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.

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 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 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.