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
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. 3 2 2 4-methyl-1-hexen-5-yne 4-methylhex-1-en-5-yne 3 2 3 4-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 2 + 2 2 + a() 2 This reaction was used to produce light for miners lamps and for the stage. Slide 9-4 2
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
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
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 9-10 5
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 9-12 6
Add to Formaldehyde Product is a primary alcohol with one more carbon than the acetylide. 3 + 3 2 + 3 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. 3 3 3 + 3 2 + 3 3 Slide 9-14 7
Product is a tertiary alcohol. Add to Ketone 3 3 3 + 3 3 3 2 + 3 3 3 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 9-16 8
Reagents for Elimination 3 2 3 K (fused) 200 3 2 3 Molten K or alcoholic K at 200 favors an internal alkyne. Sodium amide, NaN 2, at 150, followed by water, favors a terminal alkyne. 3 2 2 l 2 1) NaN 2, 150 3 2 2) 2 Slide 9-17 Migration of Triple Bond Slide 9-18 9
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 9-20 10
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 9-22 11
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. 3 3 2 3 3 + 3 3 2 3 3 Slide 9-24 12
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. 3 3 2 3 3 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 9-26 13
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 +2 3 + g + 2 3 g + 3 + 3 an enol g + 3 + 2 Slide 9-28 14
Enol to Keto (in Acid) Add + to the = double bond. Remove + from of the enol. 3 3 + 3 3 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. 3 3 3 3 B 3 3 Slide 9-30 15
ydroboration - xidation B and add across the triple bond. xidation with basic 2 2 gives the enol. 3 Sia 2 B 3 BSia 2 2 2 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 3 3 3 Slide 9-32 16
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 4 3 2 3 2, neutral arsher conditions, warm, basic 3 2 3 3 2 3 KMn 4, K 2, warm 3 + 2 3 Slide 9-34 17
zonolysis zonolysis of alkynes produces carboxylic acids (alkenes gave aldehydes and ketones). (1) 3 3 2 3 3 + (2) 2 2 3 Used to find location of triple bond in an unknown compound. Slide 9-35 End of hapter 9 Slide 9-36 18
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 9-38 19
Name these: 3 propyne 3 2 2 5-bromo-2-pentyne 5-bromopent-2-yne 3 3 3 2 3 2,6-dimethyl-3-heptyne 2,6-dimethylpept-3-yne Slide 9-39 20