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SN1 vs. SN2 vs. E1 vs. E2 Factors affecting the type of reaction an alkyl halide undergoes include: Type of alkyl halide o o o methyl, 1 , 2 , 3 , allylic or benzylic Strength of nucleophile or base SN1 vs. SN2 vs. E1 vs. E2 Guidelines for Predicting Products of Substitution and Elimination Reactions: The strength of the nucleophile or base determines the order of the reaction. SN2 and E2 reactions require strong nucleophiles (or bases) SN1 and E1 reactions generally use weak nucleophiles (or bases) SN1 vs. SN2 vs. E1 vs. E2 Nucleophilicity (nucleophile strength): a measure of the rate at which a nucleophile attacks an electrophilic carbon of a standard substrate A species with a negative charge is a stronger nucleophile than a similar neutral species: nucleophilicity of base > conjugate acid - > CH OH CH3O 3 SN1 vs. SN2 vs. E1 vs. E2 Nucleophilicity decreases from left to right in the periodic table look at the element with lone pair of e more electronegative elements hold on to electrons more tightly OH- > F- SN1 vs. SN2 vs. E1 vs. E2 Nucleophilicity increases from top to bottom size increases electrons held more loosely polarizability increases bond can begin forming when atoms are farther apart - > I Br- Polarizable: having electrons that can move more freely toward a positive charge SN1 vs. SN2 vs. E1 vs. E2 SN1 vs. SN2 vs. E1 vs. E2 Example: Identify the stronger nucleophile in each pair. Explain why. NH3 or NH2H2O or NH3 HS- or Cl- PH3 or NH3 SN1 vs. SN2 vs. E1 vs. E2 Base Strength: Weaker acids produce stronger conjugate bases. CH3CH2OH weak acid CH3CH2O- strong base strong nucleophile Strong acids produce weak conjugate bases. HBr BrStrong acid Weak base good nucleophile SN1 vs. SN2 vs. E1 vs. E2 The presence of bulky groups on the CH nucleophile makes it less nucleophilic but3 a CH3 C O stronger base. hinders approach to the electrophilic CH3 carbon CH3 CH3CH2O CH3 C O CH3 t-butoxide ion hindered strong base weak nucleophile ethoxide ion unhindered weaker base stronger nucleophile SN1 vs. SN2 vs. E1 vs. E2 Example: Identify the stronger base in each pair. OH- or H2O CH3OH or CH3O(CH3)3CO- or CH3OWater and alcohols are amphoteric. They can serve as either an acid or a base. Water and alcohols are weak acids and weak bases. SN1 vs. SN2 vs. E1 vs. E2 Guidelines for Predicting Products of Substitution and Elimination Reactions: The type of alkyl halide is important in determining the type of reaction: Methyl halide: SN2 exclusively o 1 halide: SN2 with a strong nucleophile (favored) E2 with strong base (sometimes) SN1 or E1 are sometimes observed at high temperatures if rearrangement produces more stable carbonium ion SN1 vs. SN2 vs. E1 vs. E2 Guidelines for Predicting Products of Substitution and Elimination Reactions: The type of alkyl halide is important in determining the type of reaction: o 3 Halides: E2 with a strong base SN1 and E1 with weak nucleophile or weak base SN1 vs. SN2 vs. E1 vs. E2 Guidelines for Predicting Products of Substitution and Elimination Reactions: The type of alkyl halide is important in determining the type of reaction: o 2 Halides: Mixtures of products are common Strong base/nucleophile gives SN2 and E2 Weak base/nucleophile gives SN1 and E1 SN1 vs. SN2 vs. E1 vs. E2 Guidelines for Predicting Products of Substitution and Elimination Reactions: Strong bulky bases favor E2 reactions. t-butoxide ion Good nucleophiles with limited basicity favor SN2 reactions. Br I CH CH2OH 3 CH CH OH 3 2 SN1 vs. SN2 vs. E1 vs. E2 Example: Predict the product(s) and CH CH CH CH2OCH OCH 2 3 3 33 mechanism of the following reaction. If more CH2 CH CH2 CH than one product or mechanism is possible, 2 2 explain which is/are most likely. CH3CH2OCH CH2OH CH3CH2Br CH + NaOCH 3 CH3CH2Br + NaOCH33 CH3CH2OCH3 + CH2 CH2 CH3OH major minor CH3CH2OCH3 CH3CH2Br + N NaOCH3 = strong base and strong nucleophile CH2 CH2 2nd order kinetics CH3CH S N2 or +E2NaOCH3 Br 2 1o Halide favors SN2 over E2 SN1 vs. SN2 vs. E1 vs. E2 Example: Predict the product(s) and CH3CH CH2 If more mechanism of the following reaction. NaOCH CH 2 3 than one product or mechanism is possible, CH3CH2OH explain which is/are most likely. NaOCH CH3 3 NaOCH CH CH3CHCH 22 3 CH3CHCH3 CH3CH CH2OH OH CH 3 Br 2 Br CH3CH CH CH CH2 2 CH 3 CH3CHCH3 + NaOCH2CH3 OCH2CH3 CH3CH2OH CH3CH CH2 NaOCH2CH3CH =CH strong base/nucleophile CH3CHCH CHCH3 3 CH CHCH CHCH 3 33 33 CH3CHCH3 CH CHCH OCH CH 3 3 OCH CH 2nd order kinetics S 2 or E2 Br Br N 22 33 OCH2CH3 Br Relative amounts of SN2 vs E2 depends on temperature. SN1 vs. SN2 vs. E1 vs. E2 CH3OH CH3OH product(s) Dand D Example: Predict the mechanism of the following reaction.CHIf more 3 CH3OH CH 3 than one product or mechanism is possible, CH3OH Br D explain Br which is/are most likely. D CH3 Br CH3 CH OH CH OH 3 CH 3 3 OCH3 Br CH3 + OCH3 major CH3 CH3 major + CH2 CH2 trace CH3 CH3 CH3OH = weak nucleophile or weak base CH3 CH CH OCH CH Br 3 2 2 3 CH3 CH 1st order kinetics SN1 or E1 3 OCH3 Br Saytzeff’s orientation favored for elimination product. C O