Question

(a) Triazine, \(\mathrm{C}_{3} \mathrm{H}_{3} \mathrm{~N}_{3}\), is like benzene except that in triazine every other \(C-H\) 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.

Short answer

The Lewis structure for triazine, \(\mathrm{C}_{3} \mathrm{H}_{3} \mathrm{N}_{3}\), is as follows: H | :N - C - N: | | :N - C : | H The estimated carbon-nitrogen bond distances in the triazine ring are approximately 1.315 Å.

Step-by-step solution

Determine the number of valence electrons

First, we need to calculate the total number of valence electrons in a triazine molecule. Triazine has the chemical formula \(\mathrm{C}_{3} \mathrm{H}_{3} \mathrm{N}_{3}\). Carbon has 4 valence electrons, hydrogen has 1, and nitrogen has 5. Total valence electrons = (3 × 4) + (3 × 1) + (3 × 5) = 12 + 3 + 15 = 30

Construct the basic structure

Now we need to arrange the carbon (C) and nitrogen (N) atoms alternately in a ring-like structure. Each carbon atom should have one hydrogen atom bonded to it. H | C / \ N - - N \ / - C | H

Distribute remaining electrons to satisfy the octet rule

We now distribute the remaining electrons to satisfy the octet rule for C and N atoms in the structure. H | :N - C - N: | | :N - C : | H In this structure, each carbon atom has 4 bonding electrons (2 single bonds and 1 double bond), and each nitrogen atom has 5 bonding electrons (1 single bond, 1 double bond, and 2 lone pair electrons). As all the atoms have fulfilled the octet rule, this is the correct Lewis structure for triazine.

Estimate the Carbon-Nitrogen bond distances in the ring

To estimate the carbon-nitrogen bond distances, we can refer to the bond lengths in similar molecules. In benzene, the carbon-carbon bond length is approximately 1.40 Å (angstrom). The carbon-nitrogen single bond in ethanoic acid is approximately 1.47 Å, and the nitrogen-nitrogen single bond in hydrazine is approximately 1.45 Å. The carbon-nitrogen double bond in cyanogen is approximately 1.16 Å. In triazine, there are alternating single and double bonds between carbon and nitrogen atoms in the ring. Hence, the carbon-nitrogen bond length will be an average of the lengths of carbon-nitrogen single and double bonds: Average bond length = (1.47 Å + 1.16 Å) / 2 = 1.315 Å Therefore, the estimated carbon-nitrogen bond distances in the triazine ring are approximately 1.315 Å.

Key concepts

Valence Electrons

Valence electrons are the electrons in an atom that participate in chemical bonding. They are found in the outermost shell of an atom, known as the valence shell. The number of valence electrons an element has directly influences how it bonds with other elements in the formation of chemical compounds.

For example, in the exercise provided, we investigate a molecule of triazine, with the formula \(\mathrm{C}_{3} \mathrm{H}_{3} \mathrm{N}_{3}\). Carbon atoms have 4 valence electrons each, hydrogen atoms have 1, and nitrogen atoms have 5. In creating the Lewis structure for triazine, calculating the total number of valence electrons is a crucial step, as it dictates how the atoms will bond together. In this case, the sum total is 30 valence electrons, providing a foundation for establishing bonds and lone pairs in the structure.

Octet Rule

The octet rule is an essential concept in chemistry that explains the tendency of atoms to prefer having eight electrons in their valence shell. This configuration is similar to the electron arrangement of the noble gases, which are known for their chemical stability. Atoms will often form bonds by sharing or transferring electrons to achieve a complete octet, leading to the formation of molecules.

In the triazine molecule, the octet rule guides us in distributing the 30 valence electrons among the carbon (C) and nitrogen (N) atoms. Each atom strives for an octet by forming bonds—either through single or double bonds—and, in some cases, by having lone pairs of electrons. This principle helps us achieve a stable structure where each carbon is bonded to one hydrogen and to two nitrogen atoms, while each nitrogen atom forms bonds with two carbon atoms and may have one or more lone pairs.

Bond Distances

Bond distance, or bond length, is the average distance between the nuclei of two bonded atoms. Factors influencing bond lengths include the atomic size, the bond order (whether it's a single, double, or triple bond), and the electronic environment surrounding the atoms. Shorter bonds are generally stronger and more energy-dense than longer bonds.

In the context of triazine, estimating the bond distances involves comparing with known bond lengths in similar structures. As the exercise solution indicates, we consider the bond lengths of related molecules like benzene and cyanogen. The carbon-nitrogen bond lengths in triazine will fall between the lengths of a typical C-N single bond and a C-N double bond. As calculated in the solution, the average bond length is about 1.315 Å. Understanding bond distances is crucial as it plays a significant role in determining the physical and chemical properties of molecules.