Organic Chemistry (8th Edition)

Published by Pearson
ISBN 10: 0321768418
ISBN 13: 978-0-32176-841-4

Chapter 6 - Alkyl Halides: Nucleophilic Substitution and Elimination - Problems - Page 245: Problem 6-21 a

Answer

Bromide is replaced over fluoride because it is a better leaving group.

Work Step by Step

Bromide is a much larger atom than fluoride, as it is lower on the periodic table and therefore has a larger atomic radius. Consequently, bromide can stabilize a negative charge better and is a weaker base. The weaker the base, the better the leaving group. This can also be determined by relative pKa of conjugate acids: HBr has a lower pKa than HF so it has a weaker conjugate base.
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