Physical Properties of Alkenes – Melting & Boiling Point, Polarity, Solubility

Alkenes contain a carbon-carbon double bond which changes the physical properties of alkenes. Alkenes are unsaturated carbon compounds which have a general formula of C_nH_2n. These compounds are also known as olefins.

Alkenes are a family of compounds containing hydrogen and carbon only (hydrocarbons) with a carbon-carbon double bond. Ethene and Propene are the first two hydrocarbons.

1. Physical State

• These double-bonded compounds are colourless and odourless in nature.
• However, ethene is an exception because it is a colourless gas with a faintly sweet odour.
• The first three members of the alkene group are gaseous in nature, the next fourteen members are liquids and the remaining alkenes are solids.

2. Solubility

• The alkenes are insoluble in water due to their nonpolar characteristics.
• But are completely soluble in nonpolar solvents such as benzene, ligroin, etc.

3. Boiling Point

• The boiling points of the compounds increase as the number of carbon atoms in the compound increases.
• When alkenes are compared with alkanes, it is found that the boiling points of both are almost similar, as if the compounds are made up of the same carbon skeleton.
• The boiling point of straight-chain alkenes is more than branched-chain alkenes just as in alkanes.

4. Melting Point

• The melting points of these double-bonded compounds depend upon the positioning of the molecules.
• The melting point of alkenes is similar to that of alkanes.
• However, cis-isomer molecules have a lower melting point than trans- isomers as the molecules are packed in a U-bending shape.

5. Polarity

• Alkenes are weakly polar just like alkanes but are slightly more reactive than alkanes due to the presence of double bonds.
• The π electrons which make up the double bonds can easily be removed or added as they are weakly held.
• Hence, the dipole moments exhibited by alkenes are more than alkanes.
• The polarity depends upon the functional group attached to the compounds and the chemical structures.

Frequently Asked Questions – FAQs

Q1

Why alkenes are insoluble in water?

Due to their non-polar properties, alkenes are lighter than water and insoluble in water. Alkenes can only be dissolved in nonpolar solvents.

Q2

What are the first 6 alkenes?

The following is a list of the first six Alkenes:

• Etheme (C₂H₄)
• Propene (C₃H₆)
• Butene (C₄H₈)
• Pentene (C₅H₁₀)
• Hexene (C₆H₁₂)
• Heptene (C₇H₁₄)

Q3

Why does cis have lower melting point than trans?

Trans isomers pack better than cis isomers. Cis isomers pack poorly, which means that the intermolecular forces aren’t as effective as they should be and thus less amount of energy is required to melt the cis molecule. Hence, cis molecules have lower melting point.