9.3.5. Properties

Alkenes as a class resemble alkanes in physical properties, except in types of isomerism and difference in polar nature. The first three members are gases, the next fourteen are liquids and the higher ones are solids. Ethene is a colourless gas with a faint sweet smell. All other alkenes are colourless and odourless, insoluble in water but fairly soluble in non-polar solvents like benzene, petroleum ether. They show a regular increase in boiling point with increase in size i.e., every – CH2 group added increases boiling point by 20–30 K. Like alkanes, straight chain alkenes have higher boiling point than isomeric branched chain compounds.

Alkenes have properties that are similar to alkanes, but with some differences due to their double bonds. For the first three alkenes (like ethene), they are gases at room temperature. As we move to larger alkenes, they become liquids, and the largest ones are solids. Alkenes are generally colorless and odorless. They do not mix well with water because they lack polar groups but can dissolve in non-polar solvents, which is similar to how oil does not mix with water but does mix with other oils or fats. Additionally, the boiling point of alkenes increases as their molecular size grows; adding a -CH2 unit typically raises the boiling point by 20-30 K. Also, alkenes with a straight chain structure tend to have higher boiling points than their branched chain isomers. This is due to the way molecules pack together, where straight chains can align closely and exhibit stronger van der Waals forces.

Think of how a straw is flexible and can be easily bent when it’s straight, allowing for smooth fluid flow. Now, if you were to crumple the straw into a ball, it would still hold its shape, but fluid flow would be hindered. Just like the straight-chain alkenes allow for better interaction (like smoother fluid flow), branched alkenes don't fit together as well, which impacts things like boiling points.

Alkenes are the rich source of loosely held pi (π) electrons, due to which they show addition reactions in which the electrophiles add on to the carbon-carbon double bond to form the addition products. Some reagents also add by free radical mechanism. There are cases when under special conditions, alkenes also undergo free radical substitution reactions. Oxidation and ozonolysis reactions are also quite prominent in alkenes.

Alkenes have a special feature: the double bond (C=C) between the carbon atoms is made up of one strong sigma bond and one weaker pi bond. The pi bond is easily broken, making alkenes reactive. When alkenes react, they often do so by adding other molecules across the double bond. This is called an addition reaction. For example, when alkenes react with hydrogen, they can turn into alkanes. Sometimes, they can also undergo free radical reactions, especially in the presence of certain conditions, where the reaction proceeds through radical intermediates. For instance, alkenes can react with ozone in a reaction called ozonolysis, where the double bond is cleaved to produce aldehydes or ketones.

Imagine a rubber band that can stretch. The double bond in alkenes is like a rubber band: it’s strong when you pull it gently, but if you stretch it too far, it breaks easily. This is how the weak pi bond in alkenes allows for them to undergo reactions easily, unlike alkanes which have only sigma bonds that are harder to break.

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