6-6 Consequences of Inversion in S.2 Reactions g8meatesizeapecicenantiomerbyusng (5)- changed to H (S)-2Octanethll HeRagtoeo2.ea5enio0ioa62temocoa0ee8r nomet8etanaeR8oteto2ateerg3 as2cy9waoemhrteqecanw NC一H +B Note that in the first case a meso product is formed 6-7 Structure and S,2 Reactivity:The Leaving Group Sg592dlearnngroupsthatcanbedspecadbynuceophe inroupability is a measure of the ease of aby品cag2gesambecmeado (best)B F(worst) 4 4 Halfway through the course of an SN2 reaction, the sp3 hybridization of the carbon atom has changed to the planar sp2 hybridization (transition state). As the reaction proceeds to completion the carbon atom returns to the tetrahedral sp3 hybridization. The transition state of the SN2 reaction can be described in an orbital picture. 6-6 Consequences of Inversion in SN2 Reactions We can synthesize a specific enantiomer by using SN2 reactions. When (R)-2-Bromooctane is reacted with HS- , only (S)-2- octanethiol is obtained: If we had started with the S enantiomer of 2-bromooctane, only the R enantiomer of 2-octanethiol would have been produced. In order to retain the R configuration of the starting 2- bromooctane, a sequence of two SN2 reactions is used: The double inversion sequence of two SN2 processes results in a net retention of configuration. When a substrate contains more than one stereocenter, inversion takes place only at the stereocenter being attacked by the nucleophile. Note that in the first case a meso product is formed. 6-7 Structure and SN2 Reactivity: The Leaving Group The rates of SN2 reactions depend upon: •Nature of the leaving group. •Reactivity of the nucleophile •Structure of the alkyl portion of the substrate. Leaving-group ability is a measure of the ease of displacement. The leaving group ability of a leaving group can be correlated to its ability to accommodate a negative charge. For halogens, iodide is a good leaving group, while fluoride is a poor leaving group in SN2 reactions. SN2 reactions of fluoroalkanes are rarely observed. Leaving-Group Ability (best) I- > Br- > Cl- > F- (worst) Other good leaving groups that can be displaced by nucleophiles in SN2 reactions are: