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° to the leaving group, since this provides the best overlap between the nucleophile's lone pair and the C-X σ* antibonding orbital. The leaving group is then pushed off the opposite side and the product is formed.
If the substrate under nucleophilic attack is chiral, this leads to an inversion of stereochemistry, called the Walden inversion.
SN2 reaction of bromoethane with hydroxide ion
SN2 reaction of bromoethane with hydroxide ion
In an example of the SN2 reaction, the attack of OH− (the nucleophile) on a bromoethane (the electrophile) results in ethanol, with bromide ejected as the leaving group.
SN2 attack occurs if the backside route of attack is not sterically hindered by substituents on the substrate. Therefore this mechanism usually occurs at an unhindered primary carbon centre. If there is steric crowding on the substrate near the leaving group, such as at a tertiary carbon centre, the substitution will involve an SN1 rather than an SN2 mechanism, (an SN1 would also be more likely in this case because a sufficiently stable carbocation intermediary could be formed.) The rate of an SN2 reaction is second order, as the rate-determining step depends on the nucleophile concentration, [Nu−] as well as the concentration of substrate, [RX].
J = k[RX][Nu−]
This is a key difference between the SN1 and SN2 mechanisms. In the SN1 reaction the nucleophile attacks after the rate-limiting step is over, whereas in SN2 the nucleophile forces off the leaving group in the limiting step. In cases where both mechanisms are possible (for example at a secondary carbon centre), the mechanism depends on solvent, temperature, concentration of the nucleophile or on the leaving group.
SN2 reactions are generally favoured in primary alkyl halides or secondary alkyl halides with an aprotic solvent. They occur at a negligible rate in tertiary alkyl halides due to steric hindrance.
It is important to understand that SN2 and SN1 are two extremes of a sliding scale of reactions, it is possible to find many reactions which exhibit SN2 and some SN1 character in their mechanisms. For instance, it is possible to get a contact ion pairs formed from an alkyl halide in which the ions are not fully separated. When these undergo substitution the stereochemistry will be inverted (as in SN2) for many of the reacting molecules but a few may show retention of configuration.
An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism. Either the unsaturation of the molecule increases (as in most organic elimination reactions) or the valence of an atom in the molecule decreases by two, a process known as reductive elimination. An important class of elimination reactions are those involving alkyl halides, or alkanes in general, with good leaving groups, reacting with a Lewis base to form an alkene in the reverse of an addition reaction. When the substrate is asymmetric, regioselectivity is determined by Zaitsev's rule. The one and two-step mechanisms are named and known as E2 reaction and E1 reaction, respectively.
E2 mechanism
In the 1920s, Sir Christopher Ingold proposed a model to explain a peculiar type of chemical reaction: the E2 mechanism. E2 stands for bimolecular elimination and has the following specificities.
* It is a one-step process of elimination with a single transition state.
* Typical of secondary or tertiary substituted alkyl halides. It is also observable with primary alkyl halides if a hindered base is used.
* The reaction rate both influenced by the alkyl halide and the base is second order.
* Because E2 mechanism results in the formation of a Pi bond, the two leaving groups (often a hydrogen and a halogen) need to be coplanar. An antiperiplanar transition state has staggered conformation with lower energy and a synperiplanar transition state is in eclipsed conformation with higher energy. The reaction mechanism involving staggered conformation is more favourable for E2 reactions.
* Reaction often present with strong base.
* In order for the pi bond to be created, the hybridization of carbons need to be lowered from sp3 to sp2.
* The C-H bond is weakened in the rate determining step and therefore the deuterium isotope effect is larger than 1.
* This reaction type has similarities with the SN2 reaction mechanism.
Saturated (sp3-hyrbridized) carbons will not react as readily with E2 and it will with E1 due to the steric hindrince. If SN1 and E1 are competing for the reaction, the E2 can be achieved by increasing the heat.
The reaction fundamental elements are
* Breaking of the carbon-hydrogen and carbon-halogen bonds in one step.
* Formation of a carbon=carbon Pi bond.
Scheme 1. E2 reaction mechanism
An example of this type of reaction in scheme 1 is the reaction of isopropylbromide with potassium ethoxide in ethanol. The reaction products are isobutylene, ethanol and potassium bromide.
E1 mechanism
E1 is a model to explain a peculiar type of chemical elimination reaction. E1 stands for unimolecular elimination and has the following specificities.
* It is a two-step process of elimination ionization and deprotonation.
o Ionization, Carbon-halogen breaks to give a carbocation intermediate.
o Deprotonation of the carbocation.
* Typical of tertiary and some secondary substituted alkyl halides.
* The reaction rate is influenced only by the concentration of the alkyl halide because carbocation formation is the slowest, rate-determining step. Therefore first order kinetics apply.
* Reaction mostly occurs in complete absence of base or presence of only weak base.
* E1 reactions are in competition with SN1 reactions because they share a common carbocationic intermediate.
* Deuterium isotope effect is absent.
* accompanied by carbocationic rearrangement reactions
Scheme 2. E1 reaction mechanism
An example in scheme 2 is the reaction of tert-butylbromide with potassium ethoxide in ethanol.
E1 eliminations happen with highly substituted alkyl halides due to 2 main reasons.
* Highly substituted alkyl halides are bulky, limiting the room for the E2 one-step mechanism; therefore, the two-step E1 mechanism is favored.
* Highly substituted carbocations are more stable than methyl or primary substituted. Such stability gives time for the two-step E1 mechanism to occur.
CHECK THE LINK:
http://www.freelance-teacher.com/organic_chemistry_sn2_sn1_e2_e1.pdf
No comments:
Post a Comment