Why are the reactions of alkenes described as addition reactions?
Addition reactions are limited to chemical compounds that have multiple bonds, such as molecules with carbon–carbon double bonds (alkenes), or with triple bonds (alkynes). Addition reactions are also encountered in polymerizations and called addition polymerization.
Alkenes and alkynes are unsaturated - they have π -bonds, so don't have the full number of hydrogen that they could have. The alkenes and alkynes want to form more σ -bonds and have a structure more like an alkane, so they undergo addition reactions.
- It should be noted that alkenes and alkynes react with bromine to form dibromides. When the potassium permanganate is added, styrene and benzene are oxidized. Alkenes are oxidized to diols and alkynes are oxidized to diones. The alkanes in this situation do not react with the potassium permanganate.
- Alkenes are unsaturated molecules, which means they do not have all the hydrogen they could have. This is because there is at least one double bond between carbons. This is a stable structure, but not the most stable, so when certain compounds or elements are added, like fluorine, they undergo an addition reaction.
- It's smoky, indicating incomplete combustion due to the presence of C=C bonds, which appears as soot (carbon.) It will be black, due to the production of soot from incomplete combustion. Alkenes burn with a sooty flame to produce carbon dioxide, soot (carbon) and water.
All of the products are now saturated, as they contain only single carbon-to-carbon bonds. The addition of hydrogen, reaction (2), is also known as hydrogenation. The addition of water, reaction (4), is a very important reaction to remember, because it produces alcohols. Addition of water is also known as hydration.
- In chemistry, a hydration reaction is a chemical reaction in which a substance combines with water. In organic chemistry, water is added to an unsaturated substrate, which is usually an alkene or an alkyne. This type of reaction is employed industrially to produce ethanol, isopropanol, and 2-butanol.
- In chemistry and the biological sciences, a dehydration reaction, also known as Zimmer's hydrogenesis, is a chemical reaction that involves the loss of a water molecule from the reacting molecule. Dehydration reactions are a subset of condensation reactions.
- Bromine water is a dilute solution of bromine that is normally orange-brown in colour, but becomes colourless when shaken with an alkene. Alkenes can decolourise bromine water, while alkanes cannot.
Alkenes undergo electrophilic addition reactions. bond electrons forming a bond with an electrophile. For unsymmetrical alkenes, the electrophile adds to the sp2 carbon that is bonded to the most hydrogens.
- They also undergo many other chemical reactions that alkanes do not - the double bond in alkenes makes them more reactive than alkanes. Forexample, bromine can be added to the alkene double bond. This is called an addition reaction - a typical addition reaction occurs with bromine.
- This means that they are more unstable than alkanes, since π -bonds aren't as strong as σ -bonds. The alkenes and alkynes want to form more σ -bonds and have a structure more like an alkane, so they undergo addition reactions.
- Ethene and the other alkenes are attacked by electrophiles. Electrophiles are strongly attracted to the exposed electrons in the pi bond and reactions happen because of that initial attraction - as you will see shortly. You might wonder why fully positive ions like sodium, Na+, don't react with ethene.
Updated: 17th September 2018