
Fish oil	190-205
Linseed oil	170-204
Olive oil	75-94
Peanut oil	82-107
Sesame oil	100-120
Sunflower oil	110-145