Question

Draw Lewis structures for each of the following species and predict the hybridization at each carbon atom: (a) \(\mathrm{H}_{2} \mathrm{CCH}^{+}\); (b) \(\mathrm{H}_{2} \mathrm{CCH}_{3}{ }^{+}\); (c) \(\mathrm{H}_{3} \mathrm{CCH}_{2}{ }^{-}\).

Short answer

For (a) \text{H}_2\text{CCH}^{+}, both carbon atoms are sp hybridized. For (b) \text{H}_2\text{CCH}_3^{+}, the first carbon is sp hybridized and the second carbon is sp2 hybridized. For (c) \text{H}_3\text{CCH}_2^{-}, both carbon atoms are sp3 hybridized.

Step-by-step solution

Draw Lewis Structure for \text{H}_2\text{CCH}^{+}

Carbon atoms should be connected since they are the least electronegative. Place two hydrogens on the first carbon atom. Since there's a positive charge, one less electron is present, implying a triple bond between carbon atoms to fulfill octet rule. Hydrogens have a single bond with carbon.

Predict Hybridization for \text{H}_2\text{CCH}^{+}

For the first carbon in H2CCH+, with two single bonds and two parts of the triple bond, the hybridization is sp. The second carbon, part of a triple bond and no additional bonds or unshared electrons, is also sp hybridized.

Draw Lewis Structure for \text{H}_2\text{CCH}_3^{+}

Connect the carbon atoms. The first carbon atom has two hydrogens, and a triple bond to the second carbon atom, which in turn is singly bonded to three hydrogen atoms. To account for the positive charge, we don't add any non-bonding electrons on the carbons.

Predict Hybridization for \text{H}_2\text{CCH}_3^{+}

The first carbon atom, with two hydrogen single bonds and a triple bond to another carbon, is sp hybridized. The second carbon is attached to three hydrogens and another carbon, making it sp2 hybridized.

Draw Lewis Structure for \text{H}_3\text{CCH}_2^{}

Connect carbon atoms with a single bond. The first carbon has three single bonds with hydrogen, and the second carbon has two single bonds with hydrogen and a negative charge, suggesting one non-bonding pair.

Predict Hybridization for \text{H}_3\text{CCH}_2^{}

The first carbon with three single bonds to hydrogen and one single bond to carbon is sp3 hybridized. The second carbon, with two single bonds to hydrogen, and one lone pair, due to the negative charge, is also sp3 hybridized.

Key concepts

Chemical Bonding

Chemical bonding is the process by which atoms combine to form molecules. The primary goal in bonding is to attain a stable electron configuration. To understand a molecule's stability and reactivity, you must first comprehend its bonding.

Atoms form bonds in several ways, including ionic, covalent, and metallic bonding, but for organic molecules like the ones in the exercise, covalent bonding is the main focus. In covalent bonds, atoms share electrons to fulfill the octet rule, which states that atoms are the most stable when they have eight electrons in their valence shell.

In the given examples,

• H₂CCH⁺
• H₂CCH₃⁺
• H₃CCH₂^-

we see covalent bonding between carbon (C) and hydrogen (H) atoms and between carbon atoms themselves. The presence of charges (+/-) in the molecules directly affects the electron count, influencing how many bonds each atom must form to achieve a stable configuration.

Electron Configuration

Electron configuration refers to the distribution of electrons among the orbitals of an atom. It's essential for predicting an atom's reactivity and the types of bonds it can form. For carbon atoms in organic compounds, the electron configuration is especially important because it determines the kind of hybridization a carbon atom undergoes, which in turn affects the molecule's overall shape and properties.

In simpler terms, carbon likes to have eight electrons around it, mimicking the nearest noble gas configuration. In the molecules described in the exercise, the carbon atoms either lose an electron (indicated by a positive charge) or gain an electron (indicated by a negative charge). These changes in the electron configuration directly influence the carbon atom's bonding capabilities and the types of bonds they will form in these particular structures.

Molecular Geometry

Molecular geometry describes the three-dimensional arrangement of atoms within a molecule. It's crucial for understanding the molecule's physical properties, reactivity, and how it interacts with other molecules. The geometry is determined by both the electron pairs bonding the atoms together and the electron pairs that are not involved in bonding (lone pairs).

In the exercise's context, molecular geometry is predicted after determining the hybridization of the carbon atoms. The hybridization tells us about the electron orbitals that carbon is using to share electrons with its neighbors. For instance:

• sp hybridization typically leads to a linear geometry.
• sp2 hybridization creates a trigonal planar shape.
• sp3 hybridization results in a tetrahedral structure.

By evaluating the hybridization states of carbon in each molecule from the exercise, you can predict that the linear or slightly linear structures will arise from sp and sp2 hybridized carbons while a more three-dimensional, tetrahedral shape will result from sp3 hybridization.