Question

Cyanic acid, HOCN, and isocyanic acid, HNCO, dissolve in water to yield the same anion on loss of \(\mathrm{H}^{+}\). (a) Write a Lewis structure for cyanic acid. (b) Write a Lewis structure for isocyanic acid. (c) Account for the fact that each acid gives the same anion on loss of \(\mathrm{H}^{+}\). 1.71 In Chapter 6, we study a group of organic cations called carbocations. Following is the structure of one such carbocation, the tert-butyl cation. (a) How many electrons are in the valence shell of the carbon bearing the positive charge? (b) Using VSEPR, predict the bond angles about this carbon. (c) Given the bond angle you predicted in (b), what hybridization do you predict for this carbon?

Short answer

Question: Draw the Lewis structures for cyanic acid (HOCN) and isocyanic acid (HNCO), explain why both yield the same anion after losing an H+ ion, and analyze the tert-butyl cation by finding the number of valence shell electrons on the positively charged carbon, predicting the bond angles using VSEPR theory, and determining the hybridization of the carbon. Answer: The Lewis structures for cyanic acid (HOCN) and isocyanic acid (HNCO) are H-O=C≡N and H-N-C≡O, respectively. Both acids yield the same anion (OCN-) after losing an H+ ion because the remaining atoms have the same connections (O double-bonded to C and C triple-bonded to N). The tert-butyl cation has a positively charged carbon with three valence shell electrons. Using VSEPR theory, the bond angles around this carbon are approximately 120°, and the hybridization of the carbon is sp2.

Step-by-step solution

Draw Lewis structure for cyanic acid (HOCN)

To draw the Lewis structure for cyanic acid, we first count the total number of valence electrons: H (1) + O (6) + C (4) + N (5) = 16 valence electrons. Then, we connect the atoms as follows: H - O - C ≡ N. Now, complete the octet rule for all atoms except hydrogen. So, the Lewis structure for cyanic acid (HOCN) is: O || H - O - C ≡ N

Draw Lewis structure for isocyanic acid (HNCO)

Again, let's count the total number of valence electrons for isocyanic acid: H (1) + N (5) + C (4) + O (6) = 16 valence electrons. We connect the atoms in this order: H - N - C ≡ O and complete the octet rule for all atoms except hydrogen. Hence, the Lewis structure for isocyanic acid (HNCO) is: H - N - C ≡ O

Explain why both acids yield the same anion

In both cyanic acid (HOCN) and isocyanic acid (HNCO), when the H+ ion is lost, the remaining anion in both cases is OCN-. The oxygen atom in both structures has a double bond with the carbon, while the carbon has a triple bond to the nitrogen. After hydrogen dissociates, the oxygen atom forms a negative charge. Thus, the same anion OCN- is formed from both cyanic and isocyanic acids.

Find electrons in the valence shell of positively charged carbon

The positively charged carbon in the tert-butyl cation has lost one electron from its valence shell. The carbon in organic cations typically has four valence electrons. In this case, the positively charged carbon has only three electrons in its valence shell.

Predict bond angles using VSEPR theory

Using VSEPR (Valence Shell Electron Pair Repulsion) theory, with three electron groups around the positively charged carbon, we predict a trigonal planar electron geometry, which corresponds to bond angles of approximately 120°.

Determine the hybridization of the carbon

Since the positively charged carbon has a trigonal planar electron geometry, which means it has three surrounding atoms and no lone pairs, the hybridization of the carbon is sp2.

Key concepts

Cyanic Acid

Cyanic acid (HOCN) is a chemical compound that plays an intriguing role in organic chemistry. Its molecular structure is captured through its Lewis structure, which is a straightforward visual representation of how atoms are bonded within in the molecule. To get this structure, count all the valence electrons involved: Hydrogen contributes 1, Oxygen 6, Carbon 4, and Nitrogen 5, summing up to 16 valence electrons.
The structure is arranged by forming bonds between Hydrogen and Oxygen, and a triple bond between Carbon and Nitrogen, rendering it as H - O - C ≡ N.
Cyanic acid's array of single, double, and triple bonds helps fulfill the octet rule (all except for Hydrogen, which follows the duet rule), stabilizing the structure for its dynamic processes in chemical reactions.

Isocyanic Acid

Isocyanic acid (HNCO) is closely related to cyanic acid but has a different structural arrangement. The significance lies in its ability to rearrange its atoms yet offer identical reactivity outcomes in chemical reactions. Like cyanic acid, we start by counting the total valence electrons:

• Hydrogen: 1
• Nitrogen: 5
• Carbon: 4
• Oxygen: 6

Giving us a total of 16 electrons.
In contrast to cyanic acid, arrange the bonds as H - N - C ≡ O, highlighting different atom connections yet maintaining the same electron count. The key aspect is the formation of the OCN- anion upon losing a hydrogen ion (H+). Both cyanic and isocyanic acid release the same ion, showcasing a fascinating aspect of chemistry, where different structural forms can lead to identical results through a process called isomerization.

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) Theory is a cornerstone in predicting molecular geometry. The principle behind VSEPR is that electron pairs around a central atom will arrange themselves to minimize repulsion, thus dictating the shape of the molecule.
For the positively charged carbon in a tert-butyl cation, VSEPR theory is used to predict its geometry. Given that there are three groups of electrons around this carbon and no lone pairs, VSEPR theory designates a trigonal planar shape. This electron count and absence of lone pairs lead to bond angles of approximately 120°, giving insights into how these molecular structures align in space.

Carbocations

Carbocations are fascinating entities in organic chemistry, representing ions with a positively charged carbon atom. This charge is a result of the carbon atom having fewer electrons in its valence shell than the typical four found in neutral carbon atoms.
For instance, in the tert-butyl cation, the carbon directly tied to the positive charge only has three electrons. This deficiency greatly affects the atom's reactivity and stability. Understanding this electron deficiency provides insights into the molecule's geometry and reactivity, vital for rationalizing reaction mechanisms and predicting the behavior of molecules in various chemical environments.

Hybridization

Hybridization is a concept that explains the mixing of atomic orbitals to form new hybrid orbitals. These are essential for forming covalent bonds in molecules.
In the example of a positively charged carbon in tert-butyl cation, it portrays sp2 hybridization. Because this carbon is bound to three groups and lacks lone pairs, it involves the hybridization of one s and two p orbitals to form three equivalent sp2 orbitals.
This sp2 setup is consistent with the trigonal planar geometry predicted by VSEPR theory and the roughly 120° bond angles. Understanding hybridization is fundamental to predicting molecular structure, enabling chemists to grasp how atoms within a molecule bond and interact.