Using alkenes and any other necessary inorganic reagents, write reactions showing the formation of (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHClCH}_{2} \mathrm{Cl}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}\)

Halogen addition reactions are a type of chemical reaction where diatomic halogens, such as chlorine (Cl₂) or bromine (Br₂), add across the double bond of an alkene to form a vicinal dihalide. These reactions are a key method for transforming unsaturated hydrocarbons into more reactive halogenated compounds.

In our example, the alkene 2-butene reacts with Cl₂ in what is known as an 'anti-addition', which means that the two chlorine atoms attach on opposite sides of the plane of the alkene's double bond. This can be visualized with molecular models or drawing the reaction mechanism showing the attack of the halogen on the less hindered side of the molecule. It is important for students to recognize that the reactivity of the alkene comes from the electrons in the double bond, which act as a nucleophile, or electron pair donor, towards the electrophilic halogen molecules.

The synthesis of vicinal dihalides involves the addition of halogen atoms to adjacent carbon atoms in an alkene. 'Vicinal' means neighboring, which in chemistry, refers to two atoms or groups of atoms attached to adjacent carbon atoms. When the halogenation reaction occurs with an alkene, the resulting compound is a vicinal dihalide, characterized by two halogen atoms bonded to two adjacent carbons.

Using our exercise as a case study, we see the conversion of 2-butene to 1,2-dichlorobutane through the process of halogen addition. The transformation demonstrates the core principle of creating more complex molecules from simpler ones. Educators often stress the importance of selecting the correct starting alkene to yield the desired dihalide, and in the case of our example, 2-butene is strategically chosen due to the position of its double bond coinciding with where the chlorine atoms need to be added.

An electrophilic addition reaction occurs when an electrophile, which is an electron-loving species with a deficiency of electrons, reacts with an electron-rich unsaturated compound, such as an alkene. The double bond in the alkene has high electron density and is therefore attractive to electrophiles.

The formation of bromoethane from ethene and hydrogen bromide (HBr) is an excellent illustration of this kind of reaction. The positively charged hydrogen (H⁺) part of the HBr molecule is the electrophile that first attacks the electron-rich double bond. The electrons that made up the double bond move to form a bond with the hydrogen, creating a carbocation intermediate. Then, the negatively charged bromide ion (Br^-), which is a nucleophile, quickly attacks the positively charged carbon, resulting in the addition of both the H and Br across the double bond. It's vital to comprehend why the reaction happens this way, because it's determined by the polarity of the HBr and the existing double bond's ability to support positive charges.