Susceptibility of Alkenes to Electrophilic Attack
High Electron Density of C=C
- The presence of a carbon-carbon double bond in alkenes means there's a sigma bond and a pi bond. The pi bond is particularly interesting because it's formed by the sideways overlap of p orbitals, resulting in an electron-rich region above and below the plane of the bonded atoms.
- This pi bond is more exposed compared to a sigma bond, making it relatively weak and susceptible to attacks by electrophiles. The high electron density associated with the pi bond acts as a magnet for electrophilic species.
Structure of ethene (alkene) showing carbon-carbon double bond.
Image courtesy of McMonster
Nature of Electrophiles
- Electrophiles, as the name suggests, love electrons. These are species that have a deficiency of electrons, either due to a positive charge or because of an electron-withdrawing group.
- They seek areas of high electron density to achieve a more stable state. Given the exposed and electron-rich nature of the pi bond in alkenes, it becomes a prime target for electrophiles.
Reactions of Alkenes
With Water: Hydration
- In the presence of an acid catalyst, alkenes can undergo a hydration reaction. This is an addition reaction where water adds across the double bond.R-CH=CH2 + H2O -> R-CH(OH)-CH3
- The acid catalyst (commonly sulphuric acid) provides the necessary protons to facilitate the reaction.
Image courtesy of JeanMi
With Halogens: Halogenation
- Alkenes can react with halogens like chlorine or bromine in a non-polar solvent (like tetrachloromethane) to undergo halogenation. This is another addition reaction where both atoms of the halogen molecule add across the double bond of the alkene.R-CH=CH2 + Br2 -> R-CHBr-CH2Br
- This reaction is particularly useful in determining the presence of double bonds in unknown molecules due to the distinct colour change associated with bromine.
Image courtesy of JeanMi
With Hydrogen Halides: Hydrohalogenation
- Alkenes also react with hydrogen halides (like HCl, HBr) in a hydrohalogenation reaction. Here, the hydrogen halide molecule adds across the double bond, with the hydrogen attaching to one carbon and the halide to the other.R-CH=CH2 + HBr -> R-CHBr-CH3
- Markovnikov's rule can be applied to predict the major product in the reaction of unsymmetrical alkenes with hydrogen halides.
Image courtesy of JeanMi
Bromine Water and Alkenes
Decolourisation Test
- Bromine water serves as a useful test for the presence of carbon-carbon double bonds. In the presence of an alkene, the orange-brown colour of bromine water quickly disappears, producing a colourless solution. This is due to the halogenation reaction mentioned above.
- This decolourisation does not occur with alkanes since they lack the reactive pi bond that alkenes possess. This distinction offers a simple qualitative test to differentiate between alkenes and alkanes.
Image courtesy of Science Ready
Alkenes in Industry
Foundational Building Blocks
- Alkenes are foundational in the chemical industry, serving as starting materials for a vast range of chemicals. Their reactivity profile, particularly their ability to form various addition products, makes them versatile and invaluable.
Polymer Production
- One of the most renowned uses of alkenes is in the realm of polymer chemistry. Simple alkenes like ethene can be polymerised under the right conditions to produce widely used plastics such as polyethene.
Intermediate Compounds
- Beyond their direct applications, alkenes are also instrumental in producing other significant compounds. Through various reactions, alkenes can be transformed into alcohols, halides, ethers, and a myriad of other compounds. These, in turn, find applications in producing dyes, pharmaceuticals, detergents, and other everyday products.
While alkenes might seem like simple organic compounds, their rich chemistry and wide-ranging applications underline their importance in both academic studies and real-world applications.
FAQ
While alkenes, in general, are susceptible to electrophilic attack due to the high electron density of the pi bond, some alkenes are more reactive than others. The reactivity often depends on the electronic and steric environment around the double bond. Electron-donating groups attached to the carbon-carbon double bond can enhance the electron density and make the alkene more reactive, while electron-withdrawing groups can do the opposite. Additionally, steric hindrance, caused by bulky substituents, can reduce reactivity by physically hindering the approach of the electrophile.
During an electrophilic attack on an alkene, the electron-rich pi bond interacts with an electrophile. This interaction results in the pi electrons being used to form a new sigma bond with the electrophile. Consequently, the carbon-carbon pi bond is broken. This breaking of the pi bond and the creation of new sigma bonds transform the alkene's double bond into a single bond in the resulting product. This is why addition reactions typically occur with alkenes during electrophilic attacks.
Alkenes are referred to as 'unsaturated hydrocarbons' because they possess carbon-carbon double bonds, while 'saturated hydrocarbons', like alkanes, contain only single bonds between carbon atoms. The term "unsaturated" implies that these molecules can take up additional atoms, typically hydrogen, due to the presence of the double bond. When an alkene reacts, for example in hydrogenation, the double bond can break, allowing the alkene to bond with more atoms, thus becoming "saturated".
An electrophile is a species that seeks electrons, typically having a positive charge or a polarisable bond. They "love" electrons and often react with areas of high electron density. A nucleophile, on the other hand, is electron-rich and seeks areas of positive charge or electron deficiency. Alkenes are rich in electron density due to their carbon-carbon double bond, specifically the exposed pi bond. Because of this electron-rich nature, they become prime targets for electrophiles, which are attracted to regions of high electron density. Thus, alkenes predominantly undergo electrophilic attacks.
The pi bond in alkenes is formed by the sideways overlap of the p orbitals of the carbon atoms. This overlap creates an electron cloud above and below the plane of the molecule. Rotating the pi bond would require breaking this overlap, which demands energy. Therefore, the pi bond remains fixed and doesn't rotate freely, unlike the sigma bonds. This lack of rotation gives rise to geometric isomerism in alkenes, where substituents can be on the same side (cis) or opposite sides (trans) of the double bond.
Practice Questions
Alkenes possess a carbon-carbon double bond, consisting of a sigma bond and a more exposed pi bond. This pi bond is electron-rich due to the sideways overlap of p orbitals, leading to high electron density above and below the plane of the bonded atoms. As a result, the pi bond is an inviting target for electrophiles. In contrast, alkanes only have sigma bonds, lacking this electron-rich pi bond. Using halogenation as an example, when alkenes are exposed to bromine, the electron-rich pi bond attracts the bromine molecules, leading to the addition of bromine across the double bond. This is why bromine water decolourises when mixed with an alkene. Alkanes, lacking the reactive pi bond, do not undergo such a reaction with bromine under normal conditions.
Alkenes, owing to their carbon-carbon double bond, display a unique reactivity pattern that renders them versatile and invaluable in the chemical industry. This C=C double bond, particularly the pi bond, is electron-rich and thus can participate in a variety of addition reactions. When alkenes react with water in the presence of an acid catalyst, they undergo hydration. The double bond breaks, and the water molecule adds across it, forming an alcohol. This transformation from a simple alkene to an alcohol showcases their reactivity and the vast potential for producing a wide range of chemicals from them. The adaptability of alkenes makes them excellent starting materials for myriad industrial processes.
