Chapter 3: COMMON REACTION MECHANISMS potx

ORGANIC CHEMISTRY Dr Nam T S Phan Faculty of Chemical Engineering HCMC University of Technology Office: room 211, B2 Building Phone: 8647256 ext 5681 Email: ptsnam@hcmut.edu.vn Chapter 3: COMMON REACTION MECHANISMS Reaction mechanism: the description of the step-by-step process by which reactants are changed / converted into products Nucleophilic substitution Nucleophilic addition Electrophilic substitution Electrophilic addition Elimination NUCLEOPHILIC SUBSTITUTION REACTIONS (SN) • A nucleophile: an electron-rich species that can form a covalent bond by donating electrons to a positive center • A nucleophile is any negative / neutral molecule that has unshared electron pair • Substitution reaction: chemical reaction in which atom / group replaces another atom / group in the structure of a molecule • In a nucleophilic substitution reaction, a nucleophile attacks / bonds with the positive center BIMOLECULAR NUCLEOPHILIC SUBSTITUTION REACTION (SN2) Stereochemistry of SN2 reactions • The nucleophile attacks from the back side / the side directly opposite the leaving group • This attacks causes an inversion of configuration UNIMOLECULAR NUCLEOPHILIC SUBSTITUTION REACTION (SN1) Note: slow step is rate-determining step 10 Stereochemistry of AE reactions 48 49 Markovnikov’s rule 50 Carbocation rearrangement in AE More stable 51 More stable 52 Relative reactivities of alkenes in AE O2N H H HOOC < H H CH3 H H H H3C < H H H H3C < Cl H H H H < H3C < H CH3 H3C < CH3 H3C CH3 CH3 53 NUCLEOPHILIC ADDITION REACTIONS (AN) The carbonyl group is polar because the oxygen, being more electronegative, has greater share of double-bond electrons The partial positive carbon can be attacked by nucleophiles The addition of nucleophiles to the carbon atom of the carbonyl group in nucleophilic addition reactions 54 Reaction mechanism Positive center A nucleophile Rate-determining step slow fast 55 Relative reactivities in AN NO2 CH2 C O H > H3C C CH3 o > H3C C Oo > Cl CH2 C > H3C C OR o O H > CH3 C O H > H3C C NH2 o Eletronic & steric effetcs 56 ELECTROPHILIC SUBSTITUTION REACTIONS (SE) In an electrophilic substitution reaction, an electrophile substitutes for a hydrogen of an aromatic compound Although benzene has double bonds, the overall reaction is electrophilic substitution rather than electrophilic addition 57 Reaction mechanism Rate-determining step An electrophile 58 Nonaromatic, not stable, not formed 59 +I, +C, +H Relative reactivity - I, -C, Electron-donating Eelectron-withdrawing Activating Deactivating ortho/para-directing meta-direcring 60 61 62 .. .Chapter 3: COMMON REACTION MECHANISMS Reaction mechanism: the description of the step-by-step process by which reactants... E2 reactions • Anti-elimination is highly favored in an E2 reaction groups / atoms are removed from opposite sides of C-C bond 31 Regioselectivity of E2 reactions Zaitsev’s rule for an E2 reaction: ... UNIMOLECULAR NUCLEOPHILIC SUBSTITUTION REACTION (SN1) Note: slow step is rate-determining step 10 11 Stereochemistry of SN1 reactions 12 However, few SN1 reactions occur with complete racemization