Nucleophilic Substitution and Beta-Elimination

reaction mechanism describes how a reaction occurs, specifically…

• which bonds are broken and which bonds are formed

• order (and relative rates) of the bond-breaking and bond-forming steps

• role of solvent

• role of catalyst

  • position of all atoms and energy of the entire sustem during the reaction f

nucleophilic substitution

any reaction in which one nucleophile substitutes for another at a tetravalent (sp3 hybridized) carbon

nucleophile

a molecule or ion that donates a pair of electrons to another molecule or ion to form a new covalent bond; aka a lewis base

one-step mechanism

both reactants are involved in the transition state of the rate-determining step (only one intermediate)

Sn2 reactions

• relative ease of approach of a nucleophile to the reaction site

• governed by steric factors

dielectric constant

a measure of a solvent’s ability to insulate opposite charges from one another

• polar solvent: ε > 15

• nonpolar solvent: ε < 15

protic solvent

• a solvent that is a hydrogen bond donor

• many common protic solvents have -OH groups

aprotic sovent

• a solvent that cannot serve as a hydrogen bond donor

• nowhere in the molecule is there a hydrogen bonded to an atom of high electronegativity

nucleophilicity

a kinetic property measured by the rate at which a Nu causes a nucleophilic substitution

• increases from right to left within a period

• the stronger the base = increase in nucleophilicity

• increase in atomic size - increase in nucleophilicity (for neutral species)

basicity

a equilibrium (thermodynamic) property measured by the position of equilibrium in an acid-base reaction

• because all nucleophiles are also bases, we study correlations between nucleophilicity and basicity

• increases from right to left within a period