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.

Short answer

The Lewis structure for the simplest carbene, H₂C, is as follows with the carbon atom connected to two hydrogen atoms and having a lone pair: ``` H | C -- H : (lone pair on carbon atom) ``` If two H₂C molecules reacted with each other by a combination reaction, we would expect the length of the carbon-carbon bond to be approximately 1.34 Å, which is similar to the carbon-carbon double bond length in ethylene.

Step-by-step solution

Draw the Lewis structure of the Carbene H₂C

To draw the Lewis structure of H₂C: 1. Calculate the total number of valence electrons: Carbon has 4 valence electrons and each hydrogen atom has 1 valence electron. So together, they have a total of 6 valence electrons. 2. Place the carbon atom in the center and surround it with hydrogen atoms. 3. Create single bonds between the carbon and each hydrogen atom. This will use up 4 of the 6 valence electrons. 4. The remaining 2 valence electrons will be assigned as a lone pair on the carbon atom since there are no other atoms to bond with. The Lewis structure of H₂C is: ``` H | C -- H : (lone pair on carbon atom) ```

Predict the carbon-carbon bond length in the reaction

Now that we have the Lewis structure, we can predict the carbon-carbon bond length if two H₂C molecules reacted with each other by a combination reaction. When two H₂C molecules combine, the reaction will most likely form a double bond between the two carbon atoms, as both carbon atoms have a lone pair of electrons available for bonding. To predict the carbon-carbon bond length, we can consider the length of the carbon-carbon double bond in a known molecule, such as ethylene (C₂H₄). In ethylene, the carbon-carbon double bond has a length of approximately 1.34 Å (angstroms). Thus, if two H₂C molecules reacted with each other by a combination reaction, we would expect the length of the carbon-carbon bond to be approximately 1.34 Å.

Key concepts

Uncovering the Mystery of Lewis Structures

The concept of the Lewis structure is pivotal in understanding the nature of chemical bonding and the arrangement of electrons around atoms. To make sense of this for carbenes, let's break it down with the simplest carbene, H₂C, as our example.

Imagine the Lewis structure as a map that outlines how atoms are connected in a molecule and where the electrons are located. In our example, a carbon atom is at the center of it all, since it's the one with the capacity to form multiple bonds due to its four valence electrons. Put simply, valence electrons are the ones in the outermost shell of an atom that it uses for bonding.

Each hydrogen atom has a single valence electron itching to pair up, and they do so by each forming a simple, single bond with carbon. Now, here's the catch with our H₂C carbene: after the hydrogen atoms have had their handshakes with carbon, we find that carbon is left with two extra electrons. Instead of looking for another atom to bond with, they remain on the carbon as a lone pair. This unique scenario gives carbenes their highly reactive nature and sets the stage for diverse chemical interactions.

Valence Electrons: The Passport of Atomic Interaction

Valence electrons are akin to a passport that allows atoms to travel through the world of chemistry, interacting and binding with other atoms. For our friend, the carbene H₂C, this represents the four valence electrons of carbon and the one valence electron that each hydrogen brings to the party; bringing the total to six electrons that are up for grabs in the bonding process.

The importance of valence electrons cannot be overstated, as they dictate how an atom will bond, and consequently, the properties of the resulting molecule. In the context of carbene chemistry, understanding that carbon has two valence electrons left after bonding with hydrogen sets the stage for further chemical reactions, particularly how these free electrons can attract other carbon atoms to form new types of bonds.

Carbon-Carbon Bond Length: A Measure of Molecular Intimacy

Carbon-carbon bond lengths provide valuable insight into the strength and stability of a bond between two carbon atoms. In carbene chemistry, predicting these bond lengths helps us understand the potential for molecular interactions and reactions.

Why is this important? Well, these measurements can tell us a lot about how a molecule will behave. Bond length generally corresponds to bond strength - the shorter the bond, the stronger and more stable it is. For instance, in our exercise where we predict the bond length of two H₂C carbene molecules coming together, it's likely they'll form a robust double bond. This bond would be modeled after similar known double bonds, like the one in ethylene, which measures approximately 1.34 Å. It's understanding these intimate details that give chemists insights into the reactivity of molecules like carbenes and further our ability to predict and harness their chemical behavior.