Physical Properties Alkenes: Melting and Boiling Point, Polarity

Physical properties of alkenes is an important aspect of the double bonded compounds. Alkenes or olefins are in reality, hydrocarbons, which consist of one or more double bonds. These compounds are colourless, and non-polar, and are often linked to alkanes because of their similarities. It should be remembered that alkenes are way more reactive. The alkenes possess these double bonds, are actually stronger than covalent bonds, and may be sigma bond, or pi bond. Let us know more about alkenes, their physical properties, so as to get a better understanding of the topic.

Key takeaways: alkenes, hydrocarbons, melting and boiling point, polarity, solubility.

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What are Alkenes?

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The unsaturated forms of hydrocarbon having one or more double bonds is known as alkenes. Also known as olefins, Alkenes are classified into 4 major categories which are named as monosubstituted alkenes, disubstituted alkenes, trisubstituted alkenes, and tetrasubstituted alkenes. Ethene and propene are the first two hydrocarbons. The general formula for alkenes is CnH₂n. The most important alkene is ethene which burns in air to give carbon dioxide and water. They serve many uses in the Chemical industry as fuels. 

Alkene

Although alkenes cannot be found in the crude oil industry, they are formed by the process of cracking of alkanes. The molecular formula increases by a fixed number with each and every carbon atom coming into the picture. The isomers of alkenes get formed because of the unique structural formula. 

Also check: IUPAC Nomenclature of Organic Compounds

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Properties Of Alkenes

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The properties of alkenes are as follows:

Physical state 

• Alkenes are double-bonded compounds which are colourless in nature. They are odourless too, with the only exception of ethene, which is colourless, but comes with a faint sweet smell attached to it. 
• The first three members of the alkene group which have lower carbon atoms in the range of C2-C4 are gaseous in nature, the fourteen atoms which range in between in the range C3-C17 are all liquids, and the remaining ones with higher carbon atoms are present in the form of solid in room temperature. 
• Alkenes can burn in air and emit a luminous flame. Ethylene has a special function where it assists in the ripening of the fruits and vegetables.
• Alkenes occur in several natural forms. These are 1-octene which is present in lemon oil, in the form of butadiene in coffee, and many more.
• Isomeric polyenes can be found in vegetables like tomatoes, and carrots. The vibrant colour of these vegetables comes from several isomeric polyenes which share the composition of C₄₀H₅₆. 

Also check: Classification of Haloalkanes and Haloarenes

Polarity

• Polarity of an alkene is dependent upon factors like functional groups, and the double-bonded compound’s chemical structure. 
• Alkenes possess a weaker dipole interaction because of the presence of sp2 carbon, which is a double bond. It also is more electrophilic in contrast to sp3 hybridised orbitals. 
• The double bond electrons, alkenes have a more reactive nature as compared to alkanes. These electrons can be easily eliminated as they are held weakly. This makes it possible for an efficient dipole bonding of the alkenes. 
• Alkenes have a relatively nonpolar nature. They can't dissolve in water, but are soluble in nonpolar solvents like hexane. 
• When it comes to the polarity of alkenes and alkanes, the former is more polar than the latter. This happens because of two primary reasons. First, pi bonds are more polarizable, and second, vinylic bonds tend to be slightly polar. The first case contributes to instant dipole moments, whereas the second leads to permanent dipole moments.

Solubility 

• Alkenes have a non-polar nature. It is because of this they are insoluble in water. 
• Just like alkanes, they have the ability to dissolve in organic solvents like benzene, ligroin, etc.

Boiling point 

• Alkanes and alkenes share quite some similarity when the boiling point is concerned. The boiling point of alkenes is directly proportional to the number of carbon atoms present in alkenes. This means when the number increases, the boiling point automatically rises. 
• Alkenes get formed in a structure of straight chains, which are much more than the branch-like chained alkenes. This basically depends on factors like the molecular mass of the compounds. 
• It appears that both compounds, alkanes and alkenes share the same carbon skeleton, which is why both of their boiling points are considered the same.
• The intermolecular force increases when more carbon atoms come together. This elevates the molecule’s size and structure, thereby giving rise to Van Der Waals dispersion forces. This in turn pumps up a force, which contributes to increasing the boiling point.

Also check: Halogenation

Melting point

• Melting point of alkenes is decided by the position and packaging of the molecules in these double-bonded compounds. It should be remembered that even the melting point of alkenes is similar to that of alkanes. 
• Cis-isomer differ from trans-isomers as the fomer possesses a U-bending shape. This is the reason they have a lower melting point when it comes to trans-isomers. 

Alkenes Melting Points:-

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Things to Remember

• Alkenes are basically defined as an unsaturated form of hydrocarbons which are present in the form of a double bond between carbon atoms. 
• The general formula of alkenes is CnH₂n. These are also known by the term olefins, and are capable of having isomers depending upon their physical structure. 
• The physical properties of alkenes are divided into major categories like physical state, melting point, boiling point, solubility, polarity, all of which decide the basic characteristics of the double-bonded compound. 
• Ethene and propene are known as the first two hydrocarbons in Chemistry. 
• When it comes to geometric isomerism, alkenes don’t indulge in any rotations in their structure because of having double bonded carbon atoms. 
• The CH₃ functional group on both sides of the molecule gets locked up in the same or opposite direction. 
• The former are called cis, where the groups are on the same side, whereas in the latter, its called trans. 

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