Question

The incorrect statement for following compounds is/are (I) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3}\) (II) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{C}=\mathrm{CH}-\mathrm{CH}_{3}\) (A) Both can show stereoisomerism (B) Both are position isomers (C) Number of stereoisomers of I is greater than number of stereoisomers of II (D) II is more stable than I

Short answer

The incorrect statements are (A) Both can show stereoisomerism and (D) II is more stable than I.

Step-by-step solution

(A) Stereoisomerism

Both compound I and compound II have double bonds, which restrict the rotation around these bonds. This may lead to cis-trans (E-Z) isomerism. Compound I has two double bonds, while compound II has one double bond and one triple bond. Thus, compound I can show stereoisomerism due to its two double bonds. The triple bond in compound II doesn't allow for cis-trans isomerism, so compound II can't show stereoisomerism. Statement (A) is incorrect.

(B) Position isomers

Both compounds have the same molecular formula (\(C_5H_8\)). Compound I has two double bonds, while compound II has one double bond and one triple bond. As they differ in the location of multiple bonds, they are position isomers. Statement (B) is correct.

(C) Number of stereoisomers

As we saw earlier, compound I can show stereoisomerism due to its two double bonds, allowing for cis-trans (E-Z) isomerism. This results in 4 possible stereoisomers. Compound II doesn't show stereoisomerism as it has a triple bond. So, statement (C) is correct.

(D) Stability

The stability of conjugated dienes like compound I is higher than that of isolated dienes or compounds with triple bonds like compound II. The conjugation makes the compound less reactive and more stable. Therefore, statement (D) is incorrect. In conclusion, the incorrect statements are (A) and (D).

Key concepts

E-Z Isomerism

E-Z isomerism, also known as cis-trans isomerism, is a type of stereoisomerism unique to alkenes and certain other types of molecules featuring double bonds. This form of isomerism arises because of the restricted rotation around double bonds, making the relative positioning of substituent groups significant.

For a compound to exhibit E-Z isomerism, two conditions must be met. First, it must have a double bond. Second, each carbon atom of the double bond must have two different substituent groups. If these criteria are met, isomers known as the 'E' (from the German 'Entgegen', meaning opposite) and 'Z' (from the German 'Zusammen', meaning together) forms can exist. The E isomer has the highest priority substituents on the opposite sides of the double bond, while the Z isomer has them on the same side.

In the context of the exercise, compound I \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3}\) has two double bonds and therefore the potential for E-Z isomerism. However, compound II \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{C}=\mathrm{CH}-\mathrm{CH}_{3}\), with a triple bond and only one double bond, cannot exhibit such isomerism because triple bonds do not allow for cis-trans configurations.

Conjugated Dienes Stability

Conjugated dienes are compounds with alternating single and double bonds, which grants them a unique stability advantage over non-conjugated dienes. This stability stems from the conjugation, which allows the \(\pi\) electrons from the double bonds to delocalize over the adjacent single bonds. Delocalization spreads the electrons over a larger volume, lowering the overall energy of the molecule.

The resonance that results from this electron delocalization is particularly stabilizing. In compound I from the exercise, the presence of two conjugated double bonds \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3}\) allows for such resonance, resulting in enhanced stability compared to isolated dienes or ones with triple bonds like compound II. This extra stability is also the reason why conjugated dienes can be less reactive and more resistant to addition reactions under certain conditions.

Position Isomers

Position isomers, also known as regioisomers, are compounds with the same molecular formula but different structural formulas due to variations in the positions of functional groups, multiple bonds, or substituents on a carbon chain. They are a subset of structural isomers, which means they only differ in the placement of their atoms or groups within the molecule, not in their connectivity.

Compound I and II from the exercise share the same molecular formula, specifically \(C_6H_8\), and thus can be considered isomers. However, they are position isomers because they differ in the location of their multiple bonds—one arrangement has two double bonds while the other has a double bond and a triple bond. This slight change in the position of the bonds can lead to differences in the compounds' physical and chemical properties, making the study of position isomers significant in organic chemistry.