Chapter 9 Alkynes. Introduction

Transcription

1 hapter 9 Alkynes Introduction Alkynes contain a triple bond. General formula is n 2n-2. Two elements of unsaturation for each triple bond. MST reactions are like alkenes: addition and oxidation. Some reactions are specific to alkynes. Slide 9-2 1

2 Alkyne Functional Group Priority All other functional groups, except ethers and halides have a higher priority than alkynes. For a complete list of naming priorities, look inside the back cover of your text methyl-1-hexen-5-yne 4-methylhex-1-en-5-yne hexyn-2-ol hex-4-yn-2-ol Slide 9-3 Synthesis of Acetylene Acetylene is simplest alkyne eat coke with lime in an electric furnace to form calcium carbide. Then drip water on the calcium carbide. 3 + a a 2 + coke lime a a() 2 This reaction was used to produce light for miners lamps and for the stage. Slide 9-4 2

3 Electronic Structure The sigma bond is sp-sp overlap. The two pi bonds are unhybridized p overlaps at 90, which blend into a cylindrical shape. Slide 9-5 Bond Lengths More s character, so shorter length. Three bonding overlaps, so shorter. Bond angle is 180, so linear geometry. Slide 9-6 3

4 Acidity of Alkynes Terminal alkynes, R- -, are more acidic than other hydrocarbons (pk a approx 20-25). Acetylene acetylide by N 2-, but not by - or R -. More s character, so pair of electrons in anion is held more closely to the nucleus. Less charge separation, so more stable. Slide 9-7 Acidity Table Slide 9-8 4

5 Forming Acetylide Ions + can be removed from a terminal alkyne by sodium amide, NaN 2. NaN 2 is produced by the reaction of ammonia with sodium metal. Slide 9-9 Alkynes from Acetylides Acetylide ions are good nucleophiles. S N 2 reaction with 1 alkyl halides lengthens the alkyne chain. Slide

6 Substrate Must be 1 Acetylide ions can also remove + If back-side approach is hindered, elimination reaction happens via E2. Slide 9-11 Addition to arbonyl Acetylide ion + carbonyl group yields an alkynol (alcohol on carbon adjacent to triple bond). R + R Work up 2 + R Slide

7 Add to Formaldehyde Product is a primary alcohol with one more carbon than the acetylide Slide 9-13 Add to Aldehyde Product is a secondary alcohol, one R group from the acetylide ion, the other R group from the aldehyde Slide

8 Product is a tertiary alcohol. Add to Ketone Slide 9-15 Synthesis by Elimination Removal of two molecules of X from a vicinal or geminal dihalide produces an alkyne. First step (-X) is easy, forms vinyl halide. Second step, removal of X from the vinyl halide requires very strong base and high temperatures. Slide

9 Reagents for Elimination K (fused) Molten K or alcoholic K at 200 favors an internal alkyne. Sodium amide, NaN 2, at 150, followed by water, favors a terminal alkyne l 2 1) NaN 2, ) 2 Slide 9-17 Migration of Triple Bond Slide

10 Addition Reactions Similar to addition to alkenes. Pi bond becomes two sigma bonds. Usually exothermic. ne or two molecules may add. Slide 9-19 Addition of ydrogen Three reactions: Add lots of 2 with metal catalyst (Pd, Pt, or Ni) to reduce alkyne to alkane, completely saturated. Use a special catalyst, Lindlar s catalyst, to convert an alkyne to a cis-alkene. React the alkyne with sodium in liquid ammonia to form a trans-alkene. Slide

11 Lindlar s atalyst Powdered BaS 4 coated with Pd, poisoned with quinoline. 2 adds syn, so cis-alkene is formed. Slide 9-21 Na in Liquid Ammonia Use dry ice to keep ammonia liquid. As sodium metal dissolves in the ammonia, it loses an electron. The electron is solvated by the ammonia, creating a deep blue solution. N 3 + Na + Na + N 3 e - Slide

12 Mechanism Step 1: An electron adds to the alkyne, forming a radical anion Step 2: The radical anion is protonated to give a radical Step 3: An electron adds to the alkyne, forming an anion Step 4: Protonation of the anion gives an alkene Slide 9-23 Addition of alogens l 2 and 2 add to alkynes to form vinyl dihalides. May add syn or anti, so product is mixture of cis and trans isomers. Difficult to stop the reaction at dihalide Slide

13 Addition of X l,, and I add to alkynes to form vinyl halides. For terminal alkynes, Markovnikov product is formed. If two moles of X is added, product is a geminal dihalide Slide 9-25 with Peroxides Anti-Markovnikov product is formed with a terminal alkyne. 3 3 RR RR mixture of E and Z isomers 3 Slide

14 ydration of Alkynes Mercuric sulfate in aqueous sulfuric acid adds - to one pi bond with a Markovnikov orientation, forming a vinyl alcohol (enol) that rearranges to a ketone. ydroboration-oxidation adds - with an anti- Markovnikov orientation, and rearranges to an aldehyde. Slide 9-27 Mechanism for Mercuration Mercuric ion (g 2+ ) is electrophile. Vinyl carbocation forms on most-sub.. Water is the nucleophile. 3 g g g an enol g Slide

15 Enol to Keto (in Acid) Add + to the = double bond. Remove + from of the enol A methyl ketone 3 2 Slide 9-29 ydroboration Reagent Di(secondary isoamyl)borane, called disiamylborane. Bulky, branched reagent adds to the least hindered carbon. nly one mole can add B 3 3 Slide

16 ydroboration - xidation B and add across the triple bond. xidation with basic 2 2 gives the enol. 3 Sia 2 B 3 BSia Na 3 Slide 9-31 Enol to Keto (in Base) + is removed from of the enol. Then water gives + to the adjacent carbon. 3 An aldehyde Slide

17 xidation of Alkynes Similar to oxidation of alkenes. Dilute, neutral solution of KMn 4 oxidizes alkynes to a diketone. Warm, basic KMn 4 cleaves the triple bond. zonolysis, followed by hydrolysis, cleaves the triple bond. Slide 9-33 Reaction with KMn 4 Mild conditions, dilute, neutral KMn , neutral arsher conditions, warm, basic KMn 4, K 2, warm Slide

18 zonolysis zonolysis of alkynes produces carboxylic acids (alkenes gave aldehydes and ketones). (1) (2) Used to find location of triple bond in an unknown compound. Slide 9-35 End of hapter 9 Slide

19 Nomenclature: IUPA Find the longest chain containing the triple bond. hange -ane ending to -yne. Number the chain, starting at the end closest to the triple bond. Give branches or other substituents a number to locate their position. Slide 9-37 Physical Properties Nonpolar, insoluble in water. Soluble in most organic solvents. Boiling points similar to alkane of same size. Less dense than water. Up to 4 carbons, gas at room temperature. Slide

20 Name these: 3 propyne bromo-2-pentyne 5-bromopent-2-yne ,6-dimethyl-3-heptyne 2,6-dimethylpept-3-yne Slide