Essential Organic Chemistry Paula Yurkanis Bruice Chapter 7 Delocalized Electrons and Their Effect on Stability, Reactivity, and pKa

Localized Electrons vs. Delocalized Electrons Electrons that are restricted to a particular region. Either belong to a single atom or are share by two stoms

Localized Electrons vs. Delocalized Electrons Neither belong to a single atom nor are confined to a bond between two atoms shared by three or more atoms. 離域電子並非侷限於分子中的單一原子或單一共價鍵上，它是在一原子與其鄰近的數個原子所共同組成的分子軌域範圍內的電子。離域電子在雙鍵的共軛系統(conjugated systems)、芳香族(aromatic)和中離子(mesoionic)系統內可以見到。

7.1 The Structure of Benzene The Lewis representation of benzene (C6H6) : six-membered ring of carbon atoms that are held together by alternating single and double bonds. unusually stable compound C: 6 • 4 = 24 valence electrons H: 6 • 1 = 6 valence electrons Total = 30 electrons = 15 bonds 12 molecules share 15 bonds

What is the Structure of Benzene? C6H6 compare with acyclic alkane (C6H14) Each ring and π bond results in two fewer hydrogen than an acyclic alkane Total number of rings and π bonds in benzene is four

Benzene This implies that we should observe alternating short (1.33 Å) and long (1.54 Å) bond lengths. Modern analytical techniques showed that all bond lengths are the same (1.39 Å). must have the same number of electrons between the carbon atoms only if the π electrons are delocalized around the ring.

Benzene Since both representations are equal, in average each C-C bond has a bond character in between a single and a double bond. Measured bond length is: 1.39 Å

Only Two Disubstituted Products

Benzene H 2 x X A B This also explains why replacing two of the six hydrogens produces only three isomers In case of alternating double and single bonds, A and B should be different and we should have four isomers.

7.2 Benzene Has Delocalized Electrons Two sp2 orbital to bond to two other C and has bond angles of 120o. The third sp2 orbital overlap the s orbital of H. Benzene is planar and six p orbitals are parallel. P orbitals are close enough for side-to-side overlap. Overlapping p orbitals form a doughnut-shaped cloud of electrons above and below the benzene ring. All C-C bonds have the same electron density. six π electrons of benzene are delocalized

7.3 Resonance Contributors and Resonance Hybrid Resonance (electrons are delocalized). Dashed lines inside benzene do not show how many π electrons are in the ring. The approximate structure with localized electrons is called a resonance contributor. The actual structure with delocalized electrons is called a resonance hybrid.

Resonance Contributors and Resonance Hybrid Electron delocalization occurs only if all the atoms sharing the delocalized electrons lie in or close to the same plane, so that their p orbitals can effectively overlap. 135o Cyclooctatetraene is not planar. Eight electrons are localized in four double bonds C-C bonds do not all have the same length

7.4 How to Draw Resonance Contributors Delocalized electrons result from a p orbital overlapping the p orbitals of more than one adjacent atom. Two carbon-oxygen bonds are identical 共振混成體

Rules for Drawing Resonance Contributors Only electrons move. Aatoms never move. Only π electrons or lone-pair electrons can be moved (Never move σ electrons). The total number of electrons in the molecule doesn’t change. All resonance contributors have the same net charge.

Rules for Drawing Resonance Contributors Electrons are always moved toward an sp2 carbon. An sp2 carbon is either a positively charged carbon or a double-bonded carbon. Move π Electrons to an sp2 Carbon

Rules for Drawing Resonance Contributors Electrons cannot be moved toward an sp3 carbon, because an sp3 carbon cannot accept any more electrons, it has a complete octet.

Rules for Drawing Resonance Contributors

Rules for Drawing Resonance Contributors Moving lone-pair electrons toward an sp2 carbon. The sp2 carbon can accommodate the new electrons by breaking a π bond.

Rules for Drawing Resonance Contributors electrons move toward the most electronegative atom (O). Moving Electrons away from the most Electronegative Atom: the only way electrons can be moved.

Problem 1: a. Which of the following compounds have delocalized electrons? A. C3CH=CHCH=CHCH2 B. CH3CH2NHCH2CH=CH2 C. D. E. b. Draw the resonance contributors for the compounds that have delocalized electrons. B, D, E: lone-pair electrons cannot be moved toward an sp3 carbon + ..

7.5 Predicted Stabilities of Resonance Contributors All resonance contributors do not necessarily contribute equally to the resonance hybrid. The greater the predicted stability of the resonance contributor, the more it contributes to the structure of resonance hybrid, and the more similar the contributor is to the real molecule.

Predicted Stabilities of Resonance Contributors More stable Structure B is less stable than A because: 1. One of its oxygen has a positive charge. 2. It has “separated charges” (it takes energy).

Predicted Stabilities of Resonance Contributors Structures C and D are equally stable.

Predicted Stabilities of Resonance Contributors More stable E is more Stable than F F has separated charges, with N holding a positive charge.

Predicted Stabilities of Resonance Contributors More stable H is more Stable than G Oxygen better accommodates the negative charge

Problem 2 and 3: Draw resonance contributors for each of the following species and rank them in order of decreasing contribution to the hybride. Draw the resonance hybrid for each of the species. a. A is more stable than B. A: positive charge is on a tertiary carbon B: positive charge is on a secondary carbon

Problem 2 and 3: b. c. A is more stable than B. CH3 OCH3 O ll l A is more stable than B. B: has separated charges and has a positive charge on an oxygen A is more stable than B. A: negative charge on an oxygen (electronegative) B: negative charge on a carbon

Problem 2 and 3: d. e. A is stable than B and B is stable than C. A: do not have separated charges C: electronegative oxygen is closer to the positive charge C CH3 NHCH3 +OH ll l A is more stable than B. A: positive charge is on the less electronegative atom (N)

Problem 2 and 3: f. A and B are equally stable

7.6 Delocaliztion Energy is The Additional Stability Delocalized Electrons Give to a Compound Delocalized electrons stabilize a compound. Electron delocalization is also called resonance. Delocalization energy is also called resonance energy. The resonance hybrid is more stable than any of its resonance contributors is predicted to be.. The greater the number of relatively stable resonance contributor, the greater is the resonance stabilization. The delocalization energy is greater for the carboxylate ion than for the carboxylic acid.

The greater the number of relatively stable resonance contributor, the greater is the resonance stabilization. the more nearly equivalent the structures of the resonance contributors, the greater the delocalization energy Carbonate dianion is very stable

Problem 4: a. Predict the relative bond lengths of the three carbon-oxygen bonds in the carbonate ions (CO32-). b. What would you expect the charge to be on each oxygen? a. All the carbon-oxygen bonds in the carbonate ions should be the same length. b. The two negative charges are shared equally by the oxygens, each oxygen will have 2/3 of a negative charge.

Problem 5: Rank the following species in order of decreasing delocalization energy Two equivalent resonance contributors The greatest delocalization energy Three equivalent resonance contributors The least delocalization energy Two resonance contributors have different stabilities

Problem 6: Which species has the greater delocalization energy CH2=CH-CH=CH2 The acetate ion has the greater delocalization energy (Two equivalent resonance contributors)

7.7 Delocalized Electrons Increase Stability Stability of Dienes Dienes二烯 : hydrocarbons with two double bonds. Isolated deines: are separated by more than one single bond. Conjugated dienes: have conjugated double bonds, each separated by one single bond. π Electrons in an isolated diene are localized, while π electrons in a conjugated diene are delocalized. Conjugated dienes are more stable than isolated dienes.

Problem 7: Which is more stable, 2,4-heptadinen or 2,5 heptadiene 2,4-heptadinen is more stable because it has conjugated double bond. Problem 8: Name the following dienes and rank them in order from most stable to least stable. 2,5-dimethyl-2,4-hexadiene > 2,4-hexadiene > 1,3-ptntadiene > 1,4-ptntadiene

The Effect of Delocalized Electrons on Stablility Stability of Allylic and Benzylic Cations allylic cation (烯丙基碳陽離子): carbocation with a positive charge on an allylic carbon which is adjacent to an sp2 carbon of an alkene benzylic cation: carbocation with a positive charge on an benzylic carbon which is adjacent to an sp2 carbon of a benzene ring

The Effect of Delocalized Electrons on Stablility Because allylic cation and benzylic cation have delocalized electrons, they are more stable than other primary carbocations. Two resonance contributors Five resonance contributors

The Effect of Delocalized Electrons on Stablility Relative stabilities of carboncations

Problem-solving Strategy Which carbocation is more stable? The carboncation is shared by a primary allylic carbon and a secondary allylic carbon The carboncation is shared by a primary allylic carbon and a tertiary allylic carbon more stable

Problem 9: Which carbocation in each of the following pairs is more stable? a. b. c. Secondary allylic Secondary benzylic tertiary benzylic

Problem 10: Which species is more stable? a. b. More stable Nitrogen is less electronegative More stable Negative charge is delocalization

7.8 Delocalized Electrons Affect pKa Value Carboxylic acid is stronger acid than an alcohol (2.7) Carboxylate ion (conjugate base of the carboxylic acid) is more stable (weaker) base than the alkoxide ion (conjugate base of the an alcohol) > Inductive electron withdrawal & electron delocalization  Stabilized the carboxylate ion All the electrons are localized

7.8 Delocalized Electrons Affect pKa Value Phenol (酚), OH group is bounded to a benzene ring, is a stronger acid than an alcohol such as cyclohexanol or ethanol No Electron Delocalization Separated charges More stable The base (phenolate ion) has greater delocalization energy than the conjugate acid (phenol).

7.8 Delocalized Electrons Affect pKa Value Protonated aniline (苯胺) is a stronger acid than protonated cyclohexylamine No Electron Delocalization Lack a lone pair can be delocalized The base (aniline) has greater delocalization energy than the conjugate acid (Protonated aniline ).

7.8 Delocalized Electrons Affect pKa Value

> Problem-solving Strategy Determining Relative Acidities Which is a strong acid? CH3CH2OH Ethyl alcohol CH2=CHOH Vinyl alcohol > CH3CH2-O-: : CH2=CH-O-: : -CH2-CH=O: All the electrons in ethanol’s conjugate base are localized Vinyl alcohol’s conjugate base is stabilized by electron delocalization.

Problem 11: Which member of each pair is the stronger acid? a. b. c. Electron delocalization stabilized the base

Problem 12: Which member of each pair is the stronger base? a. Ethylamine or aniline b. Ethylamide or ethoxideion c. Phenolate ion or ethoxide ion CH3-CH2-NH2 More stable, less base More stable, less base Negative charge oxygen is stronger base than the negative charge nitrogen

7.9 Electronic effects Electron-withdrawing groups stabilize a base Increase the strength of its conjugate acid Electron-donating groups destabilize a base decrease the strength of its conjugate acid (section 2.7) The stronger the acid, the more stable its conjugate base

7.9 Electronic effects Inductive electron-withdrawal A substituent more electronegative than a hydrogen withdraws σ electrons inductively from the benzene ring Electron donation by hyperconjugation An alkyl group (CH3) donates electrons into the ring by hyperconjugation Electron donation by resonance A lone pair on an atom directly attached to the ring donates electrons by resonance Electron withdrawal by resonance An atom directly attached to the ring that is doubly or triply bonded to an electronegative atom withdraws electrons by resonance .

Substituents on the Benzene Ring Affect the pKa of Benzoic Acid Electron donation by resonance Hyperconjugation Weaker acid Inductive electron-withdrawal Electron withdrawal by resonance electron donating groups decrease the acidity (destabilize the conjugate base) electron withdrawing groups increase the acidity (stabilize the conjugate base)

electron donating groups decrease the acidity (destabilize the conjugate base) electron withdrawing groups increase the acidity (stabilize the conjugate base)

Substituents on the Benzene Ring Affect the pKa of Protonated Aniline electron donating groups decrease the acidity (destabilize the conjugate base) electron withdrawing groups increase the acidity (stabilize the conjugate base)

Problem 13:

Problem 14: