What are the physical properties of alkenes and their periodic trends
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 Explained Clearly
1. What are the physical properties of alkenes?
The physical properties of alkenes include low boiling points, non-polar nature, insolubility in water, and increasing density and boiling point with molecular mass.
Key physical properties of alkenes are:
- They are non-polar hydrocarbons due to C=C and C–H bonds.
- They are insoluble in water but soluble in organic solvents like benzene and ether.
- Lower members (ethene, propene, butene) are gases; middle members are liquids; higher members are solids.
- Boiling point increases with molecular mass due to stronger van der Waals forces.
2. Why are alkenes insoluble in water?
Alkenes are insoluble in water because they are non-polar molecules while water is a polar solvent.
This happens because:
- Alkenes contain only C–C and C–H bonds, which have very small electronegativity differences.
- Water molecules form strong hydrogen bonds with each other.
- Alkenes cannot form hydrogen bonds with water, so they do not dissolve.
This follows the rule: "like dissolves like".
3. How does the boiling point of alkenes change with chain length?
The boiling point of alkenes increases as the carbon chain length increases.
This is because:
- Larger alkenes have greater molecular mass.
- They experience stronger London dispersion (van der Waals) forces.
- More energy is required to separate the molecules.
For example, ethene (C2H4) has a much lower boiling point than hexene (C6H12).
4. Are alkenes polar or non-polar molecules?
Alkenes are generally non-polar molecules because they contain only carbon and hydrogen atoms.
Although the C=C bond contains a region of high electron density:
- The overall molecule usually has symmetrical charge distribution.
- There is no significant permanent dipole moment in simple alkenes.
Therefore, alkenes behave as non-polar hydrocarbons in terms of physical properties.
5. What is the general formula of alkenes?
The general formula of open-chain alkenes is CnH2n, where n ≥ 2.
This formula applies to:
- Hydrocarbons containing one carbon–carbon double bond (C=C).
- Acyclic (non-ring) alkenes.
For example:
- Ethene: C2H4
- Propene: C3H6
- Butene: C4H8
6. What is the physical state of alkenes at room temperature?
The physical state of alkenes at room temperature depends on their molecular size.
Generally:
- Lower alkenes (C2–C4) are gases.
- Medium alkenes (about C5–C17) are liquids.
- Higher alkenes are waxy solids.
This trend is due to increasing intermolecular forces with increasing molar mass.
7. How does branching affect the boiling point of alkenes?
Branching decreases the boiling point of alkenes compared to their straight-chain isomers.
This occurs because:
- Branched molecules have a more compact shape.
- They have smaller surface area for intermolecular contact.
- Weaker van der Waals forces act between molecules.
Thus, a branched alkene boils at a lower temperature than its straight-chain counterpart with the same molecular formula.
8. Are alkenes less dense than water?
Yes, alkenes are generally less dense than water and float on its surface.
This is because:
- They are composed only of carbon and hydrogen.
- Their density is typically less than 1 g cm-3.
- They do not mix with water due to non-polar nature.
As a result, alkenes form a separate layer above water.
9. What type of intermolecular forces are present in alkenes?
Alkenes primarily exhibit London dispersion forces, a type of van der Waals force.
These forces arise because:
- Temporary dipoles form due to electron movement.
- Alkenes are non-polar and lack hydrogen bonding.
- Intermolecular attraction increases with molecular size.
These weak forces explain their relatively low melting and boiling points.
10. How do the physical properties of alkenes compare to alkanes?
Alkenes and alkanes have similar physical properties, but alkenes generally have slightly lower boiling points for similar molecular mass.
Comparison:
- Both are non-polar hydrocarbons.
- Both are insoluble in water and soluble in organic solvents.
- Both show increasing boiling point with increasing chain length.
- The presence of a C=C double bond makes alkenes slightly more reactive but does not drastically change physical properties.
Thus, their physical behavior is largely governed by molecular size and intermolecular forces.
