PowerPoint to accompany. Chapter 23. Alkenes and Alkynes

Transcription

1 PowerPoint to accompany hapter 23 Alkenes and Alkynes

2 Electrophilic addition and Substitution reactions The presence of carbon-carbon double or triple bonds in a compound markedly increases its chemical reactivity. Most characteristic reactions of alkene and alkynes are addition reactions: Halogenation Hydrohalogenation Hydration The electron-rich nature of a multiple bond leads to enhanced reactivity with electrophiles (electron-loving species).

3 Alkenes reactions Figure own, LeMay, Bursten, Murphy, Langford, Sagatys: hemistry 2e 2010 Pearson Australia

4 Mechanism for HX Addition rxn Two-steps mechanism 1. First step is slow, rate-determining step. 2. Second step is fast. slow H carbocation Fast carbocation

5 Addition of HX In the first step, -bond breaks and new H bond and cation form. Figure In the second step, a new bond forms between negative bromide ion and positive carbon. own, LeMay, Bursten, Murphy, Langford, Sagatys: hemistry 2e 2010 Pearson Australia

6 Regioselectivity A occurs when a non-symmetric alkene is used. The preference based on intermediate carbocation stability: Primary < Secondary < Tertiary (Less Stable) (Most Stable) The regioselectivity is stated as Markovnikov s Rule: In the addition of HX to an alkene, the hydrogen adds to the carbon atom of the double bond bearing the greater number of hydrogen atoms bonded directly to it. own, LeMay, Bursten, Murphy, Langford, Sagatys: hemistry 2e 2010 Pearson Australia

7 Addition of HX H 1 2 H 3 H H 3 H Major product Minor product own, LeMay, Bursten, Murphy, Langford, Sagatys: hemistry 2e 2010 Pearson Australia

8 Exercise 1. Which of the following is the most stable carbocation and why? + H 2 (A) + H 2 (B) Soln: (D) It is a tertiary (3 ) carbocation. H 3 H 3 + H 3 Inductive effect from electron rich groups H 3 + () (D) 2. What will be the major product from the reaction by reflux in dark of 2-methylbut-2-ene with hydrochloride in a carbon tetrachloride solution? Name and draw the structure of the product.

9 Soln: H 3 H 3 H l (A) H 3 H minor product H 3 H 3 H 3 H 2-methylbut-2-ene Hl l 4 2-chloro-3-methylbutane l H 3 H 3 H 3 H H (B) major product 2-chloro-2-methylbutane

10 Addition of H 2 O The hydration of alkenes and alkynes requires the addition of a strong acid catalyst e.g. H 2 SO 4 The first step is rate determining. hydronium ion (electrophile) Leads to the formation of a carbocation.

11 Second step is fast arbocation reaction with excess H 2 O (nucleophile) leads to the formation of an oxonium ion. oxonium ion Loss of H + from the oxonium ion leads to formation of an alcohol and regenerates the hydronium ion, hence it is catalytic reaction. butan-2-ol 2-butanol hydronium ion regenerated

12 Addition of H 2 O (Reversible) The fact that the reaction is always described by equilibria means that the overall reaction is reversible. That is, in the presence of H 2 SO 4, alcohols can be dehydrated to yield alkenes.

13 Formation of Alkenes Previously, we have discussed the formation of alcohols from alkenes. The reverse process is also possible. This dehydration reaction is also known as an elimination reaction. I - > H 2 O > - > l - >> F - > H 3 OO - > OH - Requires the elimination of a good leaving group. own, LeMay, Bursten, Murphy, Langford, Sagatys: hemistry 2e 2010 Pearson Australia

14 Formation of Alkenes

15 Exercise 1. Draw and name the major product in the following reactions (a) H + / H 2 O H OH pentan-2-ol 2-pentanol (b) OH H 2 SO 4 Heat cyclohexene

16 Halogenation One of the easiest and most dramatic reactions is the addition of 2 to an alkene. olour change Useful test for alkenes Figure own, LeMay, Bursten, Murphy, Langford, Sagatys: hemistry 2e 2010 Pearson Australia B

17 Many similar reactions as alkenes. Reactions of Alkynes Reactions involve the replacement of -bonds with -bonds. Example H 3 H 3 H 3 2 H 2 l 2 H 3 (E)-2,3-dibromobut-2-ene H 3 2 2,2,3,3-tetrabromobutane H 3 H 2 l 2

18 Problems 1. What will be the product for a reaction between hex-2-yne and one mole of chlorine in dichloromethane under anhydrous condition? Draw the structure of the product and provide it IUPA name. 2. How would you form 1,1,2,2-tetrabromo-1,2-difluoroethane using bromine as one of your reagent? Solutions (1) hex-2-yne H 2 H 3 l 2 l H 2 H 2 H 3 H 3 H 2 H 2 l 2 H 3 l (E)-2,3-dichlorohex-2-ene (2) F F 1,1,2,2-tetrabromo- 1,2-difluoroethane F 2 2 H 2 l 2 F H 2 l 2 (E)-1,2-dibromo-1,2-difluoroethene F F 1,2-difluoroethyne

19 atalytic Hydrogenation Reaction requires molecular hydrogen (H 2 ) and a metal catalyst Ni, Pd, Pt, Ru typical metals used Because a gas, liquid and solid metal are used, this reaction is known as a heterogenous catalysis own, LeMay, Bursten, Murphy, Langford, Sagatys: hemistry 2e 2010 Pearson Australia D

20 Problems Provide the major organic product(s) in the reactions below (1) H 2 3-ethylpent-1-ene Pd/ 3-ethylpentane (2) 2 H 2 l 2 3-ethylpent-1-ene 1,2-dibromo-3-ethylpentane (3) 3-ethylpent-1-ene 2 H 2 O HO 1-bromo-3-ethylpentan-2-ol