What are 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.
FAQs on Physical Properties of Alkenes
1. What are the key physical properties of alkenes as per the CBSE Class 11 syllabus for 2025-26?
According to the syllabus, the most important physical properties of alkenes that students should focus on are:
- Physical State: The first three alkenes (ethene, propene, butene) are gases at room temperature, the next fourteen are liquids, and higher alkenes are solids.
- Solubility: Alkenes are nearly non-polar and are therefore insoluble in water but readily dissolve in non-polar organic solvents like benzene or ether.
- Boiling Point: The boiling point of alkenes increases with the increase in molecular mass. Straight-chain alkenes have higher boiling points than their branched-chain isomers.
- Density: Alkenes are less dense than water.
- Melting Point: The melting point trend is similar to the boiling point, increasing with molecular mass. However, trans-isomers often have a higher melting point than cis-isomers due to more symmetrical packing in the crystal lattice.
2. Why are alkenes insoluble in water?
Alkenes are insoluble in water because they are non-polar molecules. Water is a polar solvent and operates on the principle of "like dissolves like." Since alkenes lack significant polarity and cannot form hydrogen bonds with water molecules, they do not dissolve. They are, however, soluble in non-polar organic solvents.
3. How does the boiling point of an alkene change as its molecular size increases?
The boiling point of alkenes steadily increases as the molecular size or the number of carbon atoms increases. This is because larger molecules have a greater surface area, leading to stronger intermolecular van der Waals forces. More energy is required to overcome these stronger forces, resulting in a higher boiling point.
4. How do the physical properties of cis- and trans-isomers of an alkene differ?
Cis- and trans-isomers (geometrical isomers) of an alkene have different physical properties due to their different shapes and molecular polarities. Typically:
- Boiling Point: The cis-isomer usually has a higher boiling point because it has a net dipole moment, leading to stronger dipole-dipole intermolecular attractions. The trans-isomer is more symmetrical, and its bond dipoles often cancel out, making it less polar.
- Melting Point: The trans-isomer generally has a higher melting point. Its symmetrical shape allows it to pack more efficiently into a crystal lattice, requiring more energy to break the structure.
5. What is the typical physical state of the first few members of the alkene series at room temperature?
The physical state of alkenes depends on their molecular mass. The first three members, ethene (C₂H₄), propene (C₃H₆), and butene (C₄H₈), are colourless gases at room temperature. Alkenes with 5 to 18 carbon atoms are typically liquids, while those with more than 18 carbons are waxy solids.
6. How does the structure of the carbon-carbon double bond influence the physical properties of alkenes?
The C=C double bond significantly influences the physical properties of alkenes. The presence of the pi (π) bond restricts free rotation around the double bond, which gives rise to geometrical isomerism (cis-trans isomers). This structural rigidity leads to isomers with distinct shapes, polarities, and molecular packing efficiencies, which in turn causes them to have different boiling points, melting points, and densities.
7. Are alkenes good conductors of electricity, and why?
No, alkenes are poor conductors of electricity. This is because they are covalent molecules that do not contain free-moving charged particles like ions or delocalised electrons. An electric current requires the flow of charge, which is absent in the structure of alkenes, making them electrical insulators.
