Electronic Structure The sigma bond is sp-sp overlap. Copyright2005 Pearson Prentice Hall.Inc The two pi bonds are unhybridized p overlaps at90°，which blend into a cylindrical shape. H Ho bond G hond Co bond H => overlap of p orbitals cylinder of electron density 11 Copyright 2005 Pearson Prentice Hall,Inc
Chapter 9 11 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. =>
Bond Lengths More s character,so shorter length. Three bonding overlaps,so shorter. 1.54A 1.33A -1.20A H H一C三C一H H 1.09A 1.08A 1.06A ethane ethene ethyne Copyright2005 Pearson Prentice Hall,Inc. Bond angle is180°，so linear geometry. Chapter 9 12
Chapter 9 12 Bond Lengths • More s character, so shorter length. • Three bonding overlaps, so shorter. Bond angle is 180, so linear geometry. =>
Acidity of Alkynes Terminal alkynes,R-C=C-H,are more acidic than other hydrocarbons. ·Acetylene-→acetylide by NH2,but not by OH-or RO. More s character,so pair of electrons in anion is held more closely to the nucleus.Less charge separation,so more stable. > Chapter9 13
Chapter 9 13 Acidity of Alkynes • Terminal alkynes, R-CC-H, are more acidic than other hydrocarbons. • Acetylene → acetylide by NH2 - , but not by OH- or RO- . • More s character, so pair of electrons in anion is held more closely to the nucleus. Less charge separation, so more stable. =>
H Acidity Table H weakest H-C-C-H H-C-C© sp3 25% 50 acid HH H、 H H C=C C=C p2 33% 44 H H H H :NH3 :NH2 (ammonia) 35 H一C≡C一H H-C=C⑧ sp 50% 25 stronger R一OH R-0: (alcohols) 16-18 acid Copyright2005 Pearson Prentice Hall,Inc. => Chapter 9 14
Chapter 9 14 Acidity Table =>