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35-452: The name S N 2 refers to the Hughes-Ingold symbol of the mechanism: "S N " indicates that the reaction is a nucleophilic substitution , and "2" that it proceeds via a bimolecular mechanism, which means both the reacting species are involved in the rate-determining step . What distinguishes S N 2 from the other major type of nucleophilic substitution, the S N 1 reaction , is that

70-409: A C−H bond adjacent to a ketone or aldehyde . The nucleophilic center for simple alkoxides is located on the oxygen, whereas the nucleophilic site on enolates is delocalized onto both carbon and oxygen sites. Ynolates are also unsaturated alkoxides derived from acetylenic alcohols. Phenoxides are close relatives of the alkoxides, in which the alkyl group is replaced by a phenyl group. Phenol

105-438: A base rather than as a nucleophile, abstracting a proton and leading to formation of the alkene . This pathway is favored with sterically hindered nucleophiles. Elimination reactions are usually favoured at elevated temperatures because of increased entropy . This effect can be demonstrated in the gas-phase reaction between a phenolate and a simple alkyl bromide taking place inside a mass spectrometer : With ethyl bromide ,

140-579: A negatively charged oxygen atom. They are written as RO , where R is the organyl substituent . Alkoxides are strong bases and, when R is not bulky , good nucleophiles and good ligands . Alkoxides, although generally not stable in protic solvents such as water, occur widely as intermediates in various reactions, including the Williamson ether synthesis . Transition metal alkoxides are widely used for coatings and as catalysts . Enolates are unsaturated alkoxides derived by deprotonation of

175-403: A prospective approach possessing an advantage of capability of obtaining functional materials with increased phase and chemical homogeneity and controllable grain size (including the preparation of nanosized materials) at relatively low temperature (less than 500–900 °C) as compared with the conventional techniques. Sodium methoxide, also called sodium methylate and sodium methanolate,

210-417: A significant impact on the intrinsic strength of the nucleophile, in which strong interactions between solvent and the nucleophile, found for polar protic solvents , furnish a weaker nucleophile. In contrast, polar aprotic solvents can only weakly interact with the nucleophile, and thus, are to a lesser extent able to reduce the strength of the nucleophile. The rate of an S N 2 reaction is second order , as

245-509: Is a better nucleophile than water, and I is a better nucleophile than Br (in polar protic solvents). 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. I would therefore be a weaker nucleophile than Br because it is a weaker base. Verdict - A strong/anionic nucleophile always favours S N 2 manner of nucleophillic substitution. Good leaving groups on

280-516: Is a white powder when pure. It is used as an initiator of an anionic addition polymerization with ethylene oxide , forming a polyether with high molecular weight. Both sodium methoxide and its counterpart prepared with potassium are frequently used as catalysts for commercial-scale production of biodiesel . In this process, vegetable oils or animal fats, which chemically are fatty acid triglycerides, are transesterified with methanol to give fatty acid methyl esters (FAMEs). Sodium methoxide

315-467: Is an elimination reaction , an S E 2 reaction involves electrophilic substitution , and an S N 1 reaction is unimolecular. The system is named for British chemists Edward D. Hughes and Christopher Kelk Ingold . This chemical reaction article is a stub . You can help Misplaced Pages by expanding it . Methoxide In chemistry , an alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to

350-534: Is frequently a halogen (often denoted X). The formation of the C–Nu bond, due to attack by the nucleophile (denoted Nu), occurs together with the breakage of the C–X bond. The reaction occurs through a transition state in which the reaction center is pentacoordinate and approximately sp-hybridised. The S N 2 reaction can be viewed as a HOMO–LUMO interaction between the nucleophile and substrate. The reaction occurs only when

385-490: Is more acidic than a typical alcohol; thus, phenoxides are correspondingly less basic and less nucleophilic than alkoxides. They are, however, often easier to handle and yield derivatives that are more crystalline than those of the alkoxides. Alkali metal alkoxides are often oligomeric or polymeric compounds, especially when the R group is small (Me, Et). The alkoxide anion is a good bridging ligand , thus many alkoxides feature M 2 O or M 3 O linkages. In solution,

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420-414: Is one S N 2 reaction in which the leaving group can also act as a nucleophile. In this reaction, the substrate has a halogen atom exchanged with another halogen. As the negative charge is more-or-less stabilized on both halides, the reaction occurs at equilibrium. The solvent affects the rate of reaction because solvents may or may not surround a nucleophile, thus hindering or not hindering its approach to

455-427: The rate-determining step depends on the nucleophile concentration, [Nu] as well as the concentration of substrate, [RX]. 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

490-485: The S N 1 mechanism invariably involve the use of bromide (or other good nucleophile) as the leaving group have confused the understanding of alkyl nucleophilic substitution reactions at secondary carbons for 80 years. Work with the 2-adamantyl system (S N 2 not possible) by Schleyer and co-workers, the use of azide (an excellent nucleophile but very poor leaving group) by Weiner and Sneen, the development of sulfonate leaving groups (non-nucleophilic good leaving groups), and

525-424: The addition of sodium metal to methanol : Other alkali metals can be used in place of sodium, and most alcohols can be used in place of methanol. Generally, the alcohol is used in excess and left to be used as a solvent in the reaction. Thus, an alcoholic solution of the alkali alkoxide is used. Another similar reaction occurs when an alcohol is reacted with a metal hydride such as NaH. The metal hydride removes

560-424: The alkali metal derivatives exhibit strong ion-pairing, as expected for the alkali metal derivative of a strongly basic anion. Alkoxides can be produced by several routes starting from an alcohol . Highly reducing metals react directly with alcohols to give the corresponding metal alkoxide. The alcohol serves as an acid , and hydrogen is produced as a by-product. A classic case is sodium methoxide produced by

595-509: The carbon atom. Polar aprotic solvents, like tetrahydrofuran , are better solvents for this reaction than polar protic solvents because polar protic solvents will hydrogen bond to the nucleophile, hindering it from attacking the carbon with the leaving group. A polar aprotic solvent with low dielectric constant or a hindered dipole end will favour S N 2 manner of nucleophilic substitution reaction. Examples: dimethylsulfoxide , dimethylformamide , acetone , etc. In parallel, solvation also has

630-565: The carbon center prior to nucleophilic attack. Halides ( Cl , Br , and I , with the exception of F ), serve as good anionic leaving groups because electronegativity stabilizes additional electron density; the fluoride exception is due to its strong bond to carbon. Leaving group reactivity of alcohols can be increased with sulfonates , such as tosylate ( OTs ), triflate ( OTf ), and mesylate ( OMs ). Poor leaving groups include hydroxide ( OH ), alkoxides ( OR ), and amides ( NR 2 ). The Finkelstein reaction

665-542: The central carbon, i.e. those that do not have as much sterically hindering substituents nearby. Methyl and primary substrates react the fastest, followed by secondary substrates. Tertiary substrates do not react via the S N 2 pathway, as the greater steric hindrance between the nucleophile and nearby groups of the substrate will leave the S N 1 reaction to occur first. Substrates with adjacent pi C=C systems can favor both S N 1 and S N 2 reactions. In S N 1, allylic and benzylic carbocations are stabilized by delocalizing

700-518: The concentration of the substrate while the S N 2 reaction rate depends on the concentration of both the substrate and nucleophile. It has been shown that except in uncommon (but predictable cases) primary and secondary substrates go exclusively by the S N 2 mechanism while tertiary substrates go via the S N 1 reaction. There are two factors which complicate determining the mechanism of nucleophilic substitution reactions at secondary carbons: The examples in textbooks of secondary substrates going by

735-422: The demonstration of significant experimental problems in the initial claim of an S N 1 mechanism in the solvolysis of optically active 2-bromooctane by Hughes et al. have demonstrated conclusively that secondary substrates go exclusively (except in unusual but predictable cases) by the S N 2 mechanism. A common side reaction taking place with S N 2 reactions is E2 elimination : the incoming anion can act as

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770-416: The displacement of the leaving group, which is the rate-determining step, is separate from the nucleophilic attack in S N 1. The S N 2 reaction can be considered as an organic-chemistry analogue of the associative substitution from the field of inorganic chemistry . The reaction most often occurs at an aliphatic sp carbon center with an electronegative , stable leaving group attached to it, which

805-484: The hydrogen atom from the hydroxyl group and forms a negatively charged alkoxide ion. The alkoxide ion and its salts react with primary alkyl halides in an S N 2 reaction to form an ether via the Williamson ether synthesis . Aliphatic metal alkoxides decompose in water as summarized in this idealized equation: In the transesterification process, metal alkoxides react with esters to bring about an exchange of alkyl groups between metal alkoxide and ester. With

840-636: The hydrolysis, often accidentally, and via ether elimination: Many metal alkoxides thermally decompose in the range ≈100–300 °C. Depending on process conditions, this thermolysis can afford nanosized powders of oxide or metallic phases. This approach is a basis of processes of fabrication of functional materials intended for aircraft, space, electronic fields, and chemical industry: individual oxides, their solid solutions, complex oxides, powders of metals and alloys active towards sintering. Decomposition of mixtures of mono- and heterometallic alkoxide derivatives has also been examined. This method represents

875-450: The leaving group, resulting in the leaving group being pushed off the opposite side and the product formed with inversion of tetrahedral geometry at the central atom. For example, the synthesis of macrocidin A, a fungal metabolite , involves an intramolecular ring closing step via an S N 2 reaction with a phenoxide group as the nucleophile and a halide as the leaving group, forming an ether . Reactions such as this, with an alkoxide as

910-467: The metal alkoxide complex in focus, the result is the same as for alcoholysis, namely the replacement of alkoxide ligands, but at the same time the alkyl groups of the ester are changed, which can also be the primary goal of the reaction. Sodium methoxide in solution, for example, is commonly used for this purpose, a reaction that is used in the production of biodiesel . Many metal alkoxide compounds also feature oxo- ligands . Oxo-ligands typically arise via

945-480: The methyl iodide to spin around once before the actual S N 2 displacement mechanism takes place. Hughes-Ingold symbol A Hughes–Ingold symbol describes various details of the reaction mechanism and overall result of a chemical reaction . For example, an S N 2 reaction is a substitution reaction ("S") by a nucleophilic process ("N") that is bimolecular ("2" molecular entities involved) in its rate-determining step . By contrast, an E2 reaction

980-469: The nucleophile, are known as the Williamson ether synthesis . If the substrate that is undergoing S N 2 reaction has a chiral centre , then inversion of configuration ( stereochemistry and optical activity ) may occur; this is called the Walden inversion . For example, 1-bromo-1-fluoroethane can undergo nucleophilic attack to form 1-fluoroethan-1-ol, with the nucleophile being an HO group. In this case, if

1015-424: The occupied lone pair orbital of the nucleophile donates electrons to the unfilled σ* antibonding orbital between the central carbon and the leaving group . Throughout the course of the reaction, a p orbital forms at the reaction center as the result of the transition from the molecular orbitals of the reactants to those of the products. To achieve optimal orbital overlap, the nucleophile attacks 180° relative to

1050-400: The positive charge. In S N 2, however, the conjugation between the reaction centre and the adjacent pi system stabilizes the transition state. Because they destabilize the positive charge in the carbocation intermediate, electron-withdrawing groups favor the S N 2 reaction. Electron-donating groups favor leaving-group displacement and are more likely to react via the S N 1 pathway. Like

1085-517: The reactant is levorotatory, then the product would be dextrorotatory, and vice versa. The four factors that affect the rate of the reaction, in the order of decreasing importance, are: The substrate plays the most important part in determining the rate of the reaction. For S N 2 reaction to occur more quickly, the nucleophile must easily access the sigma antibonding orbital between the central carbon and leaving group. S N 2 occurs more quickly with substrates that are more sterically accessible at

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1120-464: The 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. With the less basic benzoate substrate, isopropyl bromide reacts with 55% substitution. In general, gas phase reactions and solution phase reactions of this type follow

1155-420: The same trends, even though in the first, solvent effects are eliminated. A development attracting attention in 2008 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 initial collision of it with the methyl iodide molecule causes

1190-541: The substrate lead to faster S N 2 reactions. A good leaving group must be able to stabilize the electron density that comes from breaking its bond with the carbon center. This leaving group ability trend corresponds well to the p K a of the leaving group's conjugate acid (p K aH ); the lower its p K aH value, the faster the leaving group is displaced. Leaving groups that are neutral, such as water , alcohols ( R−OH ), and amines ( R−NH 2 ), are good examples because of their positive charge when bonded to

1225-545: 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. tert -Butoxide , on the other hand, is a strong base, but a poor nucleophile, because of its three methyl groups hindering its approach to the carbon. Nucleophile strength is also affected by charge and electronegativity : nucleophilicity increases with increasing negative charge and decreasing electronegativity. For example, OH

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