In many books, it is mentioned that aldehydes are easily oxidized even by mild oxidizing agents. However, I have never seen bromine water explicitly mentioned. But in the carbohydrates chapter, it is mentioned that glucose etc. are oxidized by bromine water.
So is it that only carbohydrates are oxidized by bromine water or all aldehydes?
Also, do they react with bromine water under the conditions of the bromine water test for unsaturation? Or do they react only under vigorous conditions?
Answer
Aldehydes, including aldoses, are oxidized to their respective carboxylic acids in the presence of $\ce{Br2}$ in $\ce{H2O}$. The reason this reaction is often discussed with carbohydrates is that it is useful for differentiating aldoses from from ketoses, which cannot be further oxidized. A solution of $\ce{Br2}$ and $\ce{H2O}$ will lose its characteristic brown color in the presence of aldoses, but will not in the presence of ketoses.
The general mechanism for the oxidation of an aldehyde under these conditions is:
Though ketones will be able to form a hydrate like aldehydes, no deprotonation can occur to eliminate the bromide ion as we see in aldehydes. Hence no further oxidation can occur.
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