SN2
Home ]

 

Help end world hunger

 

 

 

 

Definition:

  • SN2 stands for Substitution Nucleophilic Bimolecular.
  • This means that the rate of these reactions depends not only on the substrate concentration but also on the concentration of the substituting nucleophile.
  • The rate of the disappearance of substrate = d[ substrate ]/dt = -k[ substrate ][ nucleophile ], negative slope asymptotic to both axis.

Mechanism:

  • The SN2 substitution occurs by a direct backside attack by a nucleophile on an electrophilic carbon to avoid the electrons on the front side and to interact with a sigma anti-bond.
  • The nucleophile attaches itself while the group that made the carbon electrophilic leaves in a concerted fashion.
  • The electrophilic carbon must be sp3 hybridized.
  • The hybridization of the transition state of this process is sp2.

Stereochemistry:

  • This reaction leads to inversion of configuration (R goes to S and vice a versa) at carbons that are chiral centers.
  • This control is called stereoselectivity.

Factors:

  • The rate of this reaction is affected by:
    • Steric hindrance, rate greater for methyl > primary > secondary >> tertiary, neopentyl
    • Solvent, works best with polar aprotic solvents like acetone, dimethylformamide, etc.
      • A polar solvent is required for solubility.
      • A protic solvent would hydrogen bond to the nucleophile and reduce its effectiveness.
    • Nucleophile, works best with less electronegative, more basic but not too basic nucleophiles
      • NH3 > OH2 > HF
      • HO- > H2O
      • P > N
      • Se > S > O
      • I > Br > Cl > F
      • left-down, more left than down
      • anion > neutral
      • Too basic reagents lead to elimination reactions.
    • Leaving group, works best with leaving groups that are weak bases because this correlates to having weaker carbon leaving group bonds.
      • F > HO > H2N > C
      • I > Br > Cl > F
      • right-down, more right than down
      • cation > neutral but cations may lead to other mechanisms including eliminations and rearrangements.
  • Alkyl halides are most often used in the SN2 substitutions but other leaving groups are possible:
    • Alcohols to alkyl halides with HX, ZnCl2, SOCl2, PX3, or via arylsulfonates
    • Ethers to alkyl halides with HX (Br, I)
    • Epoxides (a strained ether) to alcohols with Grignard Reagents, Cyanide, Acetylides, Amines, Azides, Hydroxide, Alkoxides, Carboxylates, Thiolates, mineral acids, etc.

    SN2 Substitution Products with Alkyl Halides

    (Product in Parenthesis)
    *with primary or methyl alkyl halides or arylsulfonates because the nucleophile is too basic

better leaving group

 

better nucleophile

C

-:CN, cyanide (nitriles)
*-:CCR, acetylide (alkynes)
*Enolates(alpha-substituted carbonyl compounds)
 

N

:NH3 (primary amines)
:NH2R (secondary amines)
:NHR2 (tertiary amines)
:NR3 (quaternary ammonium salts)

-:N3, azide, followed by LiAlH4 (primary amines)
phthalimide and base followed by hydrazine (primary amines)

O

*-:OH(alcohols)
*-:OR , alkoxides, Williamson (ether) synthesis
-:OCOR, carboxylates, (esters)

F

fluorides

better
nucleophile
better
leaving
group
  P

:PR3, phosphine (phosphonium salts)

S

-:SH, sulfide (thiols, mercaptans)
-:SR, thiolate (dialkyl sulfides)

Cl

NaCl (chlorides)

    Se

selenides

Br

NaBr (bromides)

      I

NaI (iodides)

 

 

 

 

Up ] SN1 ] [ SN2 ]

Copyright 2003 and beyond intergrader.net

Thursday, April 06, 2017 13:18