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The S N 2 mechanism There are two mechanistic models for how an alkyl halide can undergo nucleophilic substitution. In the first picture, the reaction takes place in a single step, and bond-forming and bond-breaking occur simultaneously.
In all figures in this section, 'X' indicates a halogen substituent. This is called an ' S N 2' mechanism. In the term S N 2, S stands for 'substitution', the subscript N stands for 'nucleophilic', and the number 2 refers to the fact that this is a bimolecular reaction : the overall rate depends on a step in which two separate molecules the nucleophile and the electrophile collide. A potential energy diagram for this reaction shows the transition state TS as the highest point on the pathway from reactants to products.
If you look carefully at the progress of the S N 2 reaction, you will realize something very important about the outcome. The nucleophile, being an electron-rich species, must attack the electrophilic carbon from the back side relative to the location of the leaving group.
Approach from the front side simply doesn't work: the leaving group - which is also an electron-rich group - blocks the way. The result of this backside attack is that the stereochemical configuration at the central carbon inverts as the reaction proceeds. In a sense, the molecule is turned inside out. At the transition state, the electrophilic carbon and the three 'R' substituents all lie on the same plane. What this means is that S N 2 reactions whether enzyme catalyzed or not, are inherently stereoselective: when the substitution takes place at a stereocenter, we can confidently predict the stereochemical configuration of the product.
Below is an animation illustrating the principles we have just learned, showing the S N 2 reaction between hydroxide ion and methyl iodide. Notice how backside attack by the hydroxide nucleophile results in inversion at the tetrahedral carbon electrophile. Predict the structure of the product in this S N 2 reaction. Be sure to specify stereochemistry. The rate of an SN2 reaction is significantly influenced by the solvent in which the reaction takes place. The use of protic solvents those, such as water or alcohols, with hydrogen-bond donating capability decreases the power of the nucleophile, because of strong hydrogen-bond interactions between solvent protons and the reactive lone pairs on the nucleophile.
A less powerful nucleophile in turn means a slower SN2 reaction.
SN2 reactions are faster in polar, aprotic solvents: those that lack hydrogen-bond donating capability. Below are several polar aprotic solvents that are commonly used in the laboratory:.If you want to do well in this class, there are several things you need to work hard at: Being attentive in class, studying the notes and this textbook especially before examspracticing problems, and completing the quizzes and homeworks.
So there are many different factors that can affect your grade. In the same way, the outcome of a reaction such as nucleophilic substition depends on many different things — reactants, solvent, etc. When we want to make a chemical in a lab or on a chemical plant, we need to design the reaction so that it works well, and gives a good yield of the product in a reasonable time. In this section, we examine what factors will help an S N 2 or S N 1 reaction be successful.
As we saw in the previous section, in the S N 2 reaction the rate of reaction depends on both the electrophile usually an alkyl halide and the nucleophile.
In practice, the rates of S N 2 reactions vary enormously, and for a practicable procedure we need to make sure that the reaction will happen at a reasonable rate. So what makes for a good S N 2 reaction? We need to consider what makes a suitable nucleophile, and what makes a suitable electrophile. In section 6. Anything which removes electron-density from the nucleophilic atom will make it less nucleophilic. We summarized the main points from 6.
Regarding the solvent, polar aprotic solvents such as DMSO, DMF, acetone or acetonitrile are popular choices for S N 2 reactions, because rates are generally faster than with polar protic solvents water, alcohols, etc.
If we have a strong nucleophile, the S N 2 reaction will happen faster; a weak nucleophile will react more slowly and may not even react. So in general we want a strong nucleophile. As long as the two of the groups attached to the carbon being attacked are small hydrogens, the repulsions which happen do not require much energy.
If the groups attached to the carbon are larger, though, like methyl groups, the transition state energy increases, the activation energy increases, and the reaction becomes much slower.
Designing a “good” nucleophilic substitution
This means that the reactivity order for alkyl halides in S N 2 reactions is:. There are two main factors: The strength of the C-X bond, and the stability of the X group after it has left.
It turns out that the two factors lead to the same prediction for halogen leaving group ability:. Since the bond between the carbon and the leaving group is being broken in the transition state, the weaker this bond is the lower the activation energy and the faster the reaction.
This leads to the following reactivity order for alkyl halides. Practically, alkyl fluorides are not used for S N 2 reactions because the C-F bond is too strong. Often alkyl iodides are reactive enough to be difficult to store, so the the common choices for reactions are alkyl chlorides and alkyl bromides. When the C-X bond breaks in a nucleophilic substitution, the pair of electrons in the bond goes with the leaving group.The main focus here was at the substrate and the strength of the nucleophile.
This, as you know, is more complicated and there are separate posts devoted to this subject. There are so many factors to consider when choosing between S N 1, S N 2, E1 and E2 that the solvent is often overlooked. The solvent is what we use to carry out the reaction so, the main requirement for it is to dissolve the reactants. And because the reactants in nucleophilic substitution and elimination reactions are polar molecules, the solvent needs to be polar as well.
The dipole-dipole interaction of these polar molecules with the reactants is what helps to solvate them. There are two types of polar solvents; polar protic and polar aprotic. Polar protic solvents are capable of making hydrogen bonding i. The most common polar protic solvents are the water and alcohols. For example, when NaCl is dissolve in water, the sodium ion is solvated, through a dipole-dipole interaction, by the oxygen and the Cl — ions are solvated by hydrogen bonding with water molecules:.
Polar aprotic solventson the other hand, are the ones without a hydrogen connected to an electronegative atom and the key difference compared to polar protic solvents is the lack of intermolecular hydrogen bonding.
For example, when the salt is added to a solution of DMSO, only the sodium ions are solvated and the Cl — stays now as a naked ion:.
Now, the important question: How does this behavior of polar protic and polar aprotic solvents affect the nucleophilicity and basicity? Therefore, if we leave the solvent alone for a moment, we can say that the basicity and nucleophilicity increase as we go up the periodic table:. Check this article about the factors affecting the p K a to refresh the concept of stronger acids and bases. For example, the ethoxide ion is an excellent nucleophile and a strong base.
However, as we make the molecule bulkier sterically hinderedit becomes a weak nucleophile but still a strong base even though the negative charge is still on the same atom:.
To understand this pattern, remember that the base needs to only access a small proton, while the nucleophile needs to access a carbon atom which often hindered too:. This is why larger molecules lose their nucleophilicity while retaining the base strength.
The effect of a polar protic solvent is similar to this since it forms hydrogen bonds with the nucleophile thus putting it in a cage and making it sterically hindered. As we have seen above, this enlarged species are now weaker nucleophiles, yet, their basicity is largely intact:. If we compare this with basicity which is not affected, the result of a polar protic solvent is that the nucleophilicity is revered and it is now opposite to basicity.
A polar aprotic solvent, on the other hand, does not interact with the nucleophile since there is no hydrogen bonding. Therefore, the basicity and nucleophilicity are not affected and they still go parallel:.
If you need to choose between S N 1 and S N 2, then remember that polar aprotic solvents favor S N 2while polar protic solvents favor S N 1 mechanism since the nucleophilicity in this case is decreased. I have also addressed the effect of solvent in substitution reactions is this detailed post about the S N 2 mechanism. To keep this as simple as it can be, first remember that the solvent is not the first factor you consider when choosing between SN1, SN2, E1, E2.
For any of these pairs, remember protic solvents will favor elimination over substitution since caging the nucleophile by hydrogen bonding decreases the nucleophilicity since it makes the nucleophile bulkier and more difficult to reach the carbon with the leaving group. This effect is not significant on the basicity since even protonated and caged species can access the b-proton and thus serve as a base.
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Determine, based on the identity of the alkyl halide and the nucleophile, if the reaction goes through S N 1 or S N 2 mechanism and circle the right solvent to make the reaction faster. Draw the products of each reaction.
Polar Protic and Polar Aprotic Solvents There are two types of polar solvents; polar protic and polar aprotic.The nature of the nucleophile, the solvent, and the alkyl halide determine whether nucleophilic substitution takes place by the S N 1 or the S N 2 mecha-nism.
With polar aprotic solvents, primary alkyl halides react faster than sec-ondary halides by the S N 2 mechanism, whereas tertiary alkyl halides hardly react at all. With polar protic solvents and nonbasic nucleophiles, tertiary alkyl halides react faster than secondary alkyl halides by the SN1 mechanism, and primary halides do not react.
The reactivity of primary, secondary, and tertiary alkyl halides is controlled by electronic and steric factors. Polar, aprotic solvents are used for S N 2 reactions since they solvate cations but not anions. The rate of the S N 2 reaction increases with the nucleophilic strength of the incoming nucleophile. The rate of the S N 1 reaction is unaffected by the nature of the nucleophile. The reaction rates of both the S N 1 and the S N 2 reaction is increased if the leaving group is a stable ion and a weak base.
Iodide is a better leaving group than bromide and bromide is a better leaving group than chloride. Alkyl fluorides do not undergo nucleophilic substitution. Primary alkyl halides only have one alkyl group attached to this center and so access is easier. Formation of a planar carbocation in the first stage of the S N 1 mechanism is favored for tertiary alkyl halides since it relieves the steric strain in the crowded tetrahedral alkyl halide. The carbocation is also more accessible to an incoming nucleophile.
The formation of the carbocation is helped by electronic factors involving the inductive and hyperconjugationeffects of the three neighboring alkyl groups. Such inductive and hyperconjugation effects are greater in carbocations formed from tertiary alkyl halides than from those formed from primary or secondary alkyl halides.
Measuring how the reaction rate is affected by the concentration of the alkyl halide and the nucleophile determines whether a nucleophilic substitution is S N 2 or S N 1. Measuring the optical activity of products from the nucleo-philic substitution of asymmetric alkyl halides indicates the type of mecha-nism involved. A pure enantiomeric product indicates an S N 2 reaction. A partially or fully racemized product indicates an S N 1 reaction.
Primary alkyl halides react more quickly than secondary alkyl halides, with tertiary alkyl halides hardly reacting at all. Under protic solvent conditions with nonbasic nucleophiles e. Tertiary alkyl halides are more reactive than secondary alkyl halides, and primary alkyl halides do not react at all.
There are several factors which determine whether substitution will be S N 1 or S N 2 and which also control the rate at which these reactions take place.
These include the nature of the nucleophile and the type of solvent used. The S N 2 reaction works best in polar aprotic solvents i. Anions are solvated by hydrogen bonding and since the solvent is incapable of hydrogen bonding, the anions remain unsolvated.
Polar, protic solvents such as water or alcohols can also dissolve ionic reagents but they solvate both the metal cation and the anion.
This stabilizes the anion, makes it less nucleo- philic and makes it less likely to react by the S N 2 mechanism. As a result, the S N 1 mechanism becomes more important.
The S N 1 mechanism is particularly favored when the polar protic solvent is also a nonbasic nucleophile.
Therefore, it is most likely to occur when an alkyl halide is dissolved in water or alcohol. Protic solvents are bad for the S N 2 mechanism since they solvate the nucleophile, but they are good for the S N 1 mechanism. This is because polar protic solvents can solvate and stabilize the carbocation interme- diate. If the carbocation is stabilized, the transition state leading to it will also be stabilized and this determines whether the S N 1 reaction is favored or not.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. An S N 1 reaction speeds up with a good leaving group. This is because the leaving group is involved in the rate-determining step. A good leaving group wants to leave so it breaks the C-Leaving Group bond faster. Once the bond breaks, the carbocation is formed and the faster the carbocation is formed, the faster the nucleophile can come in and the faster the reaction will be completed.
A good leaving group is a weak base because weak bases can hold the charge. They're happy to leave with both electrons and in order for the leaving group to leave, it needs to be able to accept electrons.
Strong bases, on the other hand, donate electrons which is why they can't be good leaving groups. As you go from left to right on the periodic table, electron donating ability decreases and thus ability to be a good leaving group increases. Halides are an example of a good leaving group whos leaving-group ability increases as you go down the column. The two reactions below is the same reaction done with two different leaving groups. One is significantly faster than the other.
This is because the better leaving group leaves faster and thus the reaction can proceed faster. Other examples of good leaving groups are sulfur derivatives such as methyl sulfate ion and other sulfonate ions See Figure below.
What are the products of the following reaction and does it proceed via S N 1 or S N 2? Indicate the expected product and list why it occurs through S N 1 instead of S N 2? Polar aprotic solvents, a weak leaving group and primary substrates disfavor S N 1 reactions.
Reaction proceeds via SN1 because a tertiary carbocation was formed, the solvent is polar protic and Br- is a good leaving group. This reaction occurs via S N 1 because Cl - is a good leaving group and the solvent is polar protic.
This is an example of a solvolysis reaction because the nucleophile is also the solvent. Cation Rearrangements.Factors affecting rate of SN1. The reactivity of a SN1 reaction is determined by 3 factors:. Note that the nucleophilicity is not considered here but a weak nucleophile is usually preferred. Post a Comment. Popular posts from this blog E1 E2 Comparison - July 12, E2: A strong base is indeed needed to promote the one-step reaction.
Substrate E1: A more substituted substrate stabilizes the carbocation intermediate. E2: A more substituted substrate forms a more substituted alkene. Solvent E1: A polarizing solvent enhances the rate of ionization as it pulls the cation and anion apart. E2: The transition state is less sensitive to the solvent as the transition state has its negative charge shared over the whole molecule.
SN1 and SN2 Reaction of Haloalkanes
Leaving Group Both reactions need a good leaving group. Kinetics E1: The rate is only determined by the substrate as it is the only molecule that ionizes.
E2: Both the substrate and the base affects the rate of the concerted reaction. Stereochemistry E1: It actually d…. Read more. Here is a table summarized all we need to know about them.
The are several factors that affect the reaction rate of SN2: Nucleophilicity strength of nucleophile Substrate the guy being attacked by the nucleophile while there are 2 factors affecting the nucleophilicity i.
Let's go over it one by one. Nucleophilicity Nucleophile is a guy who loves nucleus as he has extra electrons around him. So generally, a nucleophile is negatively charged and the more negatively charge, the better nucleophile it is. We can then make a generalization that a conjugate base is a better nucleophile than its conjugate acid. For example, an ethoxide ion is more nucleophilic than its conjugate acid, ethanol.
Common Pitfall: it is tempting to say that the ethoxide ion is more nucleophilic because it is more basic. Basicity and Nucleophilicity is not the same.
Basicity is the ability ….The two symbols SN1 and SN2 refer to two reaction mechanisms. Even though both SN1 and SN2 are in the same category, they have many differences including the reaction mechanism, nucleophiles and solvents participated in the reaction, and the factors affecting the rate determining step.
In SN1 reactions, 1 indicates that the rate determining step is unimolecular. Thus, the reaction has a first-order dependence on electrophile and zero-order dependence on nucleophile. A carbocation is formed as an intermediate in this reaction and this type of reactions commonly occur in secondary and tertiary alcohols.
SN1 reactions have three steps. In SN2 reactions, one bond is broken, and one bond is formed simultaneously. In other words, this involves the displacement of the leaving group by a nucleophile. This reaction happens very well in methyl and primary alkyl halides whereas very slow in tertiary alkyl halides since the backside attack is blocked by bulky groups. The general mechanism for SN2 reactions can be described as follows.
SN2 Reactions: SN 2 reactions are single step reactions where both nucleophile and substrate are involved in the rate determining step. Therefore, the concentration of the substrate and that of the nucleophile will affect to the rate determining step.
The rate of the reaction is proportional to the stability of the carbocation. Therefore, the formation of the carbocation is the greatest barrier in SN1 reactions. The stability of the carbocation increases with the number of substituents and the resonance. This happens only if the empty orbitals are accessible.Factor affecting SN1 AND SN2 REACTION IN HINDI- REACTION FACTOR for Nucpeophlic substitution reactio
When more groups are attached to the leaving group, it slows the reaction. SN2 Reactions: SN 2 reactions require strong nucleophiles. Examples are water, alcohols, and carboxylic acids. They can also act as the nucleophiles for the reaction. Nucleophile : a chemical species that donates an electron pair to an electrophile to form a chemical bond in relation to a reaction.
Electrophile : a reagent attracted to electrons, they are positively charged or neutral species having vacant orbitals that are attracted to an electron rich centre. Coming from Engineering cum Human Resource Development background, has over 10 years experience in content developmet and management.
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