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Chapter 9 Alkynes

Chapter 9 Alkynes. 9.1 Sources of Alkynes. +. H 2. +. H 2. HC. CH. CH 2. H 2 C. Acetylene. Industrial preparation of acetylene is by dehydrogenation of ethylene. 800°C. CH 2. H 2 C. CH 3 CH 3. 1150°C. cost of energy makes acetylene a more

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Chapter 9 Alkynes

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  1. Chapter 9Alkynes

  2. 9.1Sources of Alkynes

  3. + H2 + H2 HC CH CH2 H2C Acetylene • Industrial preparation of acetylene isby dehydrogenation of ethylene 800°C CH2 H2C CH3CH3 1150°C cost of energy makes acetylene a more expensive industrial chemical than ethylene

  4. 9.2Nomenclature

  5. Acetylene and ethyne are both acceptableIUPAC names for HC CH HC CCH3 HC CCH2CH3 Propyne (CH3)3CC CCH3 Nomenclature Higher alkynes are named in much the sameway as alkenes except using an -yne suffixinstead of -ene. 1-Butyne 4,4-Dimethyl-2-pentyne

  6. 9.3Physical Properties of Alkynes The physical properties of alkynes are similar to those of alkanes and alkenes.

  7. 9.4Structure and Bonding in Alkynes:sp Hybridization

  8. 120 pm H C C H 106 pm 106 pm Structure • linear geometry for acetylene 121 pm C CH3 C H 146 pm 106 pm

  9. Cycloalkynes • Cyclononyne is the smallest cycloalkyne stable enough to be stored at room temperaturefor a reasonable length of time. • Cyclooctyne polymerizeson standing.

  10. sp Hybridization in Acetylene Mix together (hybridize) the 2s orbital and one of the three 2p orbitals 2p 2p 2sp 2s

  11. sp Hybridization in Acetylene Mix together (hybridize) the 2s orbital and one of the three 2p orbitals 2p Each carbon has two half-filled sp orbitalsavailable to form s bonds. 2sp

  12. s Bonds in Acetylene Each carbon isconnected to ahydrogen by as bond. The twocarbons are connectedto each other by as bond and two p bonds. Figure 9.2 (a)

  13. p Bonds in Acetylene One of the twop bonds in acetylene isshown here.The second pbond is at rightangles to the first. Figure 9.2 (b)

  14. p Bonds in Acetylene This is the secondof the twop bonds in acetylene. Figure 9.2 (c)

  15. Figure 9.3 Electrostatic Potential in Acetylene The region of highest negative charge encirclesthe molecule around itscenter in acetylene. The region of highest negative charge lies aboveand below the molecular plane in ethylene.

  16. Ethane Ethylene Acetylene C—C distance 153 pm 134 pm 120 pm C—H distance 111 pm 110 pm 106 pm H—C—C angles 111.0° 121.4° 180° C—C BDE 368 kJ/mol 611 kJ/mol 820 kJ/mol C—H BDE 410 kJ/mol 452 kJ/mol 536 kJ/mol hybridization of C sp3 sp2 sp % s character 25% 33% 50% pKa 62 45 26 Table 9.1Comparison of Ethane, Ethylene, and Acetylene

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