Introduction to Core Practicals 4-7

Welcome! In this chapter, we are diving into the "hands-on" side of Organic Chemistry. These four core practicals are essential for Paper 3 because they bridge the gap between theory and the real world. We will look at how halogenoalkanes react, how we transform alcohols, and how we play "Chemistry Detective" to identify unknown substances. Don't worry if organic chemistry feels like a lot of symbols—we will break it down into simple steps!

Note: For general lab safety and apparatus details, please see the chapter on "Practical techniques, apparatus and safety".

Core Practical 4: Rates of Halogenoalkane Hydrolysis

In this experiment, we compare how quickly different halogenoalkanes (chloro-, bromo-, and iodoalkanes) react with water. This reaction is called hydrolysis.

The Setup

We mix the halogenoalkane with aqueous silver nitrate. Because halogenoalkanes don't mix well with water, we use ethanol as a common solvent so everything dissolves together.

  • The Reaction: \(RX + H_{2}O \rightarrow ROH + H^{+} + X^{-}\)
  • The Detection: As soon as the halide ion (\(X^{-}\)) is released, it reacts with the silver ions (\(Ag^{+}\)) to form a colored precipitate: \(Ag^{+}(aq) + X^{-}(aq) \rightarrow AgX(s)\)

What to Observe

We measure the time it takes for the precipitate to appear. The faster it appears, the faster the rate of hydrolysis.

  • 1-chlorobutane: White precipitate (very slow).
  • 1-bromobutane: Cream precipitate (medium speed).
  • 1-iodobutane: Yellow precipitate (fastest).

The Chemistry Behind It

You might think the \(C-Cl\) bond would react fastest because it's the most polar, but bond enthalpy is the decider here. The \(C-I\) bond is the weakest (lowest bond enthalpy), so it breaks most easily, leading to the fastest reaction.

Key Takeaway: Rate of hydrolysis increases down Group 7: \(Chloro < Bromo < Iodo\). Bond strength is more important than bond polarity in this trend.

Core Practical 5: The Oxidation of Ethanol

Alcohols can be oxidized to form different products depending on how we set up the experiment. We use acidified potassium dichromate(VI) (\(K_{2}Cr_{2}O_{7} / H_{2}SO_{4}\)) as the oxidizing agent.

Method A: Distillation (To make Ethanal)

If we want to make an aldehyde (ethanal), we use distillation. We gently heat the mixture and "boil off" the ethanal as soon as it forms. This prevents it from staying in contact with the oxidizing agent for too long.

Method B: Reflux (To make Ethanoic Acid)

If we want to make a carboxylic acid (ethanoic acid), we use reflux. A condenser is placed vertically over the flask. Any vapors that boil off are cooled, condensed, and drip back into the flask to react further.

The Visual Change

In both cases, the orange dichromate(VI) ions (\(Cr_{2}O_{7}^{2-}\)) are reduced to green chromium(III) ions (\(Cr^{3+}\)). This "Orange to Green" color change is the classic test for a primary or secondary alcohol being oxidized.

Quick Review Box:
Distillation = Partial oxidation (Alcohol \(\rightarrow\) Aldehyde)
Reflux = Full oxidation (Alcohol \(\rightarrow\) Carboxylic Acid)

Core Practical 6: Chlorination of 2-methylpropan-2-ol

In this practical, we turn an alcohol into a halogenoalkane using concentrated hydrochloric acid (\(HCl\)). This is a nucleophilic substitution reaction.

The Process Step-by-Step

This practical is famous for its "purification" steps, which are frequently examined in Paper 3.

  1. Reaction: Mix the tertiary alcohol with conc. \(HCl\) in a flask and shake.
  2. Separation: Pour the mixture into a separating funnel. Two layers will form (organic and aqueous). The organic product is usually the top layer. Run off the bottom waste layer.
  3. Washing: Add sodium hydrogencarbonate solution (\(NaHCO_{3}\)) to the funnel. This reacts with any leftover \(HCl\). Watch out! This produces \(CO_{2}\) gas, so you must open the tap frequently to release the pressure.
  4. Drying: Transfer the organic layer to a flask and add a drying agent like anhydrous sodium sulfate (\(Na_{2}SO_{4}\)) or calcium chloride (\(CaCl_{2}\)). The liquid is dry when it turns from cloudy to clear.
  5. Final Purity: Perform a final distillation. Collect the liquid that boils at the specific boiling temperature of your product.

Did you know? Tertiary alcohols react much faster with \(HCl\) than primary alcohols. This is why we can do this reaction at room temperature for 2-methylpropan-2-ol!

Core Practical 7: Analysis of Unknowns

This practical is like being a detective. You are given "Mystery Samples" and must use specific tests to identify the ions or functional groups present. This combines Topic 4C (Inorganic Analysis) and Topic 6 (Organic Tests).

Inorganic Analysis (Cations and Anions)

  • Carbonates (\(CO_{3}^{2-}\)): Add dilute acid (like \(HNO_{3}\)). Bubbling (effervescence) indicates \(CO_{2}\) gas.
  • Sulfates (\(SO_{4}^{2-}\)): Add acidified barium chloride (\(BaCl_{2}\)). A white precipitate of \(BaSO_{4}\) forms.
  • Halides (\(Cl^{-}, Br^{-}, I^{-}\)): Add silver nitrate (\(AgNO_{3}\)).
    - \(Cl^{-}\) = White ppt (dissolves in dilute ammonia)
    - \(Br^{-}\) = Cream ppt (dissolves in conc. ammonia)
    - \(I^{-}\) = Yellow ppt (insoluble in ammonia)
  • Ammonium (\(NH_{4}^{+}\)): Add \(NaOH\) and warm gently. Test the gas with damp red litmus paper; it will turn blue (ammonia gas).
  • Flame Tests: Use a nichrome wire dipped in \(HCl\).
    - Lithium: Red
    - Sodium: Yellow/Orange
    - Potassium: Lilac
    - Calcium: Brick-red
    - Strontium: Red
    - Barium: Apple-green

Organic Analysis (Functional Groups)

  • Alkenes (\(C=C\)): Shake with bromine water. Color change: Orange to colorless.
  • Alcohols (\(-OH\)): Add phosphorus(V) chloride (\(PCl_{5}\)). Steamy white fumes of \(HCl\) gas are produced.
  • Aldehydes: Warm with Benedict’s/Fehling’s solution (Blue to Red ppt) or Tollens’ reagent (Silver mirror forms).
  • Carboxylic Acids: Add sodium carbonate. Effervescence (\(CO_{2}\)) occurs.

Key Takeaway: Always describe the reagent, the observation, and the inference (what it proves). For example: "Adding silver nitrate produced a yellow precipitate, which indicates the presence of iodide ions."

Common Mistakes to Avoid

  • Forgetting the Ethanol: In CP4, if you don't use ethanol, the halogenoalkane won't mix with the water/silver nitrate, and the reaction will be incredibly slow.
  • Pressure Build-up: In CP6, forgetting to vent the separating funnel while washing with \(NaHCO_{3}\) can cause the stopper to fly off due to \(CO_{2}\) pressure!
  • Acidifying incorrectly: When testing for halides, always use nitric acid (\(HNO_{3}\)), not \(HCl\). If you use \(HCl\), you are adding chloride ions to your own test, which will give a false positive white precipitate!
  • Reflux setup: Never put a stopper in the top of a reflux condenser. This creates a sealed system under heat, which is essentially a bomb.

Note: For more advanced identification techniques using Spectra (IR and Mass Spec), refer to the "Modern Analytical Techniques" chapters.