Question

What hybridization do you expect for the atom indicated in red in each of the following species? (a) \(\mathrm{CH}_{3} \mathrm{CO}_{2}^{-}\); (b) \(\mathrm{PH}_{4}^{+} ;\)(c) \(\mathrm{AlF}_{3} ;\) (d) d) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}_{2}^{+}\)

Short answer

The hybridizations of the central atoms in red for each species are: (a) sp2, (b) sp3, (c) sp2, and (d) sp2.

Step-by-step solution

Identify the central atom

For species (a), the central atom is the carbon attached to the oxygen atoms (the second carbon in the formula).

Count the regions of electron density

The second carbon is bonded to one carbon and two oxygens, giving us three bond regions. It also has no lone pairs. Therefore, there are three regions of electron density around the central carbon atom.

Determine the hybridization

Since there are three regions of electron density around the carbon atom, the hybridization is sp2. **Species (b): \(\mathrm{PH}_{4}^{+}\)**

Identify the central atom

In this species, the central atom is phosphorus (P).

Count the regions of electron density

Phosphorus is bonded to four hydrogens, and there are no lone pairs. Therefore, there are four regions of electron density around the phosphorus atom.

Determine the hybridization

Since there are four regions of electron density around the phosphorus atom, the hybridization is sp3. **Species (c): \(\mathrm{AlF}_{3}\)**

Identify the central atom

In this species, the central atom is aluminum (Al).

Count the regions of electron density

Aluminum is bonded to three fluorine atoms and has no lone pairs. Therefore, there are three regions of electron density around the aluminum atom.

Determine the hybridization

Since there are three regions of electron density around the aluminum atom, the hybridization is sp2. **Species (d): \(\mathrm{H}_{2}\mathrm{C}=\mathrm{CH}-\mathrm{CH}_{2}^{+}\)**

Identify the central atom

In this species, the central atom is the carbon attached to the positively charged carbon (the second carbon in the formula).

Count the regions of electron density

This carbon is bonded to one carbon through a double bond and a single bond to the positively charged carbon. It also has no lone pairs. Therefore, there are three regions of electron density around the central carbon atom.

Determine the hybridization

Since there are three regions of electron density around the central carbon atom, the hybridization is sp2. In summary, the hybridizations of the central atoms in red for each species are: (a) sp2 (b) sp3 (c) sp2 (d) sp2

Key concepts

sp2 Hybridization

Understanding the concept of sp2 hybridization enables us to predict the geometry and reactivity of molecules with double bonds or with atoms bonded to three groups without any lone pairs. When we discuss sp2 hybridization, we are looking at a situation where an atom mixes one 's' orbital with two 'p' orbitals to create three new equivalent hybrid orbitals.

This rearrangement allows for the formation of three σ (sigma) bonds and creates an electron density distribution that favors angles of approximately 120° between the bonded atoms. This is the classic trigonal planar structure. When you identified the central carbon in species (a) and the aluminum in species (c) as having sp2 hybridization, you deduced that they are involved in making three σ bonds, with one of the carbon bonds in species (a) being a double bond that contains an additional π (pi) bond.

sp3 Hybridization

When we shift our focus to sp3 hybridization, we consider an atom that is mixing one 's' orbital and three 'p' orbitals to generate four new hybrid orbitals of equal energy. This rearrangement allows formation of four σ bonds with the hybrid orbitals pointing towards the corners of a tetrahedron, leading to the familiar tetrahedral geometry.

The central phosphorus atom in species (b) was identified as sp3 hybridized, indicating its bond geometry forms a tetrahedral shape, which corresponds to four regions of electron density. No lone pairs are present on the phosphorus, fulfilling the condition for sp3 hybridization. These bonds are evenly spaced at an ideal bond angle of 109.5°, suggesting that the molecule should exhibit a stable configuration.

Electron Density Regions

Key to identifying hybridization states in atoms is recognizing the electron density regions around an atom. These regions are not just about the bonds an atom forms with its neighbors but also about the lone pairs of electrons that may not be involved in bonding but still exert influence on an atom's geometry.

In our exercises for species (a), (b), (c), and (d), you determined the hybridizations by counting each bond region as a region of electron density. It’s essential to note that single, double, and triple bonds each count as one region of electron density. Electron density regions determine the atom's hybridization and, consequently, the molecule's shape, which directly influences its chemical and physical properties.

Chemical Bonding

At the heart of these hybridization models is the fundamental principle of chemical bonding. Bonds form when atoms share or transfer electrons to achieve more stable electron configurations. The number and type of chemical bonds that an atom can form are crucial in determining the molecule's structure and function.

In our analysis of species (a) and (c), we implicitly used the rules of chemical bonding and electron sharing to deduce the sp2 hybridization. Likewise, in examining species (b), we applied our understanding of chemical bonding to establish the sp3 hybridization of the phosphorus atom due to its four σ bonds with hydrogen atoms. These concepts are not isolated; they're interconnected and encapsulate the molecular dynamics we observe.