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Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. The reactivity of alkenes is largely influenced by the accessibility of this double bond and the surrounding chemical environment. Generally, terminal alkenes, which have their double bond at the end of the carbon chain, are more reactive than internal ones. This is due to less steric hindrance around the double bond, allowing easier approach and reaction with reagents. For instance, in a comparison between 1-hexene and E-3-hexene, 1-hexene's terminal double bond allows it to react faster with bromine in acetic acid. Internal alkenes, like E-3-hexene, have more neighboring groups around the double bond, leading to increased steric hindrance and slower reactions.

Elimination reactions involve the removal of atoms or groups from a molecule, typically resulting in the formation of a double or triple bond. Strong bases often promote elimination reactions, especially when bulky, non-polar solvents are used. An example of this is seen with cis-4-(t-butyl)cyclohexylmethyl bromide reacting with potassium tert-butoxide in tert-butyl alcohol. In this scenario, the reagent prefers to remove a hydrogen atom and a leaving group (such as a bromide), favoring the formation of a double bond. The cis isomer, due to its higher steric hindrance and lower stability, is more prone to undergo elimination rapidly compared to the trans isomer. Therefore, elimination reactions are typically faster in molecules with high steric strain, leading to the rapid expulsion of leaving groups and formation of new double bonds.

In electrophilic addition reactions, an electrophile is attracted to an electron-rich site on a molecule, such as the double bond in alkenes. The double bond acts as a nucleophile, attacking the electrophile to form a more stable, often saturated product. This process is observed when 2-phenylpropene reacts with sulfuric acid in water. The election-rich phenyl group on the alkene enhances the electron density, promoting rapid addition of an electrophile. On the other hand, 4-(1-methylethenyl)benzoic acid, having an electron-withdrawing carboxyl group, does not facilitate the addition as effectively because it reduces electron density on the alkene. Thus, 2-phenylpropene reacts faster in electrophilic addition reactions due to its favorable electron environment and enhanced nucleophilicity offered by the electron-donating phenyl group.

Stereochemistry involves the spatial arrangement of atoms in molecules and how this affects chemical reactions. In specific reactions, stereochemistry can dictate which isomer reacts faster. For example, cis and trans isomers may behave differently in chemical environments. In elimination reactions, the cis-4-(t-butyl)cyclohexylmethyl bromide has axial interactions causing more strain. These steric interactions make the molecule less stable, thus more reactive in elimination reactions. While the trans isomer generally is sterically and energetically more favorable, the specific geometry of the cis isomer aids its participation in elimination. This concept is crucial, as it underlines the importance of molecular geometry and conclusion that even slight changes in spatial arrangement can significantly influence reaction rates and pathways. Therefore, understanding how stereochemistry affects reactivity helps us predict product formation more accurately.