Why is this Alkene structure:
CH3 - CH = C - CH3
|
Cl
called 3, chloro - 2, butene
and not 2, chloro - 2, butene?
I thought that the counting starts at the end closes to the first substituent? Wouldn't that make it start from the right?
Thanks!
The correct name for the given alkene structure is indeed 3-chloro-2-butene. Let me explain the process of determining the numbering and naming of this compound so you can understand why it is named this way.
The numbering of carbon atoms in an alkene chain is based on the principle of giving the substituents the lowest possible numbers. The first rule is to find the longest continuous chain containing the double bond. In this case, it is a butene, which has four carbon atoms. Now we need to determine how to number the carbon atoms in this chain.
According to the IUPAC nomenclature rules, the double bond takes precedence over any other functional group. Therefore, we begin to number the carbon chain from the end closest to the double bond, giving priority to the double bond even over substituents.
In the given alkene structure, the double bond starts at the third carbon atom from the left side of the chain. Therefore, the numbering starts from the left and proceeds to the right, with the third carbon atom being numbered as "3". The carbon atom to which the chlorine (Cl) substituent is attached is the second carbon atom in the chain, which is numbered as "2".
So, the systematic name for this alkene structure becomes 3-chloro-2-butene, indicating that the chlorine substituent is attached to the third carbon atom in the chain, while the double bond is between the second and third carbon atoms.
Remember, when numbering the chain, the substituents are given the lowest possible numbers, regardless of their alphabetical order.