Core Concepts
In this article, you will learn about some of the most important addition reactions and mechanisms of alkenes, highlighting regioselectivity and stereoselectivity. Understanding these processes is crucial for the design and synthesis of a wide array of organic compounds, ranging from simple hydrocarbons to complex pharmaceuticals and polymers.
Introduction
Alkene addition reactions are a fundamental class of chemical reactions central to organic synthesis. These reactions involve the addition of various atoms or groups across the carbon-carbon double bond of an alkene, leading to the formation of more complex molecules. You can convert alkenes to alkyl halides, epoxides, alcohols, aldehydes, ketones, carboxylic acids, and other functional groups. The reactions of alkenes arise from the reactivity of their carbon-carbon double bonds. Organic chemists enjoy the challenge of taking a simple carbon-carbon double bond and manipulating it in all possible ways to produce other compounds, often mimicking biological reactions that occur in cells. These are some of the reactions with their mechanisms covered in this article:
Addition of Hydrogen Halides
Hydration
Oxymercuration–Demercuration
Alkoxymercuration–Demercuration
Hydroboration-Oxidation
Addition of Halogens
Formation of Halohydrins
Hydrogenation
Epoxidation
Syn Hydroxylation
Cleavage by Permanganate
Ozonolysis
Addition of Hydrogen Halides to Alkenes
In this reaction, a hydrogen halide (HX) adds across the double bond of an alkene, resulting in the formation of an alkyl halide. This reaction typically follows Markovnikov’s rule, where the hydrogen atom attaches to the carbon with the greater number of hydrogen atoms, and the halide attaches to the carbon with fewer hydrogen atoms. Let’s look at the reaction mechanism to understand better why this occurs.
As you can see, the mechanism involves the formation of a carbocation. Adding the proton to the less substituted end of the double bond gives the more substituted carbocation, which is more stable and forms faster. Therefore, for this type of addition, you will only see Markovnikov’s product. The addition of hydrogen halides is regioselective because in each case, one of the two possible orientations of addition results preferentially over the other. Some other examples:
Free-Radical Addition of HBr: Anti-Markovnikov Addition
It is possible to generate an Anti-Markovnikov variation of the previous addition in the presence of peroxides as free radical initiators. However, this can only be performed with HBr. The product of this reaction would contain the halide attached to the least substituted carbon of the initial alkene. Let’s look at the mechanism and some examples!
Here are some more examples:
In both HBr addition mechanisms to alkenes (with and without peroxides), the electrophile adds to the less substituted end of the double bond, forming the more stable intermediate (carbocation or free radical).
Hydration of Alkenes
An alkene may react with water in the presence of a strongly acidic catalyst to form an alcohol. Like the addition of hydrogen halides, hydration is regioselective: It follows Markovnikov’s rule, giving a product in which the new hydrogen adds to the less substituted end of the double bond. Let’s look at the mechanism of the acid-catalyzed hydration of propane.
Other examples!
Like other reactions that involve carbocation intermediates, hydration may take place with rearrangement. The last example in the previous picture is an example of that. During that reaction, there was a methyl shift rearrangement of the intermediate to yield the shown product. It would be great practice to write out the mechanism on your own!
Oxymercuration–Demercuration of Alkenes
Many alkenes do not readily hydrate in aqueous acid. Some are almost insoluble in these conditions, while others experience side reactions like rearrangement, polymerization, or charring under the strong acidity. Additionally, in some instances, the equilibrium tends to favor the formation of the alkene over the alcohol. Oxymercuration–demercuration is another method for converting alkenes to alcohols with Markovnikov orientation. Since no free carbocation is formed, there is no opportunity for rearrangements or polymerization. Let’s look at the oxymercuration–demercuration mechanism of 3,3-dimethylbut-1-ene, which gives the Markovnikov product, 3,3-dimethylbutan-2-ol. This would contrast the rearranged product formed in the acid-catalyzed hydration of this molecule shown before.
Alkoxymercuration–Demercuration of Alkenes
Alkoxymercuration–demercuration converts alkenes to ethers by adding an alcohol across the double bond of the alkene. When mercuration takes place in an alcohol solvent, the alcohol serves as a nucleophile to attack the mercurinium ion. The resulting product contains an alkoxy (-O-R) group. This reaction proceeds with Markovnikov regioselectivity and avoids carbocation rearrangements, providing a straightforward and reliable method for ether synthesis.
The mechanism is very similar to what we saw before for Oxymercuration–Demercuration. An alkene reacts to form a mercurinium ion that is attacked by the nucleophilic solvent. Attack by an alcohol solvent gives an organomercurial ether that can be reduced to the ether.
The solvent attacks the mercurinium ion at the more substituted end of the double bond, giving Markovnikov orientation of addition. The Hg(OAc) group appears at the less substituted end of the double bond. Reduction gives the Markovnikov product, with hydrogen at the less substituted end of the double bond.
Hydroboration-Oxidation of Alkenes
We have seen two methods for hydrating an alkene with Markovnikov orientation. What if we need to convert an alkene to the anti-Markovnikov alcohol? Hydroboration is characterized by its anti-Markovnikov selectivity, where the boron atom attaches to the less substituted carbon atom of the double bond, while the hydrogen atom attaches to the more substituted carbon. The hydroboration process is typically followed by oxidation, converting the organoborane intermediate into an alcohol. Below is the mechanism:
The boron atom is removed by oxidation, using aqueous sodium hydroxide and hydrogen peroxide (H2O2) to replace the boron atom with a hydroxy (-OH) group.
Here are some more examples:
Addition of Halogens to Alkenes
Halogen addition to alkenes is a reaction where diatomic halogens, such as chlorine (Cl₂) or bromine (Br₂), add across the carbon-carbon double bond of an alkene, forming a vicinal dihalide. Let’s explore the mechanism of this addition through the addition of Br2 to propane.
Any solvents used must be inert to the halogens; methylene chloride (CH2Cl2), chloroform (CHCl3), and carbon tetrachloride (CCl4) are the most frequent choices. This reaction is stereospecific, producing vicinal dihalides with anti-stereochemistry, and it is a valuable tool in organic synthesis for introducing halogen functionalities into molecules.
Formation of Halohydrins
A halohydrin is an alcohol with a halogen on the adjacent carbon atom. In the presence of water, halogens add to alkenes to form halohydrins. This reaction differs from the one before in that water acts as a nucleophile to open the halonium ion and form the halohydrin. Let’s look at the addition of Cl2 to propene in water.
Because the mechanism involves a halonium ion, the stereochemistry of addition is anti, as in halogenation.
Catalytic Hydrogenation of Alkenes
In this reaction, hydrogen (Hâ‚‚) is added across the carbon-carbon double bond of an alkene to produce an alkane. This process typically requires a metal catalyst, such as palladium (Pd), platinum (Pt), or nickel (Ni), which facilitates the breaking of the hydrogen-hydrogen bond and the subsequent addition of hydrogen atoms to the carbon atoms of the double bond.
Examples:
The stereochemistry of catalytic hydrogenation of alkenes is characterized by syn addition, meaning that both hydrogen atoms are added to the same side of the plane of the double bond.
Epoxidation of Alkenes
Epoxidation of alkenes is a reaction that converts a carbon-carbon double bond into an epoxide, which is a three-membered cyclic ether with an oxygen atom. The process is stereospecific, maintaining the stereochemistry of the original alkene and resulting in a product where the oxygen adds across the double bond in a manner that preserves the relative positions of any substituents on the alkene. An alkene is converted to an epoxide by a peroxyacid, a carboxylic acid that has an extra oxygen atom in a -O-O- (peroxy) linkage. Let’s observe the mechanism:
Epoxidation of Propene by Peroxyacetic Acid
The following examples use m-chloroperoxybenzoic acid (mCPBA), a common epoxidizing reagent, to convert alkenes to epoxides having the same cis or trans stereochemistry.
Acid-Catalyzed Opening of Epoxides
Through this reaction, an epoxide ring can be cleaved in the presence of an acid to form a diol. This process coupled with an epoxidation would convert an alkene into a diol. Let’s see how this would happen starting from the epoxide!
Because diol formation involves a back-side attack on a protonated epoxide, the result is anti-orientation.
Syn Hydroxylation of Alkenes
We have seen that the epoxidation of an alkene, followed by acidic hydrolysis, causes anti-dihydroxylation of the double bond. Syn hydroxylation of alkenes is a reaction that adds two hydroxyl (OH) groups to the same side of the carbon-carbon double bond, resulting in the formation of a vicinal diol in a syn configuration. The two most common reagents for this purpose are osmium tetroxide in the presence of hydrogen peroxide and potassium permanganate in basic medium.
Osmium tetroxide (or osmic acid) reacts with alkenes in a concerted step to form a cyclic osmate ester. Oxidizing agents such as hydrogen peroxide (H2O2) or tertiary amine oxides are used to hydrolyze the osmate ester and reoxidize osmium to osmium tetroxide. The regenerated osmium tetroxide catalyst continues to hydroxylate more molecules of the alkene.
Like osmium tetroxide, permanganate adds to the alkene double bond to form a cyclic ester: a manganate ester in this case. The basic solution hydrolyzes the manganate ester, liberating the glycol and producing a brown precipitate of manganese dioxide, MnO2.
Oxidative Cleavage of Alkenes
We just described two methods for oxidizing alkenes to glycols. Stronger conditions can further oxidize glycols to cleave the bond that was originally the double bond. Potassium permanganate and ozone are two of the most common reagents for such oxidative cleavages.
Cleavage by Permanganate
In this reaction, potassium permanganate (KMnOâ‚„) oxidizes an alkene, resulting in the breaking of the carbon-carbon double bond and forming carbonyl compounds such as ketones, aldehydes, or carboxylic acids, depending on the structure of the alkene and the reaction conditions.
Examples:
Ozonolysis
Like permanganate, ozone cleaves double bonds to give ketones and aldehydes. However, ozonolysis is milder, and both ketones and aldehydes can be recovered without further oxidation.
This is how the mechanism works:
Ozone reacts with an alkene to form a cyclic compound called molozonide. This reaction is usually cooled with dry ice (−78 °C) to minimize overoxidation and other side reactions. The molozonide has two peroxy (-O-O-) linkages, so it is quite unstable. It rearranges rapidly, even at low temperatures, to form an ozonide.
Ozonides are not very stable, and they are rarely isolated. In most cases, they are immediately reduced by a mild reducing agent such as dimethyl sulfide. The products of this reduction are ketones and aldehydes.
More examples:
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