Question

Question: Name the following alkenes, including a cis or trans designation:

Short answer

The name of the compound is$\mathrm{trans}-6-\mathrm{Methyl}-3-\mathrm{heptene}$.

Step-by-step solution

Step-by-Step Solution Step 1: Naming of alkene

There are rules for naming alkene:

• Name the parent hydrocarbon: Look for the longest carbon chain having the double bond, then name the compound accordingly. The suffix –ene is used for alkenes

• Number the carbon atoms in the chain:Start numbering from the carbon nearer to the double bond but when the double bond is equidistant from both sides, start numbering from carbon nearer to the first branch point. So, double-bond carbons receive the lowest possible numbering.

• Write the full name:Number the substituents according to their positions in the chain and write them in alphabetical order. The position of the double bond is indicated by the number of the first alkene carbon and placing that number directly before the parent name.

• If more than one double bond is present, use one of the suffixes as -diene, -triene, and so on based on a number of double bonds and indicate the position of each immediately before the suffix ene.

• Cycloalkene is named the same way, but because there is no end so number the cycloalkene, so the double bond is taken between C1and C2. It is unnecessary to indicate the double bond's position in the name because it’s always between C1 and C2.

Cis and Trans configuration

Cis configuration is when two groups are on the same face of the double bond, and trans configuration is when the groups are on opposite faces.

Both isomers are stable, and neither can be converted into the other without breaking and reforming chemical bonds.

Cis alkenes are less stable than their trans form as there is steric strain between the two bulky substituents on the same side of the double bond.

Condition for cis and trans configuration

The condition for cis-trans isomerism in alkenes is that their carbons must be bonded to two different groups. Only then is cis-trans isomerism possible.

If one of their carbons bonded to two identical groups, then cis-trans isomers can’t exist.

Structure of given compound in cis and trans form

The structure of a given compound can exist in cis and trans forms as both carbons have two different groups attached.

Here, it can be seen that hydrogen is on a different side, and the alkyl group is also on a different side, so it is a trans isomer.

IUPAC name

Based on the above steps, the name of the compound is$\mathrm{trans}-6-\mathrm{Methyl}-3-\mathrm{heptene}$.