Nucleophilic substitution at the tetravalent carbon is a fundamental reaction. The understanding of this reaction was developed in England by C.K. Ingold and E.D. Hughes in the 1930s. Nucleophilic substitution reactions (SN1 vs SN2) have a different number of molecules. SN1 is a unimolecular substitution reaction whereas SN2 is a bimolecular substitution reaction.
SN1 and SN2 reactions are used to synthesize organic molecules. Both reactions differ in their mechanism, speed, and selectivity. This article discusses the differences between SN1 and SN2 reactions, including how they’re used in synthesis and some examples of each type of reaction.
Most chemical reactions have only one way they can happen, right? Nucleophilic substitution reactions are more nuanced than that, with two different mechanisms happening depending on the structure of the starting materials and the conditions.
They contain a distant combination of charged or uncharged species as reactants. Such as neutral halides or sulfonates. They can also react with neutral or anionic nucleophiles. Reaction with anionic nucleophiles has a variety of species such as cyanide and azide.
Difference between Nucleophilic substitution (SN1 Vs SN2)
| Unimolecular Substitution Reaction (SN1) | Bimolecular Substitution Reaction (SN2) |
| It is a nucleophilic substitution reaction that involves one molecule | It is a nucleophilic substitution reaction that involves two molecules |
| It follows first order Kinetic mechanism | It follows the second order Kinetic mechanism |
| They complete in two steps | They complete in a single step |
| The rate of reaction depends on the concentration of substrate | The rate of reaction depends on the concentration of both substrate and nucleophile |
| Carbocation intermediate formed in this reaction | No intermediate carbocation formed in this reaction |
| They never show inversion | They involved in inversion |
| There are no partial bonds form with carbon during the reaction | Carbon forms partial bonds with the nucleophile and leaving group |
| This reaction starts with the removal of the group while attacking nucleophile | This process takes place in only one cycle with a single intermediate stage |
| They occur in an alcoholic solution | They never occur in an alcoholic solution |
| It has weak or neutral nucleophile | It has strong nucleophile |
| The product can be a racemic mixture since retention or inversion can take place | Inversion takes place only |
| Low temperature favors this reaction | Low temperature favors reactions but high temperature lead to E2 reaction |
| The rate of reaction is K = [RX] | The rate of reaction is K = [RX][Nu] |
| A tertiary carbocation is more stable | A primary carbocation is more stable |
| There is carbocation stability | It has steric hindrance |
| It is a slower reaction | It is a faster reaction |
| The second step is the rate-determining step | The first step is the rate-determining step |
| They favor polar protic solvent | They favor polar aprotic solvent |
| It has an ionization mechanism | It has a direct displacement mechanism |
Nucleophilic Substitution Unimolecular SN1
SN1 reactions are proceeded by the ionization mechanism. It is a two-step process. Ionization occurs in the first step that is very slow. The rate-determining heterolytic dissociation of the reactant results in a carbocation and leaving group. This dissociation occurs due to the combination of the electrophilic carbocation with a nucleophile.
The second step is the rate-determining step. The reaction has first-order kinetics. The rate of decomposition of reactant is independent of the concentration and nucleophile identity.
Carbocations are known as carbonium ions. It is a generic name for carbon cations. For example, Methyl and butyl cations are carbocations. There are specific names such as methylium, ethylium, 1-methylethylium for methyl, ethyl, 2-propyl, etc.
According to the SN1 mechanism, When the same carbocation generates more than one precursor. As a result, the reaction becomes independent of its origin. This exception can be compressed by the fact that ionization initially produces an ion pair. The reaction takes place from ion pairs, rather than from the dissociated or solvated ions.
Nucleophilic Substitution Bimolecular SN2
SN2 reactions are followed by the direct displacement mechanism. It completes the reaction in a single step that is rate-determining (Transition State). According to this mechanism, the reactant is attacked by the nucleophile (Lewis base) from the opposite side of the leaving group. During the reaction, bonds are broken down and new bonds are formed simultaneously. The geometry of the transition state is trigonal bipyramidal with a pentacoordinate carbon. It exhibits second-order kinetics.
According to the mechanism, the explanation of bonding interaction is given by the frontier molecular orbitals. These orbitals are filled with nonbonding orbitals that are associated with nucleophiles. Sigma-star antibonding orbitals are associated with the leaving group and the carbon that undergo substitution. The backside approach by the nucleophile is most suitable. This is due to the strongest initial interaction between the filled orbital of the nucleophile and the sigma-star antibonding orbital.
During the transition state, the orbitals that are involved in substitution have (p) character. The molecular orbitals also predict that this reaction will proceed with the inversion of configuration. This is because the transition state is established by the rehybridization of the carbon to the trigonal bipyramidal geometry. When the product is obtained, sp3 hybridization is reestablished with inversion in it.
Key Points (SN1 vs SN2)
Concepts Berg
SN1 vs SN2 carbocation?
SN1 is a two-step mechanism. The first step has ionization that is a slow and rate-determining step. A carbocation is the product of ionization in SN1. On the other hand, SN2 has a transition state. There is no carbocation formed as a product. TS has a pentacoordinate carbon with bipyramidal geometry.
SN1 vs SN2 examples?
Nucleophiles e.g. hydroxyl group (OH–), alkoxy group (RO–), halide ions (X–), water (H2O), ammonia (NH3), are examples of SN1 and SN2.
What is the difference between SN1 and SN2 reactions?
SN1 reactions followed by the ionization method and first-order kinetic. There is also the formation of a carbocation. While SN2 reactions are preceded by the direct displacement method and second-order kinetic. There is the formation of transition states.
What is the difference between SN1 vs SN2 and E1 vs E2?
SN1 and SN2 are nucleophilic substitution reactions while E1 and E2 are elimination reactions. It depends on the nucleophile that reaction gives substitution or elimination. The use of a strong base gives elimination while bulky nucleophile gives substitution.
What do SN1 and SN2 mean in organic chemistry?
It is a substitution reaction in which a nucleophile adds to an atom that’s connected to a leaving group. In most cases, S stands for substitution, N stands for nucleophilic, 1 stands for first order, and 2 stands for bimolecular.
What are the orders of SN1 and SN2 reactions?
Unimolecular substitution followed by the first order. On the other hand, bimolecular substitution is second order.
Why is SN2 faster than SN1?
SN2 reactions are usually faster than SN1 reactions because they occur by a different mechanism. In SN1 reactions, one of two things can happen: either an electrophile (e.g., Cl−) attacks an intermediate carbocation or it reacts with a nucleophile (e.g., water). If a nucleophile such as water has already attacked, then any additional electrophiles will attack that intermediate carbon-carbon bond instead of attacking another carbon-carbon bond in the same molecule.
What are the factors that affect SN1 and SN2 reactions?
Five main factors affect SN1 and SN2 reactions. They are a steric hindrance, induction effect, nucleophilicity, polarity, and concentration.
Steric hindrance refers to how crowded an area is with other molecules. The crowdedness depends on how many neighboring groups surround a given functional group at any given time.
What are the properties of SN1 and SN2 reactions?
SN1 reactions are fast, exothermic (give off heat), and associate with bases. SN2 reactions are slow, endothermic (absorbs heat), do not associate with bases, but disassociate with acids. Each reaction has its own set of chemical properties that make it unique from others in its group.
How do I decide on a better substrate for SN1 and SN2 mechanisms?
There are many things to consider when choosing a substrate as SN1 vs SN2 reactions. The two most important factors are a steric hindrance and electron-withdrawing/donating ability of R groups. Usually, bulky substrates will favor SN2 reactions, while small substrates will favor SN1 reactions.
Is Swarts reaction SN1 or SN2?
Both are types of chemical reactions in which a nucleophile reacts with an electrophile to form a product. When it comes to Swarts reaction, both these reactions can be said to occur. According to some chemists, SN2 reactions are swarts reactions.
Do benzyl halides react mainly via SN1 or SN2 mechanisms?
The mechanism of substitution reaction that a benzyl halide undergoes depends on what substituent group is present on benzyl halide. If it has an electron-withdrawing group attached to benzyl halide, then it will have an SN2 reaction in which the new product will be formed via a dissociative mechanism. However, if there are no such groups present, then we can expect to see an SN1 reaction.
How to identify E1, E2, SN1, and SN2 reactions?
The key to identifying an organic reaction as E1, E2, SN1, or SN2 lies in knowing how a chemical can be changed so that it cannot participate in that reaction type. If a molecule cannot be converted into an unrecognizable form by losing any atoms from its skeleton (ring or chain), then it will proceed by either E1 or E2 mechanisms. Likewise, if a molecule’s bonds are too strong for it to lose any atoms through free-radical substitution, then it must follow radical substitution mechanisms, specifically SN1 vs SN2.
What product is obtained when benzaldehyde undergoes an SN2 reaction?
Benzaldehyde’s reaction with hydroxide follows an SN2 mechanism. We can predict that it will yield a tertiary carbocation. To know whether it yields phenol or anisole. We need to know what other groups are bonded to carbon on the benzene ring. Benzene’s carbons do not have any double bonds between them, so there isn’t another carbon atom nearby for either group to be moved. So there are two possible products phenol or anisole.
