This chapter explores haloalkanes and haloarenes, focusing on their formation, properties, and applications.
Haloalkanes and Haloarenes - Quick Look Revision Guide
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Key Points
Define haloalkanes and haloarenes.
Haloalkanes have halogens attached to sp³ carbon, while haloarenes attach to sp² carbon.
Classify halogenated compounds.
Halogen compounds are mono-, di-, or polyhalogenated; based on number of halogens.
Nomenclature
Use IUPAC rules to name haloalkanes; common names derive from alkyl + halide.
Physical properties - boiling points.
Boiling points increase: RI > RBr > RCl > RF, due to increased molecular size and polarity.
Reactivity of alkyl halides.
Reactivity follows order: 3° > 2° > 1° for both S_N1 and S_N2 mechanisms.
S_N2 mechanism.
Bimolecular reaction leading to inversion of configuration during nucleophilic substitution.
S_N1 mechanism.
Unimolecular reaction involving carbocation formation; typically results in racemization.
Zaitsev's rule.
In dehydrohalogenation, more substituted alkenes are generally the major product.
Grignard reagents.
Formed from haloalkanes and Mg in dry ether; highly reactive and valuable in synthesis.
Preparation from alcohols.
Haloalkanes can be synthesized by converting -OH in alcohols using PCl₃, SOCl₂, or HCl.
Free radical halogenation.
Halogenation of alkanes leads to a complex mixture of isomers; yields depend on the radical pathway.
Physical state of haloalkanes.
Lower haloalkanes are gases, higher members are liquids or solids at room temperature.
Solubility of haloalkanes.
Slightly soluble in water due to weak attractions, but soluble in organic solvents.
Electrophilic Aromatic Substitution.
Haloarenes undergo electrophilic substitution; reactions require stronger conditions than for benzene.
Role of electron withdrawing groups.
Electron-withdrawing groups increase the reactivity of haloarenes towards nucleophiles.
Environmental concerns.
Polyhalogenated compounds like DDT and Freons are environmentally hazardous.
Common uses of haloalkanes.
Used as solvents, reagents in organic synthesis; some compounds are used as anesthetics, drugs.
Stereochemistry in reactions.
Chirality influences reactivity; S_N1 leads to racemization while S_N2 leads to inversion of configuration.
Ambident nucleophiles.
These can attack from different sites; example includes cyanide which can form nitriles or isocyanides.
Nucleophilic substitution reactions.
Nucleophiles replace halides in alkyl halides; examples include OH⁻ leading to alcohols.
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