Final answer:
Alcohols react with PBr₃ through SN1 or SN2 mechanisms to form alkyl bromides. Tertiary and secondary alcohols typically react via SN1, while primary alcohols react via SN2. In biochemical systems, alcohols are activated with ATP to participate in nucleophilic substitution reactions.
Step-by-step explanation:
The reaction of alcohol with phosphorus tribromide (PBr₃) involves a mechanism whereby the alcohol is converted into an alkyl bromide. Depending on the structure of the alcohol, the reaction can proceed through either an SN1 mechanism or an SN2 mechanism. Tertiary alcohols react via the SN1 mechanism due to the stability of the formed carbocation. For example, (CH₃)₃C-OH reacts with HBr to form (CH₃)₃C-Br and H₂O. Secondary alcohols also react via the SN1 mechanism as seen with (CH₂)₂CH-OH forming (CH₃)₂CH-Br and water. However, primary alcohols such as CH₂CH₂-OH undergo the SN2 mechanism, which involves a backside attack by bromide leading directly to CH₂CH₂-Br and water without carbocation formation.
It is important to note that elimination reactions may compete with substitution reactions. The alcohol might undergo dehydration to form an alkene, which can then react with HBr to produce the same alkyl bromide product as if it had undergone a substitution reaction.
In biodhemical systems, alcohols are activated not by strong acids like HBr but by converting them into phosphate esters using adenosine triphosphate (ATP), allowing them to undergo nucleophilic substitution reactions under physiological conditions.