Chemical Properties of Alkenes

Alkenes are one of the most important types of unsaturated Hydrocarbons in Chemistry due to their major effects on everyday products and technologies. These hydrocarbons, due to the carbon-carbon double bond, have certain properties that make them different from saturated analogs called alkanes. This double bond of alkene is made of one σ and one π bond. It is due to this factor that these compounds become highly reactive and versatile to most of chemical reactions. Their reactivity usually lies in the electron-enriched nature of the π bond, which readily participates in an addition reaction with a variety of reagents.

This Story also Contains

1. Key Concept: Alkenes' Chemical Properties
2. Variable Aspects: Types of Reactions
3. Markovnikov's rule
4. Anti-Markovnikov's rule
5. Relevance and Applications
6. Some Solved Examples
7. Summary

Key Concept: Alkenes' Chemical Properties

Alkenes are hydrocarbons that contain one or more carbon-carbon double bonds. The presence of this feature differentiates the chemical properties of such compounds. The carbon-carbon double bond consists of one σ and one π bond. Due to its richness in electrons, alkenes are highly prone to addition. In this reaction, different reagents react with alkenes to form new products. This kind of reactivity is very important in organic synthesis, generating a large number of chemical compounds from alkenes.

Some of the main kinds of reactions that alkenes can go through are halogenation, hydration, and hydrogenation. Halogenation is the addition of halogens like chlorine or bromine across the double bond to form haloalkanes. Hydration consists of the addition of water across the double bond to yield alcohols, whereas hydrogenation adds hydrogen to reduce the alkene into an alkane. Each of these reactions is very significant in the synthesis of several types of organic compounds, making alkenes hence very vital for both laboratory and industrial applications.

All alkenes undergo addition reactions with the hydrogen halides. A hydrogen atom joins to one of the carbon atoms originally in the double bond and a halogen atom to another. There is a formation of a carbocation intermediate and rearrangement may occur in cases where there is a possibility of more stability.

The reaction occurs as follows:

Mechanism
The addition of hydrogen halides is one of the easiest electrophilic addition reactions because it uses the simplest electrophile: the proton. Hydrogen halides provide both an electrophile (proton) and a nucleophile (halide). First, the electrophile will attack the double bond and take up a set of π electrons, attaching it to the molecule. The resulting molecule will have a single carbon-carbon bond with a positive charge on one of them (carbocation). In case there is a possibility for the carbocation to rearrange, it will rearrange to form a more stable carbocation. The next step is when the nucleophile (halide) bonds to the carbocation, producing a new molecule with both the original hydrogen and halide attached to the organic reactant.

Variable Aspects: Types of Reactions

1. Halogenation of Alkenes: The important reaction is the halogenation of alkenes with halogens like chlorine or bromine. This process consists essentially of breaking a double bond in which each carbon atom forms a new bond with a halogen atom to form, and finally, haloalkanes. For example, ethene reacts with chlorine to result in 1,2-dichloroethane. Such reactions have applications in the preparation of halogenated compounds in pharmaceuticals, agrochemicals, and industrial processes.

Alkenes decolourises Bromine water (Br2 in CCl4)following addition of Br2across double bond. This serves as a test of unsaturation. The addition of halogens to an alkene is an anti-addition and provides an illustration for a stereoselective and stereospecific reaction. The reaction occurs as follows:

Some more examples:

2. Markovnikov and Anti-Markovnikov Addition: It is an addition reaction of reagents like hydrogen halides across the double bond of alkenes. In Markovnikov's addition, the hydrogen atom binds with the carbon-bearing more hydrogen atoms, and the halide with the carbon has fewer hydrogen atoms. Propene reacts with HCl to form 2-chloropropane. The opposite of this, called anti-Markovnikov addition—catalyzed by peroxides—means the halide attaches to the carbon with more hydrogen atoms. This is useful in forming certain products where Markovnikov's addition may not be so desirable.

Markovnikov's rule

This rule states that the acid hydrogen of the protic acid gets attached to the carbon with more hydrogen substituents and the negative part adds to the atom with less number of hydrogen atoms.

Mechanism

The addition of halogens and halogen acids takes place by electrophilic addition(EA) reaction. +E mechanism is that when electrons of the $\pi$-bonds are transferred to that atom of the multiple bonds to which the reagent finally gets attached. First, the electrophile(H⁺) adds to the positive C atom and hence this step is slow and the rate-determining step. Afterwards, the negative part of the reagent (Br^-) adds to the positive C atom. Thus, it is known as the (+E) reaction.

Rule 1: In alkene and alkyne, (+E) reaction takes place, first electrophile is added, and then the negative part of the reagent is finally added.

Rule 2: In general, when the inductively electron-withdrawing group(-I) is attached to (C=C) and has a lone pair of electrons then the +R effect is operative then the -I effect and Markovnikov's addition takes place.

Rule 3: If an inductively electron-withdrawing group(-I) is not attached to (C=C), is one or more C atoms away from (C=C), and has a lone pair of electrons, then the -I effect is more operative than the +R effect and the anti-Markovnikov's addition takes place.

Anti-Markovnikov's rule

In the presence of peroxide, such as benzoyl peroxide and light, the addition of HBr(not HCl and HI) to unsymmetrical alkenes occurs contrary to Markovnikov's rule.

Mechanism

The mechanism of this process occurs in three steps:

1. Chain initiation: Hydrogen Peroxide is an unstable molecule, if we heat it, or shine it with sunlight, two free radicals of OH will be formed. These OH radicals will go on and attack HBr, which will take the Hydrogen and create a Bromine radical. Hydrogen radicals do not form as they tend to be extremely unstable with only one electron, thus bromine radicals which is more stable will be readily formed.

2. Chain propagation: The Bromine Radical will go on and attack the substituted carbon of the alkene. This is because after the bromine radical attacked the alkene a carbon radical will be formed. A carbon radical is more stable when it is at a more substituted carbon due to induction and hyperconjugation. Thus, the radical will be formed at the more substituted carbon, while the bromine is bonded to the less substituted carbon. After a carbon radical is formed, it will go on and attack the hydrogen of HBr, and thus a bromine radical will be formed again.

3. Chain termination: In the termination step, two bromine radicals combined to give bromine. This radical addition of bromine to alkene by radical addition reaction will go on until all the alkene turns into bromoalkane, and this process will take some time to finish.

Relevance and Applications

The chemical properties of alkenes have important implications for academic research in the design and optimization of chemical reactions used to produce various materials. Halogenation of alkenes in synthesizing halogenated organic compounds, for example, is important in pharmaceuticals and agrochemicals, and also in industrial materials. Markovnikov and anti-Markovnikov addition reactions are very key concepts in organic synthesis and directly affect the design and the result of a chemical reaction.

These, in an academic setting, provide insight into the reaction mechanism and a feel for the way in which the various reagents interact with the alkenes. This is crucial in the development of new methods and the investigation of pathways toward target molecules. This impacts industry through the control and manipulation of reactions to produce certain products that will have specific properties, hence eventually affecting materials science all the way to drug development. The practical applications of alkenes underlie their importance for both theory of chemistry and real processes.

Recommended topic video on (Hydrogenation of Alkenes)

Some Solved Examples

Example 1
Question:

The major product (A) in the reaction given above is

1) (correct)

2)

3)

4)

Solution

Therefore, option (1) is correct.

Example 2
Question:

The major product form in the following reaction

CH3CH=CHCH(CH3)2→HBr

1)1CH3CH2CH(Br)CH(CH3)2
2) Br(CH2)3CH(CH3)2
3) (correct) CH3CH(Br)CH2CH(CH3)2
4) CH3CH2CH2C(Br)(CH3)2

Solution:

It follows Markonikov rule. The mechanism is given below:

Carbocation will be formed on the left side due to more stability due to a greater number of alpha hydrogens

Therefore, Option(3) is correct.

Example 3
Question:

The product of the reaction is:

1) (correct)

2)

3)

4)

Solution

Reaction Mechanism

Therefore, option (1) is correct.

Summary

Alkenes are essential participants of organic chemistry, holding a carbon-carbon double bond. The special chemical properties, basically due to the electron-rich π-bond, make them take part in a number of addition reactions. The article has reviewed the halogenation of alkenes and Markovnikov/anti-Markovnikov additions and explained how the different reagents add to the double bond in certain specified ways. Such reactions are of importance for academic study or industrial applications because they represent great power in synthesis for a huge number of products of wide diversity. Alkenes, owing to their chemical properties, find applications from the formation of plastics and pharmaceuticals to investigations into mechanisms of reactions; hence, their role in theory and practice is beyond doubt.