Major Reactions and Mechanisms of Alkenes
In organic chemistry, we study the various types of compounds formed from the carbon atom. Based on its bonding organic compounds are of three types Alkanes (single bond), Alkenes (double bonds), and Alkynes (triple bonds). When there is a single bond between the carbon-carbon atom then it forms alkanes (C-C). Whenever there is a double bond between the carbon-carbon atom then alkenes (C=C) are formed. In case if it shares a triple bond between carbon-carbon atom then it is alkynes (C≡C). A large number of pi bonds are present in alkanes that are closely held due to which they show a variety of chemical properties.
Chemical Property of Alkenes
Alkenes belong to the family of hydrocarbons containing a double bond between carbon-carbon atoms.
Alkenes are less stable than alkanes and more stable than alkynes.
Single bond > Double bond >Triple bond
Alkenes exist in all three solid-liquid and gaseous states.
Alkenes are less soluble in water due to weak Van-Der-Waal forces
The boiling point of alkenes depends on the molecular structure, the longer the molecular chain, the higher will be its boiling point.
Functional groups are responsible for the polarity of alkenes.
Reactions of Alkenes
Ozonolysis
Another significant chemical property of alkene compounds which, in the addition of ozone or three molecules, led to the formation of ozone at which upon reduction with zinc dust and water produces aldehydes and ketones respectively. This reaction is also considered a method of preparation for aldehydes and ketones.
Oxidation reaction and Ozonolysis are prominently exhibited in the chemical properties of alkenes.
Reactions
Alkenes are less stable than alkanes, but they are more reactive. The majority of alkene reactions involve additions to this pi bond, resulting in the formation of new single bonds. Because alkenes may engage in a wide range of processes, including polymerization and alkylation, they are used as feedstock in the petrochemical industry.
With the exception of ethylene, alkenes have two reactivity sites: the carbon-carbon pi-bond and the presence of allylic CH centres. The former predominates, while the allylic site is also significant.
Reactions to addition
Many additional reactions occur in alkenes, which occur by opening up the double-bond. The majority of these addition reactions follow the electrophilic addition mechanism. Hydrohalogenation, halogenation, halohydrin production, oxymercuration, hydroboration, dichlorocarbene addition, the Simmons–Smith reaction, catalytic hydrogenation, epoxidation, radical polymerization, and hydroxylation are some examples.
Hydrogenation and its associated hydro elementation
The equivalent alkanes are produced through the hydrogenation of alkenes. The reaction is sometimes performed under pressure and at high temperatures. Almost often, metallic catalysts are necessary. Platinum, nickel, and palladium are the most common metals used in industrial catalysts. The manufacture of margarine is a large-scale application.
Many more H-can X's can be added in addition to the insertion of H-H across the double bond. These procedures are frequently of significant commercial importance. The addition of H-SiR3, or hydrosilylation, is one example. This process produces organosilicon compounds. Another reaction is hydrocyanation, which occurs when H-CN is added across a double bond.
Hydration
Alcohols are produced through hydration, which is the addition of water across the double bond of alkenes. Phosphoric acid or sulfuric acid catalyses the process. This process is carried out on a large scale in order to manufacture synthetic ethanol.
CH₂ = CH₂ + H₂O → CH₃ - CH₂OH
Alkenes can also be transformed to alcohol by the oxymercuration–demercuration reaction, the hydroboration–oxidation reaction, or Mukaiyama hydration.
Halogenation
The addition of elemental bromine or chlorine to alkenes results in vicinal dibromo- and dichloro alkanes (1,2-dihalides or ethylene dihalides) in electrophilic halogenation. The decolourization of a bromine solution in water is used to detect the presence of alkenes:
CH₂ = CH₂ + Br₂ → BrCH₂ - CH₂Br
Related processes are also used to calculate the bromine and iodine numbers of a chemical or mixture as quantitative measurements of unsaturation.
Hydrohalogenation
The addition of hydrogen halides, such as HCl or HI, to alkenes, produces the equivalent haloalkanes:
CH₃ - CH = CH₂ + HI → CH₃ - CHI - CH₂ - H
If the two carbon atoms at the double bond are attached to a differing number of hydrogen atoms, the halogen preferentially occurs at the carbon with fewer hydrogen substituents. This pattern is referred to as Markovnikov's rule. The employment of radical initiators or other chemicals might result in the opposite product outcome. In the presence of different contaminants or even ambient oxygen, hydrobromic acid, in particular, is prone to generating radicals, resulting in a reversal of the Markovnikov result:
CH₃ - CH = CH₂ + HBr → CH₃ - CH - CH₂ - Br
Polymerization
Polymerization techniques use terminal alkenes as precursors to polymers. Some polymerizations are economically significant because they produce polymers polyethene and polypropylene. Although they contain no olefins, polymers derived from alkene are commonly referred to as polyolefins. Polymerization can occur through a variety of methods. Natural rubber is produced by conjugated dienes such as buta-1,3-diene and isoprene (2-methylbuta-1,3-diene).
Bonding and structure
Bonding
A carbon-carbon double bond is made up of two bonds: a sigma bond and a pi bond. This double bond is stronger than a single covalent bond (611 kJ/mol for C=C vs. 347 kJ/mol for C–C), but it is not twice as strong. The average bond length of a double bond is 1.33 (133 pm) compared to 1.53 for a normal C-C single bond.
Each carbon atom in the double bond forms sigma bonds with three additional atoms by using its three sp2 hybrid orbitals (the other carbon atom and two hydrogen atoms). The pi bond is formed by the unhybridized 2p atomic orbitals that lie perpendicular to the plane formed by the axes of the three sp2 hybrid orbitals. This bond is located outside of the main C–C axis, with half of it on one side of the molecule and the other on the other. The pi bond is substantially weaker than the sigma bond, with a strength of 65 kcal/mol.
Because breaking the alignment of the p orbitals on the two carbon atoms incurs an energy penalty, rotation about the carbon-carbon double bond is limited. As a result, cis and trans isomers interconvert so slowly that they may be handled freely at room temperature without isomerization.
For compounds with three or four distinct substituents, more complicated alkenes may be denoted using the E–Z notation (side groups). For example, in (Z)-but-2-ene (a.k.a. cis-2-butene), the two methyl groups occur on the same side of the double bond, but in (E)-but-2-ene (a.k.a. trans-2-butene), the methyl groups appear on opposing sides. Butene's two isomers have differing characteristics.
Shape
The molecular geometry of alkenes contains bond angles of around 120° around each carbon atom in a double bond, as predicted by the VSEPR model of electron pair repulsion. The angle may change due to steric strain caused by nonbonded interactions between functional groups linked to the double bond's carbon atoms. Propylene, for example, has a C–C–C bond angle of 123.9°.
Bredt's rule asserts that a double bond cannot develop at the bridgehead of a bridged ring system unless the rings are sufficiently massive.
Using Fawcett's definition of S as the total number of non-bridgehead atoms in the rings, bicyclic systems need S ≥7 while tricyclic systems require S ≥ 11.
Physical characteristics
Alkenes and alkanes have many physical features, such as being colourless, nonpolar, and flammable. The physical state is determined by molecular mass: the simplest alkenes (ethylene, propylene, and butene), like the equivalent saturated hydrocarbons, are gases at room temperature. Higher alkenes are waxy solids, while linear alkenes with five to sixteen carbon atoms are liquids. The melting point of solids rises with increasing molecular mass.
Alkenes have stronger odours than their comparable alkanes. The odour of ethylene is pleasant and musty. Cupric ion binding to olefin in the human olfactory receptor MOR244-3 is involved in the smell of alkenes (as well as thiols). Strained alkenes, in particular, norbornene and trans-cyclooctene, have strong, disagreeable scents, which is consistent with the stronger complexes they form with metal ions such as copper.
Focus on the features of alkenes and find out how they are unique from the rest. Learn how they form a family of compounds by showing similar reactions and other chemical properties.
FAQs on Chemical Properties of Alkenes: Explained with Examples
1. What are the main types of chemical reactions that alkenes undergo?
Due to the presence of a reactive pi (π) bond, the characteristic chemical property of alkenes is the electrophilic addition reaction. The main reactions include:
Addition Reactions: Addition of hydrogen (hydrogenation), halogens (halogenation), hydrogen halides (hydrohalogenation), and water (hydration).
Oxidation: Cleavage of the double bond using agents like KMnO₄ (Baeyer's Test) or ozone (ozonolysis).
Polymerisation: Formation of large polymer chains from simple alkene monomers.
2. Why are alkenes chemically more reactive than alkanes?
Alkenes are more reactive than alkanes because they contain a carbon-carbon double bond (C=C), which consists of one strong sigma (σ) bond and one weaker pi (π) bond. The pi bond's electrons are located above and below the plane of the carbon atoms, making them more exposed and easily available to attacking electrophiles. Alkanes only possess strong, stable sigma bonds, which require significantly more energy to break, thus making them relatively inert.
3. What is the outcome of adding a halogen like bromine to an alkene?
When an alkene reacts with a halogen like bromine (Br₂) or chlorine (Cl₂), it undergoes an addition reaction to form a vicinal dihalide. For example, ethene reacts with bromine to form 1,2-dibromoethane. This reaction is significant as it serves as a test for unsaturation; the reddish-orange colour of bromine water is discharged when added to an alkene, indicating the presence of a C=C double bond.
4. Explain Markownikov's rule for the addition of hydrogen halides to alkenes.
Markownikov's rule governs the addition of unsymmetrical reagents (like H-X) to unsymmetrical alkenes. The rule states that the negative part of the adding molecule (e.g., the halide ion, X⁻) gets attached to the carbon atom of the double bond which has the fewer number of hydrogen atoms. This occurs because the reaction proceeds through the formation of the more stable carbocation intermediate (tertiary > secondary > primary).
5. How does the peroxide effect cause Anti-Markownikov addition in alkenes?
The peroxide effect, also known as the Kharasch effect, leads to Anti-Markownikov addition specifically when HBr is added to an unsymmetrical alkene in the presence of a peroxide. The peroxide initiates a free-radical mechanism instead of an electrophilic addition. The bromine free radical (Br•) adds to the double bond first to form the more stable carbon free radical. Consequently, the hydrogen atom attaches to the carbon with fewer hydrogen atoms, which is the opposite of Markownikov's rule.
6. What is the ozonolysis of an alkene and why is this reaction useful?
Ozonolysis is a chemical reaction where alkenes are treated with ozone (O₃) followed by a reductive workup (e.g., with zinc and water) to cleave the carbon-carbon double bond. This process forms smaller carbonyl compounds like aldehydes or ketones. The key importance of ozonolysis is in structural determination; by identifying the carbonyl products, chemists can deduce the exact location of the double bond in the original alkene molecule.
7. How does Baeyer's test help distinguish between an alkene and an alkane?
Baeyer's test is a simple qualitative test used to detect the presence of unsaturation (double or triple bonds). It involves adding a cold, dilute, alkaline solution of potassium permanganate (KMnO₄), which is purple, to the substance. If an alkene is present, the KMnO₄ will oxidise the double bond to form a diol, and the purple colour will disappear, leaving a brown precipitate of manganese dioxide (MnO₂). Alkanes do not react under these conditions, so the purple colour persists.
8. What is the hydrogenation of alkenes and what is its main industrial application?
Hydrogenation is a chemical reaction where hydrogen gas (H₂) is added across the double bond of an alkene in the presence of a metal catalyst like nickel, platinum, or palladium. This reaction converts the unsaturated alkene into a corresponding saturated alkane. A major industrial application of this process is the manufacturing of margarine and vanaspati ghee from vegetable oils, which contain unsaturated fatty acids.
9. How do alkenes form polymers? Explain with the example of ethene.
Alkenes undergo addition polymerisation, where many simple alkene molecules (monomers) link together to form a long chain (polymer) without the loss of any atoms. For example, under high temperature and pressure, many ethene (C₂H₄) molecules react with each other. The pi bond in each ethene monomer breaks, and new sigma bonds form between adjacent monomer units, creating a long saturated chain called poly(ethene) or polythene, a widely used plastic.
10. What are some of the most important industrial uses of alkenes?
Alkenes are crucial starting materials in the chemical industry due to their reactivity. Their major uses include:
Production of Polymers: Ethene is used to make polythene, and propene is used to make polypropylene, two of the most common plastics.
Synthesis of Alcohols: Ethene is hydrated to produce ethanol, an important solvent and industrial chemical.
Manufacturing other Chemicals: They are used to synthesise a range of products like antifreeze (ethylene glycol), detergents, and synthetic rubbers.
