Electronic Structure H The sigma bond is sp-sp overlap H H spX sp The two pi bonds are unhybridized p overlaps at 90, which blend into a cylindrical shape H H =(H bond>C C<o bond H overlap of p orbitals cylinder of electron density
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. =>
H H Bond Lengths More s character, so shorter length Three bonding overlaps so shorter 1.54A 1.33A 1.20A H日 H C=C H一C≡C—H H HH HH 1.09A 1.08A 1.06A ethane ethene ethyne Bond angle is 180, 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. =>
H H Acidity of Alkynes Terminal alkynes, R-C=C-H, are more acidic than other hydrocarbons Acetylene>acetylide by Ni2 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 > Chapter 9 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 H Acidity Table Compound Conjugate Base Hybridization s Character HH HH akes H-C-C-H H-C-C<D 25% 50 acid HH H H H H H C=C 33% 44 H H H :NH3 (ammonia) 3 H-C≡C-HH-C≡C<9 50% 25 stronger R-OH R-O: (alcohols 16-18 Chapter 9 14
Chapter 9 14 Acidity Table =>
H H Forming Acetylide lons H+ can be removed from a terminal alkyne by sodium amide, NaNH2 CHC≡C-H+NaNH2 CH3C=≡C:Na+NH3 NaNH2 is produced by the reaction of ammonia with sodium metal H H H-n-h ys H一:+j5H G +3 Chapter 9 15
Chapter 9 15 Forming Acetylide Ions • H+ can be removed from a terminal alkyne by sodium amide, NaNH2 . CH3 C C H + NaNH2 CH3 C C: - Na + + NH3 • NaNH2 is produced by the reaction of ammonia with sodium metal. N H H H + Na Fe +3 N + 1/ 2 H2 H H Na + =>