These questions are about the substitution reactions of alcohols. This is in the OCR course but not in the AQA course. The mechanisms questions below are relevant for everyone, however.
Write the equations given the following reactants.
a) Methanol and phosphorus (V) chloride.
CH3OH + PCl5 → CH3Cl + POCl3 + HCl
b) Propan-1-ol and phosphorus (III) bromide.
3C3H7OH + PBr3 → 3C3H7Br + H3PO3
c) Propan-2-ol, sodium chloride and sulfuric acid.
CH3CH(OH)CH3 + NaCl + H2SO4 → CH3CHClCH3 + NaHSO4 + H2O
d) Phosphorus and bromine.
The mechanism for the reaction between methylpropan-2-ol and concentrated hydrochloric acid is outlined below.
a) Complete the mechanism by adding any relevant lone pairs and one curly arrow to each step on the outline.
b) State the role that the alcohol plays in the first step.
Base (It accepts a proton).
c) State the role that the chloride plays in the third step.
Nucleophile (It donates an electron pair to the carbocation).
d) Explain why this reaction is feasible for methylpropan-2-ol but not butan-1-ol.
The carbocation formed in the reaction above is a tertiary cation, whereas butan-1-ol would involve the formation of a primary carbocation. The tertiary carbocation is more stable than the primary carbocation due having more alkyl groups attached to it. More alkyl groups results in a stronger positive inductive effect, stabilising the carbocation.
e) Write an equation for the reaction.
CH3C(CH3)(OH)CH3 + HCl → CH3C(CH3)ClCH3 + H2O
The mechanism for the reaction between ethanol and phosphorous pentachloride is outlined below.
a) Complete the mechanism by adding any relevant lone pairs and two curly arrows to each step on the outline.
b) State how the ethanol behaves in the first step of the reaction.
c) State how the chloride behaves in the second step of the reaction.
d) Write an equation for the reaction.
C2H5OH + PCl5 → C2H5Cl + POCl3 + HCl
Ethanol, sulfuric acid and potassium chloride are reacted together.
a) Write the overall reaction that occurs.
C2H5OH + KCl + H2SO4 → C2H5Cl + KHSO4 + H2O
b) The reaction occurs in two steps. The first step is between the sulfuric acid and the potassium chloride. The second step is a reaction between one of the products of the first reaction and the alcohol.
i) Write the equation for first step.
H2SO4 + KCl → KHSO4 + HCl
ii) Write the equation for the second step.
HCl + C2H5OH → C2H5Cl + H2O
