ORGANIC CHEMISTRY Dr Nam T S Phan Faculty of Chemical Engineering HCMC University of Technology Office: room 211, B2 Building Phone: 38647256 ext 5681 Email: ptsnam@hcmut.edu.vn Chapter 3: INTRODUCTION TO 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) UNIMOLECULAR NUCLEOPHILIC SUBSTITUTION REACTION (SN1) Note: slow step is rate-determining step ELIMINATION REACTIONS In an elimination reaction: + Groups / atoms are eliminated from a reactant + A double bond is formed between the carbons from which atoms are eliminated BIMOLECULAR ELMINATION (E2) Strong base 10 UNIMOLECULAR ELMINATION (E1) 11 Weak base 12 ELETROPHILIC ADDITION REACTIONS (AE) • Electrophilic: electron-seeking / loving • Most electrophiles: + Are positively charged + Have an atom which carries a partial positive charge + Have an atom which does not have an octet of electrons An electrophilic addition reaction is an addition reaction where carbon-carbon double bonds or triple bonds are attacked by an electrophile 13 14 •Not a carbocation, but a cyclic halonium ion • More stable than carbocation 15 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 16 Reaction mechanism Positive center A nucleophile Rate-determining step slow fast 17 Examples: Nucleophiles 18 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 19 Reaction mechanism Rate-determining step An electrophile 20 Examples: An electrophile Rate-determining step 21