Reaction Mechanism Conditions

Flashcard maker : Candace Young

What conditions are required for:

 

SN1

 

  • 1o¬†with C+¬†stabilisation or 3o
  • Polar protic solvent
  • Non-basic nucleophile

What conditions are required for:

 

SN2

 

 

  • Unhindered 2o or 1o carbon
  • Good non-basic nucleophile
  • Polar aprotic solvent

What conditions are required for:

 

E2

  • 3o, unhindered 2o, unhindered 1o
  • Hindered strong base present
  • Non-polar solvent (3o)

What conditions are required for:

 

E1

  • 3o
  • Strong base
  • Polar solvent (with heat)
  • Competes with Sn1

What makes a good:


Nucleophile

  • Electron density
  • Electronegativity

  • High electron density – “the conjugate base is always a better nucleophile”
  • Low electronegativity – electrons are less tightly held so are more easily donated (move across periodic table)

What makes a good:

 

Nucleophile

 

  • Solvent
  • Steric hindrance

  • Solvent – in¬†polar protic (O-H, N-H, F-H) nucleophilicity increases¬†down the periodic table due to capacity for hydrogen bonding (F has high capacity for hydrogen bonding – more hindered, electrons are less free moving). In¬†polar aprotic nucleophilicity decreases¬†down the periodic table because no hydrogen bonding
  • Nucleophilicity¬†decreases with bulkiness

What makes a good:

 

Leaving Group

Good leaving groups are weak bases

Strong acids give weak conjugate bases

Low pka, weak base

Must be a stable anion

What conditions are required for:

 

E1cb

 

Acidic C-H

Moderate/poor leaving group

Stable carbanion (established by resonance)

What does the reaction coordinate diagram look like for:

 

Sn1 and E1

[image]

What does the reaction coordinate diagram look like for:

 

Sn2 and E2

[image]
Hoffman Elimination

Poor leaving group

Positively charged substrate (biased towards cb through E2)

Bulky base in an E2 reaction causes it to preferentially remove the most accessible hydrogen due to steric effects

Least substituted (more hydrogens) alkene is formed as the major product

 

Saytzev Elimination

Always gives the most substituted alkene

E1: Because the reaction proceeds via the lower energy T.S

E2: Because the T.S is passed through during the RDS of the reaction these are more regioselective BUT the requirement for the removed proton to be anti-periplanar overrides all other considerations

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