Question

Each question in this section has four suggested answers out of which ONE OR MORE answers will be correct. Which of the following statements is correct? (a) Cyclopropane will react with chlorine or bromine at room temperature, in diffused light to give a dihalogen derivative. (b) A hydrocarbon of molecular formula \(\mathrm{C}_{4} \mathrm{H}_{6}\) should contain always a triple bond. (c) The trans isomer of an alkene is generally less polar than the cis isomer. (d) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{CH}_{2}\) is an example of cumulated diene.

Short answer

a) Cyclopropane reacts with halogens to form dihalogen derivatives. b) A hydrocarbon with the molecular formula C4H6 must have a triple bond. c) Cis and trans isomers of alkenes differ in their polarity. d) H2C=CH-CH2-CH=CH2 is a cumulated diene. Answer: a) Cyclopropane reacts with halogens to form dihalogen derivatives.

Step-by-step solution

Statement (a) Analysis

Cyclopropane is a small, strained 3-membered ring and reacts with halogens like chlorine (Cl2) and bromine (Br2) to form dihalogen derivatives. These reactions typically occur at room temperature and in diffused light to control the reaction rate. So, statement (a) is correct.

Statement (b) Analysis

A hydrocarbon with the molecular formula C4H6 indicates either the presence of a triple bond or two double bonds (as the degree of unsaturation is 2). It does not necessarily mean that there must always be a triple bond in the molecule. So, statement (b) is incorrect.

Statement (c) Analysis

The polarity of a molecule is determined by the presence of polar bonds and the molecular geometry. In the case of cis and trans isomers of alkenes, the distribution of the polar bonds is different, but there is no net dipole moment in either case because the C=C bond is nonpolar. Thus, statement (c) is incorrect.

Statement (d) Analysis

The compound given in the statement, H2C=CH-CH2-CH=CH2, is a conjugated diene. A cumulated diene would have two double bonds directly connected, as in H2C=C=CH2. Therefore, statement (d) is incorrect. So, the correct answer is only statement (a).

Key concepts

Cyclopropane Halogenation

Cyclopropane is a unique structure due to its small, three-membered ring which creates an angle strain because the bond angles deviate from the ideal tetrahedral angle of 109.5 degrees. This strain makes cyclopropane highly reactive, particularly in halogenation reactions. When halogens such as chlorine or bromine are introduced, they react with cyclopropane at room temperature in the presence of diffused light to yield dihalogen derivatives. This reaction is a form of addition where the halogens will add across the double bond of the cyclopropane, breaking the ring structure and relieving strain. These dihalogen derivatives are important in synthetic organic chemistry as they can be used to create more complex molecules.

It is crucial that the reaction occurs under controlled conditions, like diffused light, to prevent rapid reaction rates which could lead to non-selective reactions and unwanted by-products. The choice of halogen can also influence the selectivity and outcomes of these reactions. Chlorination and bromination are most common due to their practicality and the relative stability of the resulting halocarbon products.

Hydrocarbon Unsaturation

Unsaturation in the context of organic chemistry refers to the presence of double or triple bonds in the carbon chain of a hydrocarbon. These multiple bonds reduce the number of hydrogen atoms that a particular compound can contain. The molecular formula of a hydrocarbon provides clues to its level of unsaturation. The formula \(\mathrm{C}_{4} \mathrm{H}_{6}\) indicates a degree of unsaturation of two. This might suggest the presence of a triple bond, as that would involve a decrease of two hydrogen atoms, however, it can also mean that there are two double bonds, or even a ring structure involved.

To accurately determine the structure of such a compound, additional analytical techniques like Infrared Spectroscopy, Nuclear Magnetic Resonance, or even simple chemical tests for unsaturation such as the Baeyer's test or the Bromine test can be employed. It is essential to recognize that the formula given does not dictate a specific type of multiple bond and various structural isomers are possible with the same formula.

Alkene Isomer Polarity

Isomers are molecules with the same molecular formula but different structural arrangements. Alkenes can exhibit geometric isomerism, famously known as cis-trans isomerism. This arises due to the restricted rotation around the double bond. The isomers have distinct physical and chemical properties, one of which is polarity. Contrary to the statement in the exercise, the cis isomer of an alkene, where the substituents are on the same side of the double bond, will typically be more polar due to the asymmetric distribution of charge. In contrast, the trans isomer, with substituents on opposite sides of the double bond, tends to have a more symmetrical structure leading to cancelation of dipole moments and hence less polarity.

This difference in polarity affects not only the physical properties, like boiling points and solubility, but also the reactivity of these isomers in certain chemical reactions. Understanding the implications of isomer polarity is crucial for predicting outcomes and methodology in both laboratory and industrial settings of organic chemistry.

Conjugated and Cumulated Dienes

Dienes are hydrocarbons that contain two carbon-carbon double bonds. There are different types of dienes based on the position of these double bonds. Conjugated dienes have alternating single and double bonds, which enables a level of stability through the delocalization of electrons across the pi system. The compound \(\mathrm{H}_{2}\mathrm{C}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{CH}_{2}\) represents a typical conjugated diene where the double bonds are separated by a single bond allowing for this electron delocalization.

Cumulated dienes, on the other hand, have consecutive double bonds, like in the molecule \(\mathrm{H}_{2}\mathrm{C}=\mathrm{C}=\mathrm{CH}_{2}\). These lack the stability of conjugated systems due to the increased strain and lack of resonance. The distinction between these types of dienes is significant because they exhibit different chemical behaviors. Conjugated dienes typically participate in reactions like the Diels-Alder reaction, which are not as favorable for cumulated dienes. Appreciating the difference between these structures helps to predict reactivity and the types of products that can be formed in synthetic pathways.