Physical Properties of Alkenes

Alkenes and its Physical Properties

Alkenes are among the unsaturated forms of hydrocarbons that exist when there is a double bonding between Carbon atoms at least once in their structure. These are also known as olefins. The first and purest form among other alkenes is that of ethene having the composition C2H4 and is found to be helpful for many industrial purposes. Alkenes can have isomers because of their physical structure.

Since isomeric alkenes have striking boiling points to natural alkenes, they are often difficult to differentiate by their boiling points. However, as the cis isomers of alkenes generally have lower melting points to that of trans isomers, these properties can be useful for discerning between alkenes. The Alkenes generally contain double bonds of Carbon atoms, named as Sigma (σ) and Pi (π) bonds. It is because of the sp2hybridization that the alkenes have a planar structure, with stable isomers, either on the same side (known as cis isomers) or on the opposite sides (known as the trans) isomers. Such isomers, in general, are called diastereoisomers. 

The general formula of alkenes is: CnH2n


Structural Isomerism

Found in alkenes with four or more carbon atoms in them, these isomers get formed because of the distinct structural formula with which these molecules can be represented. One such example would be that of C4H8 where there are three structural isomers present for it. 

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Geometric Isomerism

For cis-trans, or geometric isomerism, alkenes have doubly bonded carbon atoms that do not take any rotations in their structure. Therefore, the CH3 functional group on each side of the molecule gets locked up in either the same or opposite side of each other. For such isomers, the nomenclature exists in the form of cis/trans-(no. of carbon)-ene, where 'cis' refers to the groups locked on the same side, while 'trans' refers to the groups on either side of the atoms. 

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Properties of Alkenes

Alkenes are generally colourless in nature with no inherent odour, instead ethene comes with a pleasant smell. The name 'olefin' comes from the ethylene that was previously known as the olefiant gas. Therefore, alkenes can also form an oily compound upon treatment with chlorine or bromine. Overall, alkenes' physical properties are similar to that of alkanes because of its weaker Van Der Waals forces of attraction between molecules. 


Physical State of Alkenes

  • For alkenes, the compounds with lower Carbon atoms in the range of C2-C4 , are all gases, mid-range Carbon atoms like C3-C17 are all liquids, and the higher ones exist in solid forms at room temperature. 

  • The alkenes can burn in air, and produce a luminous flame.

  • They occur in several natural forms, like that of 1-octene, commonly found in lemon oil, butadiene in coffee, and more. The isomeric polyenes found in tomato and carrots' vibrant colour are because of several isomeric polyenes having composition C40H56

  • Ethylene also helps in the ripening process for fruits and vegetables. 


Polarity

The polarity of alkene is defined by the functional groups and the alkene's chemical structure. 

  • In general, the alkenes come with a weaker dipole interaction because of its sp2 carbon that is more electrophilic in nature when compared to the sp3 hybridized orbitals. 

  • Similarly, the trans isomers of alkenes come with no dipole moment as the net dipole cancels each other completely. 

  • Because of the presence of π bonds, the alkenes are more reactive than that of alkanes, yielding a better dipole bonding of the former than the latter. 


Density and Solubility

  • The alkenes are generally lighter than water and are virtually insoluble in it because of their nonpolar features.

  • They dissolve easily in organic solvents like benzene and ligroin, much like alkanes. 


Boiling Point

  • The boiling point of alkenes is likened to that of the alkanes, where its increase is directly proportional to the number of carbon atoms present in the alkenes. 

  • The boiling point of the straight-chained alkenes is more than that of the branch-chained alkenes, as it depends on the molecular mass of the compounds. 

  • With a higher number of carbon atoms in the compound, the intermolecular forces increase in strength, causing an increase in the molecules' overall size. It also creates a change in respective Van Der Waals dispersion forces and thus contributes to the higher boiling point temperature in higher alkenes. 

Considered as the most important physical property of alkene, here are some of the boiling points of alkenes: 

Boiling Points of Alkanes

Alkenes

Boiling Point

Ethene

-104°C

Trans-2-Butene

0.9°C

Cis-2-butene

3.7°C

1-Pentene

30°C


Melting Point

Among other alkenes' physical properties, the melting point of alkenes depends entirely on the packaging of the molecules present in them. Like alkanes, the alkenes too, represent similar melting point trends, however:

  • The cis isomers of alkenes have a U-bending shape than that of the trans isomers, and thus have a lower melting point than the trans-isomers. 

Here are some of the examples of how the melting point of different alkenes varies in temperature:

Melting Points of Alkanes

Alkenes

Melting Point

1-Pentene

-165°C

Trans-2-Pentene

-135°C

Cis-2-Pentene

-180°C

1-Heptene

-119°C

FAQ (Frequently Asked Questions)

1. Are Alkenes Reactive In Nature?

Alkenes are comparatively more reactive than their respective alkanes since there's a relative instability present between the doubly bonded carbon atoms, in the bond. They undergo various chemical reactions like combustion, addition, hydrogenation, and halogenation, in general, to yield several useful compounds. The alkenes can also be converted into polymers with the help of catalysts in specific chemical processes. 

2. What Are The Applications Of Alkenes?

Alkenes are generally used in bulk amounts to produce ethylene from natural gas via thermal cracking, where the complex organic molecules (mainly heavy hydrocarbons) are further broken down into simpler, lighter hydrocarbons via breaking the carbon-carbon bonds in between them. In the synthesis of plastics, alkenes are a crucial raw material.