SN2 reaction of Alkyl halides

In this subject Second-Order Nucleophilic Substitution: The SN2 Reaction of Alkyl halides will be discussed

Reactions of Alkyl Halides: Substitution and Elimination

Alkyl halides are easily converted to many other functional groups.

– The halogen atom can leave with its bonding pair of electrons to form a stable halide ion; we say that a halide is a good leaving group.

– When another atom replaces the halide ion, the reaction is a substitution.

– When the halide ion leaves with another atom or ion (often H+) and forms a new pi bond, the reaction is an elimination.

– In many eliminations, a molecule of  H-X is lost from the alkyl halide to give an alkene.

– These eliminations are called dehydrohalogenations because a hydrogen halide has been removed from the alkyl halide.

– Substitution and elimination reactions often compete with each other.

– In a nucleophilic substitution, a nucleophile (Nuc:) replaces a leaving group (X) from a carbon atom, using its lone pair of electrons to form a new bond to the carbon atom

Nucleophilic substitution

SN2 reaction of Alkyl halides

In an elimination, both the halide ion and another substituent are lost. A new bond is formed


SN2 reaction of Alkyl halides

– In the elimination (a dehydrohalogenation), the reagent reacts as a base, abstracting a proton from the alkyl halide.

– Most nucleophiles are also basic and can engage in either substitution or elimination, depending on the alkyl halide and the reaction conditions.

– Besides alkyl halides, many other types of compounds undergo substitution and elimination reactions.

– Substitutions and eliminations are introduced in this subject using the alkyl halides as examples.

– In later subjects, we encounter substitutions and eliminations of other types of compounds.

Second-Order Nucleophilic Substitution: The SN2 Reaction

– A nucleophilic substitution has the general form.

SN2 reaction of Alkyl halides

where Nuc: is the nucleophile and X is the leaving halide ion. An example is the reaction of iodomethane (CH3I) with hydroxide ion. The product is methanol.

SN2 reaction of Alkyl halides

– Hydroxide ion is a strong nucleophile (donor of an electron pair) because the oxygen atom has unshared pairs of electrons and a negative charge.

– Iodomethane is called the substrate, meaning the compound that is attacked by the reagent.

– The carbon atom of iodomethane is electrophilic because it is bonded to an electronegative iodine atom.

– Electron density is drawn away from carbon by the halogen atom, giving the carbon atom a partial positive charge.

– The negative charge of hydroxide ion is attracted to this partial positive charge

SN2 reaction of Alkyl halides

– Hydroxide ion attacks the back side of the electrophilic carbon atom, donating a pair of electrons to form a new bond. (In general, nucleophiles are said to attack electrophiles, not the other way around.)

– Notice that curved arrows are used to show the movement of electron pairs, from the electron-rich nucleophile to the electron poor carbon atom of the electrophile.

– Carbon can accommodate only eight electrons in its valence shell, so the carbon–iodine bond must begin to break as the carbon–oxygen bond begins to form.

– Iodide ion is the leaving group; it leaves with the pair of electrons that once bonded it to the carbon atom.

– This one-step mechanism is supported by kinetic information. One can vary the concentrations of the reactants and observe the effects on the reaction rate (how much methanol is formed per second).

– The rate is found to double when the concentration of either reactant is doubled.

– The reaction is therefore first order in each of the reactants and second order overall.

– The rate equation has the following form:

– This rate equation is consistent with a mechanism that requires a collision between a molecule of methyl iodide and a hydroxide ion.

– Both of these species are present in the transition state, and the collision frequency is proportional to both concentrations.

– The rate constant  Kr depends on several factors, including the energy of the transition state and the temperature

– This one-step nucleophilic substitution is an example of the SN2 mechanism.

– The abbreviation SN2 stands for Substitution, Nucleophilic, bimolecular.

– The term bimolecular means that the transition state of the rate-limiting step (the only step in this reaction) involves the collision of two molecules.

– Bimolecular reactions usually have rate equations that are second order overall.

– The SN2 reaction of methyl iodide (iodomethane) with hydroxide ion is a concerted reaction, taking place in a single step with bonds breaking and forming at the same time.

– The middle structure is a transition state, a point of maximum energy, rather than an intermediate.

– In this transition state, the bond to the nucleophile (hydroxide) is partially formed, and the bond to the leaving group (iodide) is partially broken.

– Remember that a transition state is not a discrete molecule that can be isolated; it exists for only an instant.

– The reaction-energy diagram for this substitution (see Figure) shows only one transition state and no intermediates between the reactants and the products.

SN2 reaction of Alkyl halides
The reaction-energy diagram for the reaction of methyl iodide with hydroxide shows only one energy maximum: the transition state. There are no intermediates. The electrostatic potential maps of the reactants, transition state, and products show that the negatively charged nucleophile (red) attacks the electrophilic (blue) region of the substrate. In the transition state, the negative charge (red) is delocalized over the nucleophile and the leaving group. The negative charge leaves with the leaving group.

– The reactants are shown slightly higherin energy than the products because this reaction is known  to be exothermic.

– The transition state is much higher in energy because it involves a fivecoordinate carbon atom with two partial bonds.

– The following mechanism shows a general SN2 reaction.

– A nucleophile attacks the substrate to give a transition state in which a bond to the nucleophile is forming at the same time as the bond to the leaving group is breaking.

MECHANISM: The SN2 Reaction

– The SN2 reaction takes place in a single (concerted) step.

– A strong nucleophile attacks the electrophilic carbon, forcing the leaving group to leave

The order of reactivity for substrates is:

CH3 X > 1° > 2°

(3° alkyl halides cannot react by this mechanism.)


Reaction of 1-bromobutane with sodium methoxide gives 1-methoxybutane

Generality of the SN2 Reaction

– Many useful reactions take place by SN2 the mechanism.

– The reaction of an alkyl halide, such as methyl iodide, with hydroxide ion gives an alcohol.

– Other nucleophiles convert alkyl halides to a wide variety of functional groups.

– The following table summarizes some of the types of compounds that can be formed by nucleophilic displacement of alkyl halides

SUMMARY: SN2 Reactions of Alkyl Halides

SN2 reaction of Alkyl halides

SN2 reaction of Alkyl halides

Halogen Exchange Reactions

– The SN2 reaction provides a useful method for synthesizing alkyl iodides and fluorides, which are more difficult to make than alkyl chlorides and bromides.

– Halides can be converted to other halides by halogen exchange reactions, in which one halide displaces another.

– Iodide is a good nucleophile, and many alkyl chlorides react with sodium iodide to give alkyl iodides.

– Alkyl fluorides are difficult to synthesize directly, and they are often made by treating alkyl chlorides or bromides with KF under conditions that use a crown ether to dissolve the fluoride salt in an aprotic solvent, which enhances the normally weak nucleophilicity of the fluoride ion.

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