Olefins

What is an Olefin?

Olefins, also known as alkenes, are hydrocarbons that feature one or more double bonds between two adjacent carbon atoms. This term is generally used to denote the hydrocarbons that contain at least one double bond. As per the International Union of Pure and Applied Chemistry (generally abbreviated to IUPAC), the term “alkene” denotes only the acyclic hydrocarbons containing only one double bond. For the acyclic hydrocarbons containing two double bonds, IUPAC recommends the term “alkadiene”. Similarly, the International Union of Pure and Applied Chemistry recommends the use of the term “alkatriene” to denote acyclic hydrocarbons featuring three double bonds.

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For the cyclic hydrocarbons containing one double bond, IUPAC recommends the use of the term “cycloalkene”. The International Union of Pure and Applied Chemistry also recommends the use of the term “cycloalkadiene” to denote the cyclic hydrocarbons containing two double bonds. It is important to note that the term “olefin” is used to broadly refer to all these categories of hydrocarbons. Therefore, all acyclic and cyclic hydrocarbons containing at least one double bond can be broadly categorized as olefins (since the term doesn’t differentiate them based on the total number of double bonds in their chemical structures).

General Formula of Olefins

The general formula of an acyclic olefin that contains one double bond (alkene) is CnH2n. This implies that there are twice as many hydrogen atoms as carbon atoms in such olefins. For a cyclic olefin containing only one double bond, the general formula is CnH2n-2. It can be noted that acyclic alkenes that only contain a single double bond and which do not contain any other functional groups in their chemical structures form a homologous series, and are generally referred to as mono-enes.

As the value of ‘n’ increases in the homologous series, the total number of possible structural isomers of the chemical compound also increases. This is because the value of n denotes the number of carbon atoms in the compound. Therefore, the greater the value of ‘n’, the greater the number of carbon atoms, and the greater the possible variations in the chemical structures (the length of parent chain, the extent of branching, the position of the pi bond, etc).

Properties of Olefins

It can be noted that there are many similar physical properties between alkenes (olefins) and alkanes. Both classes of chemical compounds are known to be colourless, combustible, and non-polar. The physical state of these compounds generally depends on their molecular mass. For example, the simplest alkenes (such as butene, propene, and ethene) are known to be gases at room temperature, much like the corresponding saturated hydrocarbons. The ones that exist in the liquid state are usually linear alkenes that contain approximately five to sixteen carbons. The chemical compounds belonging to this category that are known to exist as waxy solids are greater alkenes. The melting point of the solids also increases as the molecular mass increases.

In general, olefins (or more specifically, alkenes) are known to have relatively stronger odours than their alkane counterparts. For example, the chemical compound ethylene is known to have an odour that is quite sweet and musty. Strained alkenes, in particular, are known to possess extremely solid, unpleasant odours. Common examples of such compounds include norbornene and trans-cyclooctene. It is interesting to note that this fact is consistent even with the stronger π complexes that they form with metal ions such as copper.

The thermal cracking of petroleum oils into gasoline used to be a very popular method of generating olefins. Other methods for the production of olefins include fluid catalytic cracking and hydrocracking. Post the 1970s, the synthesis of linear alpha olefins has mostly done through olefin metathesis and polymerization.

The growth of the chemical compounds in the olefin oligomerization process takes places via the mixing of mono-olefins of relatively low molecular weights. In addition, olefin metathesis requires the exchange of chemical substituents with the resulting re-formation of double bonds.

It can be noted that acyclic diolefins are generally called acyclic dialkenes or acyclic dienes. In addition, CnH2n-2 is their general formula, since they contain two double bonds.

In addition, they undergo reactions so that the mono-olefins are close to them. Isoprene and butadiene are the most common dienes, which are very useful in the manufacture of synthetic rubber.

Applications of Olefins

Olefins are known to naturally exist in many life forms. For example, the nutrient known as beta-carotene is an olefin that can be found naturally in carrots. Olefins are known to have several applications in the day-to-day lives of human beings. For example, the olefin known as ethylene is widely used to promote the ripening of fruit, effectively reducing the waiting time required before the consumption of the fruit.

The most important commercial application of olefins lies in the petroleum industry, where they are employed for the production of gasoline with a high octane value. Olefins are also used as feedstock in the production of alkylate (they’re fed as a feedstock into the alkylation unit in this process).

Recommended Videos

Understanding Carbon

Preparation of alkenes from alkynes

Oxidation of Alkenes


Frequently Asked Questions on Olefin

Q1

How are olefins synthesized industrially?

Alkenes, or olefins, are formed by the cracking of hydrocarbons. To generate a mixture of mainly aliphatic alkenes and lower molecular weight alkanes, reactant alkanes are broken apart at high temperatures, generally in the presence of a zeolite catalyst. The mixture, which is a temperature-dependent feedstock separated by fractional distillation.

Q2

What are some examples of addition reactions undergone by olefins?

Olefins, specifically the acyclic olefins containing only one double bond, are known to participate in several addition reactions. The most notable of these are halogenation reactions, hydrohalogenation reactions, oxymercuration reactions, halohydrin formation reactions, the Simmons-Smith reaction, and radical polymerization reactions.

Q3

What happens when olefins undergo hydration?

When olefins undergo hydration, water is usually added across the double bond, resulting in the formation of alcohols. It can be noted that such reactions are usually catalyzed by sulfuric acid or phosphoric acid.

Q4

What happens when elemental chlorine or bromine are used in halogenation reactions with olefins?

The products formed from the electrophilic halogenation of alkenes with the specified halogens are usually vicinal dichloro- or dibromo- alkanes.

Q5

What happens when alkenes are reacted with peroxy acids?

When these olefins are reacted with peroxy acids, the products formed are usually epoxides.

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