Problem 75 - Chemistry: The Central Science in SI Units, Global Edition

Question

A carbene is a compound that has a carbon bonded to two atoms and a lone pair remaining on the carbon. Many carbenes are very reactive. (a) Draw the Lewis structure for the simplest carbene, \(\mathrm{H}_{2} \mathrm{C}\). (b) Predict the length of the carbon-carbon bond you would expect if two \(\mathrm{H}_{2} \mathrm{C}\) molecules reacted with each other by a combination reaction.

Step-by-Step Solution

Answer

The Lewis structure for H2C is: H | C: | H When two H2C molecules react with each other, they will form a single carbon-carbon bond with an approximate bond length of 1.54 Å.

Step 1: Draw the Lewis structure for H2C

To draw the Lewis structure, we'll start by counting the total number of valence electrons. Carbon has 4 valence electrons, while each hydrogen atom has 1 valence electron. Therefore, H2C has a total of (2 x 1) + 4 = 6 valence electrons. Place carbon in the center, then surround it with hydrogen atoms and a lone pair of electrons. H | C: | H

Step 2: Predict the carbon-carbon bond length for two H2C molecules

If two H2C molecules reacted with each other by a combination reaction, the carbon with the lone pair would bond with the carbon of the other H2C molecule. Since both carbons have a lone pair, they would share these electrons to form a bond. The bond formed between the carbons in this reaction will be a single bond. The length of a single carbon-carbon bond is approximately 1.54 Å. So, we would expect the carbon-carbon bond formed between two H2C molecules to be approximately 1.54 Å in length.

Key Concepts

Lewis Structure

The Lewis structure of a molecule is a diagram that represents the valence electrons of atoms within the molecule. When drawing the Lewis structure for the simplest carbene molecule, \(\text{H}_2\text{C}\), you first need to understand how many valence electrons are present. Carbon has 4 valence electrons, and each hydrogen atom has 1 valence electron, resulting in a total of 6 valence electrons for \(\text{H}_2\text{C}\).

To construct the structure:

• Place the carbon atom in the center as it can form multiple bonds.
• Position the hydrogen atoms on either side of the carbon, each forming a single sigma bond.
• Include a lone pair of electrons on the carbon atom, which is characteristic of carbenes.

This Lewis structure helps visualize the molecular structure as:\[\text{H} - \text{C} : \text{H}\]Here, the lone pair on carbon is crucial for its highly reactive nature in forming bonds during chemical reactions.

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a key role in chemical bonding. In the context of carbenes like \(\text{H}_2\text{C}\), understanding valence electrons is crucial. Carbon has 4 valence electrons, each hydrogen has 1, thus giving \(\text{H}_2\text{C}\) a total of 6 valence electrons.

These electrons are used to form bonds and influence the molecule's reactivity:

• Carbon uses two of its electrons to form single bonds with each hydrogen.
• The remaining two electrons on carbon exist as a lone pair.

This lone pair makes carbenes highly reactive as they seek to participate in reactions to stabilize by forming more bonds. Understanding how these 6 electrons are distributed in carbenes provides insights into their chemical behavior and reactivity.

Carbon-Carbon Bond Length

A crucial aspect of understanding the interaction between two \(\text{H}_2\text{C}\) molecules is predicting the carbon-carbon bond length in the combination reaction. When two carbenes come together:

• The lone pair from each carbon can combine to form a single bond.
• This process involves sharing the pair of electrons, leading to bond formation.

Typically, the length of a carbon-carbon single bond is approximately 1.54 Å. This measure is a result of the electron sharing which dictates the equilibrium distance ensuring the balance between repulsive and attractive forces within the molecule's structure.

Combination Reaction

In chemistry, a combination reaction, also known as a synthesis reaction, involves two or more simple substances combining to form a more complex compound. Applying this concept to \(\text{H}_2\text{C}\) molecules:

• Two carbene molecules would react, each contributing a lone pair of electrons for bond formation.
• The result is a single carbon-carbon bond, leading to a dimer of the original carbene compound.

The synthesis of these molecules illustrates the reactivity of carbenes and their propensity to form more stable structures via a combination reaction. This specific reaction example shows how carbenes can evolve from very reactive species into larger, more stable molecules.