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.