Step 1
+ OH
H
H fast C
CH 3
CH 3
O H H 3 C O+ H
H
C
CH 3
CH 3
H 3 C + OH
H
+
Step 2
C slow
CH 3
CH 3
H 3 C OH+ OH
H
H
H 3 CC +
CH 3
CH 3
+
Step 3
+
fast C
CH 3
CH 3
H 3 CC Cl^ − H 3 C Cl
CH 3
CH 3
+
i) The first two steps are the same as in the mechanism for the dehydration of an
alcohol ⇒ the alcohol accepts a proton and then the protonated alcohol
dissociates to form a carbocation and water.
ii) In step 3, the carbocation reacts with a nucleophile (a halide ion) in an SN 1
reaction.
- Comparison of dehydration and RX formation from alcohols:
- Dehydration is usually carried out in concentrated sulfuric acid.
i) The only nucleophiles present in the reaction mixture are water and hydrogen
sulfate (HSO 4 – ) ions.
ii) Both are poor nucleophiles and both are usually present in low concentrations
⇒ The highly reactive carbocation stabilizes itself by losing a proton and
becoming an alkene ⇒ an E1 reaction.
- Conversion of an alcohol to an alkyl halide is usually carried out in the presence
of acid and in the presence of halide ions.
i) The only nucleophiles present in the reaction mixture are water and hydrogen
sulfate (HSO 4 – ) ions.
ii) Halide ions are good nucleophiles and are present in high concentrations ⇒
Most of the carbocations stabilize themselves by accepting the electron pair of
a halide ion ⇒ an SN1 reaction.
