Electrophilic Addition of Hydrogen Halides
Due to the nature of the pi bond, pi bonds can act like a nucleophile and undergo addition of electrophiles. The pi electrons have a high electron density in the pi electron cloud which can be easily polarized (give up the electrons) and can act like a nucleophile (nucleus-loving due to too many electrons; wants to give them away to electrophile). This behavior is very similar to the behavior that the lone pair electrons have on a Lewis base.
The addition of hydrogen halides is one of the easiest electrophilic addition reactions because it uses the simplest electrophile: the proton. Hydrogen halides provide both a electrophile (proton) and a nucleophile (halide). First, the electrophile will attack the double bond and take up a set of pi electrons, attaching it to the molecule (1). This is basically the reverse of the last step in the E1 reaction (deprotonation step). The resulting molecule will have a single carbon- carbon bond with a positive charge on one of them (carbocation). The next step is when the nucleophile (halide) bonds to the carbocation, producing a new molecule with both the original hydrogen and halide attached to the organic reactant (2). The second step will only occur if a good nucleophile is used.
Mechanism of Electrophilic Addition of Hydrogen Halide to Ethene
Mechanism of Electrophilic Addition of Hydrogen Halide to Propene
All of the halides (HBr, HCl, HI, HF) can participate in this reaction and add on in the same manner. Although different halides do have different rates of reaction, due to the H-X bond getting weaker as X gets larger (poor overlap of orbitals).
The regiochemistry of this reaction can be explained through Markovnikov's Rule. The basics of this rule states that the proton will add to the less substituted carbon and the halogen goes to the more substituted carbon. This may seem odd that the larger atom is going to the more sterically hindered spot, but this rule is more about the stability of the intermediate. The addition will occur in a way that produces the most stable carbocation after the initial protonation (the intermediate needs to have a stable carbocation). Tertiary and secondary carbocations are the most stable due to hyperconjugation, so the more substituted the carbon is the more stable the carbocation will be.
Examples of regioselective behavior in Electrophilic Addition: Markovnikov's Rule
Since the intermediate of this reaction is a carbocation the carbocation wants to be the most stable it can be. In some case this will mean that the carbocation will rearrange. This is very likely when there are no good nucleophiles present to trap the carbocation in a certain position. The rearrangement of the carbocation depends on many things but generally rearrangements will occur when there are no good nucleophiles present such as in a strong acid.
The stereochemistry for this reaction is random. The important thing to understand is the regiochemistry and the Markovnikov's Rule (this will come up a lot!).
Write out the following reactions and think about the rate of the reaction:
Good job! The answers should be clear if you put effort in and think about it.
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