Alkenes

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What is an Alkene?

The common family of hydrocarbons, which is found in crude oil, is the alkenes. In this particular family, there is one carbon-carbon double bond, at least. This double bond makes a major difference in the chemistry of the compounds of the family.

In the chemical industry, the alkenes and particularly ethene, are tremendously essential. They cannot be found in crude oil in very large quantities but they are produced by cracking of the alkanes. The alkenes, like all the hydrocarbons, burn in the air to produce water and carbon dioxide. Ethene reacts in oxygen explosively, and so, it is not much good as a fuel. At the same time, alkenes are also much useful in the chemical industry in the manufacturing of plastics and various other chemicals to be used as fuels.


General Properties of Alkenes

  • Physical State - The members containing either two or four carbon atoms are gases; five to seventeen are liquids, and eighteen onwards, solids at room temperature, and also, they can burn in air with a luminous smoky flame.

  • Solubility - Alkenes are soluble in organic solvents and insoluble in water, such as benzene and more.

  • Density - Alkenes are lighter compared to water.

  • Boiling Point - The boiling points of alkenes represent a gradual increase with an increase in the chain length or molecular mass; this indicates the intermolecular attractions become stronger with the increase in the molecule size.


Classification of Alkenes

Alkyl groups bonded to the sp2 hybridized carbon atoms of alkenes affect the double bond stability. The alkene’s chemical reactivity is also often affected by the different alkyl groups that are bonded to the carbon atoms of sp2 hybridized. Therefore, it is very useful to classify alkenes by more number of alkyl groups attached to the C=C structural unit. This feature is known as the degree of substitution.

An alkene, which has a single alkyl group that is attached to the sp2 hybridized carbon atom of the double bond, is monosubstituted. Sometimes, an alkene whose double bond is at the end of the carbon atoms’ chain is also called a terminal alkene. Alkenes having two, three, and four alkyl groups bonded to the carbon atoms of the double bond are disubstituted, trisubstituted, and tetrasubstituted, respectively.

Monosubstituted

RCH=CH2

Disubstituted

R2C=CH or RCH=CHR

Trisubstituted

R2C=CHR

Tetrasubstituted

R2C=CR2


Physical Properties of Alkenes

The physical properties of alkenes are very similar to that of alkanes. Let us look at a few physical properties:

  • Alkenes naturally exist in all three states. The first three are gases, the next fourteen are liquids, and the higher than these are all solids.

  • Because of the weak van der Waal forces, all alkenes are insoluble in water.

  • But alkenes are soluble in organic solvents such as acetone or benzene because here, the van der Waal forces will be replaced by the new ones, making alkenes fully soluble.

  • The alkenes’ boiling points depend on their molecular structure. The bigger the alkane’s molecular chain, the higher the boiling points. Therefore, higher alkenes have very high boiling points.

  • The polarity of alkenes depends on their functional groups.


Chemical Properties of Alkenes

Alkenes are unsaturated compounds that make them highly reactive. Where most of these chemical reactions happen at the double bonds of Carbon-Carbon, this makes alkenes far more reactive compared to alkanes. Alkenes undergo three types of primary reactions, which are given as follows.


Addition Reactions

Addition of Hydrogen

In the presence of platinum or nickel, alkenes will react to add to its molecular chain one diatomic molecule of hydrogen (or dihydrogen). And they become alkanes in this process due to the rearrangement of atoms.


Addition of Halogens

Halogens will react with alkenes to produce vicinal dihalides. Iodine will not react with alkenes from the halogens. In contrast, bromine reacts with alkenes and will attach at the unsaturated site. Also, the reaction is used as proof of unsaturation.

C2H4(g) +Br2(aq) → C2H4Br2(aq)


Addition of Halides

These reactions follow a specific rule, which is the Markovnikov rule. This rule states the reactant’s negative portion (the molecule that gets added to the chain) will attach itself to the carbon attached with the least number of hydrogen atoms. So, when a hydrogen halide reacts with an alkene, and the hydrogen attaches at the double bond to the atom attached with more hydrogen atoms. On the other hand, the halide ion will attach to that carbon atom, which has the lesser hydrogen atoms attached.


Uses of Alkenes

The list of uses of alkenes of different ones such as propene, ethene, and more is given below.

  • Alkene used in the manufacture of polythene bags, and plastics as polythene for making bowls, buckets, bags, and more.

  • Making ethane-1,2-diol, to use as an anti-freezing for motor car radiators.

  • Polystyrene manufacturing is used in making parts of the refrigerator and car battery cases.

  • Synthetic fibre terylene and ethanol manufacturing.

  • Manufacture of acrylic fibres.

  • Manufacturing polypropene, plastic for making packaging material, and ropes.

  • Making an anti-knock for car engines.

  • Manufacture of propanol, which can be used in making acetone.

FAQ (Frequently Asked Questions)

1. What are Oxidation Reactions?

Let us look at a few oxidation reactions given below:

Oxidation by Potassium Permanganate

When the alkenes react with cold dilute KMnO4, which is also called Baeyer’s reagent, it forms vicinal glycols. Also, it will decolorize the pink color of KMnO4. Thus, it is used for unsaturation in compounds and testing.


Combustion Reaction

The combustion of alkenes is exothermic and will give out huge amounts of thermal energy. One of the practical examples of this reaction can be seen in the welding of metals. It is called oxy-ethylene welding.

CH2=CH2 + 3O2→ C2O2+2H2O

2. What are alkenes used for, and where are they found?

The application of alkanes can be found in many different manufacturing processes. They are used in the synthesis of plastics, alcohols, lacquers, detergents, and also used as fuel for starting materials. The most important alkenes in the chemical industry are propene, 1,3-butadiene, and ethene.


The production of a hydrogen molecule and ethene can be produced by the thermal cracking of ethane. Alkenes are also the raw materials for plastics such as PVC, polyethene, polystyrene, and polypropylene, among others. Chemistry of alkenes is present in beta-carotene, unsaturated fats, and light through vision.