Question

Which of the following compounds can exist as cis and trans isomers? (a) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\)

Short answer

Only compound (a) \(\mathrm{CH}_3 \mathrm{CH}=\mathrm{CHCH}_3\) can exist as cis and trans isomers.

Step-by-step solution

Identify compounds with double bonds.

Look at each compound. Is there a double bond present? The double bond will look like '='. All three compounds (a, b, c) have double bonds, so we will consider all three for the next step.

Check for different groups on both sides of the double bond.

Besides having a double bond, the compound should also have two different types of groups on either side of the double bond. For compound (a), we have \(\mathrm{CH}_{3}\) and \(\mathrm{H}\) on one side of the double bond, and \(\mathrm{H}\) and \(\mathrm{CH}_{3}\) on the other side. On the double bond of compound (b), there's \(\mathrm{CH}_{3}\) and \(\mathrm{H}\) on one side, but only 2 \(\mathrm{H}\)'s on the other side. For compound (c), the same groups, \(\mathrm{CH}_{3}\), are attached to each carbon atom of the double bond.

Identify cis and trans isomers.

From step 2, compound (a) is the only one with different groups on both sides of the double bond, fulfilling the conditions for cis and trans isomers. Compound (c) has the same groups adjacent to the double bond and compound (b) has two Hydrogens on one side, hence, both cannot show cis-trans isomerism.

Key concepts

Double Bond

In organic chemistry, a double bond refers to a chemical bond where two pairs of electrons are shared between two atoms. This bond is stronger and shorter than a single bond, due to the additional electron sharing. The general symbol for a double bond is '='.

Double bonds are often found in carbon compounds, such as alkenes — the simplest of unsaturated hydrocarbons. They are significant because they restrict rotation around the bonded atoms. Unlike single bonds, double bonds do not allow the bonded groups to rotate freely, which is pivotal for certain types of isomerism. This restricted rotation is a key reason for the existence of geometric or cis-trans isomers.

Therefore, whenever you spot a double bond within an organic molecule, it is an indicator to explore further if the molecule can display geometric isomerism, especially if different groups are attached to the double-bonded carbons.

Geometric Isomerism

Geometric isomerism, also known as cis-trans isomerism, is a form of stereoisomerism. It occurs when molecules with a double bond have different spatial arrangements of groups around the bond. Here, the restricted rotation of the double bond plays a critical role.

For a molecule to exhibit geometric isomerism, there must be two different groups attached to each carbon atom involved in the double bond. These groups can be atoms or other molecular fragments, but they must differ from each other for isomerism to occur. In simpler terms:

• **Cis isomer**: Two similar or identical groups are on the same side of the double bond.
• **Trans isomer**: Two similar or identical groups are on opposite sides of the double bond.

For instance, in compound (a) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{3}\), the presence of different groups (\(\mathrm{CH_3}\) and \(\mathrm{H}\)) on both ends of the double bond allows for cis and trans configurations.

Organic Compounds

Organic compounds are chemical compounds that primarily contain carbon and hydrogen. The ability of carbon to form stable bonds with many elements, including itself, allows for the formation of an immense variety of structures. These compounds range from simple molecules like methane (\(\mathrm{CH_4}\)) to complex polymers and biological molecules like proteins and DNA.

One of the significant characteristics of organic compounds is their diverse bonding capabilities, including single, double, and even triple bonds. This versatility allows for varied chemical behavior, including isomerism. For instance, alkenes, a group of hydrocarbons with at least one double bond, show geometric isomerism due to the spatial restrictions imposed by the double bond.

• Characteristics: Organic compounds often have covalent bonds, may contain functional groups, and can exhibit a range of chemical reactions.
• Applications: They form the basis of many pharmaceuticals, plastics, fuels, and other important materials.

Understanding the nature and behavior of organic compounds is essential for connecting molecular structure to function, laying the foundation for designing new materials and medicines.