Halogenoarenes: properties, reactions

Halogenoarenes (Aryl Halides)

Halogenoarenes are aromatic compounds in which a hydrogen atom of a benzene ring is replaced by a halogen (F, Cl, Br, I). They are a core part of Cambridge International AS & A Level Chemistry (9701) – Learning Outcomes 15.1 & 15.2. The notes below give a concise but complete revision tool.

1. Structure and Nomenclature (LO 15.1)

• General formula: C₆H₅X (X = F, Cl, Br, I).
• Systematic name (IUPAC): halobenzene. When more than one substituent is present, the carbon bearing the halogen is given the lowest possible locant and the word “halo” is placed before the parent name, e.g. 1‑bromo‑4‑nitrobenzene or 4‑bromophenol.
• Positional descriptors: ortho (2‑), meta (3‑) and para (4‑) are used when the halogen is described relative to another substituent.
• Numbering rule: Number the ring so that the halogen receives the lowest possible number; if two substituents are present, give the halogen the lower locant unless the other substituent has priority (e.g. –NO₂, –COOH).

2. Physical Properties and Trends

• Boiling‑point trend: Increases down the group because molecular weight and London‑dispersion forces increase.
• Melting‑point & vapour‑pressure trend: Melting points fall slightly down the group, while vapour pressure drops markedly – a direct consequence of stronger intermolecular forces.
• Solubility: All are only sparingly soluble in water; the non‑polar aromatic ring dominates the overall polarity.

3. Bond‑Strength, Leaving‑Group and Reactivity Trends (LO 15.1)

• Strong C–F bond makes fluorobenzenes essentially inert toward substitution.
• Reactivity order for nucleophilic displacement follows the opposite trend to bond strength: I > Br > Cl > F.

4. Why SN1 and SN2 Do Not Occur for Aryl Halides (LO 15.1)

• SN1: Requires a stable carbocation. In an aryl system the carbon bearing the halogen is sp²‑hybridised; loss of X⁻ would give a phenyl cation that cannot be stabilised by hyper‑conjugation (no β‑hydrogens) and is highly destabilised by loss of aromaticity.
• SN2: Requires a backside attack on the σ‑bond. The planar aromatic ring blocks the backside, and the C–X bond possesses partial double‑bond character due to resonance (π‑conjugation with the ring). Moreover, orbital‑symmetry considerations (the nucleophile would have to approach a node of the aromatic π‑system) make a concerted displacement impossible.
• Consequently, aryl halides react mainly by nucleophilic aromatic substitution (SNAr), metal‑halogen exchange, or copper‑catalysed couplings.

5. Nucleophilic Aromatic Substitution (SNAr) – Addition‑Elimination (LO 15.1)

1. Requirement: An electron‑withdrawing group (EWG) such as –NO₂, –CN, –COOR, –SO₂R positioned ortho or para to the halogen. The EWG stabilises the negative charge of the intermediate.
2. Step 1 – Nucleophilic attack (rate‑determining step): The nucleophile adds to the carbon bearing the halogen, forming a non‑aromatic σ‑complex (Meisenheimer complex). The negative charge is delocalised onto the EWG.
3. Step 2 – Elimination: The halide ion leaves, restoring aromaticity.

Typical conditions: Strong nucleophile (NaOH, NaOEt, NaCN, NaOAc) in a polar aprotic solvent (DMF, DMSO) at 70–100 °C. Reactions are usually carried out under reflux.

Meisenheimer complex (ASCII sketch)

        O⁻
        |
   Ar‑C‑X   +   Nu⁻   →   Ar‑C‑Nu
   \   /                \   /
    C                  C
   /   \                /   \
  X     Y            X⁻    Y

In the diagram Y represents the electron‑withdrawing group; the negative charge is delocalised onto it.

6. Copper‑Catalysed Ullmann Couplings (LO 15.2)

7. Grignard Reagents from Aryl Halides (LO 15.2)

• General reaction: C₆H₅X + Mg → C₆H₅MgX (dry ether, reflux).
• Reactivity order: Ar‑I ≈ Ar‑Br > Ar‑Cl ≫ Ar‑F.
• Why Cl is sluggish: The C–Cl bond has partial double‑bond character due to resonance, making electron transfer to Mg less favourable.
• Practical requirements: Anhydrous ether (diethyl ether or THF), nitrogen or argon atmosphere, freshly activated magnesium (iodine‑treated).
• Use: Nucleophilic addition to carbonyls, formation of C–C bonds, or conversion to other organometallics.

8. Halogen–Metal Exchange (Organolithium) (LO 15.2)

• Typical reaction: C₆H₅Br + n‑BuLi → C₆H₅Li + n‑BuBr.
• Conditions: Dry THF, –78 °C (dry‑ice/acetone bath), inert atmosphere.
• Base choice: n‑BuLi is the most common; s‑BuLi is used when higher steric bulk is required.
• Reactivity: Br > I (fast); Cl reacts only at –40 °C or with excess base; F does not undergo exchange.
• Functional‑group compatibility: Avoid protic or strongly electrophilic groups (e.g., –OH, –COOH, –CN) unless protected, because they react with the organolithium reagent.

9. Wurtz–Fittig Coupling (LO 15.2)

• Reaction: 2 C₆H₅X + 2 Na → C₆H₅–C₆H₅ + 2 NaX (dry ether, 0–50 °C).
• Outcome: Biaryl (e.g., biphenyl) together with side‑products such as reduced arene (C₆H₆) and mixed coupling products.
• Exam note: The method is low‑yielding and gives a mixture of products; it is mentioned mainly for historical context or as a revision example.

10. Elimination to Form Alkynes (E2) (LO 15.1)

• Requirement: A good leaving group on a sp³ carbon β‑to the aromatic ring (e.g., p‑bromoacetophenone).
• Base: NaNH₂ (or NaNH₃) in liquid ammonia, refluxed.
• Mechanism: Concerted E2; the β‑hydrogen must be anti‑periplanar to the leaving group for efficient overlap of the σ‑C–H and σ*‑C–X orbitals.
• Example:

p‑Bromoacetophenone  +  2 NaNH₂  →  Phenylacetylene  +  NaBr  +  2 NH₃

• Note that the halogen is on the side‑chain, not on the aromatic carbon; aryl‑C–X bonds do not undergo E2 elimination under these conditions.

11. Reactivity Summary (LO 15.1 & 15.2)

• C–X bond strength: C–F > C–Cl > C–Br > C–I (kJ mol⁻¹ values above).
• Leaving‑group ability: I⁻ > Br⁻ > Cl⁻ > F⁻.
• SN1/SN2: Not observed for aryl halides (orbital‑symmetry and carbocation instability).
• SNAr: Requires ortho/para EWG; rate‑determining nucleophilic attack.
• Metal‑halogen activation: Br and I give Grignard, organolithium and Ullmann couplings readily; Cl is sluggish; F is essentially inert.
• Substituent effect: EWGs activate toward SNAr; electron‑donating groups (–CH₃, –OMe) deactivate.
• Elimination to alkynes: Strong base + β‑halogen on side‑chain → E2, anti‑periplanar geometry required.

12. Practical Tips for A‑Level Examinations (LO 15.1)

1. Always check for an ortho/para electron‑withdrawing group before invoking SNAr.
2. Remember that aryl fluorides behave as if the C–F bond were non‑reactive; they do not undergo SNAr, Grignard formation or metal‑halogen exchange under normal conditions.
3. When a question asks for a Grignard reagent, state that bromides and iodides are reliable; chlorides need vigorous activation (e.g., iodine‑treated Mg) and fluorides do not react.
4. Distinguish clearly between SN1, SN2 and SNAr mechanisms in diagrams – show why backside attack is impossible for aryl halides.
5. For metal‑halogen exchange, write “n‑BuLi, –78 °C, dry THF, N₂ atmosphere” to demonstrate control of reactivity.
6. In Ullmann couplings, include catalyst loading (≈ 10 mol % CuI) and solvent (DMF or DMSO); note that steric bulk on either partner can lower yield.
7. When answering a coupling question, comment on side‑products and typical yields (Ullmann moderate, Wurtz–Fittig low).
8. For E2 elimination to an alkyne, emphasise that the leaving group must be on the side‑chain carbon and that anti‑periplanar geometry is essential.

13. Example Exam Question & Model Answer (LO 15.1)

Question: Predict the major product when 4‑nitro‑chlorobenzene is treated with excess aqueous NaOH in ethanol and heated at 80 °C.

Model Answer:

• The nitro group (–NO₂) is a strong electron‑withdrawing group positioned para to the chlorine, satisfying the SNAr requirement.
• Hydroxide attacks the carbon bearing chlorine, forming a Meisenheimer complex (rate‑determining step).
• Loss of Cl⁻ restores aromaticity, giving 4‑nitrophenol as the major product.

Reaction scheme (not shown) – addition of OH⁻, formation of σ‑complex, elimination of Cl⁻.

14. Further Reading

• Clayden, J., Greeves, N., & Warren, S. Organic Chemistry (2nd ed.) – Chapter on Aryl Halides.
• Cambridge International AS & A Level Chemistry Coursebook – Sections 5.4 – 5.6 (Halogenoarenes).
• Wade, L. G. Organic Chemistry (9th ed.) – Sections on SNAr, Grignard, and Ullmann reactions.