In simple terms
A friendly intro before the formal notes — no formulas yet.
Halogens: Attached but not Always Reactive
We'll explore why attaching a halogen to a carbon chain makes it reactive, but attaching it to a benzene ring makes it surprisingly stubborn. This difference in reactivity dictates how we use these important molecules.
Imagine a key in a lock. A halogen on a simple carbon chain (a halogenoalkane) is like a key that's easy to turn and remove, allowing a new key to be inserted (it's reactive). A halogen on a benzene ring (a halogenoarene) is like a key that has been partially welded into the lock; it's much harder to get out and requires a lot more force, or a completely different approach to change the lock.
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Halogenoarenes have a stronger carbon-halogen (C–X) bond than halogenoalkanes, making them less reactive. This is due to the halogen's lone pair overlapping with the ring's π-system, giving the C–X bond partial double bond character.
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Because of this strong bond, nucleophilic substitution on aryl halides requires very harsh conditions (e.g., high temperature and pressure), unlike the milder conditions needed for halogenoalkanes.
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Halogenated compounds have had significant uses, such as in pesticides (DDT) and as refrigerants (CFCs). However, their stability and effect on the ozone layer have led to environmental concerns and restrictions.
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Reactivity depends on the halogen's location. A halogen on a benzene ring is unreactive to substitution but directs incoming electrophiles. A halogen on an alkyl side-chain attached to a ring behaves just like a normal halogenoalkane.
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Key formulas
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Full topic notes
Formal explanation with the rigour you need for the exam.
Reactivity of Halogenoalkanes: Nucleophilic Substitution
In a halogenoalkane, the halogen atom is more electronegative than the carbon atom it is bonded to. This creates a polar covalent bond, . The electron-deficient carbon atom is susceptible to attack by nucleophiles, which are species with a lone pair of electrons they can donate. This leads to a nucleophilic substitution reaction, where the nucleophile replaces the halogen atom, which departs as a halide ion (a good leaving group).
General Reaction:
Conditions: Warm aqueous solution of the nucleophile, e.g., NaOH(aq) or KCN(aq).
Mechanism: Can be Sₙ1 (two-step, via carbocation, for tertiary/some secondary) or Sₙ2 (one-step, via transition state, for primary/some secondary).
Example (Hydrolysis):
Competition: Elimination Reactions
Under different conditions, halogenoalkanes can undergo an elimination reaction to form an alkene. Instead of acting as a nucleophile, the hydroxide ion can act as a base, removing a proton () from a carbon atom adjacent to the one bonded to the halogen. The C-X bond then breaks and a C=C double bond is formed.
Conditions: Hot, ethanolic solution of a strong base, e.g., KOH in ethanol.
Role of Base: The ion removes a proton.
Product: An alkene is always formed.
Example: $CH_3CHBrCH_3 + KOH(eth) \xrightarrow{Heat} CH_2=CHCH_3 + KBr + H_2O$
The Unreactive Nature of Halogenoarenes
A crucial concept in this topic is the stark contrast in reactivity between halogenoalkanes and halogenoarenes like chlorobenzene. While 1-chlorobutane readily undergoes hydrolysis, chlorobenzene is resistant to nucleophilic substitution under normal laboratory conditions. This is not because the C-Cl bond is non-polar; it is polar. The reason lies in the interaction between the halogen and the aromatic ring.
p-Orbital Overlap: A lone pair of electrons from the halogen's p-orbital is delocalised into the benzene ring's -system.
Partial Double Bond Character: This overlap gives the C-X bond some double bond character, making it stronger and shorter than the C-X bond in a halogenoalkane.
Repulsion: The electron-rich -system of the benzene ring repels approaching nucleophiles.
Harsh Conditions Required: To force a reaction, extreme conditions are needed. For example, converting chlorobenzene to phenol requires NaOH at approximately 300°C and 150 atm pressure.
When asked to explain the low reactivity of halogenoarenes, you must mention the overlap of the halogen's p-orbital with the ring's -system leading to a C-X bond with 'partial double bond character'. Simply stating 'the bond is stronger' is not enough for full marks; you must explain why it is stronger.
Worked examples
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1-bromopropane is reacted separately with (a) warm aqueous sodium hydroxide and (b) hot ethanolic potassium hydroxide. For each reaction, state the type of reaction and draw the structure of the main organic product.
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Part (a): with warm aqueous sodium hydroxide
- Reaction Type: Nucleophilic Substitution.
- Explanation: The aqueous conditions favour substitution. The ion acts as a nucleophile, attacking the carbon and displacing the ion.
- Organic Product: Propan-1-ol. Structure: .
Samples of 1-chlorobutane, 1-bromobutane, and 1-iodobutane are placed in separate test tubes. An equal volume of aqueous silver nitrate solution is added to each, and the tubes are placed in a water bath at 50°C. Describe and explain the expected observations.
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Observations:
- 1-iodobutane: A yellow precipitate (of AgI) will form almost immediately.
- 1-bromobutane: A cream precipitate (of AgBr) will form after a short time.
- 1-chlorobutane: A white precipitate (of AgCl) will form very slowly, or perhaps not be visible during the experiment.
How it all connects
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Glossary
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What is a halogenoalkane?
An alkane in which one or more hydrogen atoms have been replaced by a halogen atom (F, Cl, Br, I). General formula .
Key takeaways
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Conditions: Warm aqueous solution of the nucleophile, e.g., NaOH(aq) or KCN(aq).
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Mechanism: Can be Sₙ1 (two-step, via carbocation, for tertiary/some secondary) or Sₙ2 (one-step, via transition state, for primary/some secondary).
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Example (Hydrolysis):
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Test your understanding of Halogen Compounds
Test your understanding of Halogen Compounds
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