8.5 HOw SN2 Reactions Occur CH3(CH2)5 FIGURE 8.2 Hyb coordinate Bonding is we between carbon and changes that take 2 CH3(CH,)5 H Ormond Br (CH,)s CH3 C(sP)-Bro bond Hydroxide ion acts as a nucleophile, using an unshared electron pair to attack car bon from the side opposite the bond to the leaving group. The hybridization of the car- bon at which substitution occurs changes from sp' in the alkyl halide to sp- in the tran sition state. Both the nucleophile(hydroxide) and the leaving group(bromide)are partially bonded to this carbon in the transition state. We say that the Sn2 transition state is pentacoordinate; carbon is fully bonded to three substituents and partially bonded to both the leaving group and the incoming nucleophile. The bonds to the nucleophile and the leaving group are relatively long and weak at the transition state Once past the transition state, the leaving group is expelled and carbon becomes tetracoordinate, its hybridization returning to sp' During the passage of starting materials to products, three interdependent and syn- chronous changes take place 1. Stretching, then breaking, of the bond to the leaving group 2. Formation of a bond to the nucleophile from the opposite side of the bond that is 3. Stereochemical inversion of the tetrahedral arrangement of bonds to the carbon at hich substitution occurs Although this mechanistic picture developed from experiments involving optically active alkyl halides, chemists speak even of methyl bromide as undergoing nucleophilic substitution with inversion. By this they mean that tetrahedral inversion of the bonds to carbon occurs as the reactant proceeds to the product HH HO---C see Learning By Hydroxide Met Transition state Methyl Bromide Back Forward Main MenuToc Study Guide ToC Student o MHHE Website8.5 How SN2 Reactions Occur 309 Hydroxide ion acts as a nucleophile, using an unshared electron pair to attack car￾bon from the side opposite the bond to the leaving group. The hybridization of the car￾bon at which substitution occurs changes from sp3 in the alkyl halide to sp2 in the tran￾sition state. Both the nucleophile (hydroxide) and the leaving group (bromide) are partially bonded to this carbon in the transition state. We say that the SN2 transition state is pentacoordinate; carbon is fully bonded to three substituents and partially bonded to both the leaving group and the incoming nucleophile. The bonds to the nucleophile and the leaving group are relatively long and weak at the transition state. Once past the transition state, the leaving group is expelled and carbon becomes tetracoordinate, its hybridization returning to sp3 . During the passage of starting materials to products, three interdependent and syn￾chronous changes take place: 1. Stretching, then breaking, of the bond to the leaving group 2. Formation of a bond to the nucleophile from the opposite side of the bond that is broken 3. Stereochemical inversion of the tetrahedral arrangement of bonds to the carbon at which substitution occurs Although this mechanistic picture developed from experiments involving optically active alkyl halides, chemists speak even of methyl bromide as undergoing nucleophilic substitution with inversion. By this they mean that tetrahedral inversion of the bonds to carbon occurs as the reactant proceeds to the product. Hydroxide ion HO Methyl bromide C H H H Br Transition state HO C   Br H H H Bromide ion Br Methyl alcohol H H H HO C Potential energy Pentacoordinate carbon is sp2 - hybridized Bonding is weak between carbon and bromine and carbon and oxygen in the transition state Reaction coordinate CH3 (CH2)5 CH3 (CH2)5 CH3 CH3 CH3 Br (CH2)5 CH3 Br H H H C C HO C HO Br HO C(sp O bond σ 3 ) C(sp Br bond σ 3 ) δ δ FIGURE 8.2 Hybrid orbital description of the bonding changes that take place at carbon during nucleophilic substitution by the SN2 mechanism. For an animation of this SN2 reaction, see Learning By Modeling. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website