Why is there syn-addition in this bromination of an alkene?
In this example, it is impossible to tell from the product whether the mechanism was syn- or anti-addition. Let's look at the two possible products. After anti- or syn-addition of bromine, two compounds are produced that look different. However, facile bond rotation around the C1-C2 bond shows that they are in fact the same compound.



How would one go about proving that anti-addition is actually occurring in 4-methyl-1-pentene. One solution would be to use an isotopically labeled variant as shown below. Starting from the deuterated version of 4-methyl-1-pentene, we can draw the anti- and syn-products again. Similar rotation around the C1-C2 bond makes it clear that the products are not the same but instead are diastereomers. It would be possible to distinguish these products by some analytical technique (1H NMR) and show that the anti-product is formed selectively.