Write the mechanism of the following reaction: CH3CH2OH + HBr → CH3CH2Br + H2O

The reaction between ethanol (CH3CH2OH) and hydrobromic acid (HBr) is an example of a nucleophilic substitution reaction, specifically an SN1 or SN2 mechanism. The mechanism depends on the reaction conditions, but under typical conditions, it proceeds via an SN1 mechanism.

SN1 Mechanism:

1. Protonation of the hydroxyl group: The oxygen atom in the hydroxyl group of ethanol is protonated by HBr. This makes the hydroxyl group a better leaving group, converting it into water (H2O).

   CH3CH2OH + HBr ⇌ CH3CH2OH2+ + Br-

2. Formation of the carbocation: The protonated ethanol molecule loses a water molecule, forming a carbocation intermediate. This is the rate-determining step in the SN1 mechanism.

   CH3CH2OH2+ → CH3CH2+ + H2O

3. Nucleophilic attack: The bromide ion (Br-), which is a good nucleophile, attacks the carbocation. The bromide ion donates a pair of electrons to the positively charged carbon atom.

   CH3CH2+ + Br- → CH3CH2Br

Therefore, the overall reaction is:

CH3CH2OH + HBr → CH3CH2Br + H2O

Note: An SN2 mechanism is less likely under typical conditions because it involves a concerted reaction with simultaneous bond breaking and bond forming, and the steric hindrance around the carbon atom may prevent a backside attack by the bromide ion. However, under different conditions (e.g., using a stronger acid and a better leaving group) an SN2 mechanism might be favored. The SN1 mechanism shown here is usually the most prevalent pathway.