Mothballs are composed of naphthalene, C10 H8, a molecule...

Mothballs are composed of naphthalene, $\mathrm{C}_{10} \mathrm{H}_{8},$ a molecule that consists of two six-membered rings of carbon fused along an edge, as shown in this incomplete Lewis structure: (a) Draw all of the resonance structures of naphthalene. How many are there? (b) Do you expect the $C-C$ bond lengths in the molecule to be similar to those of $C-C$ single bonds, $C=C$ double bonds, or intermediate between $\mathrm{C}-\mathrm{C}$ single and $\mathrm{C}=\mathrm{C}$ double bonds? (c) Not all of the $\mathrm{C}-\mathrm{C}$ bond lengths in naphthalene are equivalent. Based on your resonance structures, how many $\mathrm{C}-\mathrm{C}$ bonds in the molecule do you expect to be shorter than the others?

Key Concepts

Aromaticity

Aromaticity refers to the enhanced stability observed in cyclic, planar molecules that have a fully conjugated pi electron system, satisfying Huckel's rule. In aromatic compounds like naphthalene, aromaticity explains the uniformity and intermediate bond lengths in most of the ring due to the delocalized electrons, while also allowing subtle variations in bond lengths based on differences in resonance contributions.

Delocalized Pi Electrons and Bond Order

In conjugated systems, such as aromatic compounds, pi electrons are delocalized over several adjacent atoms. This delocalization leads to bonds that are neither pure single nor pure double bonds but have an intermediate bond order. The bond lengths, consequently, become shorter than typical single bonds but longer than typical double bonds, reflecting this intermediate character.

Resonance Structures

Resonance involves the depiction of a single molecule by two or more valid Lewis structures that differ only in the placement of electrons. This concept illustrates that the actual electronic structure is a hybrid of these forms, with electron delocalization across multiple atoms, leading to an average of bond characters instead of alternating fixed single and double bonds.

Transcript

00:01 All right, guys, we're doing problem 58 of chapter 8 in chemistry, central science.

00:06 So, mothballs are composed of naphthalene, c10h8, a molecule that consists of six membered rings of carbon fused along an edge.

00:13 You can see that here.

00:16 So draw the resin and structures of naphthalene.

00:18 How many are there? there are three.

00:20 They're shown here.

00:22 Do you expect the cc bond less in the molecule to be similar to the cc single bonds, cc double bonds, or an intermediate between cc single and cc double bonds? like in benzene, the cc bonds and napoline have an intermediate path due to the resonance structure.

00:41 So they're between the normal cc bond and the cc double bond due the currents of pi conjugation.

00:48 So what you see throughout all these cc interactions, these actually have single bonds with partial double bond character.

00:55 That's a consequence of aromaticity.

00:58 So see, not all the cc bond lace and napling are equivalent.

01:02 Based on your resonance, how many cc bonds in the molecule do you expect to be shorter than the other? so let's look at our molecules.

01:15 So let's see what we're changing.

01:19 Let's see our resident structure.

01:21 So let's compare resident structures.

01:23 So in these two resident structures, we're changing between, we're not affecting the carbons on the left.

01:30 We're affecting the carbons on the right.

01:33 We're shifting it and moving these bonds around.

01:37 Now for here, we're moving.

01:41 Now for this structure, we are directly manipulated.

01:45 Manipulating the bonds...

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