Question

Following is a structural formula for guanidine, the compound by which migratory birds excrete excess metabolic nitrogen. The hydrochloride salt of this compound is a white crystalline powder, freely soluble in water and ethanol. (a) Write a Lewis structure for guanidine showing all valence electrons. (b) Does proton transfer to guanidine occur preferentially to one of its \(-\mathrm{NH}_{2}\) groups (cation A) or to its = NH group (cation B)? Explain. N=C(N)[NH3+] Guanidine \(\mathrm{A}\) B

Short answer

Answer: The proton transfer in guanidine occurs preferentially to its =NH group (cation B). This is because the positive charge is delocalized between the two nitrogen atoms in cation B through resonance, which makes it more stable than cation A.

Step-by-step solution

Draw the Lewis structure for guanidine

To draw the Lewis structure for guanidine, first, we will identify the atoms and their valence electrons. Guanidine has one Nitrogen atom with a triple bond (N), one Nitrogen atom with a double bond (=N), and two Nitrogen atoms each bonded to two Hydrogen atoms (-NH2). Next, we will arrange the atoms in a way that they share electrons to achieve a full octet (8 electrons in the outer shell) for each atom, keeping in mind the following: - Hydrogen atoms form only one bond. - Nitrogen atoms can form 3 bonds (1 triple, 1 double, and one single bond in this case). The Lewis structure can be represented as follows: H | H-N-C=N-H H In this structure, all atoms have full octets.

Determine the preferred proton transfer location

To determine whether proton transfer to guanidine occurs preferentially to one of its \(-\mathrm{NH}_{2}\) groups (cation A) or to its =NH group (cation B), we will examine the effect of protonation on the stability of the resultant ion. When a proton is added to an \(-\mathrm{NH}_{2}\) group, it will form cation A: H | H-N-C=N-H H+ \\ N / H On the other hand, if the proton is added to the =NH group, it will form cation B: H | H-N-C=N \ N-H H+ For the proton transfer to occur preferentially to one of these groups, the stability of the resulting ion must be considered. In general, the more delocalized the positive charge, the more stable the ion. In cation A, the positive charge is localized on the nitrogen atom with the \(\mathrm{NH}_{2}\) group. On the other hand, in cation B, the positive charge is delocalized between the two nitrogen atoms through resonance: Resonance structures of cation B: 1. H | H-N-C=N \ N-H+ 2. H | H-N+-C=N \ N-H As we can see that due to the resonance effect, the positive charge is delocalized in cation B. This delocalization of charge in cation B makes it more stable than cation A. Therefore, proton transfer to guanidine occurs preferentially to its =NH group (cation B).

Key concepts

Lewis Structure

The Lewis structure is a visual representation of how atoms in a molecule are bonded together, and it shows the placement of valence electrons, including any lone pairs. When drawing a Lewis structure for guanidine, we identify each atom's valence electrons and create bonds to satisfy the octet rule. Guanidine consists of nitrogen and hydrogen atoms. Nitrogen typically forms three bonds, corresponding to its five valence electrons: three are used for bonding, and the remaining two make up a lone pair.

Starting with the central carbon atom, we bond it to the different nitrogen atoms, taking into account that one nitrogen forms a triple bond, another forms a double bond, and the two remaining are single-bonded to hydrogen atoms. Hydrogen, with its single electron, forms one single bond. The result is a structure where all atoms, including the carbon, are surrounded by a stable eight-electron arrangement.

It is vital to ensure that in this arrangement, each nitrogen is bonded appropriately to fulfill not just its own octet, but also to maintain overall molecular stability.

Proton Transfer

Proton transfer in chemistry refers to the movement of a proton (H⁺) from one atom to another within a molecule or between different molecules, resulting in the formation of new ions. This process is highly influenced by the stability of the resultant ions.

In the case of guanidine, the question is whether a proton will preferentially attach to one of the amine groups (-NH₂), creating cation A, or to the imine group (=NH), resulting in cation B. The answer lies within the stability provided by the distribution of the positive charge in the parts of the molecule. For guanidine, resonance stabilization plays a significant role in this determination. By examining the resonance structures, it's clear that the positive charge is more delocalized in the structure of cation B, making it more stable and thus, the preferred site for protonation.

Resonance Stability

Resonance stability is a concept in chemistry that describes the delocalization of electrons within a molecule, providing it with greater structural stability. Resonance occurs when there are multiple ways to arrange the electrons in a molecule that cannot be represented by a single Lewis structure. Instead, multiple structures, known as resonance structures, are used to depict the molecule's electron distribution.

In guanidine, when a proton attaches to the imine group, the positive charge can be spread over two nitrogen atoms, as shown in the resonance structures for cation B. This delocalization of charge leads to a lower energy state, making the structure more stable compared to cation A, where the positive charge is confined to one nitrogen atom. The increased resonance stability in cation B is a decisive factor in making it the preferred site for protonation.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are involved in forming bonds with other atoms. In the context of guanidine, understanding the distribution of valence electrons among the carbon, nitrogen, and hydrogen atoms is essential for constructing the correct Lewis structure and predicting how proton transfer occurs.

Nitrogen typically has five valence electrons, which it can share or use to complete its octet -- three bonds and a lone pair. The carbon, sitting at the center of guanidine, bonds with nitrogen and hydrogen atoms to fill its own valence shell with eight electrons. The hydrogen atoms, with one valence electron each, form single bonds with nitrogen. During proton transfer, the added proton (a new valence electron) interacts with the molecule, seeking to form a stable arrangement with the existing valence electrons, ultimately favoring resonance-stabilized structures.