The phrase "whether not" is commonly used as a way to express doubt or uncertainty regarding a decision or outcome. However, there are several synonyms that can be used in place of "whether not" to convey a similar meaning. For example, "if not" and "or not" can both be used to indicate a possible alternative or uncertainty. Another option is "in case," which suggests preparing for a potential outcome or contingency. "Doubtful" and "ambiguous" are also useful synonyms when discussing uncertain situations. Overall, using synonyms for "whether not" can make your writing more clear and concise, while still conveying the same message.