Unsaturated Hydrocarbons

What are Unsaturated Hydrocarbons?

Unsaturated hydrocarbons are hydrocarbons have multiple bonding i.e. double or triple covalent bonds,  between adjacent carbon atoms. Unsaturated hydrocarbons having double bonds are called alkenes while those having triple bonds are known as alkynes.

We would study about unsaturated hydrocarbons in more details under following subtopics:

• Methods of Preparation of Alkenes

• Properties of Alkenes

• Methods of Preparation of Alkynes

• Properties of Alkynes

What are Alkenes?

In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond. The simplest acyclic alkenes, with only one double bond and no other functional groups, form a homologous series of hydrocarbons with the general formula C_nH_2n.

The simplest alkene is ethylene (C2H4), which has the International Union of Pure and Applied Chemistry (IUPAC) name ethene. Alkenes are also called olefins (an archaic synonym, widely used in the petrochemical industry). Aromatic compounds are often drawn as cyclic alkenes, but their structure and properties are different and they are not considered to be alkenes.

Alkenes (olefins) contains the structural unit  >C=C< and have the general formula CnH2n. These unsaturated hydrocarbons are isomeric with the saturated cycloalkanes.

Carbon atoms of alkenes involved in double bond are sp2 hybridized having trigonal planar structure with an angle of 120oC.

The coplanar structure of C=C has been described below, e.g. C2H4

Alkenes have the following characteristic bond lengths and bond energy.

IUPAC nomenclature of Alkenes :

The IUPAC name is derived from the IUPAC name of alkanes by replacing ending 'ane' by 'ene' alongwith the position of double bonds.

(e.g. alkane - ane + ene = alkene). CH3CH=CH-CH3 is known as but-2-ene or butene-2 or 2-butene.

In case of two double or two triple bonds, the ending 'ne' of alkanes is suitably replaced by diene or triene accordingly.

CH3CH=C=CH2 : buta-1, 2-diene; CH2=CH-CH=CH2: buta-1. 3-diene

Alkenyl groups : Residual part left after the removal of one H atom from alkene is known as alkenyl group. According to IUPAC nomenclature, these groups are named by replacing terminal 'e' of alkene by 'yl' e.g.

The numbering in alkenyl group is started from the carbon with free valencies.

Isomerism in Alkenes

The cis -trans system for alkenes:

The C=C consists of a s bond and a p bond, in a plane at right angles to the plane of the single s bonds to each C. The p bond is weaker and more reactive than the s bond. The reactivity of the p bond imparts the property of unsaturation to alkenes; alkenes therefore readily undergo addition reactions. The p bond prevents free rotation about the C=C and therefore an alkene having two different substituents on each doubly bonded C has geometric isomers. For example, there are two 2-butenes.

Geometric (cis - trans) isomers are stereoisomers because they differ only in the spatial arrangement of the groups. They are diastereomers and have different physical properties (m.p., b.p., etc.). In place of cis-trans, the letter Z is used if the higher - priority substituents on each C are on the same side of the double bond. The letter E is used if they are on opposite sides.

The (E) - (Z) System for Designating Alkene Diastereomers:

The term cis- and trans-, where, when used to designate the stereochemistry of alkene diastereomers, are unambiguous, only when applied to disubstituted alkenes. If the alkene is trisubstituted or tetrasubstituted, the terms cis and trans are either ambiguous or do not apply at all. In the following alkene

it is impossible to decide whether A is cis or trans since no two groups are the same.

A newer system is based on the priorities of groups in the Cahn - Ingold - Prelog convention. This system called the (E) - (Z) system, applies to alkene diastereomers of all types. In the (E) - (Z) system, we examine the two groups attached to one carbon atom of  the double bond and decide which has the higher priority.  Then we repeat the operation at the other carbon atom.

We take the group of higher priority on one carbon atom and compare it with the group of higher priority on the other carbon atom. If two groups of higher priority are on the same side of the double bond, the alkene is designated (Z). If the two groups of higher priority are on opposite sides of the double bond, the alkene is designated (E). The following examples illustrate this :

Refer to the following video for naming of unsaturated hydrocarbons

What are  Alkynes or Acetylenes?

?In organic chemistry, an Alkynes or Acetylenes are  unsaturated chemical compounds containing at least one carbon-to-carbon triple bond. The simplest acyclic alkenes, with only one double bond and no other functional groups, form a homologous series of hydrocarbons with the general formula C_nH_2n-2.

e.g.                  C₂H₂                                       C₃H₄                           C₄H₆

                    acetylene or ethyne             allylene or propyne              butyne

Acetylene is known experimentally to have a linear structure. The C º C distance of 1.20 Å is the shortest carbon-carbon bond length known. The carbon-hydrogen bond length of 1.06 Å is shorter than that in ethylene (1.08 Å) or in ethane (1.10 Å) . These structural details are readily interpreted by an extension of the s-p electronic structure of double bonds. In acetylene the s-framework consists of C_sp-hybrid orbitals as indicated in .

The sp²-s s-bonds are shorter than are sp³-s s-bonds. The trend also holds for the sp-s bonds in acetylene. 

Acetlyenes are highly reactive and thus do not exist freely in nature. Carbon atoms involved in triple bond are sp hybridized having linear nature with an angle of 180^o.  

 CH≡CH                       CH₃—C≡CH

 2σ   2σ                        4σ     2σ   2σ

sp   sp                         sp³    sp   sp   

The linear structure of C≡C has been described below, e.g. C₂H₂

Alkynes have the following characteristic bond lengths and bond energy.   

In acetylene the C≡C involves one σ bond formed by head on overlapping of sp-sp orbital as well as two π bonds formed by lateral overlapping of p–p orbitals. The comparative chart of bond energy and bond length of alkane, alkene and alkyne is given below :  

Some important orders for alkanes, alkenes and alkynes are given below:  

The more reactivity of C≡C inspite of higher bond energy is due to the presence of pbonds.  

Acetylene and all terminal alkynes are acidic in nature i.e. H atom attached on triple bonded C atom behaves as acidic. The acidic character is due to the fact that in acetylene or triply bonded C atom, an increase in s character (sp hybridization) give rise to higher electronegativity to C atom and thus H atom in C—H (i.e. sp—1s) bond acquires more polarity to show acidic nature.  

The acidic nature order however shows  H₂O > C₂H₂ > C₂H₄ > C₂H₆  

Furthermore, a decreasing trend of bond lengths (in C—C & C—H bonds) from alkane to alkyne can also be explained due to increasing s character from alkane to alkyne which leads to smaller size of hybrid orbitals and therefore showing more tendency for overlapping leading to smaller bond lengths.  

UPAC Nomenclature : The IUPAC name is derived from the IUPAC name of alkanes by replacing ending ‘ane’ by ‘yne’ alongwith the position of triple bond e.g. Alkane–ane + yne = Alkyne  

CH₃—CH₂C≡CH but-1-yne   

Alkynyl groups : Residual part left after the removal of one H atom from alkyne is known as alkynyl group. According to IUPAC nomenclature, these groups are named by replacing terminal (e) of alkyne by yl e.g.

Test For Unsaturation

By Br₂ in CCI₄ : The decolorization of 5% Br₂ in CCI₄ by a compound confirms the presence of unsaturation in molecule.

By Baeyers reagent : The decolorization of pink colour of 1% cold alkaline KMnO₄ by a compound confirms the presence of unsaturation in molecule.

Note :

1. Alkenes without any hydrogen atom on olefinic bond do not show these tests e.g.

2. Aldehydes, primary and secondary alcohols which are readily oxidized by alk. KMnO₄ and thus decolorize alk. KMnO₄

To locate the position of unsaturation : By ozonolysis

To distinguish

1. Alkane and alkene : By Br₂ in CCI₄ or By Baeyers reagent

2. Alkane and alkyne : As above

3. Alkene-1 and alkene-2 : By ozonolysis

4. Alkyne-1 and alkyne-2 : By Amm. AgNO₄ or Amm. Cu₂CI₂

To separation a mixture of alkane, alkene and alkyne-1

The mixture is passed through amm. Cu₂CI₂ or amm. AgNO₃ where alkyne-1 are retained in it and alkane, alkene mixture comes out. The mixture is then passed through conc. H₂SO₄ which absorbs alkene and alkane comes out.

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