140B ch13 lecture

140B ch13 lecture - CHAPTER 13 Alkynes: The Carbon-Carbon...

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CHAPTER 13 Alkynes: The Carbon-Carbon Triple Bond
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Properties and Bonding in the Alkynes 13-2 Alkynes are relatively nonpolar. Corresponding alkynes, alkenes and alkanes have very similar boiling points. Ethyne: sublimes at -84 o C Propyne: b.p. -23.2 o C 1-Butyne: b.p. 8.1 o C 2-Butyne: b.p. 27 o C Medium sized alkanes: distillable liquids Alkynes polymerize easily, frequently with violence.
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Ethyne is linear and has strong, short bonds. The two carbons in ethyne are sp 2 hybridized. The ! bonds are diffuse and resemble a cylindrical cloud:
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The strength of a C-C triple bond is about 229 kcal mol -1 . As with alkenes, the strength of the ! bonds is much weaker than that of the " bond which gives rise to much of the chemical reactivity of the alkyknes. The C # C bond length is 1.20 Å (C=C is 1.33 Å). The C-H bond lengths are also shorter than in ethene due to the larger degree of s character in the sp hybrid bonds (as compared to sp 2 ).
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Alkynes are high-energy compounds. Alkynes often react with the release of considerable amounts of energy (prone to explosive decomposition). Ethyne has a heat of combustion of 311 kcal mol -1 which is capable of generating a flame temperature >2500 o C, sufficient for use in welding torches.
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The heats of hydrogenation of alkyne isomers can be used to determine their relative stabilities: The greater relative stabililty of internal alkynes is due to hyperconjugation.
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Terminal alkynes are remarkably acidic. The electronegativity of a carbon atom depend upon its hybridization. The more s character in its hybrid orbitals, the greater the electronegativity. The acidity of a C-H bond is directly related to the electronegativity of the carbon atom:
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alkyllithiums and Grignard reagents can deprotonate terminal alkynes to the corresponding alkynyl anions. Alkynyl anions can react as bases and nucleophiles, much like
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140B ch13 lecture - CHAPTER 13 Alkynes: The Carbon-Carbon...

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