Problem 93 - Chemistry The Central Science

Question

(a) Triazine, \(\mathrm{C}_{3} \mathrm{H}_{3} \mathrm{N}_{3},\) is like benzene except that in triazine every other \(\mathrm{C}-\mathrm{H}\) group is replaced by a nitrogen atom. Draw the Lewis structure(s) for the triazine molecule. (b) Estimate the carbon-nitrogen bond distances in the ring.

Step-by-Step Solution

Answer

The triazine molecule has two resonance Lewis structures that represent the delocalization of π electrons in the ring. The estimated carbon-nitrogen bond distance in the triazine ring is approximately 1.355 Å.

Step 1: (a) Drawing the Lewis structure(s) for triazine

To draw the Lewis structure of triazine, follow these steps: 1. Count the total number of valence electrons: Triazine, \(\mathrm{C}_{3}\mathrm{H}_{3}\mathrm{N}_{3}\), has 3 carbon atoms (4 valence electrons each), 3 hydrogen atoms (1 valence electron each), and 3 nitrogen atoms (5 valence electrons each). This gives a total of \(3(4) + 3(1) + 3(5) = 12 + 3 + 15 = 30\) valence electrons. 2. Place the atoms in the skeleton structure: Since triazine is an aromatic molecule (like benzene) with a ring structure, alternate the carbon and nitrogen atoms in a six-membered ring with a hydrogen atom bonded to each carbon atom. 3. Distribute the remaining valence electrons: Now, we need to complete the octet for each atom (except the hydrogen atoms that need only two electrons). Add bonds or lone pairs to each atom using the remaining valence electrons. The final structure should look like:  4. Check for resonance structures: The triazine molecule has resonance structures due to the delocalization of the π bonds between carbon and nitrogen atoms. The two resonance structures can be represented as:  In summary, the triazine molecule has two resonance Lewis structures that represent the delocalization of π electrons in the ring.

Step 2: (b) Estimating carbon-nitrogen bond distances in the ring

To estimate the carbon-nitrogen (C-N) bond distances in the triazine ring, consider that the C-N bond is a resonance hybrid of single and double bonds. This means that the C-N bond distance will be in between that of a single C-N bond and a double C-N bond. The typical bond distances for these bonds are: - C-N single bond: 1.47 Å - C-N double bond: 1.24 Å The resonance hybrid would result in a bond distance close to: \[Estimate = \frac{1.47 + 1.24}{2} = 1.355 \ Å\] The estimated carbon-nitrogen bond distance in the triazine ring is approximately 1.355 Å.

Key Concepts

Understanding Resonance Structures in Molecular Chemistry

Resonance structures are a fundamental concept in molecular chemistry that explain the nature of electron distribution within molecules that cannot be adequately represented by a single Lewis structure. Resonance is a method used to illustrate the delocalized electrons within certain molecules or ions where the bonding cannot be expressed by one single Lewis structure.

Take for example the molecule of triazine. The electrons that form the π bonds between carbon and nitrogen are not static; they are shared amongst multiple atoms, creating a more stable structure overall. By representing triazine with two different Lewis structures where the position of the double bonds varies, scientists can communicate the idea that the actual molecule is a hybrid of these structures, and that the π electrons are delocalized throughout the molecule.

Why Resonance Structures Matter

Delocalization contributes to the stability of the molecule which is especially significant in molecules called aromatic compounds, like benzene and, in this case, triazine. This delocalization can affect physical properties of a substance, such as the absorptivity in UV/Vis spectroscopy, or chemical properties like reactivity and bond strength. When evaluating resonance structures, it is important to remember that they are not real, alternating states of the molecule; instead, the molecule is a blend, or resonance hybrid, of all possible structures.

Aromatic Compounds and Ring Stability

Aromatic compounds are a class of molecules that exhibit enhanced stability due to resonance delocalization of electrons in a cyclic ring system. To be classified as aromatic, a molecule must have a continuous ring of p-orbitals, follow Hückel's rule of 4n+2 π electrons, and exhibit a planar structure.

The classic example of an aromatic compound is benzene. However, our subject molecule, triazine, is also aromatic. Aromaticity adds to the chemical stability of compounds like triazine, making them less reactive to many chemical reactions that would alter non-aromatic structures. This exceptional stability is why aromatic compounds are widely seen in nature and synthetic materials.

Impact of Aromaticity

Aromaticity affects everything from the molecule's reactivity to its spectral characteristics and even its physical properties such as melting and boiling points. For students and chemists alike, understanding the aromatic nature of compounds is crucial for predicting reactivity and designing synthetic pathways. A powerful technique to determine aromaticity is observing the degree of electron delocalization via resonance structures, as done in the case of triazine.

Bond Length Estimation Between Atoms in Resonance Hybrids

Estimation of bond lengths in molecules involves understanding the nature of the bond between any two atoms. In simple cases where bonds are strictly single or double, we have clear benchmarks. However, in the case of resonance hybrids such as triazine, bond length estimation becomes more nuanced.

Since resonance structures share characteristics of both single and double bonds, the actual bond length is an average of the two. When estimating the carbon-nitrogen bond distances in triazine, we account for resonance by calculating the bond length as an intermediate value between a typical C-N single bond (1.47 Å) and a C-N double bond (1.24 Å). This gives us an estimated bond length that is shorter than a single bond but longer than a double bond, reflective of the average distance due to electron delocalization.

Practical Relevance of Bond Length Estimation

Knowing the estimated bond length is essential in molecular modeling and understanding the physical properties of a molecule, such as reactivity, stability, and strain. Accurate estimation informs not only the molecular geometry but also the potential energy surface, which impacts how a molecule interacts with others. For instance, the estimated bond length in the triazine molecule indicates lower reactivity typical of aromatic compounds, which has practical implications in materials science and pharmaceuticals.