Why sn1 is faster than sn2

It is very common that the polar protic solvents serve as nucleophiles as well for S N 1 reactions, so usually S N 1 reactions are solvolysis reactions as we learned earlier. Since salts are insoluble in non-polar solvent, therefore non-polar solvents are not appropriate choices, and we need polar solvents that can dissolve the salts. The issue for polar protic solvent is that the nucleophile anions will be surrounded by a layer of solvent molecules with hydrogen bonds, and this is called the solvation effect.

The solvation effect stabilize or encumber the nucleophiles and hinder their reactivities in S N 2 reaction. Therefore, polar protic solvents are not suitable for S N 2 reactions.

They are polar enough to dissolve the salt format nucleophiles, and also not interact as strongly with anions to hinder their reactivities. The nucleophile anions still move around freely in polar aprotic solvent to act as nucleophile. The reaction rate for a S N 2 reaction in different solvents are provided in the table below, and the polar aprotic solvent DMF proved to be the best choice that speed up the reaction significantly. With all the knowledge about S N 1, S N 2 reactions and reaction conditions, we should be able to determine that whether a given reaction go with S N 1 or S N 2 pathway, or design a proper reaction that will produce the desired product s.

The reaction pathway predominantlydepends on the nature of the substrates primary, secondary or tertiary , and the choice of proper reaction condition serve as a way to facilitate the process.

It may seems contradictory, why a strong nucleophile for S N 2 combine with solvent for S N 1? This is an example of a solvolysis reaction because the nucleophile is also the solvent.

Just as with S N 2 reactions, the nucleophile, solvent and leaving group also affect S N 1 Unimolecular Nucleophilic Substitution reactions. Polar protic solvents have a hydrogen atom attached to an electronegative atom so the hydrogen is highly polarized.

Polar aprotic solvents have a dipole moment, but their hydrogen is not highly polarized. Polar aprotic solvents are not used in S N 1 reactions because some of them can react with the carbocation intermediate and give you an unwanted product. Rather, polar protic solvents are preferred. Since the hydrogen atom in a polar protic solvent is highly positively charged, it can interact with the anionic nucleophile which would negatively affect an SN2, but it does not affect an S N 1 reaction because the nucleophile is not a part of the rate-determining step See S N 2 Nucleophile.

Polar protic solvents actually speed up the rate of the unimolecular substitution reaction because the large dipole moment of the solvent helps to stabilize the transition state. The highly positive and highly negative parts interact with the substrate to lower the energy of the transition state. Since the carbocation is unstable, anything that can stabilize this even a little will speed up the reaction. Related questions What factors determine whether an alkyl halide will undergo an SN1 or an SN2 reaction?

What factors affect SN2 reactions? Are SN1 reactions faster in polar solvents? How are SN1 and SN2 reactions different? Why do solvents with low dielectric constants favor SN2 reactions?

The greater the negative charge, the more likely an atom will give up its pair of electrons to form a bond. If you think about it, in a substitution reaction there really are two main factors that tell you whether it's S N2 or S N1 : the leaving group propensity or the strength of an incoming nucleophile.

Two molecules react, and one displaces a substituent on the other. In the S N 2 reaction, the addition of the nucleophile and the departure of the leaving group occur in a concerted taking place in a single step manner, hence the name S N 2: substitution, nucleophilic, bimolecular. In the S N 2 reaction, the nucleophile approaches the carbon atom to which the leaving group is attached. Note — some of these substitution reactions work better than others, especially on secondary carbons — depending on conditions, elimination reactions can start to compete when strong bases are used.

If the reaction rates differ by a factor of two , then the mechanism is SN2. If the rates are the same, then the mechanism is SN1. Another method that chemists use to learn about reaction mechanisms is by isotopic labeling. Which would undergo S N 1 reaction faster in the following pair? A tertiary alkyl halide tends to undergo the S N 1 mechanism because it can form a tertiary carbocation, which is stabilized by the three alkyl groups attached to it.

Considering the steric factor, surely -CH3 group is bulkier than -H. Hence -CH3 provides greater resistance for the substitution 1st condition. Turns out that the methyl halides and the primary alkyl halide react the fastest in an SN2 mechanism. Secondary alkyl halides react very slowly and tertiary alkyl halides react so, so slowly that we say they are unreactive toward an SN2 mechanism.