Handout #3, 5.12 Spring 2003, 2/12/03 Physical Properties: Bond Length, Bond Strength & Acidity A. Bond Lengths: mostly dependent on atomic size, bond order, and hybridization Bond Lengths (Å) 3 3 sp –sp C C 1.54 sp2–sp2 C C 1.34 sp–sp C C 1.20 3 sp –sp 3 • Multiple Bonding: Bond length depends strongly on bond order (length: single > double > triple) C O 1.42 sp2–sp2 C O 1.22 Bond Lengths (Å) sp3 C H 1.09 sp2 C H 1.086 sp C H 1.06 sp3–sp3 C C 1.54 sp3–sp2 C C 1.50 sp3–sp C C 1.47 • Effect of hybridization on length of single bonds: C–H and C–C bonds shorten slightly with increased s character on carbon B. Bond Strengths/Bond Dissociation Energies (BDEs): Energy for homolytic bond cleavage to uncharged radical fragments X. X X Common Bond Strengths (kcal/mol) C C 81 C O 79 C C 145 C O 173 C C 198 C H 98 X. • Bond strengths are bond energies for a certain bond averaged over many different molecules. • Bond dissociation energies are for a particular molecule and are dependent on the specific molecular structure (Bond Strength ± 20 kcal/mol) • Multiple Bonding: Bond strength depends strongly on bond order (strength: single < double < triple) Data taken from: Advanced Organic Chemistry, 3rd Edition, F. A. Carey and R. J. Sundberg C. Acidity of Organic Molecules Functional Group Acid Approximate pKaValues Conjugate Base (in water) increasing basicity alkane-sp3 H–CH3 48 – CH3 alkene-sp2 H–CH=CH2 44 – CH=CH2 H–NH2 38 – NH2 hydrogen H–H 35 – H alkyne-sp H–C≡CH 25 – C≡CH alcohol H–OCR3 17 – OCR3 amine water H–OH 15.7 – OH thiol H–SR 10–11 – SR ammonium H–+NR3 10–11 NR3 nitrile (cyanide) H–C≡N 9.2 – C≡N phenol H–OAr 8–11 – OAr H–OC(O)R 4–5 – OC(O)R H–F 3.17 – F H–+OH2 –1.74 carboxylic acid hydronium OH2 H–Cl –7 – Cl H–I –10 – I increasing acidity • Acidity increases across a row: H–C < H–N < H–O < H–F (electronegativity) • Acidity increases down a period: H–F < H–Cl < H–Br < H–I (size) • Neutral species less acidic than corresponding positively charged species: H–OH < H–+OH2 pKa data from: Advanced Organic Chemisry, 4th Ed., J. March