The solvent plays a crucial role in the electrophilic addition reactions of alkenes, influencing both the reaction mechanism and the rate. In the case of non-polar alkenes reacting with non-polar reagents like bromine, the solvent must be able to dissolve both reactants, typically requiring a non-polar or weakly polar solvent. Solvents can influence the stability of intermediates, such as carbocations, and can also affect the nucleophilicity of the attacking species. Polar solvents, for example, can stabilize carbocations through solvation, potentially altering the rate and outcome of the reaction. Furthermore, the choice of solvent can impact the stereochemistry of the reaction. For instance, in reactions where carbocations are intermediates, polar solvents can lead to more substitution on the carbocation due to better solvation and stabilization of the intermediate. In contrast, non-polar solvents might favor less substituted carbocations due to lesser solvation effects. Therefore, the selection of an appropriate solvent is essential for achieving the desired reaction pathway and yield in electrophilic addition reactions of alkenes.

Anti-addition is observed in the reaction of alkenes with bromine due to the formation and subsequent reaction of the bromonium ion intermediate. When an alkene reacts with bromine, the initial attack of the π electrons on the bromine molecule leads to the formation of a bromonium ion, which is a three-membered cyclic structure. This intermediate is characterized by a positive charge on the bromine atom. The nucleophilic attack, which follows, occurs from the side opposite to the already bonded bromine due to steric hindrance and the nature of the cyclic bromonium ion. The bromide ion, being a nucleophile, attacks the more accessible carbon atom of the bromonium ion, which is on the side opposite to the bonded bromine. This attack mechanism results in the bromine atoms attaching to opposite sides of the former double bond, thus leading to anti-addition. This stereochemical outcome is a direct consequence of the cyclic nature of the bromonium ion intermediate and the spatial constraints it imposes on the subsequent nucleophilic attack.

Hyperconjugation is a significant concept in understanding the stability of carbocations in electrophilic addition reactions. It refers to the delocalization of electrons in σ (sigma) bonds of adjacent alkyl groups with the empty p-orbital of the carbocation. This delocalization allows for the dispersal of the positive charge over a larger area, thereby stabilizing the carbocation. In electrophilic addition reactions, when a carbocation is formed as an intermediate, its stability is crucial for the overall reaction rate and the product distribution. Carbocations adjacent to alkyl groups (such as secondary or tertiary carbocations) are more stable due to hyperconjugation. These alkyl groups effectively donate electron density through their σ bonds, reducing the electron deficiency of the carbocation. The greater the number of alkyl groups attached to the carbocation, the more pronounced the hyperconjugation effect, and thus, the greater the stability of the carbocation. This stability influences not only the rate at which the carbocation will react with the nucleophile but also the regioselectivity of the reaction, as explained by Markovnikov's rule.

The presence of substituents on the alkene significantly affects the electrophilic addition mechanism, primarily influencing the stability of the carbocation intermediate and the regioselectivity of the reaction. Substituents can be either electron-donating or electron-withdrawing, and their effects are explained through inductive and resonance effects. Electron-donating groups (EDGs), such as alkyl groups, stabilize the carbocation intermediate through the inductive effect by donating electron density towards the positively charged carbon. This stabilizing effect increases with the number of alkyl substituents, leading to a more stable carbocation and, consequently, influencing the site of nucleophilic attack in the reaction. On the other hand, electron-withdrawing groups (EWGs) destabilize the carbocation by withdrawing electron density, making the carbocation less favorable at that position. This can lead to alternative reaction pathways or lower yields of the expected product. Additionally, the presence of substituents can influence the stereochemistry of the reaction, especially in the formation of intermediates like the bromonium ion, where steric factors can dictate the direction of nucleophilic attack.

The formation of a three-membered ring bromonium ion during the addition of bromine to alkenes is a consequence of the unique interaction between the electron-rich double bond of the alkene and the bromine molecule. When the π electrons of the alkene attack one of the bromine atoms, a cyclic structure is formed because this interaction induces a temporary shift in electron density that leads to the departure of the other bromine atom as a bromide ion. This departure leaves a positively charged bromine atom bonded to both carbons of the original double bond. The three-membered ring structure is favored due to its ability to stabilize the positive charge on the bromine atom through a phenomenon known as bridging. This bridging reduces the strain on the positive charge by spreading it over the entire ring structure, which includes the two carbon atoms and the bromine atom. It's this distribution of charge that stabilizes the bromonium ion, making the three-membered ring structure a key intermediate in the reaction.