What are Alkenes?

Alkenes are a class of unsaturated hydrocarbons, meaning they are organic compounds composed solely of carbon and hydrogen atoms, and they contain at least one carbon-carbon double bond.

The presence of this carbon-carbon double bond ($C=C$) is the defining characteristic and functional group of alkenes, distinguishing them from alkanes which only possess single bonds.

The general formula for non-cyclic alkenes with one double bond is $C_nH_{2n}$, where 'n' represents the number of carbon atoms. This formula indicates that alkenes have two fewer hydrogen atoms than their corresponding alkanes ($C_nH_{2n+2}$) due to the double bond.

The carbon-carbon double bond ($C=C$) consists of one sigma ($\sigma$) bond and one pi ($\pi$) bond, making it stronger and shorter than a carbon-carbon single bond. The pi bond is formed by the sideways overlap of p-orbitals, resulting in electron density above and below the plane of the sigma bond.

This double bond introduces rigidity into the molecule, preventing free rotation around the bond axis, unlike single bonds. This rigidity can lead to geometric (cis-trans) isomerism in certain alkenes.

The presence of the double bond means that each carbon atom involved in the $C=C$ bond is bonded to only three other atoms (e.g., two carbons and two hydrogens in ethene), leading to a trigonal planar geometry around these carbon atoms with approximate bond angles of $120^\circ$.

Alkenes are named using the same root prefixes as alkanes (e.g., "eth-" for two carbons, "prop-" for three), but with the suffix "-ene" to indicate the presence of a double bond. The longest continuous carbon chain containing the double bond is chosen as the parent chain.

For alkenes with four or more carbon atoms, the position of the double bond must be indicated by a number, which corresponds to the lower-numbered carbon atom involved in the double bond. The carbon chain is numbered from the end that gives the lowest possible number to the double bond (e.g., but-1-ene vs. but-2-ene).

Positional isomerism is common in alkenes, where molecules have the same molecular formula but differ in the position of the double bond within the carbon chain. For example, $C_4H_8$ can be but-1-ene or but-2-ene, which are structural isomers.

Alkenes are significantly more reactive than alkanes due to the presence of the pi ($\pi$) bond within the $C=C$ double bond. This pi bond is weaker than the sigma bond and has exposed electron density.

The pi bond is a region of high electron density, making it susceptible to attack by electrophiles (electron-seeking species). This characteristic reactivity is central to alkene chemistry.

This high reactivity allows alkenes to undergo addition reactions, where the pi bond breaks, and new single bonds are formed with incoming atoms or groups across the original double bond carbons. This process converts the unsaturated alkene into a saturated compound.

Alkenes vs. Alkanes: Alkenes are unsaturated (contain $C=C$ bonds) and follow the general formula $C_nH_{2n}$ (for non-cyclic, single double bond), while alkanes are saturated (only $C-C$ single bonds) and follow $C_nH_{2n+2}$.

The functional group in alkenes is the $C=C$ double bond, which is responsible for their characteristic reactions, whereas alkanes lack a distinct functional group and are generally less reactive.

Alkenes readily undergo addition reactions, where atoms are added across the double bond. Alkanes, conversely, typically undergo substitution reactions under specific conditions, where one atom is replaced by another.

When identifying or naming alkenes, always ensure you locate the longest continuous carbon chain that specifically includes the carbon-carbon double bond. This chain forms the basis of the parent name.

For proper nomenclature, remember to number the carbon chain from the end that gives the lowest possible number to the first carbon atom of the double bond. This ensures a unique and systematic name.

Be mindful that the general formula $C_nH_{2n}$ is specific to non-cyclic alkenes containing only one double bond. If an alkene is cyclic or contains multiple double bonds, its molecular formula will differ from this general rule.

A common mistake is forgetting to indicate the position of the double bond in alkenes with four or more carbon atoms, leading to ambiguous or incorrect naming (e.g., simply 'pentene' instead of 'pent-1-ene' or 'pent-2-ene').

Students sometimes confuse the general formula for alkenes ($C_nH_{2n}$) with that of cycloalkanes, which also share this formula but have distinct cyclic structures and different chemical properties.

Another misconception is assuming that the double bond is simply a stronger version of a single bond and therefore less reactive. In reality, the presence of the pi bond makes the double bond a site of high electron density and increased reactivity, especially towards addition reactions.