Structure and Reactivity: Prerequired Knowledge

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1 Structure and eactivity: Prerequired Knowledge!!! The concepts presented in this summary are required for lecture and examination!!! 1. Important Principles in rganic Chemistry In general, structures which can stabilize electrons are favored. 1.1 Electronegativity and "ctet" ule The electronegativity describes the ability of atoms to attract electrons. The nearest to octet (closed shell) the strongest the electronegativity. The electronegativity is weaker for larger atoms. n chemical structures and during chemical reactions, the electrons go to the more electronegative element. example:acidity 3 C 2 F pk a 53 > 36 > 16 > 3.2 The conjugate is more stable for more electronegative atoms! electronegativity Indirect effect: inductive effect F 3 C pk a 0.25 < 4.76 Transfer of the influence of electronegative atoms through sigma bonds. Effect of hybridization: electron in orbitals with more s characters are more stabilized (more probability next to the nucleus) example: acidity 3 C pk a ybridization 3 S P SP 2 SP 1.2 Stabilization through Delocalization: delocalized charges (electrons) are more stable Delocalization onto 1 atom: large atoms are more able to stabilize charges (= polarizable) examples F Br I 1) acidity: pk a 3.2 > 8 > 9 > 10 electronegativity Delocalization is more important than electronegativity in this case! 2) leaving group ability in substitution reaction F < < Br < I

2 1.2.2 Delocalization on two atoms: the chemical bond Important for organic chemistry: examples: 1) bond is stronger than bond for C=C bond, but not for C= and C= 2) Delocalization is better between atoms of the same size (orbital overlap) 3) For strong polar bonds: ionic part can become important and compensate the weaker covalent bonds: prediction is more difficult bond: energy in Kcal C 99 CC 83 C 70 C 86 CF 117 CF 117 size effects C 81 CI 52 bonds C=C C= C= energy in Kcal C= 92 C=S 49 Important practical consequence: in organic chemistry, losing C=C and making C= is often favorable! Delocalization on more than two atoms: resonance structures esonance structure = obtained by moving electron without changing position or connectivity of atoms esonance structures are essential to understand structure and reactivity in organic chemistry! good resonance structure octet rule for 1.raw elements, no charges, charges on electronegative atoms, more bonds, "better bonds", aromatic structures stabilization through resonance (delocalization) is maximal if the resonance structures are identical "reality" = weighted sums of the resonance structures key examples: best structure reactivity of carbonyls best structure conjugate reactivity identical structures acidity of carboxylic acid acidity of carbonyls and reactivity of enolates identical structures best structure structure of nitro and diazo compounds

3 Stabilization of carbocations by neighbouring heteroatoms or bonds Limitation of resonance structure: descripition of aromaticity aromatic stabilization: cyclic conjugate system with 4n2 electrons (ückel's rule) identical structure stabilized identical structure? destabilized! igher level model is needed: orbital theory Benzene Cyclobutadiene Benzene: all electrons stabilized in bonding orbitals LUM Cyclobutadiene: 2 electrons in nonbonding orbitals, no stabilization M LUM = Lowest Unoccupied Molecule rbital M = ighest ccupied Molecule rbital In organic chemistry, many observations can be explained by FM (Frontier Molecular rbital, LUM and M) considerations. This model is more powerful and precise than resonance/lewis structure considerations, but need more time to apply. Test for part 1 2 assify this carbonyl compounds in order of increasing reactivity towards nucleophile addition.

4 2. Important ucleophiles and Electrophiles in Bachelor Level rganic Chemistry 2.1 ucleophiles eteroatoms 2 S P alcohols amines thiols phosphines hydrazines S alcoholates amides thiolates halogen anions neutral charged ydrides ( ) sources ab 4 LiAl 4 borohydrides aluminium hydrides C ucleophiles M M MgBr Li M=a,K,Li Grignard alkyl lithium enolates stabilized enolates 1 2 enamines C cyanide ED ED electronrich aromatic compounds and alkenes ED = Electrondonating group 2.2 Electrophiles M proton metals alkyl halogenide carbonyls imines iminiums EWG EWG electronpoor aromatic compounds and alkenes EWG = Electronwithdrawing group 2.3 ard and Soft classification of ucleophiles (Lewis Bases) and Electrophiles (Lewis Acids) ard charged, localized electrons, highly electronegative/positive, reaction under charge control typical hard electrophiles:,mg 2,, Tf typical hard nucleophiles: MgBr,Li,,,F, atom of enolates Soft less charged, delocalized electrons, reaction under orbital (MLUM) control typical soft electrophiles: Pd 2, carbonyls, electronpoor double bonds and aromatic compounds typical soft nucleophiles: C atom of enolates, stabilized enolates, electronrich double bonds and aromatic compounds, I, 2,P 3 Principle: ardard and SoftSoft interactions are favored! Competition basicitynucleophilicity: especially hard nucleophiles are usually also strong s, because proton is hard (hardhard interaction)

5 3. Important assical eactions in Bachelor rganic Chemistry 3.1 Substitution eactions S u u 1 u 2 3 racemization S u u u 2 3 inversion S E Ar El El El better for electrondonating in orthopara S Ar u u El better for electronwithdrawing in orthopara 3.2 Elimination eactions E 1 E 2 E 1cb 3.3 Addition to double bonds In principle: all mechanisms for elimination are possible in the reverse sense! Special case: dibromination Br 2 Br Br Br bromonium ion Br

6 3.4 Chemistry of Carbonyls ucleophile Addition Grignard 1 2 MgBr BrMg ydride eduction a B ab ab 4 no reaction Li Al 3 1 LiAl 4 1 Li 1 Al 3 LiAl 4 then 2 Alcohols: acetal formation under acidic catalysis Equilibrium is on the side of the ketone It is necessary to remove water to drive reaction! Acetal Alcohols: ester formation under acidic catalysis Equilibrium is nearly 1:1 Use excess alcohol to drive reaction

7 Ester ydrolysis Stability of carboxylate completely on the side of product! Amine addition and imine formation Water removal necessary for ketones, reaction easier with aldehyde Enolate chemistry Enolate alkylation 1 1 soft nucleophile 1 hard nucleophile hard electrophile 2 Tf 2 I soft electrophile Alkylation CAlkylation Aldol eaction Aldol product or acid 2 Aldol Condensation 1 2 aisen Condensation ther related reactions: Knoevenagel condensation, Perkin condensation, Dieckmann condensation.

8 Wittig lefination 1 B A PPh PPh 3 cisolefin 1 A not stabilizing 1) PPh 3 2) PPh 3 B PPh 3 betaine intermediates 1 1 PPh 3 1 transolefin B stabilizing anion For nonstabilized ylides (A), formation of the cis betaine is favored and irreversible, leading to cis olefin. For stabilized ylides (B), an equilibrium lead to formation of the more stable trans betaine and finally to trans olefin. 3.5 xidation eactions Chrom(VI) Cr 3 2 S 4 Jones eagents VI Cr IV Cr 2 2 Cr 3 Second oxidation step possible only in presence of water! IV Cr VI Cr In practice also often used PDC (pyridinium dichromate), PCC (pyridinium chlorochromate) MoffatSwern DMS, (C) 2 chanism S S S C 2 C S S S In practice, there are many more methods!

9 Dihydroxylation, zonolysis and Epoxidation Epoxidation mcpba mcpba = metachloroperbenzoic acid Dihydroxylation s 4 zonolysis 1) 3 2) 2 S These reactions will be rediscussed in more details in the lecture

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