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POCl3 Dehydration quiz

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  • What is the main advantage of using POCl3 and pyridine for alcohol dehydration instead of acid?
    POCl3 and pyridine allow dehydration without acid, which is useful for acid-sensitive molecules that might decompose in acidic conditions.
  • What type of reaction does POCl3 and pyridine perform on alcohols?
    They perform an elimination reaction, converting alcohols into alkenes.
  • Which alkene product is favored in POCl3/pyridine dehydration reactions?
    The Zaitsev product, which is the most substituted alkene, is favored.
  • What role does pyridine play in the POCl3 dehydration mechanism?
    Pyridine acts as a base, facilitating deprotonation and the E2 elimination step.
  • Why is the oxygen in alcohol considered nucleophilic in this reaction?
    Because it has two lone pairs that can attack the electrophilic phosphorus in POCl3.
  • What makes the phosphorus atom in POCl3 highly electrophilic?
    The three chlorine atoms pull electron density away, giving phosphorus a strong partial positive charge.
  • What happens to one of the chlorine atoms in POCl3 during the reaction?
    It leaves as a chloride ion, making room for the alcohol to attach to the phosphorus.
  • How does the leaving group ability of the alcohol change after reaction with POCl3?
    The alcohol's oxygen becomes part of a much better leaving group (OPOCl2), facilitating elimination.
  • What is the first step in the POCl3 dehydration mechanism?
    The alcohol's oxygen attacks the electrophilic phosphorus atom in POCl3.
  • How many equivalents of pyridine are used in the full dehydration mechanism?
    Two equivalents: one for deprotonation after the alcohol attacks POCl3, and one for the E2 elimination.
  • What type of elimination mechanism occurs in the final step of POCl3 dehydration?
    An E2 β-elimination occurs, forming the double bond.
  • How does the reaction ensure formation of the Zaitsev product?
    The base (pyridine) abstracts a proton from the β-carbon that leads to the most substituted (Zaitsev) alkene.
  • What is the structure of pyridine?
    Pyridine is a benzene ring with one nitrogen atom replacing a CH group.
  • Why are Cl atoms in POCl3 considered good leaving groups?
    Chloride ions are stable as conjugate bases, making them excellent leaving groups.
  • What is the main organic product of the POCl3/pyridine dehydration of an alcohol?
    The main product is an alkene, specifically the most substituted one according to Zaitsev's rule.