Question

Indicate whether each of the following molecules is capable of geometrical (cis-trans) isomerism. For those that are, draw the structures: (a) \(1,1-\) dichloro1-butene, (b) 2,4 -dichloro-2-butene, (c) 1,4 -dichlorobenzene, (d) 4,5 -dimethyl-2 -pentyne.

Short answer

In conclusion, only 2,4-dichloro-2-butene can exhibit geometrical (cis-trans) isomerism due to the presence of differing substituents attached to the double bond. Its two possible structures are cis and trans isomers. The other molecules either have identical substituents attached to double bonds (1,1-dichloro-1-butene), lack the specific double bonds required for isomerism (1,4-dichlorobenzene), or possess triple bonds (4,5-dimethyl-2-pentyne) which do not exhibit geometrical isomerism.

Step-by-step solution

(a) 1,1-dichloro-1-butene

In order to analyze this molecule, let's first draw its structure: H3C-CH2-CH=CH2 | Cl Now, it's clearly visible that this molecule has a double bond between carbons 1 and 2. However, both the substituents attached to carbon 1 are chlorine atoms, making them identical. As a result, 1,1-dichloro-1-butene doesn't exhibit geometrical isomerism.

(b) 2,4-dichloro-2-butene

Let's draw the structure of 2,4-dichloro-2-butene: H3C-CH=CH-CH2-Cl | Cl Here, we have a double bond between carbons 2 and 3, and substituents attached to these carbons (a chlorine atom on carbon-3 and a hydrogen atom on carbon-2) differ from each other. This indicates that this molecule can exhibit geometrical isomerism. Therefore, we can draw the two possible structures: - Cis-isomer: H3C-CH=CH-CH2-Cl | Cl - Trans-isomer: H3C-CH=CH-CH2-Cl | Cl

(c) 1,4-dichlorobenzene

Now, let's draw the structure of 1,4-dichlorobenzene: Cl | C=C-C=C-Benzene | | C Cl Since it has a cyclic structure, we should analyze if it can have isomers. However, 1,4-dichlorobenzene does not exhibit geometrical isomerism, because the chlorine atoms are connected to benzene carbons, which form a fully conjugated aromatic system, it does not possess a double bond in the aromatic ring with differing substituents. As a result, there are no geometrical isomers for 1,4-dichlorobenzene.

(d) 4,5-dimethyl-2-pentyne

Finally, let's draw the structure of 4,5-dimethyl-2-pentyne: H3C-CH2-C≡C-CH2-CH3 | CH3 | CH3 Observing the structure, we see a triple bond between carbons 2 and 3. Since geometrical isomerism occurs in compounds with double bonds or cyclic structures, the presence of an alkyne (triple bond) in this molecule indicates that it does not exhibit geometrical isomerism. In conclusion, the only molecule capable of geometrical isomerism out of the given molecules is 2,4-dichloro-2-butene, for which we drew both the cis and trans structures.

Key concepts

Cis-Trans Isomerism

Cis-trans isomerism, a form of stereoisomerism, is a key concept in understanding molecular structures and their spatial arrangements. This type of isomerism arises due to the restricted rotation around double bonds or within ring structures, leading to isomers with unique physical and chemical properties.

When we delve into the specifics of molecules with double bonds, such as alkenes, the two substituents attached to each carbon of the double bond can arrange themselves in two distinct ways. If the substituents are on the same side of the double bond, the isomer is referred to as 'cis'. Conversely, if they lie on opposite sides, the isomer is called 'trans'. These differences can influence aspects such as the compounds' boiling points, solubility, and reactivity.

An example from the exercise involves 2,4-dichloro-2-butene, which does have the capability to form both cis and trans isomers due to the different substituents (chlorine and hydrogen atoms) attached across the double bond. The presence of substituents in distinct positions means that these molecules are not superimposable and thus exhibit geometrical isomerism.

Double Bonds

The concept of double bonds is central to organic chemistry and has significant implications for molecular behavior, including isomerism. Double bonds consist of one sigma bond and one pi bond, with the pi bond being responsible for the limited rotation that gives rise to isomerism.

In compounds with double bonds, the electron density above and below the bond plane restricts the free rotation that is typically seen in single bonds. This absence of rotation prevents the interchange of atoms or groups without breaking the pi bond, which translates into the existence of distinct geometrical arrangements of the molecule, i.e., geometrical isomerism.

The double bond in a compound like 1,1-dichloro-1-butene does not lead to cis-trans isomerism due to the identical substituents (chlorines) on one end of the bond, showcasing that not all double bonds result in isomerism. Knowledge of the types of substituents attached to a double bond is therefore essential for predicting the capacity for isomerism in organic molecules.

Stereochemistry

Stereochemistry is the study of the three-dimensional arrangements of atoms within molecules and how this arrangement affects their function and reactions. This aspect of chemistry is vital for understanding how the same atoms can form distinct molecular species based on their spatial orientation, leading to vastly different properties.

Stereoisomers, which include cis-trans isomers discussed earlier, are molecules that have the same molecular formula and sequence of bonded atoms but differ in how these atoms are organized in space. Interactions within and between molecules, such as binding to enzyme active sites or fitting into cellular receptors, highly depend on this three-dimensional structure.

The lack of geometrical isomerism in 1,4-dichlorobenzene and 4,5-dimethyl-2-pentyne from the exercise illustrates well how stereochemistry rules apply. In the former, the aromatic ring's resonance prevents the formation of cis-trans isomers, while in the latter, the triple bond does not allow for the same type of isomerism seen in alkenes. Mastery of stereochemistry is thus integral to predicting molecular behavior in various chemical and biological contexts.