Question

If a research paper appeared reporting the structure of a new molecule with formula \(\mathrm{C}_{2} \mathrm{H}_{8}\), most chemists would be highly skeptical. Why?

Short answer

The formula \( \mathrm{C}_{2} \mathrm{H}_{8} \) exceeds carbon's bonding rules, making it chemically impossible. Carbon can form only 6 hydrogen bonds in total with 2 carbon atoms.

Step-by-step solution

Understanding Chemical Formulas

The chemical formula \( \mathrm{C}_{2} \mathrm{H}_{8} \) suggests a molecule composed of two carbon atoms and eight hydrogen atoms. In organic chemistry, carbon commonly forms four covalent bonds due to its valency of four.

Applying Basic Chemistry Rules

Each carbon atom forms four bonds. In \( \mathrm{C}_{2} \), the two carbons would require a bond between them. This leaves each carbon with three bonds available to bond with hydrogen atoms. Thus, the maximum number of hydrogens is \(6\). The formula \( \mathrm{C}_{2} \mathrm{H}_{8} \) implies more hydrogens than is possible based on these bonding rules.

Evaluating with Valence Electrons

Consider the valence electrons: carbon has four, and hydrogen has one. The total valence should be satisfied by forming appropriate bonds. Every carbon atom can hold a maximum of four bonds. The configuration \( \mathrm{C}_{2} \mathrm{H}_{8} \) exceeds the total capacity of carbon to share electrons appropriately, making it a violation of basic bonding rules.

Conclusion Based on Bonding Capabilities

The molecule is chemically impossible given the basic rules of carbon bonding. Instead of \( \mathrm{C}_{2} \mathrm{H}_{8} \), a plausible alkane structure is \( \mathrm{C}_{2} \mathrm{H}_{6} \) (ethane), where each carbon shares bonds appropriately with hydrogen.

Key concepts

Valency

Valency refers to the ability of an atom to bond with other atoms. It is determined by the number of electrons an atom can gain, lose, or share to achieve a full outer shell or electron configuration.
In organic chemistry, the valency of carbon is especially significant as it dictates how carbon atoms will bond with other elements. Carbon has a valency of four, which means it can form four covalent bonds with other atoms.
This chemical behavior is crucial in the formation of organic molecules. For example, in the molecule ethane (\(\mathrm{C}_{2}\mathrm{H}_{6}\)), each carbon atom forms single bonds with three hydrogen atoms and one bond with another carbon atom. This fulfills its valency of four. Conversely, the molecule \(\mathrm{C}_{2}\mathrm{H}_{8}\) would require each carbon to form more than four bonds, which is not possible, thus demonstrating a violation of carbon's valency.

Bonding Rules

Bonding rules are fundamental guidelines in chemistry that dictate how atoms connect to form molecules. One basic rule is that atoms bond to achieve a complete outer shell of electrons, often leading to increased stability. For example, hydrogen atoms, with just one valence electron, seek one additional electron to fill their outer shell.
When considering carbon, these rules translate into each carbon atom forming four covalent bonds, which can be connections to other carbon atoms or to different elements like hydrogen.

• In a correctly structured molecule such as \(\mathrm{C}_{2}\mathrm{H}_{6}\) (ethane), carbon atoms form a single bond with each other and three more bonds with hydrogen.
• The bonding arrangement ensures all atoms satisfy their valency and bonding guidelines.

In contrast, a proposed molecule \(\mathrm{C}_{2}\mathrm{H}_{8}\) would breach these bonding principles because it suggests too many bonds originating from carbon atoms.

Organic Chemistry

Organic chemistry is a vast field focusing on the study of carbon-containing compounds, mainly those with carbon-to-carbon (C-C) and carbon-to-hydrogen (C-H) bonds. The complexity and variety of organic molecules are largely due to the ability of carbon atoms to form stable chains and rings with themselves and other elements.
Organic chemistry follows specific predictable patterns, such as the homologous series of alkanes, alkenes, and alkynes based on carbon and hydrogen atoms. Alkanes, for instance, follow the general formula \(\mathrm{C}_{n}\mathrm{H}_{2n+2}\), which would make \(\mathrm{C}_{2}\mathrm{H}_{6}\) (ethane) a member of this series.

• Each increment in the series follows established organic chemistry rules, allowing for predictions of molecular structures.
• Organic compounds tend to have specific functional groups that define their chemical properties and reactions, such as hydroxyl groups in alcohols or carbonyl groups in ketones.

The proposed \(\mathrm{C}_{2}\mathrm{H}_{8}\) molecule does not fit into any known patterns or homologous series, underlining why chemists are skeptical of such a structure.