-ene Nomenclature (semi-structural formulas)

[Rudders]

Hi,

I was just confused as to the semi-structural formula nomenclature of -ene and -yne structures. The thing that I don't understand is the = sign in the formula.

For example:
CH₃CH₂CH=CH₂

What does that equals sign mean? Does it represent where the double bond is located?

If so, why does ethene have the structural formula CH₂CH₂ instead of CH₂=CH₂

Thanks
-Rudders

 

[alxm]

What does that equals sign mean? Does it represent where the double bond is located?

Yes.

If so, why does ethene have the structural formula CH₂CH₂ instead of CH₂=CH₂

You can write it both ways. CH₂CH₂ is the 'normal' way, whereas when you use an equal sign to represent the double bond you're approximating the structure diagram in text. Formally you should really stick to either the formula or a full and proper structure diagram, but in informal contexts it can be useful.

 

[quicknote]

Hello Rudders,

Thank you for your question about -ene nomenclature. The = sign in the semi-structural formula represents a double bond between the two carbon atoms. In organic chemistry, double bonds are represented by the = sign, while single bonds are represented by a single line. Therefore, in the example you provided, CH3CH2CH=CH2, the = sign indicates that there is a double bond between the third and fourth carbon atoms.

As for your question about ethene, the structural formula CH2=CH2 is actually an abbreviated version of the more accurate semi-structural formula CH2CH2. This is because in ethene, the double bond is always located between the first and second carbon atoms, so it is not necessary to indicate its location with the = sign. However, for molecules with more than two carbon atoms, it is important to specify the location of the double bond using the = sign in the semi-structural formula.

I hope this helps clarify the -ene nomenclature for you. Please let me know if you have any further questions.

Best,