Introduction

Organic compounds are the class of compounds in which carbon atom or atoms are connected to other atoms, mostly hydrogen, oxygen, and nitrogen, through covalent bonds. Carbides, cyanides, and carbonates are the few carbon-containing non-organic compounds. Carbohydrates, lipids, proteins, and nucleotides are the organic compounds essential to human beings. Alkenes are a class of unsaturated hydrocarbons characterized by at least one carbon-carbon double bond. The presence of the double bond increases the reactivity of these compounds if compared to alkanes, allowing for an increased number of reactions. The general formula for acyclic alkenes is Cₙ+H₂ₙ.

Figure 1: Structure of simplest alkenes

Why do students struggle to learn alkenes?

1. Lack of basic knowledge

Before going for any topic, the basics of the topic must be cleared. If you have to teach the organic compound, you must clear the concepts of basic organic chemistry first. Students assume every carbon and hydrogen-containing compound is an organic compound, which is not true. They don't know about the functional groups, attacking and leaving groups in any compound. Before going further, you must clarify what saturated and unsaturated organic compounds are. They also face difficulties when they have to identify any organic compound by the IUPAC name. 

2. A large number of reactions and reaction mechanism

In organic chemistry, there is more than one possible reaction and reaction mechanism. Students face difficulties selecting the major pathway for the compounds, so you must be clear when there is more than one functional group, what the major product will be, and which mechanism it will follow. If there is more than one attacking and leaving group, students get confused about which group will attack and which one is the good leaving group. These terms must be cleared first to study any organic compound and its reactions.

3. It’s conceptually challenging 

Theoretical studies of organic compounds and reactions are tricky because most compounds have 3D structures. Organic reactions involve complex reactions mechanisms that are difficult to understand theoretically. You can use 3D models of organic molecules so that students easily understand the structures of molecules. For reactions, mechanisms must use video graphics; if possible, draw the complete mechanism along with arrows identifying the reaction sites and reacting groups. 

5 ways to make alkenes, structure, and reactions of alkenes interesting for students

1. Review the basics of organic chemistry

Before going to organic compounds and their reactions following terms must be cleared first:

• An attacking group is a part of a molecule that attacks a substrate or intermediate to produce the product. In organic reactions, it may be an electrophile or a nucleophile. Electrophiles are the attacking groups that make chemical bonds with nucleophiles by accepting the electron pair. In contrast, nucleophiles make chemical bonds by donating the electron pair to the substrate or the intermediate. 
• A leaving group is a part of a molecule that can break away (leave the molecule) during a reaction. The key factor contributing to a species' suitedness as leaving group is its basicity: The weaker the base, the better the leaving group. Halogens are often used as the leaving group, e.g., in alkyl halides. The general order of "ability to leave" for them is I > Br > Cl > F. 
• Steric effects are non-bonding molecular interactions that influence the shape and reactivity of ions and molecules. Steric effects resulting from repulsive forces between functional groups arise from overlapping electron clouds. Steric hindrance is slowing a chemical reaction rate due to steric bulk interactions. Steric hindrance usually refers to the interaction of intermolecular groups. Understanding steric hindrance can allow the design of chemical reactions to force regioselectivity or stereoselectivity in a reaction or to minimize undesirable side reactions.

2. Explain Alkenes

Alkenes are a class of unsaturated hydrocarbons characterized by at least one double bond. The presence of the double bond increases the reactivity of these compounds if compared to alkanes, allowing for an increased number of reactions.

One peculiar property of alkenes is the formation of geometric isomers. As the double bond doesn't allow rotation, the substituents can be placed above or below its plane. If both substituents are on the same side of the plane, the isomer is called cis. If one substituent is above and the other below the plane, it is called trans.

Figure 2: Cis and trans isomers (Image source: Labster theory)

Structure

Alkanes are classified as acyclic and cyclic. If a chain of carbons forms the compound with double bonds, it is considered acyclic. If instead, the compound has a cyclic structure made of carbons with at least a double bond in it, it is considered cyclic.

Alkenes are classified into four groups based on alkyl groups attached to the C=C unit. The general formula for a monosubstituted alkene is RCH=CH₂.

• Monosubstituted: when a single alkyl group is attached to the C=C structural unit, the alkene is called mono-substituted alkene. The general formula for a monosubstituted alkene is RCH=CH₂.
• Disubstituted: when two alkyl groups are attached to the C=C structural unit, the alkene is called disubstituted alkene. The general formula for a disubstituted alkene is RCH=CHR or R₂C=CH.
• Trisubstituted: when three alkyl groups are attached to the C=C structural unit, the alkene is called trisubstituted alkene. The general formula for a trisubstituted alkene is R₂C=CHR.
• Tetrasubstituted: when four alkyl groups are attached to the C=C structural unit, the alkene is called tetrasubstituted alkene. The general formula for a tetrasubstituted alkene is R₂C=CR₂.

2. Explain the reactions of alkenes

Halogenation of alkenes

This reaction results in addition of halogen across the double bond. It only occurs with Chlorine (Cl₂) and Bromine (Br₂). Iodine does not react with many alkenes. Fluorine is too reactive, causing an explosive reaction with alkenes producing hydrogen fluoride and carbon. In aqueous conditions, we can form halohydrins. In these, the groups added across the double bond are an OH group and one halogen atom.

Figure 3: Halogenation reaction of alkene (ethylene).

Hydration of Alkenes

The hydration reaction of alkenes is the net addition of water across the double bond, resulting in an alcohol. 

Figure 4: Hydration reaction of alkene (ethylene).

There are a few ways to perform the hydration of alkenes.

Acid-catalyzed hydration

Water reacts too slowly with the alkene on its own, but we can use an acid catalyst to speed it up. The mechanism involves the electrophilic addition of a proton from the acid to form a carbocation intermediate. This happens using Markovnikov's rule of deprotonation occurring at the least substituted carbon to form the most stable carbocation intermediate.

In the next step, an oxonium ion is formed through the addition of water. And simple deprotonation gives alcohol as the product. The proton in the oxonium intermediate can be deprotonated by any base present, including the conjugate base of the acid used as a catalyst. Deprotonation can even be by another alkene molecule, which would generate another carbocation intermediate and propagate the chain mechanism.

Oxymercuration

Transition metals can also aid the addition of water to alkenes by breaking the double bond. A common compound is mercuric acetate Hg(OAc)₂. The coordination of the metal to the alkene is susceptible to the nucleophilic attack from the water.

The transition metal behaves the same way as the acid catalyst. The alkene undergoes electrophilic addition, forming a bridged cyclic structure known as a mercurinium ion.

The water attacks to form a hydroxyalkyl mercury complex, and lastly, demercuration occurs via reduction with sodium borohydride.

Hydroboration

A process involving the addition of a B–H bond of borane (BH₃) to an alkene yields an organoborane intermediate, RBH₂. The BH₂ group is then replaced by an OH group using hydrogen peroxide.

Figure 5: Hydroboration of alkene (ethylene).

Oxidation of Alkenes

Oxidation is a loss of electron density. This can happen in two ways: Bond formation between carbon and a more electronegative atom or bond-breaking between carbon and a less electronegative atom.

Epoxidation (more electronegative atom)

An epoxide is a cyclic ether with an oxygen atom in a three-membered ring. The reactants are an alkene and a peroxy carboxylic acid. The common reagent used is meta-chloro-peroxybenzoic acid (MCPBA).

The double bond in the alkene reacts with the peroxycarboxcylic acid to form the cyclic ether called an epoxide. The by-product of the reaction formed from the MCPBA reagent is a carboxylic acid.

Hydroxylation

The reagents used for the hydroxylation reaction are Osmium tetroxide OsO₄ and pyridine. The Osmium tetroxide reacts with the double bond in the alkene to form a cyclic osmate intermediate. The double bond is broken at this stage, and two new carbon-to-oxygen bonds have formed. Sodium bisulfite in water is introduced to the reaction, adding hydrogen atoms to each oxygen bonded to the carbons, forcing the detachment of the osmium compound. The product is a diol (two OH groups).

Cleavage

Alkene reacts with ozone to give a carbonyl compound. A reaction with potassium permanganate can form carboxylic acids if hydrogens are present in the alkene group.

Reduction of Alkenes

The reduction of alkenes is simply breaking the double bond and forming two new C-H bonds. The product is an alkane. The reagents required for this reaction are Hydrogen H₂ and a metal catalyst. Two common catalysts are used: Platinum PtO₂ or Palladium with Carbon Pd/C. This reaction is called Catalytic Hydrogenation.

Once both carbons have formed a bond with a hydrogen atom, the product detaches from the catalyst surface. The H₂ bond is broken and is adsorbed onto the catalytic surface. Similarly, the alkene attaches to the metal catalyst. The hydrogenation of the alkene then proceeds while attached to the metal surface.

Carbenes reaction with Alkenes

Alkenes can react with carbenes. Carbenes are neutral divalent carbon structures with a reactive long pair of electrons. When they react with alkenes, they break the double bond and form a cyclopropane structure. This is a three-membered ring of carbon atoms.

4. Add some daily life uses of alkenes

There are a lot of alkenes we use in our daily life; some common uses of alkenes are;

• Alkenes like polythene are used to manufacture buckets, bags, etc.
• Alkenes like polystyrene are used in manufacturing cases for car batteries and different refrigerators parts.
• Alkenes like 1,2-diol are used as the anti-freezing liquid in automobile radiators.
• Alkenes like ethene are used to produce ethanol.
• Alkenes, like vitamin A, is used as a drug for different diseases.

5. Use virtual lab simulations

With technological advances, it is much easier to explain complex and challenging processes with the help of simulations. Now, you can simulate experiments without the need for any valuable equipment.

In this regard, you can take help from Labster’s virtual lab simulations. These simulations engage students through interactive learning scenarios. Students dive into a 3D world, where they visually learn and apply their concepts to solving real-life problems.

In Labster’s interactive Reactions and Structure: Alkenes Virtual Lab, students will learn the structures and reactions of alkenes and how they can affect the world around us.

References

1. https://theory.labster.com/welcome_rsa/
2. https://www.labster.com/simulations/reactions-and-structures-alkenes/ 
3. https://byjus.com/chemistry/alkene