Order of bond making and bond breaking of Nucleophilic Substitutions

Substitution Nucleophilic Bimolecular (SN2)

If bond making and bond breaking occur at the same time, this is a concerted.

• The bonds become inverted (stereochemistry has changed!)

• The mechanism is one step. In such a molecular reaction, the rate depends upon the concentration of both reactants, that is, the rate equation is 2nd order

• Rate law expression: rate = k[RX][Nu-]

• Units for k = L/mol*s or M-1s-1

Substitution Nucleophilic Unimolecular (SN1)

If bond breaking occurs before bond making, this is a step-wise reaction

• Nu can attack the carbon from either side so stereochemistry is lost - RACEMIC MIXTURE FORMS!

• The empty p orbital is perpendicular to the other sp3 orbitals (trigonal planar shape)

• The intermediate is NOT energetically favored, so it’s a slow step

• The mechanism has two steps and a carbocation (C+) is formed as an intermediate

• Since the first step is the rate determining step, the rate depends on the concentration of RX (alkyl halide) only; that is, the rate law equation is first order

• Rate law expression: rate = k[RX]

• Units for k = 1/s or s-1

SN1

Substitution Nucleophilic Unimolecular

SN2

Substitution Nucleophilic Bimolecular