A common reagent for these allylic brominations is N-bromosuccinamide (NBS)because it continually generates small amounts of Br2 through reaction with HBr. H-Br I-H +Br-Br Other methods for Preparation (These will be covered in detail in appropriate later chapters). From alkenes and alkynes: X-x -C=C- 2H-X_ 一C=C一 2X-X Ch06 Alkyl Halides (landscape) Page 6
Ch06 Alkyl Halides (landscape) Page 6 A common reagent for these allylic brominations is N-bromosuccinamide (NBS) because it continually generates small amounts of Br2 through reaction with HBr. Other methods for Preparation (These will be covered in detail in appropriate later chapters). From alkenes and alkynes: N-Br O O + H-Br N-H O O + Br-Br C C C C C H C X H-X C C C X C X X-X C H H C X X 2 H-X C C C X X C X X 2 X-X
From alcohols: R-OH HX.R-X From other halides R-X +X →R-X+X Reactions of Alkyl Halides The alkyl halides are chemically versatile. The halogen atom may leave with its bonding pair of electrons to give a halide ion which is stable-a halide is called a good leaving group If an atom replaces the halide the overall reaction is a substitution. If the halide loss is accompanied by the loss of another atom,the overall reaction is called an elimination Very often the other atom lost is a hydrogen(as H).The elimination of H-X is common,and is called a dehydrohalogenation. Often substitution and elimination reactions will occur in competition with each other. Ch06 Alkyl Halides (landscape) Page 7
Ch06 Alkyl Halides (landscape) Page 7 From alcohols: From other halides: Reactions of Alkyl Halides The alkyl halides are chemically versatile. The halogen atom may leave with its bonding pair of electrons to give a halide ion which is stable – a halide is called a good leaving group. If an atom replaces the halide the overall reaction is a substitution. If the halide loss is accompanied by the loss of another atom, the overall reaction is called an elimination. Very often the other atom lost is a hydrogen (as H+ ). The elimination of H-X is common, and is called a dehydrohalogenation. Often substitution and elimination reactions will occur in competition with each other. R OH H-X R X R X' + X R X - + 'X -
Nucleophilic Substitution H:X: Nuc The nucleophile Nuc:displaces the leaving group X from the carbon atom by using its lone pair to form a new bond to the carbon atom. Elimination A new nt bond is formed by the elimination of halide ion and another atom(usually H) -C-+B:→ B-H +C=C H:X: In a dehydrohalogenation,the base(B:)abstracts a proton from the alkyl halide Most nucleophiles can also act as bases,therefore the preference for elimination or substitution depends on the reaction conditions and the alkyl halide used. Ch06 Alkyl Halides (landscape) Page 8
Ch06 Alkyl Halides (landscape) Page 8 Nucleophilic Substitution The nucleophile Nuc:- displaces the leaving group Xfrom the carbon atom by using its lone pair to form a new bond to the carbon atom. Elimination A new bond is formed by the elimination of halide ion and another atom (usually H+ ). In a dehydrohalogenation, the base (B:- ) abstracts a proton from the alkyl halide. Most nucleophiles can also act as bases, therefore the preference for elimination or substitution depends on the reaction conditions and the alkyl halide used
The SN2 reaction SN2 means substitution nucleophilic bimolecular. Consider the reaction of hydroxide ion with methyl iodide,to yield methanol. H H H0:+ H H一O一C—H+ H H hydroxide iodomethane methanol iodide (nucleophile) (substrate) (product) (leaving group) The hydroxide ion is a good nucleophile since the oxygen atom has a negative charge and a pair of unshared electrons The carbon atom is electrophilic since it is bound to a(more electronegative)halogen,which pulls electron density away from the carbon,thus polarizing the bond with carbon bearing partial positive charge and the halogen bearing partial negative charge. The nucleophile is attracted to the electrophile by electrostatic charges The nucleophile attacks the electrophilic carbon through donation of 2 electrons. Carbon can only have a maximum of 8 valence electrons,so as the carbon-nucleophile bond is forming,then the carbon-leaving group bond must be breaking. lodide is the leaving group since it leaves with the pair of electrons that once bound it to carbon. Ch06 Alkyl Halides (landscape) Page 9
Ch06 Alkyl Halides (landscape) Page 9 The SN2 reaction SN2 means substitution nucleophilic bimolecular. Consider the reaction of hydroxide ion with methyl iodide, to yield methanol. The hydroxide ion is a good nucleophile since the oxygen atom has a negative charge and a pair of unshared electrons. The carbon atom is electrophilic since it is bound to a (more electronegative) halogen, which pulls electron density away from the carbon, thus polarizing the bond with carbon bearing partial positive charge and the halogen bearing partial negative charge. The nucleophile is attracted to the electrophile by electrostatic charges. The nucleophile attacks the electrophilic carbon through donation of 2 electrons. Carbon can only have a maximum of 8 valence electrons, so as the carbon-nucleophile bond is forming, then the carbon-leaving group bond must be breaking. Iodide is the leaving group since it leaves with the pair of electrons that once bound it to carbon
H H0: H0-C- H HH H H nucleophile electrophile transition state product leaving (substrate) group The reaction is said to be concerted,taking place in a single step with the new bond forming as the old bond is breaking. The transition state is a point of highest energy(not an intermediate). H HO- HH 」transition state HO H H reaction coordinate→ Ch06 Alkyl Halides (landscape) Page 10
Ch06 Alkyl Halides (landscape) Page 10 The reaction is said to be concerted, taking place in a single step with the new bond forming as the old bond is breaking. The transition state is a point of highest energy (not an intermediate)