Why is carbocation formation a slow step

S. N 1 reaction - S.N1 reaction

A substitution reaction with a carbocation intermediate


In the first step of the Sn¹ mechanism, carbonation is formed which is more planar and therefore nucleophile attack from either side (second step) to give a racemic product, but in fact complete racemization does not occur. This is because the nucleophilic species attacks the carbonation before the leaving halide ion has sufficiently removed from the carbonation. The negatively charged halide ion protects the carbonation from attack on the front and back, which leads to a reversal of the configuration. Thus, the actual product undoubtedly consists of a mixture of enantiomers, but the enantiomers with an inverted configuration would predominate and complete racemization does not take place.

The S. N 1 reaction is a substitution reaction in organic chemistry, the name of which refers to the Hughes-Ingold symbol of the mechanism. "S. N "Stands for" nucleophilic substitution, "and the" 1 "so the rate-limiting step is unimolecular. Therefore, it is often shown that the rate equation has a first order dependence on the electrophile and a zero order dependence on the nucleophile. This relationship applies to situations where the amount of nucleophile is much greater than that of the intermediate. Instead, the rate equation can be described in more detail using stationary kinetics. The reaction involves an intermediate carbocation and is commonly observed in reactions of secondary or tertiary alkyl halides under strongly basic conditions or under strongly acidic conditions with secondary or tertiary alcohols. In the case of primary and secondary alkyl halides, the alternative S N 2 reaction takes place. In inorganic chemistry, the S N 1 reaction often called termed dissociative mechanism . This dissociation path is well described by the cis effect. A reaction mechanism was first proposed by Christopher Ingold et al. In contrast to the S, this reaction depends N 2 mechanism does not depend much on the strength of the nucleophile. This type of mechanism involves two steps. The first step is the ionization of the alkyl halide in the presence of aqueous acetone or ethyl alcohol. This step provides an intermediate carbocation.

mechanism

An example of a reaction starting with an S N 1 reaction mechanism takes place is the hydrolysis of tert-butyl bromide with the formation of tert- Butanol:

This S N 1 reaction takes place in three steps:

Recombination of carbocation with nucleophile
  • Nucleophilic attack: The carbocation reacts with the nucleophile. If the nucleophile is a neutral molecule (ie, a solvent), a third step is required to complete the reaction. When the solvent is water, the intermediate is an oxonium ion. This reaction step is quick.

Rate law

Although the law of speed of the S N 1 reaction is often viewed as a first-order alkyl halide and a zero-order nucleophile, this is a simplification that only applies under certain conditions. Even if it is an approximation, the velocity law derived from the stationary approximation (SSA) offers a better insight into the kinetic behavior of the S. N 1 reaction. Consider the following reaction scheme for the mechanism shown above:

Although it is a relatively stable tertiary carbocation, it is tert- Butyl cation is a high-energy species that is present in very low concentrations and cannot be directly observed under normal conditions. Thus, SSA can be applied in this way:

  • (1) Assumption of the steady state: d [ t Bu + ] / German = 0 = k 1 [ t BuBr] - k - 1 [ t Bu + ] [Br - ] - k 2 [ t Bu + ] [H 2 O]
  • (2) Concentration of the t- Butyl cation, based on the steady state assumption: [ t Bu + ] = k 1 [ t BuBr] / ( k - 1 [Br - ] + k 2 [H 2 O])
  • (3) Overall reaction rate assuming a fast final step: d [ t BuOH] / German = k 2 [ t Bu + ] [H 2 O]
  • (4) Steady-State-Rate-Law by plugging (2) into (3): d [ t BuOH] / German = k 1 k 2 [ t BuBr] [H 2 O] / ( k - 1 [Br - ] + k 2 [H 2 O])

Under normal synthesis conditions, the entering nucleophile is more nucleophile than the leaving group and is present in excess. In addition, kinetic experiments are often carried out under initial rate conditions (5 to 10% conversion) and without the addition of bromide, so that [Br - ] is negligible. For these reasons it is common k - 1 [Br - ] ≪ k 2 [H 2 O]. Under these conditions the SSA rate law is reduced to rate = d [ t BuOH] / German = k 1 k 2 [ t BuBr] [H 2 O] / ( k 2 [H 2 O]) = k 1 [ t BuBr], the simple first-order law of interest described in introductory textbooks. Under these conditions, the concentration of the nucleophile does not affect the rate of the reaction, and changing the nucleophile (e.g. from H 2 O to MeOH) does not affect the reaction rate, although the product is of course different. In this regime, the first step (ionization of the alkyl bromide) is slow, rate-limiting and irreversible, while the second step (nucleophilic addition) is fast and kinetically invisible.

Under certain conditions, however, reaction kinetics cannot be observed of the first order. Especially when there is a large concentration of bromide while the concentration of water is limited, the reverse of the first step becomes kinetically important. As the SSA rate law indicates, there is a broken dependence (between zero and first order) on [H 2 O], while a negative fractional order dependence on [Br - ] consists. Therefore, it is often observed that S N 1 reactions become slower if an exogenous source of the leaving group (in this case bromide) is added to the reaction mixture. This is as common ion effect known, and the observation of this effect is evidence of an S. N 1 mechanism (although the lack of a common ionic effect does not preclude it).

scope

The S N 1 mechanism tends to dominate when the central carbon atom is surrounded by bulky groups, since such groups form the S N 2 -reaction sterically hinder. In addition, bulky substituents on the central carbon increase the rate of carbocation formation due to the alleviation of the steric stress that occurs. The resulting carbocation is also stabilized by both inductive stabilization and hyperconjugation of attached alkyl groups. The Hammond-Leffler postulate suggests that this too will increase the rate of carbocation formation. The S N 1 mechanism therefore dominates in reactions at tertiary alkyl centers.

An example of a response made to S N 1 process is running is the synthesis of 2,5-dichloro-2,5-dimethylhexane from the corresponding diol with concentrated hydrochloric acid:

As the alpha and beta substitutions with respect to leaving groups increase, the reaction of S N 2 to S N 1 diverted.

Stereochemistry

The carbocation intermediate is in the rate-limiting step of the reaction as sp 2 -hybridized carbon formed with trigonal planar molecular geometry. This allows for two different avenues for nucleophilic attack, one on each side of the planar molecule. If neither route is preferred, these two routes occur equally and give a racemic mixture of enantiomers when the reaction takes place in a stereocenter. This is explained below in the p N 1 reaction of S-3-chloro-3-methylhexane with an iodide ion, which gives a racemic mixture of 3-iodo-3-methylhexane:

However, an excess of one stereoisomer can be observed as the leaving group can stay near the carbocation intermediate for a short time and block nucleophilic attack. This is in contrast to the S N 2 mechanism, a stereospecific mechanism in which the stereochemistry is always inverted when the nucleophile comes in from the back of the leaving group.

Side reactions

Two common side reactions are elimination reactions and carbocation rearrangements. If the reaction is carried out under warm or hot conditions (which favors an increase in entropy), E1 elimination is likely to predominate, resulting in the formation of an alkene. At lower temperatures, S N 1 and E 1 reactions competitive reactions and it becomes difficult to prefer each other. Even if the reaction is carried out cold, some alkene can form. If an S N 1 reaction using a strongly basic nucleophile such as hydroxide or methoxide ions, the alkene is re-formed, this time via E2 elimination. This is especially true when the reaction is heated. If the carbocation intermediate can rearrange to a more stable carbocation, it will eventually result in a product that is derived from the more stable carbocation rather than the simple substitution product.

Solvent effects

As this N 1 reaction involves the formation of an unstable carbocation intermediate in the rate-limiting step, anything that can facilitate this will accelerate the reaction. The normal solvents of choice are both polar (to stabilize ionic intermediates in general) as well as protic solvents (for solvating the leaving group in particular). Typical polar protic solvents include water and alcohols, which also act as nucleophiles, and the process is known as solvolysis.

The Y scale correlates the solvolysis reaction rates of any solvent ( k ) with those of a standard solvent (80% v / v ethanol / water) ( k 0 ) by

With m a reactant constant (m = 1 for tert- Butyl chloride) and Y a solvent parameter. For example, 100% ethanol gives Y = -2.3, 50% ethanol in water gives Y = +1.65, and 15% concentration Y = +3.2.

See also

References

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