Question

Explain the difference between S_N1 and S_N2 reactions.

Answer 1

To put in more simply:

The leaving group (LG) leaves on its own in Sn1 mechs, which forms a carbocation (watch out for alkyl/hydride shifts!), and then the nucleophile (Nu) attack the C+, commonly followed by deprotonation (Sn1 has "acidic" Nu's)...(faster with tert C--LG carbons)

In Sn2, the Nu attacks the C bearing the LG displacing the LG....(faster with primary C--LG carbons)

Sn2 occurs in basic/aprotic conditions (based on nature of Nu, not so much solvent)

Sn1 occurs in acidic/protic conditions, also based largely on the Nu as opposed to solvent

Sn2 is a one-step, concerted reaction

Sn1 is two/three step deal, which in non-concerted

The numbers involved here have nothing to do with the number of steps, mind you...

Sn1: rate = k[starting materials], so first-order kinetics

Sn2: rate = k [SM}[Nu], so second-order kinetics

Hope this helps...

Answer 2

Think about that for a second. In the meantime, let

Answer 3

The SN1 reaction is a substitution reaction in organic chemistry. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is unimolecular [1], [2]. It involves a carbocation intermediate and is commonly seen in reactions of secondary or tertiary alkyl halides or, under strongly acidic conditions, with secondary or tertiary alcohols. With primary alkyl halides, the alternative SN2 reaction occurs. Among inorganic chemists, SN1 is referred to perhaps more accessibly as a dissociative mechanism.

Mechanism:

Diagram of SN1 Mechanism for hydrolysis of an alkyl halide

The SN1 reaction between a molecule A and a nucleophile B takes place in three steps:

1. Formation of a carbocation from A by separation of a leaving group from the carbon; this step is slow.
2. Nucleophilic attack: B reacts with A. If the nucleophile is a neutral molecule (i.e. a solvent) a third step is required to complete the reaction. When the solvent is water, the intermediate is an oxonium ion.
3. Deprotonation: Removal of a proton on the protonated nucleophile by a nearby ion or molecule.

An example reaction:

(CH3)3CBr + H2O ? (CH3)3COH + HBr

This goes via the three step reaction mechanism described above:

1. (CH3)3CBr ? (CH3)3C(+) + Br(?)
2. (CH3)3C(+) + H2O ? (CH3)3C-OH2(+)
3. (CH3)3C-OH2(+) + H2O ? (CH3)3COH + H3O(+)

Kinetics:

In contrast to SN2, SN1 reactions take place in two steps (excluding any protonation or deprotonation). The rate determining step is the first step, so the rate of the overall reaction is essentially equal to that of carbocation formation and does not involve the attacking nucleophile. Thus nucleophilicity is irrelevant and the overall reaction rate depends on the concentration of the reactant only.

rate = k[reactant]

In some cases the SN1 reaction will occur at an abnormally high rate due to neighbouring group participation (NGP). NGP often lowers the energy barrier required for the formation of the carbocation intermediate.

The SN2 reaction is a type of nucleophilic substitution, where a nucleophile attacks an electrophilic center and bonds to it, expelling another group called a leaving group. Thus the incoming group replaces the leaving group in one step. Since two reacting species are involved in the slow, rate-determining step of the reaction, this leads to the name bimolecular nucleophilic substitution, or SN2. The somewhat more transparently named analog to SN2 among inorganic chemists is the interchange mechanism.

The reaction most often occurs at an aliphatic sp3 carbon center. The breaking of the C-X bond and the formation of the new C-Nu bond occur simultaneously to form a transition state in which the carbon under nucleophilic attack is pentavalent, and approximately sp2 hybridised. The nucleophile attacks the carbon at 180

Answer 4

Mechanism

• The SN2 reaction is concerted. That is, the SN2 occurs in one step, and both the nucleophile and substrate are involved in the rate determining step. Therefore the rate is dependent onboth the concentration of substrate and that of the nucleophile.
• The SN1 reaction proceeds stepwise. The leaving group first leaves, whereupon a carbocation forms that is attacked by the nucleophile.

The tendencies of alkyl halides toward the two types of substitution mechanisms.

Increasing SN2 reactions 1 alkyl halide 2°alkyl halide 3°alkyl halide Increasing SN1 reactions