Chemistry (9701) | Halogen compounds

HALOGEN COMPOUNDS: HALOALKANES AND HALOGENOARENES (9701 Organic Chemistry)

Hey there, future Chemist! Welcome to the world of Halogen Compounds. These molecules, which contain a carbon-halogen (C–X) bond, are incredibly important because they act as starting materials for synthesizing almost every other functional group we study in organic chemistry. Think of them as the versatile middle managers in the molecular manufacturing industry!

In this chapter, we will break down two main types: Halogenoalkanes (Haloalkanes) and Halogenoarenes (Aryl Halides), focusing on how they are made and their characteristic reactions, especially the crucial nucleophilic substitution mechanism.

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1. Halogenoalkanes (R–X): The Alkyl Halides

Halogenoalkanes are organic compounds where one or more hydrogen atoms in an alkane (R–H) have been replaced by a halogen atom (X = F, Cl, Br, I).

1.1 Structure and Classification (15.1.2)

The core feature is the C–X bond. Halogens are highly electronegative, making the C atom slightly positive (\(\delta+\)) and the X atom slightly negative (\(\delta-\)). This polarity makes the C atom vulnerable to attack by species looking for a positive charge (nucleophiles).

Haloalkanes are classified based on the number of alkyl (R) groups attached to the carbon atom that holds the halogen:

• Primary (1°): The carbon bonded to the halogen is attached to one other alkyl group.
  Example: Chloroethane.
• Secondary (2°): The carbon bonded to the halogen is attached to two other alkyl groups.
  Example: 2-Bromopropane.
• Tertiary (3°): The carbon bonded to the halogen is attached to three other alkyl groups.
  Example: 2-Iodo-2-methylpropane.

Quick Tip: Think of the halogen-bearing carbon as a party host. If the host only invites one other alkyl guest (1°), there is plenty of room. If three alkyl guests show up (3°), it gets very crowded!

1.2 Preparation of Halogenoalkanes (15.1.1)

Halogenoalkanes can be produced using several key methods:

A. From Alkanes (Free-Radical Substitution) (14.1.2b, 15.1.1a)

Reagents: \(Cl_2\) or \(Br_2\) in the presence of UV light.
Process: The UV light provides energy for homolytic fission, creating free radicals. This leads to a substitution reaction where an H atom is replaced by a halogen atom.
(Remember the three steps: Initiation, Propagation, Termination, as seen in the alkane chapter.)

B. From Alkenes (Electrophilic Addition) (14.2.2a(iv), 15.1.1b)

Reagents: Halogen (\(X_2\), e.g., \(Br_2\)) or Hydrogen Halide (\(HX(g)\)) at room temperature.
Process: The double bond attacks the electrophile. This is a very fast and efficient way to make haloalkanes.

C. From Alcohols (Substitution) (15.1.1c, 16.1.2b)

Alcohols can be converted directly into haloalkanes, usually by substituting the hydroxyl (\(–OH\)) group with a halogen atom (–X).

• Reaction with HX(g) (e.g., concentrated \(HBr\)).
• Reaction with KCl and concentrated \(H_2SO_4\) or concentrated \(H_3PO_4\).
• Reaction with \(PCl_3\) and heat, or \(PCl_5\), or \(SOCl_2\) (thionyl chloride).

Key Takeaway (Preparation): Halogenoalkanes are central synthetic intermediates; they can be made easily from alkanes, alkenes, or alcohols.

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2. Characteristic Reactions of Halogenoalkanes

Halogenoalkanes undergo two main types of reaction: Nucleophilic Substitution and Elimination.

2.1 Nucleophilic Substitution Reactions (\(S_N\)) (15.1.3)

In this reaction, a Nucleophile (an electron-rich species, seeking a positive centre) attacks the partially positive carbon atom and replaces the halogen atom.

What is a Nucleophile?

A nucleophile literally means "nucleus loving". It has a lone pair of electrons (or is a negatively charged ion) and is ready to donate this pair to form a new covalent bond. Examples include \(OH^-\), \(CN^-\), and \(NH_3\).

Specific Substitution Reactions:

1. Hydrolysis (Formation of Alcohols)
   Reagent: Aqueous NaOH (aq) or KOH, heated.
   Nucleophile: Hydroxide ion (\(OH^-\)).
   Reaction:
   $$ R-X + OH^- \rightarrow R-OH + X^- $$
2. Formation of Nitriles (Increasing the Carbon Chain)
   Reagent: KCN in ethanol (ethanolic KCN), heated.
   Nucleophile: Cyanide ion (\(CN^-\)).
   Reaction:
   $$ R-X + CN^- \rightarrow R-CN + X^- $$ Did you know? This is a vital reaction because it increases the length of the carbon chain by one carbon atom!
3. Formation of Amines
   Reagent: Excess concentrated \(NH_3\) in ethanol, heated under pressure.
   Nucleophile: Ammonia (\(NH_3\)).
   Reaction:
   $$ R-X + 2NH_3 \rightarrow R-NH_2 + NH_4^+X^- $$
4. Identification Test (AgNO₃ in Ethanol) (15.1.3d)
   Reagent: Aqueous silver nitrate (\(AgNO_3\)) in ethanol, warmed (to speed up hydrolysis).
   Observation: The halide ion produced forms a precipitate with silver ions (\(Ag^+\)).
   • Chloroalkane: White precipitate (\(AgCl\)) (soluble in dilute \(NH_3(aq)\)).
   • Bromoalkane: Cream precipitate (\(AgBr\)) (partially soluble in dilute \(NH_3(aq)\)).
   • Iodoalkane: Pale yellow precipitate (\(AgI\)) (insoluble in dilute \(NH_3(aq)\)).

2.2 The Mechanisms of Nucleophilic Substitution (\(S_N1\) and \(S_N2\)) (15.1.5, 15.1.6)

The path a substitution reaction takes depends heavily on the structure (1°, 2°, or 3°) of the halogenoalkane.

A. \(S_N2\) Mechanism (Substitution Nucleophilic Bimolecular)

This is a one-step mechanism where the nucleophile attacks the carbon atom at the same time the C–X bond breaks. It involves a transition state where both the nucleophile and the leaving group are partially bonded to the carbon.

• Preferred by: Primary (1°) Halogenoalkanes.
• Why? 1° haloalkanes are the least 'crowded'. The nucleophile can easily access the \(\delta+\) carbon without much steric hindrance (physical obstruction from bulky alkyl groups).
• Rate-determining step: The single step involves both reactants, so Rate \(= k[R-X][Nu^-]\). It is a second-order reaction (Bimolecular).

B. \(S_N1\) Mechanism (Substitution Nucleophilic Unimolecular)

This is a two-step mechanism:

1. The C–X bond breaks slowly, forming a highly unstable intermediate called a carbocation (\(R^+\)). (This is the rate-determining step).
2. The nucleophile rapidly attacks the carbocation.

• Preferred by: Tertiary (3°) Halogenoalkanes.
• Why? The stability of the intermediate carbocation is key. Tertiary carbocations (\(3^\circ C^+\)) are the most stable due to the inductive effect of the three alkyl groups, which push electron density towards the positive charge, spreading it out and stabilizing the ion (15.1.5).
• Rate-determining step: Step 1 only involves R-X, so Rate \(= k[R-X]\). It is a first-order reaction (Unimolecular).

Summary of Trends (15.1.6):

• 3° Halogenoalkanes react via \(S_N1\) (stable carbocation).
• 1° Halogenoalkanes react via \(S_N2\) (low steric hindrance).
• 2° Halogenoalkanes react via a mixture of both mechanisms.

Analogy: Imagine two ways to leave a crowded party (R-X).
1. \(S_N2\) (1°): You and your friend (the halogen) leave together immediately, bumping into the new guest (nucleophile) who is just walking in. (One step, no waiting).
2. \(S_N1\) (3°): Your friend (halogen) leaves first, making the carbon atom very sad and positive (carbocation). But since it’s a big 3° party, the other guests (alkyl groups) make it feel stable until the new guest (nucleophile) arrives. (Two steps, stable intermediate).

2.3 Elimination Reactions (15.1.4)

Substitution isn't the only option! If conditions are changed, the halogenoalkane can lose the halogen and a hydrogen atom from an adjacent carbon, forming an alkene. This is called elimination.

Reagents: Sodium hydroxide (NaOH) dissolved in ethanol (ethanolic conditions), and heat.
Condition Contrast:

• Aqueous NaOH/heat gives Substitution (alcohol).
• Ethanolic NaOH/heat gives Elimination (alkene).

In elimination, the \(OH^-\) acts as a strong base (proton acceptor) rather than a nucleophile, stripping a proton off an adjacent carbon.

2.4 Reactivity and Bond Strength (15.1.7)

The overall reactivity of halogenoalkanes in substitution reactions (like the \(AgNO_3\) test) is determined by the strength of the C–X bond. The weaker the bond, the easier it breaks, and the faster the reaction.

Trend in C–X Bond Strength:

$$ C-F >> C-Cl > C-Br > C-I $$

Trend in Reactivity:

$$ R-I > R-Br > R-Cl > R-F $$

The C–I bond is the weakest because Iodine is the largest halogen, meaning the bonding electrons are furthest from the C and I nuclei, resulting in the lowest bond enthalpy. Therefore, iodoalkanes react fastest and fluoroalkanes react slowest.

Key Takeaway (Reactions): Substitution dominates under aqueous conditions; elimination dominates under hot, ethanolic conditions. The rate of substitution depends on both the mechanism (1°, 3°) and the C–X bond strength (I > Br > Cl).

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3. Halogenoarenes (Aryl Halides)

Halogenoarenes are compounds where a halogen atom is directly bonded to a carbon atom within a benzene ring (e.g., chlorobenzene).

3.1 Preparation of Halogenoarenes (31.1.1)

Halogenoarenes are typically made via Electrophilic Substitution of the arene (benzene or a substituted benzene ring).

Reagents: \(Cl_2\) or \(Br_2\) in the presence of a Lewis acid catalyst, such as anhydrous \(AlCl_3\) or \(FeBr_3\).

Example: Benzene reacts with \(Cl_2\) and \(AlCl_3\) to form chlorobenzene.

3.2 The Difference in Reactivity: Haloalkane vs. Halogenoarene (31.1.2, 33.3.4)

A crucial point in A-Level Chemistry is explaining why halogenoarenes (like chlorobenzene) are far less reactive towards nucleophilic substitution than halogenoalkanes (like chloroethane).

The difference stems from the environment of the C–X bond in the aromatic ring:

1. Overlap with the \(\pi\) system: The lone pair of electrons on the halogen atom overlaps with the delocalised \(\pi\) electron system of the benzene ring.
2. Partial Double Bond Character: This overlap introduces partial double bond character to the C–X bond in the halogenoarene.
3. Increased Bond Strength: A double bond is much stronger than a single bond (the C–X bond in a haloalkane). Therefore, the C–X bond in a halogenoarene is much harder to break, requiring harsher conditions or failing to react entirely with common nucleophiles (like \(NaOH(aq)\)).

The Halogenoarene is also resistant to \(S_N1\) formation because the potential aryl carbocation is unstable, as it interrupts the stabilizing delocalised ring system.

Analogy: In a haloalkane, the halogen is attached by a standard single rope (easy to cut). In a halogenoarene, the halogen is attached by a reinforced rope (partial double bond, very hard to cut).