The preparation of 2,4-dichlorobenzoic acid from benzoic acid and chlorine gas is typically carried out through electrophilic aromatic substitution. The optimal temperature and reaction time for this reaction can vary depending on the specific conditions and catalyst used.One common method for this reaction involves using a Lewis acid catalyst, such as aluminum chloride AlCl3 or ferric chloride FeCl3 , to promote the chlorination of benzoic acid. In this case, the reaction is typically carried out at a temperature of around 40-60C. The reaction time can vary depending on the specific conditions, but it generally takes a few hours e.g., 3-6 hours for the reaction to reach completion.It is important to note that the reaction conditions can be adjusted to optimize the yield and selectivity of the desired 2,4-dichlorobenzoic acid product. For example, using a lower temperature and a longer reaction time may help to minimize the formation of undesired byproducts, such as the 2,6-dichlorobenzoic acid isomer.In summary, the optimal temperature and reaction time for the preparation of 2,4-dichlorobenzoic acid from benzoic acid and chlorine gas are approximately 40-60C and a few hours, respectively. However, these conditions can be adjusted to optimize the yield and selectivity of the desired product.