Predict the major product(s), including stereochemistry where relevant, for the bromination reaction \(\left(\mathrm{Br}_{2} / \mathrm{CCl}_{4}\right)\) with each of the following alkenes: (a) 1-pentene (b) cis-2-pentene (c) is-3-hexene (d) trans \(-3\)-hexene (e) \((R)-3\)-methyl-1-pentene

In alkene bromination reactions, bromine is added across the double bond through a process known as anti-addition. This means that each bromine atom attaches to opposite sides of the double bond. Imagine the double bond as a horizontal line; during anti-addition, one bromine will add to the top side and the other to the bottom side.
This is different from syn-addition, where both atoms attach to the same side of the double bond.
For students, it's important to visualize the *anti-addition* mechanism to grasp how the stereochemistry of the product is affected. This process leads directly to the formation of vicinal dibromides, where the two bromine atoms end up on neighboring carbons.

Vicinal dibromides are the products of the anti-addition of bromine to an alkene. 'Vicinal' means 'neighboring,' so a vicinal dibromide has two bromine atoms on adjacent carbon atoms.
For example:

• When bromine is added to 1-pentene, the product is 1,2-dibromopentane, a vicinal dibromide.
• Similarly, bromination of trans-3-hexene yields (3S, 4R)-3,4-dibromohexane.

Remember, the position and stereochemistry of the starting alkene determine the specific vicinal dibromide produced.
This concept is foundational in understanding the outcome of alkene bromination reactions, especially when considering their stereochemistry.

Stereochemistry involves the 3D arrangement of atoms in molecules. In bromination reactions, *anti-addition* impacts the stereochemistry of the product, often leading to chiral centers.
Consider the following examples:

• When cis-2-pentene undergoes bromination, we get (2R, 3S)-2,3-dibromopentane.
• Bromination of (R)-3-methyl-1-pentene yields (2R, 3R)-3-methyl-1,2-dibromopentane.

Here, both the *cis* or *trans* configuration of the starting material and *anti-addition* lead to specific stereochemical outcomes. The R/S notation describes the spatial arrangement around chiral centers, helping predict the behavior and reactivity of these compounds.
Understanding stereochemistry is crucial for predicting the properties of the product and is a vital part of mastering organic chemistry.