Alkenes — Definition

Definition

Imagine hydrocarbons as the fundamental building blocks of organic chemistry, made solely of carbon and hydrogen atoms. Within this vast family, we have different types based on how these carbon atoms are connected.

Alkenes are a special type of hydrocarbon where at least two carbon atoms are joined together by a 'double bond'. Think of it like two hands holding instead of just one, making the connection stronger in some ways, but also providing a 'handle' for other molecules to grab onto.

\n\nThe simplest alkene is ethene, also commonly known as ethylene, which has two carbon atoms connected by a double bond, and each carbon is also bonded to two hydrogen atoms. Its formula is $C_2H_4$.

The general formula for alkenes with one double bond is $C_nH_{2n}$. This formula tells us that for every 'n' carbon atoms, there are '2n' hydrogen atoms. This is different from alkanes ($C_nH_{2n+2}$), which only have single bonds, and alkynes ($C_nH_{2n-2}$), which have triple bonds.

The '2n' hydrogen atoms indicate a degree of unsaturation compared to alkanes. \n\nWhat makes this double bond so important? It's composed of two types of bonds: a strong sigma ($\sigma$) bond and a weaker pi ($\pi$) bond.

The sigma bond is formed by the direct overlap of hybrid orbitals, while the pi bond is formed by the sideways overlap of unhybridized p-orbitals above and below the plane of the sigma bond. This pi bond is crucial because its electrons are more exposed and less tightly held than sigma electrons, making alkenes 'electron-rich' and highly susceptible to attack by electron-deficient species, known as electrophiles.

This is why alkenes primarily undergo 'addition reactions', where the pi bond breaks, and new atoms or groups add across the double bond, converting the unsaturated alkene into a saturated product. \n\nNomenclature, or naming, of alkenes follows specific rules.

We find the longest carbon chain that contains the double bond, number it to give the double bond the lowest possible number, and change the '-ane' suffix of the corresponding alkane to '-ene'. For example, a two-carbon alkene is ethene, a three-carbon alkene is propene, and so on.

If there are multiple double bonds, we use suffixes like '-adiene' or '-atriene'. \n\nAlkenes are incredibly important in everyday life and industry. Ethene, for instance, is a plant hormone that promotes fruit ripening and is a key starting material for making polyethylene plastic.

Propene is used to make polypropylene. Understanding alkenes is fundamental to grasping organic chemistry and its vast applications.