-ene Nomenclature (semi-structural formulas)

In summary, the = sign in a semi-structural formula represents the location of a double bond. While ethene's structural formula is typically written as CH2CH2, using CH2=CH2 is a valid approximation in text. It is important to use either the formula or a proper structure diagram, but the use of an equal sign can be useful in informal contexts.
  • #1
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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
 
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  • #2
Rudders said:
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.
 
  • #3


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,
 

What is '-ene' nomenclature?

-ene nomenclature is a system used to name organic molecules with double bonds. The suffix '-ene' is added to the end of the name to indicate the presence of a double bond.

How do you determine the position of the double bond in '-ene' nomenclature?

The position of the double bond is determined by numbering the carbon atoms in the longest carbon chain that contains the double bond. The double bond is placed between the two carbon atoms with the lowest numbers.

Can '-ene' nomenclature be used for molecules with multiple double bonds?

Yes, '-ene' nomenclature can be used for molecules with multiple double bonds. The suffix '-diene' is added for molecules with two double bonds, '-triene' for three double bonds, and so on.

How do you name branched molecules using '-ene' nomenclature?

Branched molecules using '-ene' nomenclature are named by identifying the longest carbon chain that contains the double bond and numbering the carbon atoms to indicate the position of the double bond. The branches are then named and their position on the main chain is indicated by a number before the branch name.

Are there any exceptions to the naming rules in '-ene' nomenclature?

Yes, there are a few exceptions to the naming rules in '-ene' nomenclature. For example, if the molecule contains a functional group that takes priority over the double bond, the suffix '-ene' is replaced with the appropriate functional group suffix. Additionally, if the molecule contains a triple bond, the suffix '-yne' is used instead of '-ene'.

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