Reactions of Alkenes
Alkenes are a type of molecule that have a special double bond between carbon atoms. To test if a molecule is an alkene, you can add a few drops of bromine water. If it turns from orange-brown to colorless, you know it's an alkene! This is just one way to test for alkenes.
There are many reactions alkenes can have, but we'll focus on one called electrophilic addition reactions. This is when an electrophile (a positive ion) is added to the double bond, breaking it apart and forming a new bond. We'll look at how alkenes react with hydrogen halides, halogens, hydrogen, and steam.
Finally, we'll talk about other reactions of alkenes, including how they react with manganate(VII) solution. By the end of this article, you'll have a better understanding of the reactions of alkenes and how they behave in different situations.
What are alkenes?
Alkenes are a type of organic molecule that contain only carbon and hydrogen atoms. They are also known as olefins. The defining feature of an alkene is the presence of a carbon-carbon double bond (C=C). This double bond means that they are not fully saturated with hydrogen atoms, making them unsaturated hydrocarbons.
The general formula for alkenes is CnH2n, where "n" represents the number of carbon atoms in the molecule. For example, ethene (C2H4) has 2 carbon atoms and therefore 4 hydrogen atoms. Butene (C4H8) has 4 carbon atoms and 8 hydrogen atoms.
Some other examples of alkenes include propene (C3H6), pentene (C5H10), and octene (C8H16). Alkenes are important in organic chemistry and have many practical uses, such as in the production of plastics and synthetic materials.
Alkenes are known to react in many different ways, but the most common type of reaction is electrophilic addition reactions. These reactions involve two smaller molecules coming together to form one larger molecule.
In electrophilic addition reactions, one of the molecules involved is an electrophile. Electrophiles are molecules that are electron deficient and are attracted to areas of high electron density, such as the double bond in alkenes. Examples of electrophiles include hydrogen ions (H+), hydrogen atoms in molecules like sulfuric acid or water, and positively charged chlorine ions (Cl+).
During the reaction, the electrophile forms a covalent bond with the double bond in the alkene, breaking it apart and forming two new single bonds. This creates a larger molecule overall. Alkenes commonly react in electrophilic addition reactions with hydrogen halides, halogens, sulfuric acid, water vapor, and hydrogen gas. Before discussing these reactions in more detail, it's important to understand the mechanism of alkene electrophilic addition reactions.
Alkene addition reaction mechanism
Alkene electrophilic addition reactions follow a similar mechanism, which involves the electrophile being attracted to the high electron density of the C=C double bond in the alkene.
The first step involves the electrophile accepting a pair of electrons from the double bond of the alkene, forming a new bond between the electrophile and the carbon atom that had the double bond. This step breaks the original double bond in the alkene and creates a new positive ion called a carbocation, which is electron deficient.
The carbocation is then stabilized by surrounding groups or solvent molecules. In the final step, a negatively charged molecule or ion (such as a halogen ion or a hydroxide ion) attacks the carbocation, forming a new covalent bond and neutralizing the molecule.
Overall, the reaction results in the formation of a new single bond between the electrophile and the alkene, while the original double bond in the alkene is broken. The reaction also creates a new molecule with a different set of functional groups.
Let's now apply that mechanism to specific alkene electrophilic addition reactions. Examples of alkene addition reactions
First up: the reaction of alkenes with hydrogen halides, HX.
Reaction with hydrogen halides
Hydrogen halides, such as HBr and HCl, can add across the C=C double bond in alkenes to form halogenoalkanes, which is an electrophilic addition reaction called halogenation. This reaction occurs at room temperature.
In this reaction, the hydrogen halide acts as an electrophile, attracted to the high electron density of the C=C double bond in the alkene. The hydrogen halide molecule adds to the alkene, forming a carbocation intermediate. The carbocation is then attacked by the halide ion, forming a new covalent bond between the halogen and one of the carbon atoms in the alkene.
The resulting product is a halogenoalkane, which is an organic molecule with at least one carbon-halogen bond (C-X). For example, the reaction between hydrogen bromide (HBr) and ethene (C2H4) produces bromoethane (CH3CH2Br). Overall, halogenation is an important reaction for the synthesis of halogenoalkanes, which are useful intermediates in organic chemistry.
In the case of the reaction between HBr and ethene, the partially positive hydrogen atom in HBr is attracted to the C=C double bond in ethene. The hydrogen atom forms a bond with one of the carbon atoms in the double bond, causing the H-Br bond to break. This results in the formation of a positive carbocation and a negative Br- ion. Finally, the Br- ion forms a bond with the positive carbocation, resulting in the formation of bromoethane.
You are also right that with symmetrical alkenes like ethene, the electrophile is as likely to bond with one carbon atom as the other. However, with larger, asymmetric alkenes, the position of the electrophile can matter. For example, if the carbon atoms in the double bond have different substituents, the electrophile may preferentially bond with the carbon atom that has the larger substituent due to steric effects. This can affect the stereochemistry of the product formed in the reaction.
Reaction with halogens
Alkenes can react with halogen molecules at room temperature to form dihalogenoalkanes, which are halogenoalkanes with two halogen atoms.
In this reaction, the halogen molecule is initially not an electrophile. However, as it approaches the alkene's electron-rich C=C bond, a dipole is induced, and the halogen atom nearest to the C=C bond becomes partially positively charged and can act as an electrophile. The reaction then follows the general mechanism of electrophilic addition we explored earlier.
For example, bromine (Br2) can react with ethene to form 1,2-dibromoethane (CH2BrCH2Br). This reaction enables bromine to be used as a test for the alkene functional group, as the orange-brown bromine water will be decolorized if added to a solution containing an alkene. This is because the bromine adds onto the C=C double bond, forming a dibromoalkane. Overall, the halogenation and dihalogenation reactions are important tools for the synthesis of halogenoalkanes, which have a wide range of applications in organic chemistry.
Reaction with sulphuric acid
Alkenes react with concentrated sulfuric acid (H2SO4) at room temperature in a highly exothermic reaction. The electrophile is one of the hydrogen atoms in the acid molecule.
If water is added after the reaction between sulphuric acid and ethene, the end product is an alcohol, and the sulphuric acid reforms. This means that the sulphuric acid acts as a catalyst in the reaction.
A catalyst is a substance that speeds up the rate of a reaction, without being used up in the process. In this reaction, the sulphuric acid donates a proton to the ethene molecule, forming a carbocation intermediate. The carbocation intermediate then reacts with water to form an alcohol, and the sulphuric acid is regenerated in the process.
The overall reaction between sulphuric acid and ethene can be represented as follows:
H2SO4 + C2H4 → C2H5HSO4
C2H5HSO4 + H2O → C2H5OH + H2SO4
The reaction mechanism for the electrophilic addition reaction between sulphuric acid and ethene involves the formation of a carbocation intermediate, as shown below:
Overall, the reaction between sulphuric acid and ethene is an important industrial process for the production of ethanol, which is a widely used alcohol with many applications in industry and everyday life.
Reaction with steam
Yes, that's correct. Phosphoric acid (H3PO4) is commonly used in industry as a catalyst for the reaction between ethene and steam to form ethanol (CH3CH2OH). This reaction is known as a hydration reaction and takes place under a pressure of 60 atm and a temperature of 300℃.
The mechanism for the electrophilic addition reaction between steam and ethene involves the formation of a carbocation intermediate, which is then attacked by a water molecule to form an alcohol. The overall reaction can be represented as follows:
C2H4 + H2O → CH3CH2OH
The detailed mechanism for the reaction involves the following steps:
- Protonation of ethene: The phosphoric acid donates a proton to the ethene molecule, forming a carbocation intermediate.
- Attack by water: A water molecule attacks the carbocation intermediate, forming an oxonium ion.
- Deprotonation: The oxonium ion is deprotonated by another water molecule, forming the alcohol product.
The overall reaction is exothermic, releasing energy in the form of heat. The high pressure and temperature are required to overcome the thermodynamic barrier to the reaction.
Overall, the reaction between ethene and steam using phosphoric acid as a catalyst is an important industrial process for the production of ethanol, which is a widely used alcohol with many applications in industry and everyday life.
Reaction with hydrogen
Alkenes react with hydrogen in a hydrogenation reaction to form alkanes. The reaction occurs at 140℃ in the presence of a nickel catalyst.
For example, ethene can be hydrogenated into ethane:
C2H4 + H2 → C2H6
The mechanism for the electrophilic addition reaction between hydrogen and ethene involves the following steps:
- Adsorption of hydrogen: Hydrogen molecules are adsorbed onto the surface of the nickel catalyst.
- Dissociation of hydrogen: The hydrogen molecules dissociate into hydrogen atoms on the surface of the catalyst.
- Adsorption of ethene: Ethene molecules are adsorbed onto the surface of the nickel catalyst.
- Formation of a surface intermediate: The hydrogen atoms react with the adsorbed ethene molecules to form a surface intermediate.
- Desorption of the product: The surface intermediate desorbs from the surface of the catalyst, forming the alkane product.
Hydrogenation is commonly used in margarine production to 'harden' vegetable oils, raising their melting point so that they are solid at room temperature. However, partial hydrogenation, in which some of the C=C double cis-bonds in oils are hydrogenated and some are turned into trans-bonds, has been linked to raised cholesterol levels and an increased risk of heart disease due to the formation of trans fats.
Therefore, it is important to limit the consumption of trans fats in our diet and choose healthier alternatives for cooking and food preparation.
Other reactions of alkenes
Alkenes can indeed take place in oxidation reactions with potassium manganate(VII) solution (KMnO4). In this reaction, the manganate(VII) ions act as an oxidizing agent, and the reaction is characterized by a dramatic color change. At room temperature, alkenes react with dilute, acidic manganate(VII) solution to form a diol, and the reaction also releases Mn2+ ions, which turn the purple manganate solution colorless. For example, reacting ethene with cold, dilute, acidic manganate solution to form ethane-1,2-diol.
Alkenes can also take part in polymerization reactions. In this reaction, the double bond of the alkene is broken, and the two carbon atoms form a new single bond with two other carbon atoms. This process is repeated until a long chain of carbon atoms is formed, known as a polymer. For example, the polymerization of ethene produces polyethylene.
It is correct that the reaction with alkaline manganate solution leads to the oxidation of alkenes into diols, but in this case, the manganate ions are reduced into Mn4+ ions, which turn the purple solution dark green before producing a dark brown precipitate.
Heating an alkene with concentrated, acidic manganate solution leads to further oxidation of the alcohol. In this case, the alkene's C=C double bond is completely broken, and the alkene is split into two molecules. Each of the halved molecules forms a C=O double bond with the oxidizing agent, resulting in the formation of either carbon dioxide (CO2), an aldehyde (RCHO), a carboxylic acid (RCOOH), or a ketone (RCOR'). The products depend on the groups attached to each carbon atom in the C=C double bond.
If the carbon atom is attached to two R groups, a ketone is formed. If the carbon atom is attached to one R group and one hydrogen atom, an aldehyde is formed, which then gets oxidized into a carboxylic acid. If the carbon atom is attached to two hydrogen atoms, carbon dioxide is formed. The specific products formed can be predicted by considering the groups attached to each carbon atom in the C=C double bond.
While the reactions with manganate solutions produce a characteristic color change, they are not a reliable indicator of the presence of alkenes, as potassium permanganate can also oxidize other molecules. Electrophilic addition with bromine water is a more useful test for the presence of alkenes.
Alkene polymerization reactions involve multiple alkenes joining together to form addition polymers, which are large molecules made up of repeating units called monomers. Polyalkenes, which are alkene polymers, consist of many alkene monomers connected by single covalent bonds. The C=C double bond in each alkene opens up and connects to the adjacent alkene, forming one long polymer chain. Addition polymers like polyalkenes are used to make a variety of products such as plastics, fabric, and building materials.
In addition, alkenes react in electrophilic addition reactions, which involve an electrophile combining with an alkene to form a larger molecule, breaking the alkene's C=C double bond. Examples of alkene electrophilic addition reactions include reactions with hydrogen halides, halogens, sulfuric acid, hydrogen, and steam. Alkenes also react with manganate(VII) solution in an oxidation reaction, and the products formed depend on the specific conditions.
Reactions of Alkenes
What type of reaction do alkenes undergo?
Alkenes typically undergo electrophilic addition reactions.
What are the four reactions of alkenes?
Four common reactions involving alkenes are halogenation, hydrogenation, oxidation, and hydration.
Which reagents do alkenes typically react with?
Alkenes typically react with halogens, hydrogen halides, and steam if in the presence of an acid catalyst.
What are some examples of alkenes?
The simplest alkene is ethene, which has just two carbon atoms and four hydrogen atoms. Other alkenes include propene and butene.
How do you write an addition reaction involving an alkene?
To represent addition reactions, you can write equations as with any other reaction. Remember to write out your products and reactants using structural formulae to help you identify the different functional groups and show the changes in the molecules.
