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Alkynes

Alkynes. Nomenclature Physical Properties Synthesis Reactions. Alkynes. Contain at least one carbon-carbon triple bond (C ≡ C). Also called acetylenes. Properties of C ≡ C Bonds. sigma ( σ ) bond. pi ( π ) bonds. C ≡ C consists of a σ bond and two π bonds.

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Alkynes

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  1. Alkynes Nomenclature Physical Properties Synthesis Reactions

  2. Alkynes • Contain at least one carbon-carbon triple bond (C≡C). • Also called acetylenes.

  3. Properties of C≡C Bonds sigma (σ) bond pi (π) bonds • C≡C consists of a σ bond and two π bonds. • C≡C is shorter than a C=C bond.

  4. Properties of C≡C Bonds • The pi bonds force the four atoms involved to be linear. • The e- density of the pi bonds is a cylinder surrounding the two C atoms.

  5. Alkynes • The pi bonds are relatively easy to break, which makes C≡C a functional group. • The pi bonds block nucleophilic attack.

  6. Nomenclature of Alkynes • When the chain contains more than three C atoms, use a number to give the location of the triple bond. • Terminal alkynes have one H on the triple-bonded C. Internal alkynes do not. CH3C≡CCH2CH3 2-pentyne pent-2-yne internal alkyne CH3C≡CH propyne terminal alkyne

  7. IR of Alkynes 1-octyne a terminal alkyne

  8. IR of Alkynes 4-octyne an internal alkyne

  9. Nomenclature of Alkynes • Apply the same rules you learned for the alkanes. • Use the root name of the longest chain containing the triple bond, but change -ane to -yne. • In 8th edition of Wade, alkenes and alkynes are given equal priority, but “-en” is first alphabetically, so • The numbering starts at the end closer to the alkene, and • the order of naming is “en-yne.”

  10. Nomenclature of Alkynes • Name the following: (E)-3-methylhept-2-en-4-yne 2,2-dimethylhex-3-yne

  11. Nomenclature of Alkynes • Alkynes as substituents are called alkynyl groups. ethynylbenzene

  12. Uses and Synthesis of Acetylene • Main use is as a fuel for oxyacetylene welding. • It is one of the cheapest organic elements. • Synthesized from • coal: 3C + CaO CaC2 +CO CaC2+2H2O HC≡CH + Ca(OH)2 • natural gas: 2CH4 HC≡CH + 3H2 • driven by ∆S, high T, and 0.01s heating time

  13. Stability of Acetylene • Acetylene (HC≡CH) is thermodynamically unstable: • HC≡CH(g)  2C(s) + H2(g) • produces a very hot flame when burned in pure oxygen

  14. Physical Properties of Alkynes • Similar to alkanes and alkenes of comparable molecular weight. • nonpolar • virtually insoluble in water • soluble in organic solvents

  15. Physical Properties of Alkynes • Terminal alkynes (-C≡C-H) have an acetylenic Hthat is more acidic than the H’s on other hydrocarbons due to the greater s character of the sp bond. The greater s character makes the ≡C-H bond more polar and the acetylenic H more acidic. • pKa of terminal acetylenes ≈25 • pKa of alkanes ≈ 50 • pKa of NH3 is 35 (so -:NH2 reacts with -C≡C-H) • pKa of alcohols ≈ 16 (and alkoxides don’t)

  16. Formation of Acetylide Ions • From the reaction of a terminal alkyne with sodium amide • CH3CH2C≡CH + NaNH2 CH3CH2C≡C:- Na+ + :NH3 • CH3C≡CCH3 + NaNH2 NR • Acetylide ions are strong nucleophiles. • OH- and RO- are not strong enough to remove the terminal H.

  17. Synthesis of Alkynes • from acetylides (lengthens the C skeleton) • an excellent way to make a more complex alkyne • alkylation of an acetylide • addition to carbonyl groups • by elimination reactions

  18. Synthesis of Alkynes • from acetylides (lengthens the C skeleton) - an SN2 reaction • alkylation of an acetylide • if the halide is 2°, there will also be the elimination product…which would be what?

  19. Synthesis of Alkynes • Predict the product:

  20. Synthesis of Alkynes • from acetylides (lengthens the C skeleton) • addition to carbonyl groups • the acetylide is the nucleophile • addition to aldehydes gives 2° alcohols • addition to ketones gives 3°alcohols

  21. Synthesis of Alkynes • from acetylide addition to a carbonyl group an alkoxide ion a 3° alcohol

  22. Synthesis of Alkynes • Predict the product:

  23. Synthesis of Alkynes • by elimination reactions • A vicinal dihalide or a geminal dihalide can undergo a double dehydrohalogenation to form the alkyne • This requires STRONGLY BASIC conditions, and some compounds can’t “take it.” • KOH in a sealed tube heated to 200°C • NaNH2, 150°C

  24. Dehydrohalogenation with KOH • The heated base is so strong that the triple bond can migrate along the carbon chain to form the more stable internal alkyne. terminal alkyne, will rearrange The base abstracts a proton from the neighboring sp3 C.

  25. Dehydrohalogenation with KOH an allene

  26. Dehydrohalogenation with KOH The base abstracts a proton from the allene. internal alkyne

  27. Dehydrohalogenation with NaNH2 • NaNH2 is even stronger than fused KOH. It is so strong that it traps the terminal alkyne as the sodium salt, and no rearrangement occurs. major component terminal alkyne, will not rearrange

  28. Synthesis of Alkynes • Predict the product:

  29. Reactions of Alkynes • Additions • reduction to alkanes • reduction to alkenes • addition of halogens • addition of HX • addition of water • Markovnikov • anti-Markovnikov • Oxidations • to α–diketones • cleavage • Nucleophilicattack on electrophiles • covered in synthesis

  30. Reactions of Alkynes • Reduction to alkanes by hydrogen. • second π bond energy = 226 kJ • first π bond energy = 264 kJ • Alkynes can undergo double additions • These typically go all the way to the alkane. R-C≡C-R’ + 2H2(g)  RCH2CH2R’ Pt, Pd, or Ni

  31. Reactions of Alkynes • Reduction to cis-alkenes • the syn addition of hydrogen can be stopped at the alkene stage if a poisoned catalyst is used • Ni2B • Lindlar’s catalyst: Pd/BaSO4 poisoned with quinoline (in CH3OH to dissolve C≡C)

  32. Reactions of Alkynes • Reduction to trans-alkenes • Na/NH3(l) required • Na + NH3(l)  e- •NH3 + Na+ • The H’s come from NH3. • The solvated e- leads to a vinyl radical, which is more stable in the trans geometry.

  33. Reactions of Alkynes • Addition of halogens • One mole X2, syn or anti addition leading to cis- or trans-alkenes • Two moles X2 leads to the tetrahalides

  34. Reactions of Alkynes • Addition of HX • Reaction proceeds through a vinyl cation intermediate then a carbocation intermediate, so the addition is Markovnikov. • If a peroxide is used with HBr, the anti-Markovnikov product will be formed. Why?

  35. Reactions of Alkynes • Markovnikov addition of water • HgSO4/H2SO4 catalyst • product is a ketone, not an alcohol

  36. Keto-enol conversion • Can occur in acid or base, but the mechanisms are different. • The mechanism shown is the conversion in acid.

  37. Reactions of Alkynes • anti-Markovnikov addition of water • hindered dialkylborane needed • product is an aldehyde, not an alcohol

  38. Reactions of Alkynes • Permanganate oxidations • nearly neutral conditions needed • product is an α-diketone, not a diol • terminal alkynes give keto acids

  39. Reactions of Alkynes • Permanganate oxidations • warm, basic conditions cause cleavage • products are salts of carboxylic acids • (A second step, acidification, is needed to produce the acids themselves.)

  40. Reactions of Alkynes • Ozonolysis followed by hydrolysis

  41. Reactions of Alkynes • Predict the products

  42. Reactions of Alkynes • Predict the products

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