Physical Properties of Alkenes

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 sp2 hybridization 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 hybridised 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. 

Alkenes are relatively nonpolar. They are insoluble in water but soluble in nonpolar solvents such as hexane. Alkenes are more polar than alkanes for two reasons: the pi bond electrons are more polarizable, thus contributing to instantaneous dipole moments, and the vinylic bond tends to be  slightly polar  that contributes to the permanent dipole moment.

The sum of the dipole moments in a symmetrical trans disubstituted  alkene is zero. The vector sum of the two poles is directed perpendicular to the double bond in the analogous cis alkene.This results in a non-zero molecular dipole. The permanent dipole results in an increased bp.

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. 

Due to their non polar characteristics alkenes are only soluble in nonpolar solvents and insoluble in water. Alkenes are also lighter than water. Alkenes dissolves in organic solvents but is Virtually insoluble in water.

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. 

The molecular mass besides the boiling point of the alkenes. The higher the intermolecular mass is, the higher the boiling point. With an increase in the size of the molecules the intermolecular forces of the alkenes also get stronger.

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

Boiling Points of Alkenes

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 Alkenes

Conclusion

At room temperatures Alkanes exist as liquids, gases or solids. Alkenes contain a carbon-carbon double bond. This carbon-carbon double bond changes the physical properties of alkenes. At room temperature, alkenes exist in all three phases, solids, liquids, and gases. The melting and boiling points of alkenes are similar to that of alkanes, however, isomers of cis alkenes have lower melting points than trans-isomers.