Chemistry (9701) Study Notes: Halogen Compounds (Topic 15)

Welcome to the Halogen Compounds chapter! This is one of the most important building blocks in organic chemistry because halogenoalkanes (molecules containing a halogen atom) are incredibly useful starting materials for making many other functional groups. Understanding how they react means you’re learning the essential skills for synthetic chemistry. Don't worry if the mechanisms seem tricky; we’ll break them down step-by-step!

1. Introduction to Halogenoalkanes (Haloalkanes)

1.1 What are Halogenoalkanes?

A halogenoalkane (also called an alkyl halide) is an organic compound in which one or more hydrogen atoms in an alkane molecule have been replaced by a halogen atom (\(X\), where \(X = \text{F, Cl, Br, or I}\)).

  • General Formula: \(R-X\)
  • Example: Chloroethane (\(\text{CH}_3\text{CH}_2\text{Cl}\))

1.2 Classification: Primary, Secondary, and Tertiary

Halogenoalkanes are classified based on the number of alkyl groups attached to the carbon atom that is bonded directly to the halogen (\(X\)).

Primary (\(1^\circ\)) Halogenoalkanes
  • The carbon atom bonded to \(X\) is attached to one other alkyl group (\(R\)).
  • Example: Bromoethane.
Secondary (\(2^\circ\)) Halogenoalkanes
  • The carbon atom bonded to \(X\) is attached to two other alkyl groups (\(R\)).
  • Example: 2-Chloropropane.
Tertiary (\(3^\circ\)) Halogenoalkanes
  • The carbon atom bonded to \(X\) is attached to three other alkyl groups (\(R\)).
  • Example: 2-Bromo-2-methylpropane.

Quick Review: Classification determines which reaction mechanism (\(S_N1\) or \(S_N2\)) they prefer, so make sure you can identify them correctly!

2. The Key to Reactivity: The C–X Bond

2.1 Polarity of the C–X Bond

Halogens are highly electronegative (they pull electrons towards themselves). When bonded to carbon, they pull the bonding electrons away from the carbon atom.

  • This results in a polar bond, where the carbon atom carries a partial positive charge (\(\text{C}^{\delta+}\)) and the halogen carries a partial negative charge (\(\text{X}^{\delta-}\)).
  • \(\text{C}^{\delta+}-\text{X}^{\delta-}\)
  • This positive carbon atom is an easy target for nucleophiles (electron-rich species).

2.2 Bond Strength and Reactivity Trend

The strength of the carbon-halogen bond (\(\text{C}-\text{X}\)) determines how easily the halogen can be broken off and replaced. Bond strength generally decreases as the size of the halogen atom increases:

Bond Strength Trend:
\( \text{C}-\text{F} \quad > \quad \text{C}-\text{Cl} \quad > \quad \text{C}-\text{Br} \quad > \quad \text{C}-\text{I} \)

Reactivity Trend:
Iodoalkanes (\(\text{C}-\text{I}\)) are the most reactive, and fluoroalkanes (\(\text{C}-\text{F}\)) are the least reactive (they are practically inert to many nucleophiles) in nucleophilic substitution reactions.

Memory Aid: Think of the C–I bond as a weak link because Iodine is so large. The bond length is long, making it easier to break.

3. Preparation of Halogenoalkanes

Halogenoalkanes can be made via substitution or addition reactions, depending on the starting material.

3.1 From Alkanes (Free-Radical Substitution)

Alkanes react with halogens (\(\text{Cl}_2\) or \(\text{Br}_2\)) in the presence of ultraviolet light (UV). This is a free-radical substitution reaction.

  • Example: Reaction of ethane with chlorine, exemplified by:
    \(\text{CH}_3\text{CH}_3 + \text{Cl}_2 \xrightarrow{\text{UV light}} \text{CH}_3\text{CH}_2\text{Cl} + \text{HCl}\)

3.2 From Alkenes (Electrophilic Addition)

Alkenes react quickly with halogens (\(\text{X}_2\)) or hydrogen halides (\(\text{HX}(\text{g})\)) at room temperature via electrophilic addition.

  • Reaction with Halogen: Ethene reacts with \(\text{Br}_2\) to form 1,2-dibromoethane.
  • Reaction with Hydrogen Halide: Ethene reacts with \(\text{HBr}(\text{g})\) to form bromoethane.

3.3 From Alcohols (Substitution)

Alcohols (\(R-\text{OH}\)) can be converted directly into halogenoalkanes (\(R-\text{X}\)) by replacing the hydroxyl group (\(-\text{OH}\)).

  • Reagent 1: Hydrogen Halide gas (\(\text{HX}(\text{g})\))
    Condition: Requires heat or appropriate acid catalyst.
  • Reagent 2: Potassium Halide (e.g., \(\text{KCl}\)) and Concentrated Sulfuric/Phosphoric Acid
    Reagents: \(\text{KCl}\) and concentrated \(\text{H}_2\text{SO}_4\) or concentrated \(\text{H}_3\text{PO}_4\)
  • Reagent 3: Phosphorus Halides (\(\text{PCl}_3\), \(\text{PCl}_5\), or \(\text{SOCl}_2\))
    These reagents are often used because they give a cleaner product. For \(\text{PCl}_3\), heat is required.

Key Takeaway: Halogenoalkanes are central synthetic intermediates, most easily prepared from alcohols or alkenes.

4. Reactions of Halogenoalkanes: Nucleophilic Substitution (\(S_N\))

The primary reaction type for halogenoalkanes is nucleophilic substitution. The halogen atom, the leaving group, is replaced by a nucleophile.

4.1 What is a Nucleophile?

A nucleophile is a species that possesses a lone pair of electrons and is attracted to an electron-deficient centre (like the \(\text{C}^{\delta+}\) in the halogenoalkane). Nucleophiles are electron-pair donors.

Common Nucleophiles: Hydroxide ion (\(\text{OH}^-\)), Cyanide ion (\(\text{CN}^-\)), Ammonia (\(\text{NH}_3\)), Water (\(\text{H}_2\text{O}\)).

4.2 Key Nucleophilic Substitution Reactions

| Reaction | Reagent | Conditions | Product Class | Specific Example | | :--- | :--- | :--- | :--- | :--- | | Hydrolysis | Aqueous sodium hydroxide (\(\text{NaOH}(\text{aq})\)) | Heat (reflux) | Alcohol | \(\text{R}-\text{X} \xrightarrow{\text{OH}^-} \text{R}-\text{OH}\) | | Formation of Nitriles | Potassium cyanide (\(\text{KCN}\)) | Ethanol (solvent), Heat (reflux) | Nitrile | \(\text{R}-\text{X} \xrightarrow{\text{CN}^-} \text{R}-\text{CN}\) | | Formation of Amines | Excess Ammonia (\(\text{NH}_3\)) | Ethanol (solvent), Heat under pressure | Amine | \(\text{R}-\text{X} \xrightarrow{\text{NH}_3} \text{R}-\text{NH}_2\) |

Step-by-Step Focus: Reaction with \(\text{KCN}\) (Nitrile Formation)

This reaction is important because it increases the carbon chain length by one atom, which is often a goal in organic synthesis.

Reaction: \(R-\text{X} + \text{KCN} \rightarrow R-\text{CN} + \text{KX}\)
Reagent: Potassium cyanide (\(\text{KCN}\)).
Solvent: Ethanol (This ensures the reactants are miscible and prevents hydrolysis).
Condition: Heat (usually reflux).

5. The Nucleophilic Substitution Mechanisms: \(S_N1\) and \(S_N2\)

The classification of the halogenoalkane (primary, secondary, or tertiary) dictates which of the two substitution mechanisms is preferred.

5.1 The \(S_N2\) Mechanism (Substitution Nucleophilic Bimolecular)

This is a one-step mechanism where bond breaking (\(\text{C}-\text{X}\)) and bond forming (\(\text{C}-\text{Nu}\)) happen simultaneously through a single transition state.

  • Kinetics: Bimolecular (rate depends on the concentration of both the halogenoalkane and the nucleophile).
    Rate \(= k [R-\text{X}][\text{Nu}^-]\)
  • Preference: Favoured by primary halogenoalkanes.
  • Reason: Primary halogenoalkanes have minimal steric hindrance (crowding) around the \(\text{C}^{\delta+}\) atom, allowing the nucleophile to approach easily from the back side.

Analogy: Think of a revolving door. The nucleophile pushes in as the halogen leaves, all in one smooth motion.

5.2 The \(S_N1\) Mechanism (Substitution Nucleophilic Unimolecular)

This is a two-step mechanism.

  1. Step 1 (Rate Determining Step): The \(\text{C}-\text{X}\) bond breaks heterolytically (unevenly), forming a planar intermediate called a carbocation (\(\text{R}^+\)). This is the slowest step.
  2. Step 2: The nucleophile quickly attacks the carbocation.
  • Kinetics: Unimolecular (rate depends only on the concentration of the halogenoalkane).
    Rate \(= k [R-\text{X}]\)
  • Preference: Favoured by tertiary halogenoalkanes.
  • Reason (The Inductive Effect): Tertiary carbocations are highly stable because the three alkyl groups surrounding the positive carbon atom push electron density towards it (the inductive effect), helping to disperse the positive charge. This stability makes the formation of the carbocation intermediate easier.

Important Syllabus Point (15.1.6):
- Primary halogenoalkanes react via the \(\mathbf{S_N2}\) mechanism.
- Tertiary halogenoalkanes react via the \(\mathbf{S_N1}\) mechanism.
- Secondary halogenoalkanes react via a mixture of both mechanisms.

Quick Review: \(S_N2\) (one step, primary, relies on nucleophile concentration). \(S_N1\) (two steps, tertiary, relies on carbocation stability).

6. Elimination Reactions

Halogenoalkanes can also undergo elimination to form alkenes.

6.1 Reagents and Conditions

For elimination to occur, the hydroxide ion must act as a strong base, removing a proton (\(\text{H}^+\)) rather than replacing the halogen (\(\text{X}\)).

  • Reagent: Sodium hydroxide dissolved in ethanol (\(\text{NaOH}\) in ethanol).
  • Conditions: Heat (reflux).
  • Contrast: If the solvent is aqueous, substitution (hydrolysis to form an alcohol) is favoured. If the solvent is ethanolic, elimination (to form an alkene) is favoured.
  • Example: Elimination of \(\text{HBr}\) from bromoethane:
    \(\text{CH}_3\text{CH}_2\text{Br} + \text{OH}^- (\text{ethanolic}) \xrightarrow{\text{heat}} \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O} + \text{Br}^-\)

Did You Know? In organic chemistry, you must pay close attention to the solvent! Ethanol is non-polar enough to encourage the hydroxide ion to act as a base rather than a nucleophile.

7. Reactivity Comparison and Identification Tests

7.1 The Aqueous Silver Nitrate Test (Identifying the Halogen)

Since the reactivity of halogenoalkanes depends on the C–X bond strength (\(\text{C}-\text{I}\) is weakest, \(\text{C}-\text{Cl}\) is strongest), we can compare their relative reactivities using aqueous silver nitrate in ethanol.

Method:
We add aqueous silver nitrate (\(\text{AgNO}_3(\text{aq})\)) dissolved in ethanol to the halogenoalkane and heat gently. The halide ion (\(\text{X}^-\)) is released during hydrolysis and immediately reacts with silver ions (\(\text{Ag}^+\)) to form an insoluble silver halide precipitate (\(\text{AgX}\)).

Observation & Reactivity:

  • Iodoalkane (\(\text{R}-\text{I}\)): Fastest reaction; forms a pale yellow precipitate of \(\text{AgI}\) almost immediately.
  • Bromoalkane (\(\text{R}-\text{Br}\)): Slower reaction; forms a cream/off-white precipitate of \(\text{AgBr}\) within minutes.
  • Chloroalkane (\(\text{R}-\text{Cl}\)): Slowest reaction; forms a white precipitate of \(\text{AgCl}\) only after prolonged heating or standing.

The speed of precipitation directly reflects the strength of the C–X bond breaking (i.e., reactivity). The faster the reaction, the weaker the bond.

7.2 Halogenoalkanes vs. Halogenoarenes

A halogenoarene (aryl halide), like chlorobenzene, has the halogen atom attached directly to a benzene ring. They are significantly less reactive towards nucleophilic substitution than halogenoalkanes.

Reason for Low Reactivity:

  1. Partial Double Bond Character: The lone pair of electrons on the halogen atom are partially delocalised into the benzene ring’s \(\pi\) system.
  2. This delocalisation gives the \(\text{C}-\text{X}\) bond partial double bond character.
  3. This makes the \(\text{C}-\text{X}\) bond in halogenoarenes much stronger and harder to break than the single \(\text{C}-\text{X}\) bond in halogenoalkanes, thus resisting nucleophilic attack.

Example: Chloroethane (a halogenoalkane) reacts readily with \(\text{NaOH}(\text{aq})\) and heat, while chlorobenzene (a halogenoarene) requires extreme conditions (high temperature/pressure) to react.

Final Key Takeaway: Halogenoalkane chemistry is dominated by the polar C–X bond. Whether a substitution (\(S_N1\)/\(S_N2\)) or an elimination reaction occurs depends on the structure of the R group and, critically, the reaction conditions (especially the solvent used).