Chapter 6 - Nucleophilic substitution of haloalkanes. This chapter presents the following content: Haloalkanes, the C-X bond is polarized, physical properties of R-X, nucleophilic substitution: general, remember acid-base reaction, large number of reactions,. | Chapter 6: Nucleophilic Substitution of Haloalkanes Guanine base of DNA Leaving group Nucleus Nucleophile Toxic Names: Halo- is a substituent to alkane stem 1-Chlorobutane (1S,2R)-1-Bromo-2-fluorocyclohexane 2-Iodo-2-methylpentane C + - X Cl 4 3 2 1 F Br R S I CH3 Haloalkanes The C-X Bond is Polarized Cl CH3 + - Electrophilic Dipole Physical Properties of R-X X gets larger and less electronegative Bonds get longer and weaker along the series F→I Boiling points are higher than for R-H and increase along the series F→I and London forces (polarizability) Nucleophilic Substitution: General Color scheme: Nu, E, L, and curved arrows: e-flow C X Nu + C X Nu - + Nucleophile (Nu) Leaving group (L) Electrophile (E) - Remember Acid-Base Reaction B + H A B H + A - - Conjugate acid When H is attacked, we call the e-rich species a base B. When C (or other nuclei) are attacked, we call it a nucleophile Nu. - - B = Nu Conjugate base Note: no (simple) H º calculations possible on ionic reactions; . | Chapter 6: Nucleophilic Substitution of Haloalkanes Guanine base of DNA Leaving group Nucleus Nucleophile Toxic Names: Halo- is a substituent to alkane stem 1-Chlorobutane (1S,2R)-1-Bromo-2-fluorocyclohexane 2-Iodo-2-methylpentane C + - X Cl 4 3 2 1 F Br R S I CH3 Haloalkanes The C-X Bond is Polarized Cl CH3 + - Electrophilic Dipole Physical Properties of R-X X gets larger and less electronegative Bonds get longer and weaker along the series F→I Boiling points are higher than for R-H and increase along the series F→I and London forces (polarizability) Nucleophilic Substitution: General Color scheme: Nu, E, L, and curved arrows: e-flow C X Nu + C X Nu - + Nucleophile (Nu) Leaving group (L) Electrophile (E) - Remember Acid-Base Reaction B + H A B H + A - - Conjugate acid When H is attacked, we call the e-rich species a base B. When C (or other nuclei) are attacked, we call it a nucleophile Nu. - - B = Nu Conjugate base Note: no (simple) H º calculations possible on ionic reactions; bond strengths refer to homolytic, not heterolytic, dissociation. Large Number of Reactions Tertiary halides are notably absent from this list. Mechanism How do we study it ? Kinetics (rates and their dependence on concentration and temperature) Stereochemistry (R, S, cis, trans) Modify substituents: look for electronic and steric effects Modify reagents/subtrates: Nu, E, L, solvent Kinetics For HO + CH3 Cl CH3OH + Cl - - Rate = k [CH3Cl][ OH] 2nd order Points to bimolecular mechanism, . both starting materials are incorporated in the transition state. Hence name: SN2 bimolecular, nucleophilic substitution - 7/25/2015 © Univesity of California 9 [HO···CH3Cl]‡ ? CH3Cl CH3OH E What is the TS structure? We can look at stereochemistry: Two extreme approaches of Nu : C X Back Front + Cl- + -OH The Transition State ― Bimolecular nucleophilic substitution goes in one-step (. no intermediates). Bond making occurs at the same time as bond breaking. Such a process is called a concerted .
