Aldehydes and ketones can be halogenated at their α-position by reaction with cl2, br2, or i2, under acidic conditions. using br2 under acidic conditions, an intermediate enol is formed which adds bromine at the α-position. the reaction stops after the addition of one bromine because the electron-withdrawing halogen decreases the basicity of the carbonyl oxygen, making the protonation less favorable. draw curved arrows to show the movement of electrons in this step of the mechanism.