Chapter 6

Halogen, X, is directly bonded to sp3 carbon

X is bonded to sp2 carbon of alkene

X is bonded to sp2 carbon on phenyl ring

methylene halide

haloform

carbon tetra halide

two halogen atoms are bonded to the same carbon

two halogen atoms are bonded to adjacent carbons

• Allylic radical is resonance stabilized • Bromination occurs with good yield at the allylic position (sp3 C next to C=C)

• Bimolecular nucleophilic substitution • Concerted reaction: new bond forming and old bond breaking at same time • Rate is first order in each reactant • Inversion at carbon

• The SN2 reaction is a one-step reaction • Transition state is highest in energy

SN2 stands for: substitution, nucleophilic, bimolecular

TWO species are involved in rate determining step

Substrate is PRIMARY and UNHINDERED GOOD (strongly basic) Nucleophile GOOD (weakly basic) Leaving Group POLAR, APROTIC Solvent

Second order reaction: Rate = k [:Nu] [substrate] Suggests that C-Nu bond forms as C-X bond breaks Requires backside attack of nucleophile Trigonal bipyramidal transition state

• Backside attack of nucleophile results in INVERSION of configuration • The stereochemistry is always reversed, but the R/S does not always switch. Depends on the priority of the nucleophile and substituents on the stereocenter.

Inversion of Configuration Required by Orbitals

methyl > primary > secondary >> tertiary (doesn’t happen)

As substitution increases, steric crowding increases. Approach of the nucleophile is increasingly hindered.

Nucleophilicity increases across a period (R to L).

Nucleophilicity increases down a group.

Neg. charged species a stronger nucleophile than its conjugate acid. Why? Ability to donate electrons.

More stable anion = better leaving group

Good leaving groups: I -, Br -, sulfonates Modest leaving group: Cl - Poor leaving groups: F -, - OH, CH3CO2 -, PhO -