10.5. HL: Electrophilic Addition to Alkenes (Mechanism)

As discussed earlier, alkenes are nucleophilic due to the high electron density of their π bond. They readily react with electrophiles (electron-deficient species). The mechanism of electrophilic addition typically involves two steps:

In this section, we learn that alkenes act as nucleophiles because of the high electron density found in their π bond. This means they are attracted to electrophiles, which are species that are electron-deficient. When an alkene reacts with an electrophile, the process of electrophilic addition occurs in two main steps, which we'll explore further.

Think of the alkene as a magnet (nucleophile) and the electrophile as an object that has a positive charge (like a battery). Just as a magnet attracts a battery due to its opposite charge, the alkene attracts the electrophile because of its higher electron density.

● Step 1: Electrophilic Attack and Carbocation Formation (Rate-Determining Step)

○ The electron-rich π bond of the alkene acts as a nucleophile and attacks the electrophile (E+). This causes the π bond to break, and a new σ bond forms between one of the carbon atoms of the original double bond and the electrophile.

○ This leaves the other carbon atom of the original double bond with only three bonds and a positive charge, forming a carbocation intermediate (a species with a positively charged carbon atom).

○ This step is typically the slower, rate-determining step of the reaction, as it involves breaking a bond and forming a highly unstable, high-energy intermediate.

In the first step of the electrophilic addition mechanism, the π bond of the alkene, which is rich in electrons, attacks the electrophile. This interaction breaks the π bond and forms a new σ bond with one of the carbon atoms. The carbon atom that does not bond with the electrophile now has a positive charge, resulting in a carbocation. This step is significant as it is the slowest part of the reaction, known as the rate-determining step, due to the formation of this high-energy intermediate.

Imagine a crowded room where a person (the electrophile) tries to make their way towards another person (the nucleophile). The moment they reach out to touch this person, they break their connection with others, making it a bit chaotic for a moment, akin to forming a carbocation. This moment of chaos represents the instability and energy in making that new connection.

● Step 2: Nucleophilic Attack on the Carbocation (Fast Step)

○ The highly reactive carbocation intermediate (electron-deficient) is rapidly attacked by a nucleophile (Nu−, typically the counter-ion formed in the first step, e.g., Br−).

○ The nucleophile donates a lone pair of electrons to the positively charged carbon, forming a new σ bond and completing the addition reaction. This step is usually very fast.

In the second step, the carbocation created in the first step is very reactive because it has a positive charge that seeks electrons. A nucleophile, which is often the counter-ion formed in the first step (like Br−), quickly approaches and donates a lone pair of electrons to the positively charged carbon atom. This results in the formation of a new σ bond, leading to the complete addition reaction. This step happens rapidly.

Think of the carbocation as a person who is very thirsty (electron-deficient) after running a race. When they see a bottle of water (the nucleophile) nearby, they quickly grab it to quench that thirst. This quick response mirrors how the nucleophile seizes the opportunity to bond with the carbocation.

● Explanation of Markovnikov's Rule: Markovnikov's Rule is a consequence of the two-step electrophilic addition mechanism and the relative stabilities of carbocation intermediates.

○ When an unsymmetrical alkene reacts with an unsymmetrical reagent (like HBr), two different carbocations could potentially form in the first step.

○ Stability of Carbocations: Carbocations are stabilized by adjacent alkyl groups (due to inductive effect and hyperconjugation, which delocalize the positive charge). Therefore, the order of stability is: Tertiary (3∘) > Secondary (2∘) > Primary (1∘) > Methyl

○ The reaction proceeds via the formation of the more stable carbocation intermediate because it has a lower activation energy barrier for its formation (Kinetic Control). This is why the hydrogen atom adds to the carbon of the double bond that already has more hydrogen atoms (as this leads to the formation of the more substituted, and thus more stable, carbocation).

Markovnikov's Rule explains the preferential formation of carbocations during the electrophilic addition of alkenes. When an unsymmetrical alkene reacts with an unsymmetrical reagent like HBr, two possible carbocations can form. The more stable carbocation, typically a tertiary one, will form preferentially because it possesses lower energy and therefore requires less energy to form. This stability arises from the presence of surrounding alkyl groups that help to stabilize the positive charge. Consequently, the hydrogen atom ends up on the carbon that already has more hydrogen atoms, leading to the more substituted and stable product.

Imagine choosing the most comfortable chair in a room full of different styles. If you pick the chair with the most supportive cushions (like the stable tertiary carbocation), it's easier to sit down (form) rather than selecting a flimsy stool (an unstable primary carbocation). Thus, your choice aligns with the comfort (stability) you prefer, similar to how the reaction favors forming a more stable carbocation.

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