Reaction Mechanisms Reaction mechanisms are detailed step by step description of a reaction at the molecular level. This also shows the movement of electrons as well as the formation and breaking of bonds, Most of the time there are several proposed mechanisms for a reaction, but we use the most feasible and

most reasonable. A. Energy Profiles of Reactions Concerted Reactions

Whether a reaction is exergonic or endergonic, there should always an amount of energy known as the activation energy that must be needed.

Generally, the smaller the activation energy, the faster the reaction is.

Multistep Reactions

Note that reactants attain the transition state/activated complex (peak) and eventually the intermediate. The intermediate attains another transition state and to final product.

The slow step in this multistep reaction is the first step since it requires a larger activation energy for the

reaction to proceed. B. Free Radical Substitution (FRS)

This type of reaction is limited to your alkanes. In the presence of uv light or very high temperature, free radicals may be generated from halogens.

These halogen radicals in turn abstract protons from alkanes to create alkyl radicals. Finally, the alkyl and halogen radicals react to form an alkyl halide.

Free radical substitution comprises of three stages: initiation, propagation (radical chain reaction) and

termination. 1. Mechanism

There are actually mixtures of products that can be formed in FRS as we can see that in alkanes, not all carbons and hydrogens are equivalent.

Example: butane can give rise to two possible products, but the yields are not as expected. Why?

Exercise: How many monohalogenated products can you obtain with the following?

CH3

CH3

CH3

CH3 CH3

CH3

CH3

CH3

As one can see only Cl2 and Br2 are commonly used. F2 is highly reactive and I2 is very unreactive. As seen from above, the product distribution is not even in butane. This can be accounted by the relative

stabilities of radicals.

Recall that secondary radicals are more stable than primary radicals and therefore, more 2-chlorobutane will

be formed than chlorobutane.

In fact, both chlorine and bromine follow a set of relative rates in reaction depending on the type of radical formed.

Example:

The yields are computed based on the relative rates of formation. The same is applied to bromine.

2. Benzylic and Allylic Radicals

Benzyl and allyl radicals are more stable than alkyl radicals because their unpaired electrons are delocalized. The reaction of halogens with benzylic and allylic radicals follows essentially the same mechanisms as that of

other alkanes.

Illustration:

3. Reaction with N-bromosuccinimide (NBS)

N-Bromosuccinimide (NBS) is frequently used to brominate allylic positions. It allows a radical substitution reaction to be carried out without subjecting the reactant to a relatively high

concentration of that could add to its double bond.

4. Stereochemistry of FRS FRS of alkanes that generate chiral carbons often give rise to a pair of enantiomers as the radical is a planar

structure there are equal chances of attacking from above and below.

C. Electrophilic Addition

Electrophilic addition takes place in alkenes and alkynes, their double bonds are electron rich and can react with electrophiles.

The first step of the reaction is a relatively slow addition of the electrophilic proton to the nucleophilic alkene to form a carbocation intermediate.

In the second step, the positively charged carbocation intermediate (an electrophile) reacts rapidly with the negatively charged bromide ion (a nucleophile)

1. Addition of Hydrogen Halides

If the electrophilic reagent that adds to an alkene is a hydrogen halide (HF, HCl, HBr, or HI), the product of the reaction will be an alkyl halide.

Example:

As one can see, only one product is formed by the reaction above. How is this so? Exercise: Show the mechanism of the addition of hydrogen halides. Recall: Carbocation stability

In the reaction above, there are two carbocations formed, but since tertiary carbocations are more stable, then its product would be more favored than that of the primary carbocation.

Other Examples:

The Markovnikov’s Rule This rule is used to predict the major product of an electrophilic addition reaction. ““When a hydrogen halide adds to an unsymmetrical alkene, the addition occurs in such a manner that the halogen attaches itself to the double-bonded carbon atom of the alkene bearing the lesser number of hydrogen atoms.” “The hydrogen adds to the sp2 carbon that is bonded to the greater number of hydrogens.” Example:

Reactions that do not obey Markovnikov’s Rule are Anti-Markovnikov reactions. Reactions of Alkynes with Hydrogen Halides

Essentially the same mechanism as in alkenes.

Alkynes are usually difficult to stop at the alkene level and reacts further to form alkyl halides similar to alkenes.

2. Addition of Water and Alcohols

When water is added to an alkene, no reaction takes place, because there is no electrophile present to start a reaction by adding to the nucleophilic alkene.

If, however, an acid is added to the solution, a reaction will occur because the acid provides an electrophile.

The addition of water to a molecule is called hydration; the product of the reaction is an alcohol.

Mechanism:

For the case of hydration, the Markovnikov’s Rule is obeyed. Also, the addition of the catalyst is only to increase the rate of the reaction, it does not increase nor

decreases yield.

The addition of alcohols follows the same mechanism as with water, the product is ether.

Mechanism:

Carbocation Rearrangements Some electrophilic addition reactions do not result to the addition of the nucleophile to the carbon with less

number of hydrogens.

Illustration:

Whitmore suggested that there could be migration of groups or rearrangements taking place that could give rise to the said products.

The migration allowed the formation of tertiary carbocations which are more stable and therefore much preferred.

The 1, 2 Hydride Shift

The 1, 2 Methyl Shift

Other Examples: Ring Expansion and Insignificant Rearrangements

Reactions of Alkynes with Water

Addition of water to alkynes results to ketones.

3. Addition of Halogens

The halogens Cl2 and Br2 add to alkenes though at first it is not obvious that they are electrophiles.

However, the bond joining the two halogen atoms is relatively weak and, therefore, easily broken. When the electrons of the alkene approach a molecule of the halogen atoms, it accepts the electrons and

releases the shared electrons to the other halogen atom. Mechanism:

Therefore, in an electrophilic addition reaction, Br2 behaves as if it were Br+ and Br-, and Cl2 behaves as if it were Cl+ and Cl-.

Examples:

Vicinal dihalides are formed, meaning that the halogens are located in adjacent carbons. Since no carbocation is formed, there are no rearrangements involved. However, when the solvent of the reaction is not an inert solvent like CH2Cl2 but water, another reaction

known as the Halohydrin Formation takes place. Question: Why does this reaction take place? Why does water interfere?

Mechanism:

Reaction of Alkynes with Halogens

Cl2 and Br2 also add to alkynes and this often results to a tetra halo alkane.

Examples:

Word of Caution: Do not memorize the products of alkene addition reactions. Instead, for each reaction, ask yourself, “What is the

electrophile?" and “What nucleophile is present in the greatest concentration?” 4. Oxymercuration-Reduction

Hydration reactions as seen above are often used in industry to produce alcohols. However, in the laboratory, we often use the oxymercuration-reduction reaction to produce alcohols from alkenes.

Advantages over Hydration: a) No acid catalysts are required. b) There are no carbocations formed and hence no rearrangements. Mechanism:

Reduction Reactions: Notice that sodium borohydride (NaBH4) converts the C-Hg bond into a C-H bond.

A reaction that increases the number of C-H bonds or decreases the number of C-O, C-N, or C-X bonds in a compound (where X denotes a halogen), is called a reduction reaction.

NaBH4 is a reducing agent.

Oxymercuration-Reduction of Alkynes

This reaction of alkynes if fairly similar to hydration of alkynes.

Mechanism:

5. Hydroboration-Oxidation

Borane (BH3) a neutral molecule, is an electrophile because boron has only six shared electrons in its valence shell. Boron, therefore, readily accepts a pair of electrons in order to complete its octet.

When the addition reaction is over, an aqueous solution of sodium hydroxide and hydrogen peroxide is added to the reaction mixture, and the resulting product is an alcohol.

This reaction involves the formation of alcohols from alkenes in an Anti-Markovnikov fashion. Illustration:

Mechanism:

The addition of borane to an alkene is an example of a concerted reaction. The end result is replacement of boron by an OH group. Because replacing boron by an OH group is an

oxidation reaction, the overall reaction is called hydroboration– oxidation.

An oxidation reaction increases the number of C-O, C-N, or C-X bonds in a compound (where X denotes a halogen), or it decreases the number of C-H bonds.

Carbocation rearrangements do not in this type of reaction.

Hydroboration-Oxidation Reaction of Alkynes

The mechanisms follow essentially those of the alkenes. Since the alkyne reacts in an Anti-Markovnikov fashion, it results to an aldehyde instead of a ketone if

terminal alkynes are used. Illustration:

6. Addition of Peroxides If peroxide is added to the reaction, the orientation will switch from Markovnikov to Anti-Markovnikov. Illustration:

Peroxide reverses the order of addition because it changes the mechanism of the reaction in a way that

causes Br. to be the electrophile.

Illustration:

Mechanism:

As seen from above, when the Br radical attaches to the primary carbon (violates Markovnikov’s Rule) it

creates a secondary radical which is more stable than if the result of the reverse situation.

Also, there are no carbocation rearrangements when peroxides are added.

7. Addition of Hydrogen

In the presence of a metal catalyst such as platinum, palladium, or nickel, hydrogen (H2) adds to the double bond of an alkene to form an alkane.

Without metal catalysts, tremendous amounts of energy would be needed to break the H-H bond. Illustration:

Mechanism:

The addition of hydrogen is called hydrogenation. Because the preceding reactions require a catalyst, they are examples of catalytic hydrogenation.

The metal catalysts are insoluble in the reaction mixture and therefore are classified as heterogeneous catalysts.

When hydrogenated, different alkenes will release different amounts of heat. Such heat is termed heat of hydrogenation.

In general, the smaller the heats of hydrogenation, the more stable the alkene. It is apparent that alkyl substituents bonded to the carbons of an alkene have a stabilizing effect. Therefore, the more alkyl substituents bonded to the carbons of an alkene, the greater is its stability or in

terms of hydrogens, the statement is, the fewer hydrogens bonded to the carbons of alkene, the greater is its stability.

Question: cis and trans alkenes have two groups attached. Which is more stable? Addition of Hydrogen to Alkynes The mechanism of this reaction is similar to that of alkenes. The reaction usually proceeds to the alkane level and cannot be stopped at the alkene level. Illustration:

Internal alkynes can usually form either a cis or trans product, therefore, special reactions were develop to

control product stereochemistry. Synthesis of Cis-alkenes

The reaction can be stopped at the alkene stage if a “poisoned” (partially deactivated) metal catalyst is used. The most commonly used partially deactivated metal catalyst is Lindlar catalyst, which is prepared by

precipitating palladium on calcium carbonate and treating it with lead(II) acetate and quinoline. This reaction results to a cis-alkene.

Illustration:

Synthesis of Trans-alkenes

Similarly, internal alkynes can be converted into trans-alkenes with the use of sodium (Na) in liquid ammonia at -78oC.

Illustration: