Question

Following are three contributing structures for diazomethane, \(\mathrm{CH}_{2} \mathrm{~N}_{2}\). This molecule is used to make methyl esters from carboxylic acids (Section 17.7C). CC=NC (a) Using curved arrows, show how each contributing structure is converted to the one on its right. (b) Which contributing structure makes the largest contribution to the hybrid?

Short answer

Answer: The contributing structure B makes the largest contribution to the hybrid because it has the minimum separation of charges and all atoms are following the octet rule.

Step-by-step solution

(a) Curved arrows for contributing structures

To show how each contributing structure is converted to the one on its right using curved arrows, follow the steps: Step 1: Label the contributing structures as A, B, and C. Step 2: Identify the moving electrons between the neighboring contributing structures. Step 3: Clearly show the movement of electrons using curved arrows. Step 4: Make sure that octet rule and formal charges are maintained in the structures. We get the following conversions: Between A and B: Move one of the pair of electrons between N and N into a triple bond between N and N, while pushing the other pair of electrons towards the left nitrogen. Between B and C: Move one of the triple bond electrons between N and N back to a double bond and push the other pair of electrons towards the right nitrogen.

(b) Largest contribution to the hybrid

To determine which contributing structure makes the largest contribution to the hybrid: Step 1: Compare the formal charges of each contributing structure and choose the one that has the least charges or minimum separation of charges. Step 2: Check for octet rule compliance for all atoms in the contributing structures. Step 3: Identify which contributing structure is most stable based on formal charges and octet rule. When comparing the formal charges and considering the octet rule, the contributing structure B makes the largest contribution to the hybrid, as it has the minimum separation of charges and all atoms are following the octet rule.

Key concepts

Curved Arrow Notation

When we're talking about chemical reactions, especially those involving electrons moving between atoms, visualizing how these tiny particles transfer can be a challenge. Here is where curved arrow notation comes to our rescue.

Curved arrow notation is a system used by chemists to depict the movement of electrons in chemical reactions. Electrons are the stars of the show in any chemical reaction, and their movement from one place to another can make or break bonds. The arrow starts at the electron pair being moved—either in a bond or a lone pair—and points to where the electrons will end up.

Using this notation, we can take a peek at the molecular dance floor. For instance, a single arrow is used to show the movement of a pair of electrons during the course of a reaction. Remember, in resonance structures, these arrows help indicate possible ways electrons can 'delocalize' or spread out across a molecule, allowing us to draw all possible contributing structures of a molecule like diazomethane.

Resonance Hybrid

Resonance hybrid is not a new EDM artist; it's something much cooler if you're into chemistry. It is a way to represent the real electronic structure of a molecule that can't be accurately captured by a single Lewis structure.

In the case of diazomethane, the CH₂N₂ molecule doesn't have just one rigid structure. Instead, it's got a few structures, known as contributing structures or resonance forms, that it likes to shuffle through. The true form of the molecule is actually a blend—a resonance hybrid—of these structures.

This hybrid carries characteristics from all its contributing structures, much like a smoothie takes on the flavor of all its blended fruits. The resonance hybrid is the more accurate depiction of a molecule's electron distribution because it reflects the delocalization of electrons. In a learning context, understanding this helps students better predict the molecule's reactivity and stability.

Formal Charge

When molecules share electrons in covalent bonds, they don't always do it evenly. To keep track of who's sharing more and who's sharing less, chemists use the concept of formal charge.

Formal charge is like a ledger balance for each atom in a molecule—it tells us whether the atom is hogging electrons (negative formal charge), being generous (positive formal charge), or just playing fair (zero formal charge). The formula to calculate the formal charge is quite simple: it's the difference between the number of valence electrons the atom has in its free state and the number of electrons it's assigned in the molecule (counting each shared electron once).

In resonance forms, atoms can have different formal charges, and when we compare these, it helps us to determine which structure makes the most significant contribution to the resonance hybrid. Generally, structures with the fewest and lowest formal charges are more stable and contribute more to the hybrid structure.

Octet Rule

Here we get a bit of an atomic rule of thumb known as the octet rule. This rule is the street cred for an atom desiring to be one of the cool kids—having a full set of eight valence electrons.

The octet rule states that atoms tend to form bonds until they have eight electrons in their outer shell, giving them the same electronic configuration as a noble gas, which is the epitome of chemical stability. Not every atom follows this rule (looking at you, hydrogen and helium), but for many, this is the aim.

In the context of resonance and contributing structures, like those of diazomethane, making sure atoms follow the octet rule is crucial. Structures where atoms respect the octet rule are often more stable and thus make a larger contribution to the resonance hybrid. When assessing whether a molecule follows the octet rule, remember that it applies to the second period of the periodic table (carbon, nitrogen, oxygen, fluorine), whose members are often found in organic molecules.