HOAÙ HOÏC HÖÕU CÔ

1. HOAÙ HOÏC HÖÕU CÔ CHÖÔNG 7 (t.t) HYDROCARBON CHÖA NO MAÏCH HỞ ALKYNES Organic Chemistry

2. Alkynes • Alkynes have C-C triple bond. • They are more unsaturated than alkenes. • The general formula is CnH2n-2. • The first member of alkyne family is C2H2. • IUPAC name : ethyne • Common name : acetylene • H-C C-H

3. Characteristics of alkynes • The family name ends in –yne in the IUPAC nomenclature system. • Geometric isomerism is not possible for alkynes. • Triple bond is due to the formation of sp-hybridized C. • The triple bond is composed of one strong sigma bond and two weak pi bonds. • Chemical reactions are same as alkenes but requires double the reagent as two pi bonds are present in alkynes. • Acetylene is an important industrial product. Acetylene torch used in welding to melt and vaporize steel and iron.

4. ALKYNES

6. ALKYNES

7. I. Nomenclature of Alkynes -- “yne” • Step 1: Name longest continuous chain containing the triple bond; parent is “yne” • Step 2: Number to give the carbon-carbon triple bond the lowest number.

8. I. Nomenclature of Alkynes -- “yne” • Step 3: If there is more than one triple bond, indicate by numbers where they are and use prefixes:

9. Nomenclature of Compounds containing both double and triple carbon-carbon bonds: “enyne” • Rule 1: Number from the end to give the multiple bond (double or triple) the lowest number and give each multiple bond a number. • Rule 2: The parent name is • #-alken-#-yne

10. Nomenclature of Compounds containing both double and triple carbon-carbon bonds: “enyne” • Rule 3: If number is the same the double and the triple bond, give the double bond the lower number.

11. Nomenclature of Alkynes: Substituents and Cycloalkynes • Substituents: (ethynyl group) • ethynylcyclooctane • Cycloalkyne • cyclooctyne

13. D imethylacetylene V inylacetylene Nomenclature • IUPAC: use the infix -yn- to show the presence of a carbon-carbon triple bond • Common names: prefix the substituents on the triple bond to the word “acetylene” IUPAC name: 2-Butyne 1-Buten-3-yne Common name:

14. Cycloalkynes • Cyclononyne is the smallest cycloalkyne isolated • it is quite unstable and polymerizes at room temp • the C-C-C bond angle about the triple bond is approximately 155°, indicating high angle strain

15. Classification of Alkynes • Terminal alkyne: monosubstituted alkyne: has triple bond at the end of the chain. • Internal alkyne: disubstituted alkyne: has triple bond inside chain.

16. II. Structure • Bonding: • geometry is linear • all atoms are in one line • hybridization is sp • bond angles are 180o • triple bond is one sigma (s) bond and two pi (p) bonds

17. II. Structure • Bond lengths: • H-C bond is 106 pm • C-C bond is 120 pm • both bond lengths are shorter than in alkene and alkane • percent s character is greater in alkynes (50%) vs 33% in alkenes vs 25% in alkanes: • electrons held closer to nucleus; results in shorter bond lengths

18. ALKYNES

19. Alkynes • stretch: weak absorption at 2260-2100 cm–1 - not observed for symmetrical alkynes (v. weak for ‘pseudo’ symmetric alkynes - terminal alkynes (R-C C-H) absorptions are stronger than internal (R-C C-R) absorptions • C C–H stretch: - 3333–3267 cm–1 - strong, narrow (as compared to OH or NH) • C C–H bend: - 700-610 cm–1: broad, strong absorption - 1400-1220 cm–1, overtone of above

20. Terminal Alkynes Alkyne CC stretch 2119 cm–1 Alkyne C-H bend overtone 1260 cm–1 Alkyne C-H stretch 3310 cm–1 Alkyne C-H bend 630 cm–1

21. Alkyne Synthesis

22. Alkyne Synthesis

23. Alkyne Synthesis

24. Alkylation of Alkyne Anions • Alkyne anions are both strong bases and good nucleophiles • They participate in nucleophilic substitution reactions with alkyl halides to form new C-C bonds to alkyl groups; they undergo alkylation • because alkyne anions are also strong bases, alkylation is practical only with methyl and 1° halides • with 2° and 3° halides, elimination is the major reaction

25. Alkylation of Alkyne Anions • alkylation of alkyne anions is the most convenient method for the synthesis of terminal alkynes • alkylation can be repeated and a terminal alkyne can be converted to an internal alkyne

26. Preparation of Alkynes • By double dehydrohalogenation of either • geminal dihalide (halides on same carbon) or • vicinal dihalide (halides on adjacent carbons)

27. Alkyne Synthesis • Double Elimination

28. Preparation of Alkynes: Double Elimination • Vicinal dihalides (Cl or Br on adjacent carbons) • Requires two moles of very very strong base such as NaNH2 in NH3 (NH2-) • Triple bond forms between the carbons that had the halogen

29. Preparation of Alkynes • Mechanism • Double dehydrohalogenation • Preparation: • alkene-->dihalide-->alkyne

30. Alkyne Synthesis

31. Alkyne Synthesis

32. Alkene to Alkyne • Bromination and two consecutive dehydrohalogenation reactions

33. N a N H 2 R C – C = C R - H B r R C C C R A haloalkene (a vinylic halide) Preparation from Alkenes • a side product may be an allene, a compound containing adjacent carbon-carbon double bonds, C=C=C R H C C C X H H R R An allene H R R An alkyne

34. Preparation from Alkenes • for a terminal alkene to a terminal alkyne, 3 moles of base are required

35. Physical Properties • Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton

36. Acidity of Alkynes

37. Acidity of Alkynes

38. ALKYNES

46. A Look to Why Alkynes are Less Reactive Than Alkenes

49. Predicting direction of equilibrium: • Equilibrium lies toward weaker species

50. Reduction of Alkynes • Hydrogenation reaction • Heterogeneous catalysts • Complete reduction • Partial reduction • Lindlar’s catalyst – syn addition