ORGO MECHY trends

SN1

• carbocation rearrangement

• Relies of leaving group ability to carry out reaction (Rate=k[elec])

  • better LG, faster reaction

• only tertiary and secondary substitution can occur, with tertiary as most stable

  • primary can only occur if resonance stability of carbocation is possible

• needs POOR nucleophiles and WEAK bases

• best when reacted under polar protic solvents

  • stabilizes carbocations and increases rate

• better in room temp

SN2

• Influenced by LG ability but does not rely on it

  • reaction can still occur with a strong base, but its not as fast

• the less substituted the alpha carbon, the more preference for an SN2 reaction

  • methyl, primary, secondary can occur

  • tertiary cannot occur because a tertiary carbon has too many groups around it

  • occurs fastest with methyl and slowest with secondary

• for primary and methyl, prefer GOOD nucleophile

• for secondary, needs a GOOD nucleophile and a GOOD base

• faster in polar aprotic solvents

• Better in room temp

E1

• Relies on LG ability (Rate=k[substrate])

• only tertiary and secondary can occur

  • primary can occur if carbocation can participate in resonance

  • methyl cannot occur

• needs POOR nucleophiles and POOR bases

• best under polar protic conditions

• Better in High temp

E2

• is influenced by LG ability but does not rely on it (rate=k[base][substrate]); is bimolecular

• Primary, Secondary, and Tertiary can all occur (cannot occur with methyl)

  • primary is preferred with STRONG, BULKY bases

  • secondary and tertiary are preferred with STRONG, REGULAR bases

• Needs GOOD nucleophile

• prefers polar aprotic solvents

• Better in High temp

Good nucleophiles

typically have a negative charge or are attached to a metal

• nucleophilicity follows basicity trends

  • the more basic the anion, the stronger the nucleophile

• different with protic/aprotic solvents

  • the trend for nucleophilicity is F−>Cl−>Br−>I− in aprotic solvent, but in protic solvent, it is I−>Br−>Cl−>F− (F− is now larger than others in size because it is solvated.)

good leaving groups

weak bases that can accept electrons

-as electronegativity increases, basicity increases

• which is why F- is a terrible leaving group but I- is a good one