Question

Draw the three distinct geometric isomers of \(2,4\)-hexadiene.

Short answer

The three distinct geometric isomers of \(2,4\)-hexadiene are: 1. cis-cis \(2,4\)-hexadiene: Both double bonds have neighboring hydrogens on the same side of the double bond. 2. cis-trans \(2,4\)-hexadiene: The first double bond has neighboring hydrogens on the same side (cis), and the second double bond has neighboring hydrogens on opposite sides (trans). 3. trans-cis \(2,4\)-hexadiene: The first double bond has neighboring hydrogens on opposite sides (trans), and the second double bond has neighboring hydrogens on the same side (cis). Note that the trans-trans configuration is not stable due to steric hindrance.

Step-by-step solution

Draw the base structure of 2,4-hexadiene

Begin by drawing the base structure of 2,4-hexadiene. The compound consists of a six-carbon chain with double bonds between C2-C3 and C4-C5. Create single bonds between the rest of the consecutive carbons.

Identify the possibilities for the double bonds configuration

Each of the two double bonds can exist in either the cis or trans configuration. Since there are two double bonds, there are a total of 4 possible combinations of cis and trans configurations: 1. cis-cis (both double bonds are in cis configuration) 2. cis-trans (first double bond is in cis configuration, the second is in trans configuration) 3. trans-cis (first double bond is in trans configuration, the second is in cis configuration) 4. trans-trans (both double bonds are in trans configuration)

Draw the three distinct isomers of 2,4-hexadiene

To draw the three distinct isomers, we need to create the following configurations: 1. cis-cis 2,4-hexadiene Draw a six-carbon chain with double bonds at positions 2 and 4. Make sure that both double bonds have nearby hydrogens on the same side of the double bond to represent the cis configuration. 2. cis-trans 2,4-hexadiene Draw a six-carbon chain with double bonds at positions 2 and 4. For the first double bond, make sure the nearby hydrogen atoms are on the same side of the double bond (cis configuration). For the second double bond, place the nearby hydrogen atoms on opposite sides of the double bond (trans configuration). 3. trans-cis 2,4-hexadiene Draw a six-carbon chain with double bonds at positions 2 and 4. For the first double bond, make sure the nearby hydrogen atoms are on opposite sides of the double bond (trans configuration). For the second double bond, place the nearby hydrogen atoms on the same side of the double bond (cis configuration). Note: The trans-trans configuration of 2,4-hexadiene does not exist as a stable molecule due to steric hinderance between the hydrogen atoms on each double bond. So, we don't need to draw it.

Key concepts

Cis-Trans Configuration

When talking about the cis-trans configuration, it is an essential concept in organic chemistry that pertains to the spatial arrangement of atoms around double bonds. For a molecule with a double bond, such as 2,4-hexadiene, the atoms or groups attached can be either on the same side (cis) or on opposite sides (trans) of the double bond. This distinction is crucial because it contributes to the physical properties and reactivity of the molecule.

Understanding the importance of the position of atoms in cis-trans isomers is intrinsic to predicting the behavior of substances. For instance, cis-isomers generally have higher boiling points due to stronger intermolecular forces, such as dipole-dipole interactions, when polar groups are involved. In addition, cis configurations in alkenes tend to be less stable due to the higher potential for steric interactions, which arises when atoms or groups are too close to each other, leading to repulsion.

Steric Hindrance

Steric hindrance refers to the interference that occurs when atoms within a molecule are brought too close to one another, causing repulsion and affecting the molecule’s reactivity and stability. This concept often explains why certain configurations of molecules, such as the trans-trans form mentioned in the textbook exercise for 2,4-hexadiene, are rarely observed or do not exist under normal conditions.

Imagine trying to push two large pieces of furniture through a doorway at the same time; they simply won't fit. Similarly, in chemistry, large groups or atoms can't occupy the same space without causing strain to the molecule. This strain can lead to instability or prevent a particular molecular conformation from forming. In 2,4-hexadiene, the steric hindrance between the hydrogen atoms at each end of the molecule is so significant that the trans-trans form is not stable enough to exist.

Conjugated Dienes

Conjugated dienes are compounds that contain two double bonds separated by a single bond. This arrangement allows for electron delocalization, which describes the distribution of electrons across several atoms, enhancing the stability of the molecule. In conjugated dienes such as 2,4-hexadiene, this delocalization confers unique chemical properties including the ability to participate in specific reactions like the Diels-Alder reaction, which is a [useful tool for synthesizing complex organic molecules.]

The alternating pattern of single and double bonds in conjugated dienes not only affects the molecule's reactivity but also its color and UV absorption. Such compounds can absorb light in the ultraviolet-visible spectrum, which can have implications for materials designed to resist UV degradation. Understanding the structure of conjugated dienes is essential for manipulating these properties for applications in material science, pharmaceuticals, and organic synthesis.