Presentation is loading. Please wait.

Presentation is loading. Please wait.

Substitution and Elimination Competing Reactions.

Similar presentations


Presentation on theme: "Substitution and Elimination Competing Reactions."— Presentation transcript:

1 Substitution and Elimination Competing Reactions

2 Alkyl halides can react with Lewis bases by nucleophilic substitution and/or elimination. C CHX + Y : – C C Y H X : – + C C + H Y X : – +  -elimination nucleophilic substitution Two Competing Reactions

3 How can a prediction be made which reaction pathway Is followed for a particular alkyl halide? C C H X + Y :– C C Y H X :–+ C C +HY X :–+  -elimination nucleophilic substitution Two Competing Reactions

4 Comparison

5 A systematic approach is to choose as a reference point the reaction followed by a typical alkyl halide (secondary) with a typical Lewis base (an alkoxide ion). The major reaction of a secondary alkyl halide with an alkoxide ion (a strong base similar to hydroxide) is elimination by the E2 mechanism. Elimination versus Substitution

6 CH 3 CHCH 3 Br NaOCH 2 CH 3 ethanol, 55°C CH 3 CHCH 3 OCH 2 CH 3 CH 3 CH=CH 2 + (87%) (13%) Example

7 Br E2 Figure CH 3 CH 2 O –

8 Br SN2SN2SN2SN2 Figure CH 3 CH 2 O –

9 Given that the major reaction of a secondary alkyl 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 that bears the leaving group When is Substitution Favored?

10 Question Which one of the following alkyl halides would be expected to give the highest ratio of substitution to elimination on treatment with sodium ethoxide in ethanol (50°C)? A)1-bromopentane B)2-bromopentane C)3-bromopentane D)2-bromo-3-methylbutane

11 Decreased crowding at carbon that bears the leaving group increases substitution relative to elimination. primary alkyl halide CH 3 CH 2 CH 2 Br NaOCH 2 CH 3 ethanol, 55°C CH 3 CH=CH 2 + CH 3 CH 2 CH 2 OCH 2 CH 3 (9%) (91%) Uncrowded Alkyl Halides

12 primary alkyl halide + bulky base CH 3 (CH 2 ) 15 CH 2 CH 2 Br KOC(CH 3 ) 3 tert-butyl alcohol, 40°C + CH 3 (CH 2 ) 15 CH 2 CH 2 OC(CH 3 ) 3 CH 3 (CH 2 ) 15 CH=CH 2 (87%)(13%) But a Crowded Alkoxide Base Can Favor Elimination Even with a Primary Alkyl Halide

13 Given that the major reaction of a secondary alkyl 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 that bears the leaving group 2) decreased basicity of the nucleophile When is Substitution Favored?

14 Question Which one of the following statements is true? A)CH 3 CH 2 -S - is both a stronger base and more nucleophilic than CH3CH 2- O - B)CH 3 CH 2 -S - is a stronger base but less nucleophilic than CH 3 CH 2- O - C)CH 3 CH 2- S - is a weaker base but is more nucleophilic than CH 3 CH 2- O - D)CH 3 CH 2- S - is both a weaker base and less nucleophilic than CH 3 CH 2- O -

15 Weakly basic nucleophile increases substitution relative to elimination (70%) CH 3 CH(CH 2 ) 5 CH 3 CN KCN Cl pK a (HCN) = 9.1 DMSO secondary alkyl halide + weakly basic nucleophile Weakly Basic Nucleophile

16 Weakly basic nucleophile increases substitution relative to elimination secondary alkyl halide + weakly basic nucleophile Weakly Basic Nucleophile NaN 3 pK a (HN 3 ) = 4.6 I (75%) N3N3N3N3

17 Question Which one of the following compounds gives the highest subtitution-to-elimination ratio (most substitution least elimination) on reaction with 2-bromobutane? A)NaOCH 3 B)NaNH 2 C)NaC  N D)NaC  CH

18 Tertiary alkyl halides are so sterically hindered that elimination is the major reaction with all anionic nucleophiles. Only in solvolysis reactions does substitution predominate over elimination with tertiary alkyl halides. Tertiary Alkyl Halides

19 Question Which one of the following statements is true concerning substitution and elimination in tert-butyl bromide? A)the mechanism generally believed to be available to (CH 3 ) 3 CBr are S N 1 and E1 B)the mechanism generally believed to be available to (CH 3 ) 3 CBr are S N 1, S N 2 and E1 C)the mechanism generally believed to be available to (CH 3 ) 3 CBr are S N 1, S N 2 and E2 D)the mechanism generally believed to be available to (CH 3 ) 3 CBr are S N 1, E1 and E2

20 (CH 3 ) 2 CCH 2 CH 3 Br + CH 3 CCH 2 CH 3 OCH 2 CH 3 CH 3 CH 2 =CCH 2 CH 3 CH 3 CH 3 C=CHCH 3 CH 3 + ethanol, 25°C 64% 36% 2M sodium ethoxide in ethanol, 25°C 1%99% Example

21 Question Reactions proceeding through this mechanism give a racemic mixture: A)S N 1 B)S N 2 C)E1 D)E2

22 Question Methyl bromide reacts with sodium ethoxide in ethanol by this mechanism: A)S N 1 B)S N 2 C)E1 D)E2

23 Question 3-bromo-3-methylpentane reacts with sodium ethoxide in ethanol by this mechanism: A)S N 1 B)S N 2 C)E1 D)E2


Download ppt "Substitution and Elimination Competing Reactions."

Similar presentations


Ads by Google