What are Unsaturated Hydrocarbons?

Unsaturated hydrocarbons are the molecules that have multiple bonds between their carbon atoms. Alkenes are the hydrocarbons with a double bond between two adjacent carbon atoms ${\text{(R}}_{\text{2}}{\text{C=CR}}_{\text{2}}\text{)}$ whereas alkynes are the hydrocarbon with a triple bond between two adjacent carbon atoms$\left(\text{R-C=C-R}\right)$. These two groups are together known as unsaturated hydrocarbons and they show presence of less number of hydrogen atoms when compared with alkanes with same number of carbon atoms. The general molecular formula for an unsaturated hydrocarbon can be represented as ${\text{C}}_{\text{n}}{\text{H}}_{\text{2n +2}}$. The identification of any compound becomes easy once you know the general molecular formula and on substituting the values of ‘n’, the name of the compound can be obtained.

Following are few examples of alkenes with their general molecular as well as their structural formulas: 

The two adjacent carbon atoms establish the double linkages unlike the saturated hydrocarbons which have single bonds between all the carbon atoms involved.

The well-known names of ethene and propene are ethylene and propylene respectively. The main industrial chemical polyene is ethylene. Rather than any other synthetic organic chemicals, the US chemical industry generates approximately $25$milliard kilograms per year of ethylene. About half of this ethylene is used as one of the main popular plastics in polyethylene. Propylene is also an essential compound for the industry. It is transformed to a host of several drugs including isopropyl alcohol.

Nomenclature

Here are some fundamental principles to name alkenes and alkynes from the International Union of Pure and Applied Chemistry (IUPAC) in organic chemistry:  

• According to IUPAC naming substituent classes, and their location, are specified by number, as done with saturated compounds.

Naming an alkyne is same as that for alkenes or alkanes with a -yne suffix. The name of an alkyne can be written as substituents & locations + bond location + stem + -yne.

Reactions Involving Alkenes

There are four types of common reactions:

• Combustion reactions  
• Addition reactions
• Oxidation reactions of alkenes  
• Polymerization of alkenes

Combustion reactions  

  Just like saturated compounds, unsaturated compounds can also undergo combustion reactions to produce carbon dioxide and water molecules. It can be represented as:

Just like saturated compounds these also produce CO on combustion in presence of insufficient oxygen.

Addition reactions  

These reactions refer to an addition of a substituent to the unsaturated hydrocarbons. Addition reactions of alkanes can be classified into two categories; Symmetrical and non-symmetrical reactions.
a. Symmetrical reactions  

In these reactions addition of same substituent occurs to each carbon of a double bond.  

b. Non-symmetrical reactions  

 In these reactions different substituents are added to the carbon atoms involved in the double bond.  

Examples of addition reactions are:

• Hydrogenation refers to addition of hydrogen molecule to the unsaturated compound. It generally occurs in presence of catalysts such as Pt, Pd and also in the presence of heat or pressure. Conversion of unsaturated hydrocarbons into saturated forms occurs during hydrogenation. For converting liquid oils into solid fats, hydrogenation is employed.  The melting point of the fat raises as the quantity of double bonds is decreased. This reaction is also known as hardening where Pt usually acts as a standard catalyst.  

• Addition of halogen molecules to the unsaturated compounds is called halogenation. In these reactions the double bond undergoes reaction with ${\text{Br}}_{\text{2}}$or${\text{Cl}}_{\text{2}}$ at room temperature. These reactions do not need a catalyst for the reaction to occur. The order of reaction of halogens is in the order $\text{F>> Cl > Br >>> I}$ where F reacts extensively while I does not show any reactivity. Identification of a halogenation can be carried out with the addition of a ${\text{Br}}_{\text{2}}$ solution. The occurrence of a clear solution will indicate the presence of an alkene.

• Benzene is an aromatic hydrocarbon with a ring structure made up of six carbon and six hydrogen atoms with a molecular formula of${\text{C}}_{\text{6}}{\text{H}}_{\text{6}}$. The structural formula of benzene represents a benzene ring with alternate double bonds and single bonds. Benzene also undergoes addition reactions that is characteristic feature of unsaturated hydrocarbons. Benzene when reacts in presence of a catalyst such as nickel results in the formation of cyclohexane.

Oxidation reactions of alkene

Oxidation is defined as either addition of oxygen or the removal of hydrogen.  

There are few most common oxidizing agents such as ozone, potassium dichromate, oxygen,${\text{O}}_{\text{2}}$Dihydroxy alcohol can be formed when an alkene reacts with dilute aqueous${\text{KMnO}}_{\text{4}}$. Alkene oxidation can be used to detect the presence of alkenes. If the test sample is mixed with an aqueous solution of ${\text{KMnO}}_{\text{4}}$. The disappearance of the purple color confirms the presence of an alkene.

Polymerization of alkenes

"Polymer" is the Greek term for "poly" (many) and "mer" (parts). A polymer is an addition polymer that connects several alkene compounds through additional responses. It  is a large complex molecule with recurring units called a monomer. A monomer is the substance from which polymers are repeated. A polymer formed by a polymerization reaction cannot be given a precise formula, since the various polymer molecules differ in size. Polymers made of alkenes lead to a long chain of alkanes. Therefore, alkanes are chemically inert.   Some alkenes experience self-sufficient reactions to generate polymers by using various catalysts. The reaction requires double links wherein hundreds or thousands of molecules bind to create long chains. In the below shown polymerization of ethene to polyethylene, the large amount of molecules binding is defined by 'n'. 

${\text{n CH}}_{\text{2}}{\text{=CH}}_{\text{2}}\underset{\text{Catalysis}}{\overset{\text{heat,pressure}}{\to }}{\left({\text{-CH}}_{\text{2}}{\text{-CH}}_{\text{2}}\text{-}\right)}_{\text{n}}$

Naturally Occurring Alkenes  

Pheromone  

A pheromone is a chemical used to send a signal to other individuals of the same genus by insects (and certain animals). Pheromones are frequently alkenes or derivatives of alkenes and not saturated hydrocarbons. $\text{C = C}$ links in a system of a cis or a trans form are involved in the biochemical process of pheromones.

Example: A 16-carbon alkene derivative comprising of a group, $\text{-OH}$, is the female silkworm's sex attractive substance.

Terpenes

These are organic compounds with a molecular formula that contains two or even more $5\text{-}$carbon isoprene structural units of the carbon skeleton. Isoprene is a five-carbon diene (2-methyl-1,3-butadiene). Terpene in nature is widespread. They are used in biological environments which make certain arbors and plant characteristic odors.

Common Mistakes  

Students need to be careful while identifying an alkene and alkyne as well as naming the given compound.  

Context and Applications:

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry, Biochemistry and courses in Biotechnology.