Q7-15P-c - Organic Chemistry (Mcmurry)

Question

Name the following alkenes, and tell which compound in each pair is more stable:

Step-by-Step Solution

Answer

1- Methylcyclohexene is more stable.

Step-by-Step Solution Step 1: Alkene stability

The stability order of substituted alkenes is dependent on two factors:

•  Hyperconjugation, more the substituent number on carbon of double bond more will be hyperconjugation. and more stable will be alkene.
• Bond strengths, a bond between an ${\mathrm{sp}}^2$ carbon and an ${\mathrm{sp}}^3$ carbon is stronger than a bond between two ${\mathrm{sp}}^3$ carbons. 

Step 2: Cis and Trans configuration

When both are equally substituted check for cis and trans:

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 are on same side of the double bond. 

Step 3: Condition for cis and trans configuration

Condition for cis–trans isomerism in alkenes is that both of their carbons must be bonded to two different groups only then cis–trans isomerism possible.

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

Step 4: 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. 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 double bond is equidistant from both sides then 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 chain, and write them in alphabetical order. Position of the double bond is indicated by the number of first alkene carbon and placing that number directly before the parent name.

 

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

 

• Cycloalkenes are named same way, but because here is no end so number the cycloalkene so double bond is taken between C1and C2. It is not necessary to indicate the position of the double bond in the name because it’s always between C1 and C2.

Step 5: Name of alkenes

1- Methylcyclohexene

3- Methylcyclohexene

Step 6: Checking stability of given alkene

• 1- Methylcyclohexene has alkene trisubstituted. 
• 3- Methylcyclohexene has alkene disubstituted. 

Step 7: More stable of given alkene

1- Methylcyclohexene is more stable.