S N 1 vs. S N 2 vs. E1 vs. E2

SN1 vs. SN2 vs. E1 vs. E2 • Factors affecting the type of reaction an alkyl halide undergoes include: • Strength of nucleophile or base • Type of alkyl halide • methyl, 1o, 2o, 3o, allylic or benzylic

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 very weak conjugate bases. HBr Br- strong base strong nucleophile weak acid Poor 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 stronger 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 in the presence of a weak nucleophile/base at high temperatures if rearrangement produces more stable carbocation

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.

S N 1 vs. S N 2 vs. E1 vs. E2