8.13Substitution and Eliminationas Competing Reactions,Alkyl halides can react with Lewis bases in two different ways; nucleophilic substitution or elimination.,H,X,+,Y,H,+,b-elimination,nucleophilic substitution,Two Reaction Types,How can we tell which reaction pathway is followedfor a particular alkyl halide?,H,X,+,Y,H,+,b-elimination,nucleophilic substitution,Two Reaction Types,A systematic approach is to choose as a referencepoint the reaction followed by a typical alkyl halide(secondary) with a typical Lewis base (an alkoxideion).,The major reaction of a secondary alkyl halidewith an alkoxide ion is elimination by the E2mechanism.,Elimination versus Substitution,NaOCH2CH3 ethanol, 55C,CH3CH=CH2,+,(87%),(13%),Example,Br,E2,Figure 8.11,Br,SN2,Figure 8.11,Given that the major reaction of a secondaryalkyl halide with an alkoxide ion is elimination by the E2 mechanism, we can expect the proportion of substitution to increase with:,1)decreased crowding at the carbon thatbears the leaving group,When is substitution favored?,Decreased crowding at carbon that bears the leaving group increases substitution relative to elimination.,primary alkyl halide,CH3CH2CH2Br,NaOCH2CH3 ethanol, 55C,CH3CH=CH2,+,CH3CH2CH2OCH2CH3,(9%),(91%),Uncrowded Alkyl Halides,primary alkyl halide + bulky base,CH3(CH2)15CH2CH2Br,KOC(CH3)3,tert-butyl alcohol, 40C,+,CH3(CH2)15CH2CH2OC(CH3)3,CH3(CH2)15CH=CH2,(87%),(13%),But a crowded alkoxide base can favor elimination even with a primary alkyl halide.,Given that the major reaction of a secondaryalkyl halide with an alkoxide ion is elimination by the E2 mechanism, we can expect the proportion of substitution to increase with:,1)decreased crowding at the carbon thatbears the leaving group 2) decreased basicity of the nucleophile,When is substitution favored?,Weakly basic nucleophile increases substitution relative to elimination,KCN,pKa (HCN) = 9.1,(70%),DMSO,secondary alkyl halide + weakly basic nucleophile,Weakly Basic Nucleophile,Weakly basic nucleophile increases substitution relative to elimination,NaN3,pKa (HN3) = 4.6,(75%),secondary alkyl halide + weakly basic nucleophile,Weakly Basic Nucleophile,I,Tertiary alkyl halides are so sterically hinderedthat elimination is the major reaction with allanionic nucleophiles. Only in solvolysis reactionsdoes substitution predominate over eliminationwith tertiary alkyl halides.,Tertiary Alkyl Halides,+,+,2M sodium ethoxide in ethanol, 25C,1%,99%,Example,