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S N 1 vs. S N 2 vs. E1 vs. E2 Factors affecting the type of reaction an alkyl halide undergoes include: Type of alkyl halide methyl, 1 o, 2 o, 3 o, allylic.

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Presentation on theme: "S N 1 vs. S N 2 vs. E1 vs. E2 Factors affecting the type of reaction an alkyl halide undergoes include: Type of alkyl halide methyl, 1 o, 2 o, 3 o, allylic."— Presentation transcript:

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

2 S N 1 vs. S N 2 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. S N 2 and E2 reactions require strong nucleophiles (or bases) S N 1 and E1 reactions generally use weak nucleophiles (or bases)

3 S N 1 vs. S N 2 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 3 O - > CH 3 OH

4 S N 1 vs. S N 2 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 -

5 S N 1 vs. S N 2 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

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

7 Example: Identify the stronger nucleophile in each pair. Explain why. NH 3 or NH 2 - H 2 O or NH 3 HS - or Cl - PH 3 or NH 3

8 S N 1 vs. S N 2 vs. E1 vs. E2 Base Strength: Weaker acids produce stronger conjugate bases. CH 3 CH 2 OHCH 3 CH 2 O - Strong acids produce weak conjugate bases. HBr Br- Strong acid Weak base good nucleophile weak acid strong base strong nucleophile

9 S N 1 vs. S N 2 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

10 S N 1 vs. S N 2 vs. E1 vs. E2 Example: Identify the stronger base in each pair. OH - or H 2 O CH 3 OH or CH 3 O - (CH 3 ) 3 CO - or CH 3 O - Water and alcohols are amphoteric. They can serve as either an acid or a base. Water and alcohols are weak acids and weak bases.

11 S N 1 vs. S N 2 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: S N 2 exclusively 1 o halide: S N 2 with a strong nucleophile (favored) E2 with strong base (sometimes) S N 1 or E1 are sometimes observed at high temperatures if rearrangement produces more stable carbonium ion

12 S N 1 vs. S N 2 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: 3 o Halides: E2 with a strong base S N 1 and E1 with weak nucleophile or weak base

13 S N 1 vs. S N 2 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: 2 o Halides: Mixtures of products are common Strong base/nucleophile gives S N 2 and E2 Weak base/nucleophile gives S N 1 and E1

14 S N 1 vs. S N 2 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 S N 2 reactions. Br - I -

15 S N 1 vs. S N 2 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.

16 S N 1 vs. S N 2 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.

17 S N 1 vs. S N 2 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.


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