CHM 301 2018 Lecture 7 1 Delocalization over p orbitals: resonance theory possibly useful video: https://youtu.be/Rfr0q95ln6g Continuous set of p orbitals. Electrons can be delocalized over the p orbital system Three orbital system is most common: Actual molecule is a combination of the resonance structures, weighted according to their stability. Evaluate structures: a. # of bonds b. # of charges c. stabilization of charges (EN of charged atom) OOHROHRHRcarboxylic acid ketone alkeneOORORRHHHHOORORRHHHHHHHHHHHHORHHORHHCHM 301 2018 Lecture 7 2 Extended pi systems: Structure/Acidity relationships: One more factor Especially in considering the acidity of hydrocarbons: Consider the stability of the orbital in which the electron pair of the anion resides. HYBRIDIZATION: "s character" effects HHHHHHHH•••HHHHHHHHHHHHHHHHHHHHHHHHHHHHδδδCHHHHH +CHHHCHM 301 2018 Lecture 7 3 Summary: Steric, resonance, inductive (polarizability) and hybridization effects rationalize acidity of organic compounds. Inductive effects depend on electronegativity of attached or nearby atoms. Polarizable atoms or groups can also allow charge displacement, delocalization through sigma bonds. Resonance effects depend on delocalization through pi bonds and stability of contributing structures. Hybridization effects depend on "%s character" of the orbital bearing the (-) charge. Steric effects show up as inhibition of solvation, usually destabilization of anions. The dominant effect is the delocalization or neutralization of charge. H +C CHHHHC CHHHH +HRHH +RCHM 301 2018 Lecture 7 4 Grandaddy of stabilized hydrocarbon anions: _______________________________________________________________________________ Note: Similar analysis for cation stability. But: Highly unstable—never been observed. MO theory to the rescue—in 302/304 HH-H+HHHHHδ-δ-δ-δ-δ-completely delocalized 5 equivalent resonance structuresHHHHHHHHHHdelocalized cation; stabilizedHHHHHδ+δ+δ+δ+δ+completely delocalized 5 equivalent resonance structuresCHM 301 2018 Lecture 7 5 Now: BASICITY: Lewis base: Capable of donating a pair of electrons to a Lewis acid (or H+) B: + H+ ® [H-B]+ More readily donated = stronger base Less strongly held = more readily donated :B- + H+ ® [H-B] Less stabilized charge = stronger base Measure basicity: (a) related to pKa of the conjugate acid (higher pKa = weaker conjugate acid = stronger base) i.e., pKa of [H-B] relates to the base strength of :B-. (b) can define pKb which relates directly to base strength in water B: + H HBKeqKb =[B-H] [HO ][B:] [H2O]=[B:] [H ][B-H][H2O][H-O ] [H ]Kb =Ka(BH)1Ka(H2O)pKb = - pKa(BH) + 14pKb(B:) = 14 - pKa(BH+)CHM 301 2018 Lecture 7 6 Example: ammonia is a prototype amine base Amines are the most common organic bases: (pKa of the conjugate acid) Data: pKa :NH3 9.3 (CH3)NH2 10.6 Hybridization effects, "s" character analysis: :NH3 + H [ NH4 ]pKa 9.3 [pKb 4.7]higher pKa = weaker or stronger conjugate acid? weaker or stronger base?RCNRCRNRRCRNRRRIIIIII N: pKa 5.2 N N pKa 13.5 R C N:pKa ca -10 NO NOOpKa 18CHM 301 2018 Lecture 7 7 extreme in nitriles Intermediate in imines: imine iminium ion Pyridine: RCN:H+RCNpKa -10pKb 24NRRR+H+NRRRNRRRpKa 3N:N N:CHM 301 2018 Lecture 7 8 Resonance Effects: aniline derivatives Benzyl amine: Pyrrole: NH2+ HNHHHNH2NH2NH2NH2pKa 4.6NH2+ HNH3NH+ HNHNHNHNHNHNHNHNHCHM 301 2018 Lecture 7 9 Amides: consequences: 1. very polar, highly self-associated bp 221 O 118 O mp 82 O 17 O 2. restricted rotation about C-N bond? RONRHRONRHRONRHH3CONHHH3CONHHRONRHRONRHRONRHRONRHRONRHCHM 301 2018 Lecture 7 10 3. basicity of N atom: pKa ca 0 to -1 (for protonated amide group) 4. in fact, somewhat acidic if H on N: pKa ca 16 Resonance Effects. N analog of carboxylic acid RONRHRONRHRONRHRONRHRONRHRONRHNNpKa 13.5NNNNNNNNCHM 301 2018 Lecture 7 11 Special case: NNimidazole+ HNHNNHNNHHNNHNHH2NCO2HNHNhistidineH2NNHNhistamineH2NNHCO2HNHNH2H+H2NNHCO2HNHNH2H2NNHCO2HNHNH2H2NNHCO2HNHNH2NO NOOpKa 18NO NOOCHM 301 2018 Lecture 7 12 Oxygen functional groups: Most important conclusion is that the oxygen in an ether, alcohol, or carbonyl group can serve as a (very) weak base. This will be important in considering reactions of these groups. Nitrogen anions: Where to find a strong enough base to remove a proton from an amine nitrogen???? O+ HOHO+ HOHHHO+ HOHOHNH2amide anionNHHH+ strong baseCHM 301 2018 Lecture 7 13 Special reactions to generate strong bases: Reduction of amines: Insertion of alkali metals into carbon-halogen bonds: So: Other common bases: NH3 + Na0Na NH2 + H2 (like H2O + Na0)Br CH3 + LiLiCH3=LiCH3pKa ca 50I+ MgMg-IMg-ILi CH3 + R2NHCH4 + R2N LiNH+H3CLiNLiNLilithium diisopropylamideNa HO