Alkenes – IUPAC Names and Isomers

Alkenes are acyclic hydrocarbons with one carbon to carbon double bond and the general molecular formula. They are named using a similar naming rule for alkanes; now, they have changed only the suffix to –ene. Alkenes are unsaturated hydrocarbons with the molecular formula CnH2n. 

It is essential for you to know that higher alkenes are named by calculating the sum of carbons in the longest continuous chain comprising double or triple bond and adding an –ene suffix to the base name of the unbranched alkanes that possess that number of carbons. 

We will also study IUPAC nomenclature practice exercises to understand alkenes better.

Nomenclature of alkenes

A consistent nomenclature system must be adopted with alkanes that can help separate the elements of these unsaturated chemicals. In the IUPAC nomenclature system, ethylene and acetylene are the synonyms for ethene and ethyne. 

The -ene suffix indicates an alkene, and the longest select chain for the root name must comprise carbon atoms with a double bond. Hydrocarbon’s molecular formula contains data about the potential structural models it may show. 

Examples of simple unbranched alkanes

IUPAC rules for alkene nomenclature

There are specific rules that you should know about alkene nomenclature. Here they are:

1. The root chain should be numbered from the end close to a double bond carbon atom. If you see that a double chain is placed in the middle of the chain, then the rule is to see the end from where the numbering begins.
2. The double bond is used as the double bond locator when the smaller of the two numbers designate the carbon atoms.
3. The compound is called diene or triene if there is more than one double bond, and every double bond is assigned a locator number.
4. Double bond holding substituents are:

H2C = CH– Vinyl group

H2C = CH–CH2– Allyl group.

New IUPAC Nomenclature

IUPAC refreshed their naming proposal in 1993 to change the place of the double bond prior to the –ene suffix of alkene names. The names that they changed to were, for example, Hex-2-ene instead of 2-Hexene. This new rule has been accepted but takes time to get applicable everywhere; thus, you still find people using the old names. 

The IUPAC structured approach to nomenclature

A general nomenclature system is expected to do majorly two things. The foremost thing is it should signify how carbon atoms of specific compounds bond together in a characteristic frame of chains and rings. 

The second thing is recognizing and locating any functional group in the compound. Hydrogen is one of the most common components of the organic compound. Its quantity and position can be assumed from the tetravalency of carbon. The best thing is that it is not required to be mentioned in most cases.

The IUPAC nomenclature has its own set of rules that organic chemists use to avert issues caused by arbitrary nomenclature. Once you know all its rules, you can write a separate name and structural formula for every compound.  

It is important to note that usually, you will find these three features in an IUPAC name:

1. A root signifying a significant chain or ring of carbon atoms that are found in its molecular structure
2. Names of the substituent groups, except hydrogen, which would complete the molecular structure
3. A suffix or any other element detailing functional groups that might be in the compound.

Naming alkenes

Rule 1

As mentioned above, the suffix has changed to –ene.  Here are some of the examples of the straight-chain alkenes-

Methene is not included in this list because carbon can’t have a double bond without bonding with another carbon.

Rule 2

The most extended structure is the parent structure, as it comprises both carbon atoms of the double bond. If alkene has just a single, double bond and that bond is terminal, it is not essential to place any number at the beginning of the name. 

But, if the double bond is not terminal, you should number the carbons to provide the lowest possible number to the front of the two double-bonded carbons. Plus, that number should be written with -ene.

Rule 3

If the alkene comprises more than one double bond, you should number all the bonds in the molecule’s name, even the terminal double bonds. Remember that the number should be ranked from lowest to highest and be separated using a comma.

Rule 4: Geometric Isomers

It has been seen that double bonds also exist as geometric isomers, and they are designated by using cis/trans designation or more flexible E/Z designation. Here E stands for entgegen and Z stands for zusammen. If there are 3 or 4 non-hydrogen different atoms linked to an alkene, you should use the E, Z system.