Chemistry

I understand all of the following questions expect "Suggest a reactant, other than a different acid, that could be used to improve the atom economy of making 1-bromobutane by the same metho
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The mark scheme said the answers were NaBr or LiBr, but why an earth are those the correct answers? Any advice would be appreciated!

Atom economy = 100 x (Mr of desired products)/(Sum of Mr’s of all reactants)

Since you can’t increase or decrease the Mr of the desired product, that isn’t the way to increase the atom economy. So how could you possibly change the sum of the Mr’s of all the reactants to give the atom economy its largest possible value?

Could you not just use C4H9 instead of C4H9OH, wouldn't that increase the atom economy or would it throw off the balance and not change the atom economy?