IUPAC name for Dibenzalacetone: The molecule is symmetric with 2 benzene rings on each end and a ketone in the center.

So is the "longest chain" that is to give the root of the name considered to be one of the 6C benzenes.

OR is the root of the name considered to be the 5C central chain which contains the ketone as a substituent??

In the case of Dibenzalacetone, the IUPAC name is determined by considering the longest continuous chain of carbon atoms that includes the functional group. In this molecule, the longest continuous chain is the 5-carbon chain that contains the ketone as a substituent. Therefore, the root of the name will be based on this 5-carbon central chain.

The systematic name for Dibenzalacetone is 4,4'-Diacetyl-1,3-diphenylbut-2-en-1-one.

To determine the IUPAC name for Dibenzalacetone, we need to identify the longest continuous carbon chain in the molecule. In this case, we have two benzene rings on each end and a ketone in the center.

The IUPAC nomenclature prioritizes choosing the longest continuous carbon chain as the main chain. However, since both the benzene rings and the ketone are present in the molecule, we need to decide which one should be considered the main chain.

In this case, the longest continuous carbon chain with six carbons is formed by the two benzene rings, which makes it the main chain. The ketone group is then considered a substituent on this main chain.

Therefore, the root of the name will be derived from the benzene rings, while the ketone will be indicated as a substituent. The name of the molecule is derived as follows:

- Start with the prefix "di" to indicate the presence of two substituents.
- Followed by the name of the benzene rings with the prefix "benzyl" indicating two benzene rings.
- Then, include the substituent "ketone" to indicate the presence of the ketone group.

Hence, the IUPAC name for Dibenzalacetone is "1,5-diphenyl-2,4-pentadien-1-one."