Q4(b). Rates of HCl addition: an enol ether reacts fastest

Introduction. All three are electrophilic additions of HCl that pass through a carbocation. The rate is set by the stability of the cation formed in the first step; a cation with an adjacent oxygen lone pair (an oxocarbenium ion) is far more stable than any alkyl cation.

Scheme: Step 1: H⁺ adds to the CH₂ end (slow). Step 2: Cl⁻ adds to the oxocarbenium carbon (fast).

1. (i) Fastest: Y. Protonation of 2-methoxypropene gives a tertiary cation that is also stabilised by the lone pair of the OMe oxygen (+M): the C=O⁺ resonance form has an octet on every atom. X (propene) and Z (3,3-dimethylbut-1-ene) give only secondary alkyl cations, stabilised by hyperconjugation alone. Enol ethers are electron-rich alkenes and add acids many orders of magnitude faster.
2. (ii) Major product: 2-chloro-2-methoxypropane. H⁺ goes to the terminal CH₂ (Markovnikov), because that puts the positive charge on C-2, next to oxygen. Protonation at C-2 would give a primary cation with no oxygen help, so 1-chloro-2-methoxypropane is negligible.
3. (iii) Mechanism. Step 1: the C=C π electrons attack the H of H–Cl; the H–Cl bond breaks, giving the oxocarbenium ion and Cl⁻ (rate-determining). Step 2: Cl⁻ attacks the electrophilic C-2 (fast). The product is an α-chloro ether.
4. (iv) Why step 1 is the RDS. Step 1 breaks a π bond and the strong H–Cl bond and creates two charged species from neutral molecules: it is strongly endergonic, and by the Hammond postulate its transition state resembles the cation, high in energy. Step 2 combines a cation with an anion, is strongly exergonic and has almost no barrier. Rate =k[alkene][HCl], and the rate correlates with cation stability, as found (Y≫ X, Z).
5. Note on Z. The secondary cation from 3,3-dimethylbut-1-ene undergoes a 1,2-methyl shift to a tertiary cation, so 2-chloro-2,3-dimethylbutane is also formed; Z is still much slower than Y.

Conclusion. (i) Y is fastest: its cation is an oxygen-stabilised oxocarbenium ion. (ii) 2-chloro-2-methoxypropane (Markovnikov). (iii)–(iv) Protonation of the C=C to give the oxocarbenium ion is the slow, rate-determining step; capture by Cl⁻ is fast.

Application. The same fast protonation of enol ethers is used to protect alcohols as THP and MOM-type acetals (dihydropyran or 2-methoxypropene with an acid catalyst) and explains why enol ethers hydrolyse rapidly in dilute acid.