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- SN1 stands for Nucleophilic Substitution Unimolecular.
- This means that the rate of these reactions appears to depend only on the
substrate concentration and not on the concentration of nucleophile.
- The rate of the disappearance of substrate = d[ substrate ]/dt = -k[ substrate
], negative slope asymptotic to both axis.
- The truth is that the rate of this reaction is really pseudo-first order
in substrate concentration because the concentration of solvent and acid
catalyst, which affect this reaction, don't change that much as the reaction
- These substitutions occur by a removal (ionization) of the leaving group
to make a carbocation intermediate.
- They share the same first step as an E1 reaction.
- The nucleophile attaches itself to the carbocation intermediate in an
- The electrophilic carbon must be sp3 hybridized.
- The hybridization of the carbocation intermediate of this process is sp2.
- This reaction leads towards racemization at carbons that are chiral
- sometimes not a perfect 50/50 mixture because of a cage effect or
favoring one diastereomer,
- When a neighboring group can stabilize the carbocation intermediate,
inversion of configuration is observed. This is called anchimeric
- Epoxides substitute by this mechanism at the most sterically hindered
position with inversion of configuration because the leaving oxygen is
anchored as a neighbor, SN1 with "goal tending".
- The rate of this reaction is affected by:
- The stability of the carbocation intermediate, rate greater for
tertiary > secondary >> primary, neopentyl
- Inductive effect
- Solvent, works best with polar protic solvents like water, alcohols, etc.
- A polar solvent is required for solubility.
- A protic solvent hydrogen bonds to the leaving group helping to
ionize it off.
- When the solvent is also the nucleophile the reaction is called
- This means that SN1 substitutions are really
pseudo-first order because the solvent has an effect on the rate but
its concentration does not change that much!
- Leaving group, works best with leaving groups that are weak bases
because they have a weaker bonds with carbon and can stabilize negative charge best.
- F > HO > H2N
- I > Br > Cl > F
- right-down, more right than down
- cation > neutral
- Though not affecting the rate of the reaction, less electronegative nucleophiles
(but not bases) works best:
- OH2 > HF
- Se > S > O
- I > Br > Cl > F
- left-down, more left than down
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Thursday, April 06, 2017 13:18