SN1 vs. SN2 vs. E1 vs. E2 Factors affecting the type of reaction an alkyl halide undergoes include: Type of alkyl halide methyl, 1o, 2o, 3o, 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 CH3O- > CH3OH

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 NH2- H2O or NH3 HS- or Cl- PH3 or NH3

SN1 vs. SN2 vs. E1 vs. E2 Base Strength: Weaker acids produce stronger conjugate bases. CH3CH2OH CH3CH2O- Strong acids produce weak conjugate bases. HBr Br- strong base strong nucleophile weak acid Weak base good nucleophile Strong acid

SN1 vs. SN2 vs. E1 vs. E2 The presence of bulky groups on the nucleophile makes it less nucleophilic but a stronger base. hinders approach to the electrophilic carbon - - 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 CH3O- Water 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 1o 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: 3o 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: 2o 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-

SN1 vs. SN2 vs. E1 vs. E2 Example: Predict the product(s) and mechanism of the following reaction. If more than one product or mechanism is possible, explain which is/are most likely.

SN1 vs. SN2 vs. E1 vs. E2 Example: Predict the product(s) and mechanism of the following reaction. If more than one product or mechanism is possible, explain which is/are most likely.

SN1 vs. SN2 vs. E1 vs. E2 Example: Predict the product(s) and mechanism of the following reaction. If more than one product or mechanism is possible, explain which is/are most likely.