Alkynes Nomenclature of Alkynes

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1 Chapter 7 Alkynes Alkynes - hydrocarbons containing a carbon-carbon triple bond (2 bonds) Acyclic alkanes = C n H 2n+2 Alkenes and cyclic alkanes = C n H 2n Alkynes (and cyclic alkenes) = C n H 2n-2 The Reactions of Alkynes An Introduction to Multistep Synthesis Cyclic alkynes = C n H 2n-4 Naturally occurring alkynes. Capillin - naturally occurring fungicide Ichthyothereol - convulsant used by indigenous people of Amazon for poisoned arrowheads Enediynes - class of naturally occurring compounds with powerful antibiotic and anti-cancer properties (some in clinical trials) Paula Yurkanis Bruice University of California, Santa Barbara Alkynes Nomenclature of Alkynes An alkyne is a hydrocarbon that contains a carbon carbon triple bond. Simplest alkyne. Acetylene - C 2 H 2 - combustion results in high-temperature flame useful for welding Complete combustion 2C 2 H O 2 -> 4CO 2 + 2H 2 O H<0. General formula: C n H 2n 2 (acyclic) C n H 2n 4 (cyclic) propane/air flame: 2,000 C propane/oxygen flame: 2,500 C acetylene/oxygen flame: 3,500 C 1

2 Nomenclature of Alkynes Two of the Same Functional Group The yne suffix is assigned the lowest number. The substituents are assigned so the lowest number is in the name. A Double Bond and a Triple Bond Double and Triple are Tied When the same number is obtained for both, the double bond gets the lower number. 2

3 Priority of Functional Groups Suffixes OH and NH 2 Have Higher Priority Physical Properties of Unsaturated Hydrocarbons The Structure of Alkynes The triple bond is composed of a sigma bond and two π bonds. 3

4 ALKYNES ARE LESS REACTIVE THAN ALKENES The heats of hydrogenation of alkyne isomers can be used to determine their relative stabilities: (sp 2 ) C C bonds are longer weaker less s character (sp) C C bonds are shorter stronger more s character SSS RULE Shorter - Stronger - more S character The greater relative stabililty of internal alkynes is due to hyperconjugation. Alkynes (Like Alkenes) Undergo Electrophilic Addition Reactions Alkynes Are Less Stable and Less Reactive Than Alkenes 4

5 The First Step is Addition of an Electrophile Addition of HX to Alkynes Involves Vinylic Carbocations Addition of an electrophile to an alkene Addition of an electrophile to an alkyne Addition of H-X to alkyne should produce a vinylic carbocation intermediate We are not sure it does Relative Carbocation Stability Vinyl cation has a linear structure and the cationic carbon has sp-hybrid structure: more electronegative carbon Hyperconjugation X O Secondary Vinyl Carbocation 5

6 Formation of a π-complex Evidence for Formation of a π-complex A primary vinylic cation is less sable than a primary alkyl cation. Evidence for the formation of a π-complex A Second Addition Reaction Can Occur Addition to a Terminal Alkyne is Regioselective 6

7 Mechanism for Electrophilic Addition Formation of the More Stable Transition State Accounts for the Regioselectivity The Second Electrophilic Addition Reaction is Also Regioselective Formation of the More Stable Carbocation Accounts for the Regioselectivity 7

8 Addition of excess hydrogen halides - an explanation of regioselectivity The Second Electrophilic Addition Reaction is Slower Generating a carbocation on the carbon atom directly bonded to bromine allows for stabilization of the positive charge by overlap of carbon s empty 2p orbital with one of bromine s lone-pair containing orbitals Br Br This explains why if only one equivalent of HCl is used, the reaction stops at the halo-substituted alkene. CH 3 CH 2 C CH 3 CH3 CH 2 C CH 3 An Asymmetrical Internal Alkyne Forms Two Products A Symmetrical Internal Alkyne Forms One Product 8

9 Addition of Cl 2 and Br 2 Acid-Catalyzed Addition of Water to an Alkene Acid-Catalyzed Addition of Water to an Alkene Keto Enol Interconversion 9

10 ENOL O H H 3 C C CH 2 unstable enol TAUTOMERISM KETO O H 3 C C CH 2 H TAUTOMERS : species in equilibrium that differ in the position of a proton or other group. Mechanism for Acid-Catalyzed Keto Enol Interconversion Most enols are not favored, keto enol E N E R G Y To interconvert tautomers, a proton is transferred from oxygen to carbon. Mechanism, next slide.. they rapidly change to keto. Acid-Catalyzed Addition of Water ADDITION OF WATER ALKYNES ARE LESS REACTIVE THAN ALKENES H 3 C C CH H dilute H 2 SO 4 H 2 O + O H H H 3 C + C CH H reaction is slow Water doesn t add under the usual conditions (dil. H 2 SO 4 ). A stronger electrophile than H 3 O + is required.. HgSO 4 is added. 10

11 MERCURY IS LIKE A BIG PROTON Hg 2+ is a stronger electrophile than H + H 3 C C CH H 3 C C CH A Terminal Alkyne is Less Reactive Than an Internal Alkyne slower H + Hg 2+ faster Hg 2+ is not only a stronger Lewis acid, it is large and it forms a bridged ion. So we use a mixture of H 2 SO 4, H 2 O and HgSO 4 as our reagents. ( HgSO 4 = mercuric sulfate ) Because terminal alkynes are less reactive, the rate of the reaction is increased with a mercuric ion catalyst. Mechanism for Catalysis by Hg 2+ Hydroboration Oxidation Experimental consideration - only one equivalent of borane (3 equivalents of alkyne) can be used to ensure the reaction stops at the alkene - a boron-substituted alkene can serve as a substrate for another equivalent of borane yielding an alkane with multiple boron substituents 11

12 BH 3 Is an Electrophile Solution: 9-BBN or R 2 BH Second additions of borane are generally only problematic with terminal alkynes (alkyl bulk on internal alkynes prevent second addition reactions and borane can be used) R 2 BH should be used with terminal alkynes. R 2 BH can be used with internal alkynes. Hydroboration Oxidation Addition of Hydrogen Forms an Alkane Terminal alkynes form aldehydes. Internal alkynes form ketones. 12

13 Stopping at the Alkene Lindlar Catalyst POISONING MECHANISM IS UNKNOWN A POSSIBLE HYPOTHESIS IS GIVEN HERE Quinoline binds to sites on the catalyst. Linear alkynes can approach and pick up hydrogens. Planar alkenes are blocked. Syn Addition bound quinoline QUINOLINE QUINOLINE surface of Lindlar catalyst R-C C-R alkyne fits QUINOLINE QUINOLINE R R QUINOLINE C=C R R QUINOLINE QUINOLINE QUINOLINE QUINOLINE alkene blocked ( probably over-simplified ) 13

14 Why Cis? Anti Addition The catalyst delivers the hydrogens to one side of the triple bond. Conversion of Internal Alkynes to Trans Alkenes Why Trans? 14

15 Relative Electronegativities of Carbon A Hydrogen Attached to an sp Carbon is the Most Acidic Need a Strong Base to Remove the Hydrogen HO is Not Strong Enough 15

16 Relative Acid Strengths Forming a New Carbon Carbon Bond Mechanism for Formation of the Carbon Carbon Bond Two Steps 16

17 How Can the Compound Be Prepared From the Starting Material? Synthesis Retrosynthetic Analysis and Synthesis Designing a Synthesis Costs and benefits of a synthetic analysis: Time - minimize Cost - minimize Yield maximize As a young synthetic chemist focus on minimizing the number of steps in a reaction - you don t know enough about reaction difficulty, cost, or yield to plan otherwise :) 17

18 Designing a Synthesis Retrosynthetic Analysis and Synthesis Designing a Synthesis Retrosynthetic Analysis and Synthesis 18

19 Designing a Synthesis Retrosynthetic Analysis and Synthesis Designing a Synthesis Retrosynthetic Analysis and Synthesis 19

20 Designing a Synthesis Retrosynthetic Analysis and Synthesis Designing a Synthesis Retrosynthetic Analysis and Synthesis 20

21 Designing a Synthesis Retrosynthetic Analysis and Synthesis SUMMARY OF REACTIONS Electrophilic addition reactions - the first step is the addition of an electrophile to the sp 2 carbon that is bonded to the greater number of hydrogens (least substituted) Addition of hydrogen halides SUMMARY OF REACTIONS Electrophilic addition reactions - the first step is the addition of an electrophile to the sp 2 carbon that is bonded to the greater number of hydrogens (least substituted) Acid-catalyzed addition of water/hydroboration-oxidation (H + or B (replaced by H) is electrophile) Addition of halogens (X + is electrophile; Section 6.6) 21

22 SUMMARY OF REACTIONS Addition of hydrogen Removal of a proton from a terminal alkyne, followed by alkylation 22