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Topic 3Haloalkanes

Haloalkanes (alkyl halides) have a halogen bonded to an sp3 carbon. The polarity of the C–X bond makes them the ideal substrate for studying two of the great mechanisms of Organic Chemistry: nucleophilic substitution and elimination.

Structural Features

Nomenclature examples: 2-bromo-2-methylpropane, (2-iodopropyl)cycloheptane, trans-1-bromo-2-chlorocyclohexane and 3-chloro-1,1,1-trifluoro-4,7-dimethyloctane

Nomenclature

 Separating funnels: halogenated solvents such as CH₂Cl₂ and CHCl₃ are denser than water and stay below the aqueous phase

Physical Properties

 The carbon bonded to the halogen is electrophilic, electron deficient, and is attacked by nucleophiles

Bond Properties

Reactivity

Result of the experiment: (R)-2-bromooctane gives 80% substitution (2-octanol with 94% inversion) and 20% elimination (E alkene 95%, Z 5% and traces of terminal olefin)
The Result of an Experiment

Mechanisms

Animation of the SN2 mechanism: the nucleophile attacks from the side opposite the leaving group and the configuration is inverted

Bimolecular Nucleophilic Substitution

 Animation of carbocation formation by departure of the chloride

Unimolecular Nucleophilic Substitution

 Animation of the E1 mechanism: the carbocation loses a proton from the adjacent carbon and the alkene is formed

Unimolecular Elimination

 Animation of the E2 mechanism: the base removes the proton while the halogen leaves in an antiperiplanar arrangement

Bimolecular Elimination

 Animations of the competition between substitution and elimination

Substitution vs. Elimination

 Preparation of methyllithium and methylmagnesium bromide from bromomethane

Organometallic Reagents