How to Make Chloroform in Chemistry: A Step-by-Step Lab Guide

Chloroform (trichloromethane, CHCl₃) is a volatile organic solvent historically used as an anesthetic. In a laboratory setting, you can prepare it via the haloform reaction, which involves the halogenation of a methyl ketone (usually acetone) with a halogen source (sodium hypochlorite, found in household bleach) in the presence of a base. This guide walks through the procedure, safety measures, and the underlying chemistry.

The Chemistry Behind the Haloform Reaction

The haloform reaction converts a methyl ketone (a molecule with a CH₃-CO- group) into a haloform (CHX₃, where X is a halogen) and a carboxylate salt. For chloroform, the halogen is chlorine. The reaction proceeds through three key steps:

1. Enolate formation: The base removes a proton from the methyl group adjacent to the carbonyl, forming an enolate ion.
2. Halogenation: The enolate attacks the chlorine atom in sodium hypochlorite (NaOCl), replacing a hydrogen with chlorine. This repeats until the methyl group becomes a trichloromethyl group (-CCl₃).
3. Cleavage: The trichloromethyl group is a good leaving group. Hydroxide attacks the carbonyl carbon, and the CCl₃⁻ group leaves. This anion quickly picks up a proton from water to form chloroform (CHCl₃).

The overall balanced equation using acetone and bleach is:

CH₃COCH₃ + 3 NaOCl → CHCl₃ + CH₃COONa + 2 NaOH

Acetone reacts with three equivalents of sodium hypochlorite to yield chloroform, sodium acetate, and sodium hydroxide. The reaction is exothermic, so temperature control matters.

Required Materials and Reagents

Before starting, gather the following lab-grade materials. Do not substitute consumer-grade products without verifying their composition.

• Acetone (propanone, ≥99% purity) – the methyl ketone source.
• Sodium hypochlorite solution (household bleach, typically 5-6% NaOCl by weight). Do not use “splashless” or scented varieties.
• Sodium hydroxide (NaOH, solid or concentrated solution) – to ensure the medium is sufficiently basic.
• Distilled water – for dilutions and washing.
• Ice bath – to control the exothermic reaction.
• Separatory funnel – for liquid-liquid extraction.
• Drying agent – anhydrous sodium sulfate (Na₂SO₄) or calcium chloride (CaCl₂).
• Distillation apparatus – round-bottom flask, condenser, receiving flask, heat source.
• Glass stirring rod, thermometer, pH paper.

Safety equipment: chemical splash goggles, nitrile gloves, lab coat, fume hood. Chloroform is a suspected carcinogen and a central nervous system depressant. Work only in a well-ventilated area and avoid skin contact.

Step-by-Step Procedure

1. Set Up the Reaction Vessel

Place a 500 mL round-bottom flask in an ice bath. Add 10 mL of acetone and 100 mL of distilled water. Stir gently. The ice bath will keep the mixture cool during the exothermic reaction.

2. Add the Bleach Slowly

Measure 200 mL of household bleach (5% NaOCl) into a dropping funnel. While stirring the acetone solution, add the bleach dropwise over 15-20 minutes. The solution will turn yellow and then cloudy. If the temperature rises above 30-35 °C, pause the addition and let the mixture cool.

3. Adjust the pH

After all the bleach is added, check the pH with pH paper. The mixture should be strongly basic (pH 10-12). If it is not, add a few drops of 10% sodium hydroxide solution until the pH is above 10. Stir for another 10 minutes. A white precipitate (sodium acetate) may form.

4. Extract the Chloroform

Transfer the mixture to a separatory funnel. Chloroform is denser than water (density about 1.48 g/mL) and will form a lower layer. Drain the lower organic layer into a clean flask. The upper aqueous layer can be discarded or saved for neutralization.

You may need to do a second extraction: add 20 mL of dichloromethane to the aqueous layer, shake, and drain the lower layer again. Combine the organic extracts.

5. Wash and Dry the Organic Layer

Wash the combined organic layer with 20 mL of distilled water to remove residual base and salts. Then wash with 20 mL of saturated sodium chloride solution (brine) to remove water. Separate the lower organic layer.

Add about 2-3 g of anhydrous sodium sulfate to the organic layer. Swirl and let it sit for 10-15 minutes. The drying agent will clump as it absorbs water. Decant or filter the dry chloroform into a clean round-bottom flask.

6. Distill the Chloroform

Set up a simple distillation apparatus. Heat the flask gently using a heating mantle or hot water bath – chloroform boils at 61.2 °C at standard pressure. Do not overheat; chloroform decomposes above 150 °C to form phosgene.

Collect the fraction that distills between 60-63 °C. Discard the first few drops and collect the main product in a clean, dry receiver. You should obtain a clear, colorless liquid.

Typical yield: from 10 mL acetone and 200 mL of 5% bleach, expect about 5-8 mL of crude chloroform and 3-5 mL after distillation, depending on technique.

Purification and Characterization

Distilled chloroform is usually >95% pure. For higher purity, you can dry it again over molecular sieves or redistill it. Store chloroform in a dark glass bottle with a tight cap, away from light and heat. Chloroform slowly decomposes in air and light to form phosgene and hydrogen chloride. Commercial chloroform often contains 0.5-1% ethanol as a stabilizer; you can add a small amount of ethanol (about 1% by volume) to your product for stability.

To confirm you have chloroform, measure its boiling point (61-62 °C) and refractive index (nD²⁰ = 1.445-1.446). You can also perform a simple density check: a drop of your product should sink in water but float on a dense organic solvent like carbon tetrachloride. For definitive identification, use gas chromatography or infrared spectroscopy (strong C-Cl stretch near 760 cm⁻¹).

Safety, Storage, and Legal Considerations

Chloroform is a hazardous chemical. The U.S. National Institute for Occupational Safety and Health (NIOSH) lists it as a potential occupational carcinogen. Inhalation can cause dizziness, headache, and unconsciousness. Skin contact leads to defatting and irritation. Always use a fume hood, avoid open flames, and wear gloves.

Storage: Keep in a tightly sealed amber glass bottle, in a cool, dry, ventilated cabinet away from oxidizers and strong bases. Label clearly with the chemical name, concentration, date, and hazard warnings.

Legal notes: In many jurisdictions, chloroform is a controlled precursor chemical because of its use in illicit drug synthesis. Check your local regulations before purchasing or preparing it. Academic labs typically require a chemical safety plan and institutional approval. Do not produce chloroform without proper authorization.

Common Mistakes and Troubleshooting

• Low yield: The most common cause is insufficient base. Check the pH after bleach addition; if it is below 10, the reaction stalls. Add more NaOH solution.
• Emulsion during extraction: If the layers do not separate cleanly, add a few drops of brine or gently swirl the separatory funnel. Let it sit for 5-10 minutes.
• Product smells like bleach: You may have carried over hypochlorite. Wash the organic layer with a dilute sodium thiosulfate solution (5% Na₂S₂O₃) to destroy residual oxidizer, then re-wash with water.
• Boiling point too high or too low: Impurities can shift the boiling point. Redistill and collect only the 60-63 °C fraction. Use a drying agent more thoroughly.
• Cloudy distillate: Water is present. Redry with fresh sodium sulfate and redistill.

If you see a yellow or green color in the distillate, it may contain chlorine or decomposition products. Discard that fraction and check your distillation setup for leaks or overheating.

Key Takeaways

• Chloroform is made via the haloform reaction: acetone reacts with sodium hypochlorite (bleach) in basic conditions to produce chloroform, sodium acetate, and sodium hydroxide.
• The procedure requires careful temperature control (ice bath), slow addition of bleach, and a strongly basic pH (above 10) for good yield.
• Purification involves extraction, washing, drying with anhydrous sodium sulfate, and distillation at 61-62 °C.
• Chloroform is a hazardous chemical (carcinogen, CNS depressant) and must be handled in a fume hood with proper PPE. Store it in a dark bottle away from light and heat.
• Legal restrictions may apply; always check local regulations before synthesis.
• Common problems (low yield, emulsions, impurities) can be solved by adjusting pH, using brine, or redistilling.