Cl2 or Br2 + Alkene (Adding halogens across double bonds)
When a halogen (like Cl2 or Br2) is allowed to react with an alkene, the alkene will attack one of the atoms. Normally the bond between the two halogens isn't polar, but it becomes polar as it gets near the double bond, because the double bond has so much - charge density.
So, the double bond breaks open, and one of the carbons is left without a fourth bond. This means it's a carbocation and has a positive (+) formal charge. Important to note is that tertiary carbocations are more stable than secondary ones, and secondary carbocations are more stable than primary ones. Thus, the more substituted carbocation is favoured.
When you've added a halogen (as opposed to an H or something else) to the double bond, you also get a bonus resonance structure for the carbocation. For bromine, it's called the bromonium ion and it prevents the other Br from attacking the carbocation from the same side that the Br2 originally attacked from. It's called chloronium for chlorine, iodonium for iodine, and fluoronium for fluorine.
In any case, the other halogen atom (which was left over after its brother was ripped away in the first step) has a negative charge, and will be attracted to (and will attach itself to) the carbocation ... the attack, to repeat, will be from the opposite side as the original attack. This is called an "anti" addition to the double bond.
If you care about stereochemistry: The alkene itself is planar (because it's sp2 hybridized). So, depending on how bulky the side chains are, there's about a 50/50 chance the original halogen atom would attack from either the top or bottom, and so we end up with a 50/50 mixture of R and S enantiomers. The stereochemistry can get complicated if there are two chiral centres, but remembering that it's an "anti" addition across the double bond will help you.
So, the double bond breaks open, and one of the carbons is left without a fourth bond. This means it's a carbocation and has a positive (+) formal charge. Important to note is that tertiary carbocations are more stable than secondary ones, and secondary carbocations are more stable than primary ones. Thus, the more substituted carbocation is favoured.
When you've added a halogen (as opposed to an H or something else) to the double bond, you also get a bonus resonance structure for the carbocation. For bromine, it's called the bromonium ion and it prevents the other Br from attacking the carbocation from the same side that the Br2 originally attacked from. It's called chloronium for chlorine, iodonium for iodine, and fluoronium for fluorine.
In any case, the other halogen atom (which was left over after its brother was ripped away in the first step) has a negative charge, and will be attracted to (and will attach itself to) the carbocation ... the attack, to repeat, will be from the opposite side as the original attack. This is called an "anti" addition to the double bond.
If you care about stereochemistry: The alkene itself is planar (because it's sp2 hybridized). So, depending on how bulky the side chains are, there's about a 50/50 chance the original halogen atom would attack from either the top or bottom, and so we end up with a 50/50 mixture of R and S enantiomers. The stereochemistry can get complicated if there are two chiral centres, but remembering that it's an "anti" addition across the double bond will help you.
