Reaction determination .


How to understand whether the reaction is in Sn1 or Sn2 type reaction .

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2 Answers


To determine if a reaction is s1 or sn2 ,these simple rules will help you:

1. Primary substrates undergo SN2 reactions because

  • they form unstable primary carbocations and
  • are unhindered to attack by the nucleophile

2. Tertiary substrates undergo SN1 reactions because they

  • are too hindered to undergo backside attack by the nucleophile and
  • can form stable tertiary carbocations.

3. Secondary substrates can undergo substitution by either SN1 or SN2mechanisms.

The mechanism will be

SN2 if

  • the nucleophile has a negative charge (e.g. RO-,HO-,NH-2,SH-,CN-,N-3,RCOO-,X-) or
  • the solvent is a polar aprotic solvent (DMF , DMSO, acetone) or
  • the leaving group is the conjugate base of a weak acid (pKa<0). Examples are F- and RCOO-.

  • SN1 if

  • the nucleophile is uncharged or

  • the solvent is a polar protic solvent (H2O,ROH, RCOOH) or
  • the leaving group is the conjugate base of a strong acid (pKa>14). Examples are X-,TsO-,MsO-,andTfO-.
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Experimental Methods for Measuring Reactions. In order to experimentally determine reaction rates, we need to measure the concentrations of reactants and/or products over the course of a chemical reaction.
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