Haloalkanes and Haloarenes · Lesson 4 of 16
Preparing Haloalkanes from Alcohols and Halides
“Choose alcohol-substitution and halogen-exchange routes with the necessary reagents, conditions and limitations.”
• Explain how an alcohol hydroxyl group is activated for replacement. • Write balanced reactions with HX, phosphorus halides and thionyl chloride. • Use Finkelstein and Swarts exchange with appropriate substrates and conditions. • Explain why the same procedures cannot be transferred unchanged to phenols.
A poor leaving group must first become a better one
An alcohol contains a C–O bond, but OH⁻ is not normally a good leaving group in an ordinary substitution. In acid the oxygen can be protonated so that neutral water leaves, or another reagent can convert the OH group into a more favourable leaving arrangement. Thus preparing R–X from R–OH is not accomplished merely by imagining OH disappearing. The reagent and substrate determine the pathway.
The formal net acid reaction is ROH+HX→RX+H₂O. Concentrated HCl with anhydrous ZnCl₂ assists replacement in many primary and secondary alcohols by activating the oxygen-containing group. Tertiary alcohols commonly react with concentrated HCl at room temperature without that catalyst because carbocation formation is easier. The common reactivity order 3°>2°>1° refers to comparable HX conditions favouring ionisation; it should not be imported as a universal order for every alcohol-conversion reagent.
| Route | Representative balanced reaction | Conditions and purpose |
|---|---|---|
| HCl | CH₃CH₂OH+HCl→CH₃CH₂Cl+H₂O | HCl, commonly ZnCl₂ assistance; conditions depend on alcohol |
| HBr | CH₃CH₂OH+HBr→CH₃CH₂Br+H₂O | Concentrated aqueous HBr with suitable heating |
| HI generated from iodide | CH₃CH₂CH₂CH₂OH+HI→CH₃CH₂CH₂CH₂I+H₂O | Iodide with non-oxidising acid such as H₃PO₄; heat as appropriate |
| PCl₃ or PBr₃ | 3ROH+PX₃→3RX+H₃PO₃ | X=Cl or Br; appropriate alcohol substrate |
| PCl₅ | ROH+PCl₅→RCl+POCl₃+HCl | Chlorinating conditions |
| SOCl₂ | ROH+SOCl₂→RCl+SO₂+HCl | Thionyl chloride; often a base such as pyridine is used |
The thionyl-chloride route produces gaseous SO₂ and HCl, which can help drive conversion and simplify separation of the organic chloride. This does not mean the crude product is automatically analytically pure or that any alcohol gives an unrearranged product under every condition. For primary and many secondary alcohols PBr₃ offers a pathway that avoids a freely rearranging carbocation; tertiary substrates require different expectations. Stereochemistry with SOCl₂ depends on solvent and additives, so no universal inversion or retention rule is assigned to every version of that reaction.
Problem
Choose a route from butan-1-ol to 1-iodobutane.
- 1.The substrate is primary, HOCH₂CH₂CH₂CH₃. Supply HI through iodide and a suitable non-oxidising acid, with heating as required.
- 2.The net organic conversion is CH₃CH₂CH₂CH₂OH+HI→CH₃CH₂CH₂CH₂I+H₂O. All four carbons remain in the same skeleton.
- 3.Concentrated sulfuric acid is unsuitable for iodide generation here because it can oxidise HI/iodide to iodine, consuming the required reagent. Orthophosphoric acid avoids that unwanted oxidising role.
Phosphorus tribromide or triiodide can be generated in the reaction mixture from phosphorus and the relevant halogen; this is the meaning of in situ. It describes reagent generation, not a new product rule. The ordinary tertiary-alcohol pathway may compete with elimination or rearrangement, and concentrated-acid conditions can affect other functional groups. Route selection must therefore consider more than the desired halogen letter.
Why a phenol is not simply another alcohol
In phenol, oxygen is directly attached to an aromatic sp² carbon. Lone-pair interaction with the ring gives the aryl C–O bond partial double-bond character and a stronger, electronically different connection. The simple HX replacement routes used for saturated alcohols do not generally convert phenol directly into chlorobenzene. Later aromatic methods such as diazonium replacement provide a separate route rather than treating aryl and alkyl bonds as interchangeable.
Finkelstein: a solvent can help choose the direction
An alkyl chloride or bromide can react with NaI in dry acetone: RCl+NaI→RI+NaCl(s), or RBr+NaI→RI+NaBr(s). Sodium iodide is sufficiently soluble in the medium while sodium chloride or bromide precipitates, helping remove a product from the solution equilibrium. This reaction is most useful with substrates that can undergo nucleophilic substitution efficiently, commonly primary halides. An aryl chloride or tertiary chloride is not automatically a good substrate merely because the formula contains Cl.
Problem
Prepare 1-iodobutane from 1-chlorobutane.
- 1.Choose NaI in dry acetone, using the primary chloride CH₃CH₂CH₂CH₂Cl.
- 2.Iodide replaces chlorine at the terminal carbon: CH₃CH₂CH₂CH₂Cl+NaI→CH₃CH₂CH₂CH₂I+NaCl(s).
- 3.The primary centre permits backside substitution, and precipitation helps favour conversion. The carbon skeleton and carbon count remain unchanged.
Swarts: introducing fluorine by exchange
Heating suitable alkyl chlorides or bromides with metal fluorides such as AgF, SbF₃ or other appropriate fluoride reagents can replace halogen by fluorine. This family is called the Swarts reaction. A simple balanced example is CH₃Br+AgF→CH₃F+AgBr. The reagent set is not a claim that every listed fluoride works equally well for every substrate. Fluorination chemistry can be demanding, and direct reaction with highly reactive elemental fluorine is not a generally selective substitute for a planned exchange.
Problem
Give a two-stage route from ethanol to fluoroethane.
- 1.Convert ethanol to an ethyl chloride using suitable HCl/ZnCl₂ or thionyl-chloride conditions: C₂H₅OH→C₂H₅Cl.
- 2.Apply an appropriate Swarts fluoride-exchange reagent, for example AgF with suitable heating: C₂H₅Cl+AgF→C₂H₅F+AgCl.
- 3.There is no carbon-chain extension. The first step replaces OH; the second replaces a halide. Distinguish their separate activation requirements.
A complete reaction description includes the medium and substrate, not just the reagent acronym. Aqueous hydroxide often converts an alkyl halide back toward an alcohol, whereas a dry exchange medium may favour a different nucleophile. The same structural understanding used for preparation will reappear in the substitution mechanisms.
| Starting structure and conditions | Product and reason |
|---|---|
| Cyclohexanol + SOCl₂ | Cyclohexyl chloride; OH is replaced at the saturated ring carbon. |
| p-HOC₆H₄CH₂OH + HCl, heat | p-HOC₆H₄CH₂Cl; the benzylic OH is replaced while phenolic OH remains. |
| Bromoethane + NaI in dry acetone | Iodoethane + NaBr precipitate; ordinary alkyl halide exchange. |
Quick Quiz
Quiz
Why can acid assist substitution of an alcohol?
Explanation: Neutral water is a better leaving group than unactivated hydroxide.
Which reagent gives RCl with SO₂ and HCl as byproducts?
Explanation: ROH+SOCl₂→RCl+SO₂+HCl.
What helps drive Finkelstein exchange in dry acetone?
Explanation: The insoluble salt is removed from the reacting solution.
Why avoid concentrated H₂SO₄ with iodide in the alcohol-to-iodide preparation?
Explanation: Oxidation consumes the iodide-based reagent needed for substitution.
Which describes a Swarts transformation?
Explanation: The defining transformation is halogen exchange to introduce F.
Practice Problems
- Write balanced equations for ethanol with PBr₃ and with SOCl₂.
- Explain the role of ZnCl₂ in many alcohol/HCl conversions.
- Why is benzyl alcohol replaceable at CH₂OH while phenolic OH behaves differently?
- What should accompany the instruction “NaI” when planning a Finkelstein reaction?
- Explain why an alcohol reactivity order with HX is not universal for PBr₃.
- Propan-1-ol reacts with thionyl chloride. Give the organic product, byproducts and carbon-count check.
3C₂H₅OH+PBr₃→3C₂H₅Br+H₃PO₃; C₂H₅OH+SOCl₂→C₂H₅Cl+SO₂+HCl. Count oxygen and hydrogen as well as the replaced halogen; these balanced net equations do not imply identical mechanisms.
Key Takeaways
• Alcohol substitution requires activation of the oxygen-containing group. • HX, phosphorus halides and SOCl₂ provide distinct preparation routes. • SOCl₂ gives gaseous byproducts but does not guarantee perfect purity or one universal stereochemical outcome. • Iodide preparations need an acid that does not consume iodide by oxidation. • Finkelstein exchange uses dry acetone and salt precipitation with a suitable substrate. • Swarts exchange introduces fluorine using an appropriate fluoride reagent. • Phenolic aryl C–O bonds do not follow ordinary alkyl-alcohol replacement rules.