S N 2 Mechanism. The S N 2 reaction involves displacement of a leaving group usually a halide or a tosylateby a nucleophile. This reaction works the best with methyl and primary halides because bulky alkyl groups block the backside attack of the nucleophile, but the reaction does work with secondary halides although it is usually accompanied by eliminationand will not react at all with tertiary halides.
In the following example, the hydroxide ion is acting as the nucleophile and bromine is the leaving group:. Because of the backside attack of the nucleophile, inversion of configuration occurs. Solvents : Protic solvents such as water and alcohols stabilize the nucleophile so much that it won't react.
Therefore, a good polar aprotic solvent is required such as ethers and ketones and halogenated hydrocarbons. Nucleophiles : A good nucleophile is required since it is involved in the rate-determining step. Leaving groups : A good leaving group is required, such as a halide or a tosylate, since it is involved in the rate-determining step.
Home email: chemhelper gmail. In the following example, the hydroxide ion is acting as the nucleophile and bromine is the leaving group: Because of the backside attack of the nucleophile, inversion of configuration occurs.Topic hierarchy.
Previously Physical Properties of Haloalkaneswe learned that haloalkanes contain a polarized C-X bond, leaving a carbon that is partially positive and a halogen that is partially negative. Examples of Negatively Charged and Neutral Nucleophiles.
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. As the nucleophile forms a bond with this carbon atom, the bond between the carbon atom and the leaving group breaks. The bond making and bond breaking actions occur simultaneously. Eventually, the nucleophile has formed a complete bond to the carbon atom and the bond between the carbon atom and the leaving group is completely broken.
In the image below, we introduce the concepts of arrow pushing and of a reaction mechanism. Recall that electrons compose the bonds in molecules. Hence for any reaction to occur, electrons must move. In arrow pushing, the movement of electrons is indicated by arrows.
Arrows may show electrons forming or breaking bonds or traveling as lone pairs or negative charges on atoms. A complete schematic showing all steps in a reaction, including arrow pushing to indicate the movement of electrons, constitutes a reaction mechanism. In the reaction mechanism below, observe that the electrons from a negative charge on the nucleophile "attack" or form a bond with the carbon atom to which the leaving group is attached.
In the center image, the partially formed bond is visible, as well as the partially broken bond to the leaving group. In the final image, the bond to the leaving group is broken when its electrons become a negative charge on the leaving group.
Figure 1: SN2 reaction showing concerted, bimolecular participation of nucleophile and leaving group. A consequence of the concerted, bimolecular nature of the S N 2 reaction is that the nucleophile must attack from the side of the molecule opposite to the leaving group. This geometry of reaction is called back side attack.
In a back side attack, as the nucleophile approaches the molecule from the side opposite to the leaving group, the other three bonds move away from the nucleophile and its attacking electrons. Eventually, these three bonds are all in the same plane as the carbon atom center image. As the bond to the leaving group breaks, these bonds retreat farther away from the nucleophile and its newly formed bond to carbon atom.
As a result of these geometric changes, the stereochemical configuration of the molecule is inverted during an S N 2 reaction to the opposite enantiomer. This stereochemical change is called inversion of configuration. The concerted mechanism and nature of the nucleophilic attack in an S N 2 reaction give rise to several important results:. Now let's look at two actual examples of these two general equations. In the first reaction shown below, the negative nucleophile, hydroxide, reacts with methyl iodide.
Hydroxide takes the place of the leaving group, iodide, forming neutral methanol and an iodide ion. This reaction is the same as the first type of nucleophilic substitution shown above.The SN2 reaction is a bimolecular nucleophilic substitution reaction that occurs in one step. The nucleophile performs a backside attack on the carbon to which the leaving group is attached. If the carbon is asymmetric, inversion of stereochemistry is observed.
In other words, the nucleophile makes a bond and breaks the bond of the leaving group to the carbon holding it. Notice that the net charge of the reaction stays the same. SN2 transition state. Notice that both the nucleophile and leaving group have partial negative charges. That makes sense because the nucleophile is donating an electron while the leaving group is accepting an electron.
SN2 reaction coordinate diagram. In this diagram, there are really only three parts: the reagents, the transition state, and the products. The transition state is the point in the reaction with the highest energy level, and the difference in energy between the reagents and transition state is called the activation energy often abbreviated as Ea.
Remember that the rate-determining step is the step that has the highest activation energy in the reaction. The nucleophile in SN2 reactions is generally anionic.
A great example of this is NaCN. CN is negatively charged, and Na is positively charged. Group 1 atoms like Na, Li, and K are a dead giveaway! Atoms or molecules that can easily hold a negative charge are generally good leaving groups. Bromide leaving group. When a leaving group dissociates from the substrate, it gains an electron.
The bromine in the reaction above is a good leaving group because the pKa of its conjugate acid HBr is -9, which means it can hold a negative charge well. Not all leaving groups are created equal! Degree affects reactivity toward SN2. Secondary leaving groups risk competing with E2 reactions. Try to imagine a nucleophile trying to overcome the sterics of even three methyl groups, let alone three phenyl groups.
Steric hindrance. See how hard it is for the nucleophile to bypass all the R-groups to get to the carbon holding the tertiary halogen X? Theoretical tertiary SN2 reaction coordinate diagram. Other reactions that have lower activation energies will happen instead.
SN2 reactions function best in polar aprotic reactions. Acetone, DMSO, and dimethyl chloride are commonly used polar aprotic solvents. SN2 rate law. So, what happens to the rate if we double the concentration of the nucleophile?
The leaving group? Sodium cyanide and isopropyl bromide. All that happens is that the —CN attacks the secondary carbon bonded to the Br, and the formation of that bond causes the C-Br bond to break. The NaBr salt that forms is an inorganic product, and it can generally be ignored.
Now, wait a minute. See how I specified that the alkyl halide above was achiral? What happens when the leaving group is at a chiral center like in the following example?Teachers, not yet a subscriber?
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SN1 vs SN2 Reactions: What Is Steric Hindrance?
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It is a type of nucleophilic substitutionwhere a lone pair from a nucleophile attacks an electron deficient electrophilic center and bonds to it. This expels another group called a " leaving group ". So, the incoming group replaces the leaving group in one step.
Since two reacting species are involved in the slow, rate-determining step of the reactionthis leads to the name bi molecular n ucleophilic s ubstitutionor S N 2. Among inorganic chemists, the S N 2 reaction is often known as the interchange mechanism. The reaction most often occurs at an aliphatic sp 3 carbon center with an electronegativestable leaving group attached to it - 'X' - frequently a halide atom.
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 pentacoordinateand approximately sp 2 hybridized. The leaving group is then pushed off the opposite side and the product is formed. If the substrate under nucleophilic attack is chiralthis can lead, although not necessarily, to an inversion of stereochemistrycalled the Walden inversion.
A S N 2 reaction occurs if the backside route of attack is not blocked by other atoms in the molecule sterically hindered by substituents on the substrate. So, this mechanism usually occurs at an unhindered primary carbon center. If there is steric crowding on the substrate near the leaving group, such as at a tertiary carbon center, the substitution will use an S N 1 rather than an S N 2 mechanism, an S N 1 would also be more likely with blocked molecules because a sufficiently stable carbocation intermediary could be formed.
In coordination chemistryassociative substitution proceeds by a similar mechanism as S N 2. This is a key difference between the S N 1 and S N 2 mechanisms. In the S N 1 reaction, the nucleophile attacks after the rate-limiting step is over. But in a S N 2 reaction, the nucleophile forces off the leaving group in the limiting step.
In other words, the rate of S N 1 reactions depend only on the concentration of the substrate while the S N 2 reaction rate depends on the concentration of both the substrate and nucleophile. In cases where both mechanisms are possible for example at a secondary carbon centerthe mechanism depends on solvent, temperature, concentration of the nucleophile or on the leaving group.
S N 2 reactions are generally favored 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. S N 2 and S N 1 are two extremes of a sliding scale of reactions. It is possible to find many reactions which exhibit both S N 2 and S N 1 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 S N 2 for many of the reacting molecules but a few may show retention of configuration. S N 2 reactions are more common than S N 1 reactions.
A common side reaction taking place with S N 2 reactions is E2 elimination : the incoming anion can act as a base rather than as a nucleophile, removing a proton and leading to formation of the alkene. This effect can be demonstrated in the gas-phase reaction between a sulfonate and a simple alkyl bromide taking place inside a mass spectrometer :  .
With ethyl bromidethe reaction product is predominantly the substitution product. As steric hindrance around the electrophilic center increases, as with isobutyl bromide, substitution is disfavored and elimination is the predominant reaction. Other factors favoring elimination are the strength of the base. In general, gas phase reactions and solution phase reactions of this type follow the same trends, even though in the first, solvent effects are dropped.
A development attracting attention in concerns a S N 2 roundabout mechanism observed in a gas-phase reaction between chloride ions and methyl iodide with a special technique called crossed molecular beam imaging.
When the chloride ions have sufficient velocity, the energy of the resulting iodide ions after the collision is much lower than expected, and it is theorized that energy is lost as a result of a full roundabout of the methyl group around the iodine atom before the actual displacement takes place. From Wikipedia, the free encyclopedia.Nucleophilic substitution is the reaction of an electron pair donor the nucleophile, Nu with an electron pair acceptor the electrophile.
An sp 3 -hybridized electrophile must have a leaving group X in order for the reaction to take place. Mechanism of Nucleophilic Substitution The term S N 2 means that two molecules are involved in the actual transition state:. The departure of the leaving group occurs simultaneously with the backside attack by the nucleophile. The S N 2 reaction thus leads to a predictable configuration of the stereocenter - it proceeds with inversion reversal of the configuration.
In the S N 1 reaction, a planar carbenium ion is formed first, which then reacts further with the nucleophile.
Since the nucleophile is free to attack from either side, this reaction is associated with racemization. In both reactions, the nucleophile competes with the leaving group. Because of this, one must realize what properties a leaving group should have, and what constitutes a good nucleophile.
Organic Chemistry : Help with SN2 Reactions
For this reason, it is worthwhile to know which factors will determine whether a reaction follows an S N 1 or S N 2 pathway. Very good leaving groups, such as triflate, tosylate and mesylate, stabilize an incipient negative charge.
The delocalization of this charge is reflected in the fact that these ions are not considered to be nucleophilic. Epoxides are an exception, since they relieve their ring strain when they undergo nucleophilic substitution, with activation by acid being optional:. Triflate, tosylate and mesylate are the anions of strong acids. The weak conjugate bases are poor nucleophiles. Nucleophilicity increases in parallel with the base strength.
Thus, amines, alcohols and alkoxides are very good nucleophiles. Base strength is a rough measure of how reactive the nonbonding electron pair is; thus, it is not necessary for a nucleophile to be anionic. Under substitution conditions, amines proceed all the way to form quaternary salts, which makes it difficult to control the extent of the reaction.
However, as a nucleophile's base strength and steric hindrance increase, its basicity tends to be accentuated. An additional factor that plays a role is the character of the solvent.
Increasing stabilization of the nucleophile by the solvent results in decreasing reactivity. Thus, polar protic solvents will stabilize the chloride and bromide ions through the formation of hydrogen bonds to these smaller anions.The S N 2 reaction is a type of reaction mechanism that is common in organic chemistry. In this mechanism, one bond is broken and one bond is formed synchronously, i. S N 2 is a kind of nucleophilic substitution reaction mechanism.
Since two reacting species are involved in the slow rate-determining step, this leads to the term s ubstitution n ucleophilic bi -molecular or S N 2 ; the other major kind is S N 1. The reaction type is so common that it has other names, e. The reaction most often occurs at an aliphatic sp 3 carbon center with an electronegativestable leaving group attached to it often denoted Xwhich is frequently a halide atom. The breaking of the C—X bond and the formation of the new bond often denoted C—Y or C—Nu occur simultaneously through a transition state in which a carbon under nucleophilic attack is pentacoordinateand approximately sp 2 hybridised.
The leaving group is then pushed off the opposite side and the product is formed with inversion of the tetrahedral geometry at the central atom. If the substrate under nucleophilic attack is chiralthen this often leads to inversion of configuration stereochemistrycalled a Walden inversion. S N 2 attack occurs if the backside route of attack is not sterically hindered by substituents on the substrate. Therefore, this mechanism usually occurs at unhindered primary and secondary carbon centres.
If there is steric crowding on the substrate near the leaving group, such as at a tertiary carbon centre, the substitution will involve an S N 1 rather than an S N 2 mechanism, an S N 1 would also be more likely in this case because a sufficiently stable carbocation intermediary could be formed.
SN2 Reaction Mechanism
Four factors affect the rate of the reaction:  . The substrate plays the most important part in determining the rate of the reaction. This is because the nucleophile attacks from the back of the substrate, thus breaking the carbon-leaving group bond and forming the carbon-nucleophile bond. Therefore, to maximise the rate of the S N 2 reaction, the back of the substrate must be as unhindered as possible.
Overall, this means that methyl and primary substrates react the fastest, followed by secondary substrates. Tertiary substrates do not participate in S N 2 reactions, because of steric hindrance. Structures that can form highly stable cations by simple loss of the leaving group, for example, as a resonance-stabilized carbocation, are especially likely to react via an S N 1 pathway in competition with S N 2. Like the substrate, steric hindrance affects the nucleophile's strength.
The methoxide anion, for example, is both a strong base and nucleophile because it is a methyl nucleophile, and is thus very much unhindered. Nucleophile strength is also affected by charge and electronegativity : nucleophilicity increases with increasing negative charge and decreasing electronegativity.
In a polar aprotic solvent, nucleophilicity increases up a column of the periodic table as there is no hydrogen bonding between the solvent and nucleophile; in this case nucleophilicity mirrors basicity. The solvent affects the rate of reaction because solvents may or may not surround a nucleophile, thus hindering or not hindering its approach to the carbon atom. A polar aprotic solvent with low dielectric constant or a hindered dipole end will favour S N 2 manner of nucleophilic substitution reaction.
In polar aprotic solvent, nucleophilicity parallels basicity. The stability of the leaving group as an anion and the strength of its bond to the carbon atom both affect the rate of reaction. The more stable the conjugate base of the leaving group is, the more likely that it will take the two electrons of its bond to carbon during the reaction. Therefore, the weaker the leaving group is as a conjugate base, and thus the stronger its corresponding acid, the better the leaving group. This is a key difference between the S N 1 and S N 2 mechanisms.
In the S N 1 reaction the nucleophile attacks after the rate-limiting step is over, whereas in S N 2 the nucleophile forces off the leaving group in the limiting step. In other words, the rate of S N 1 reactions depend only on the concentration of the substrate while the S N 2 reaction rate depends on the concentration of both the substrate and nucleophile.