From Klein's Organic chemistry [1, p. 780]:
The 1,2-adduct is believed to form more rapidly as a result of a proximity effect. Specifically, the carbocation and the bromide ion are initially very close to each other immediately after their formation in the first step of the mechanism. The bromide ion is simply closer in proximity to C2 than C4, so attack at C2 occurs more rapidly.
Similarly, from Vollhardt's Organic Chemistry: Structure and Function [2, pp. 345–346]:
Ring size controls the speed of cyclic ether formation
A comparison of the relative rates of cyclic ether formation reveals a surprising fact: Three-membered rings form quickly, about as fast as five-membered rings. Six-membered ring systems, four-membered rings, and the larger oxacycloalkanes are generated more slowly.
Relative Rates of Cyclic Ether Formation
$$k_3 ≥ k_5 > k_6 > k_4 ≥ k_7 > k_8$$
$k_n =$ reaction rate, $n =$ ring size
A second enthalpic phenomenon, which has been called the proximity effect, operates, especially in 2-haloalkoxides.
The proximity effect was used to explain these two reactions. Is it applicable to every reaction?
What is the proximity effect?
- Klein, D. R. Organic Chemistry, 1st ed.; John Wiley: Hoboken, NJ, 2012. ISBN 978-0-471-75614-9.
- Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry: Structure and Function, 7th ed.; W. H. Freeman: New York, 2014. ISBN 978-1-4641-2027-5.