Question

Draw the Lewis structure for each organic compound from its condensed structural formula. \begin{equation}\mathrm{ a. }{C}_{2} \mathrm{H}_{4} \quad \text { b. } \mathrm{CH}_{3} \mathrm{NH}_{2} \quad\mathrm{ c. }{HCHO}\end{equation}\begin{equation} \mathrm{ d. }{CH}_{3} \mathrm{COOH} \text { e. } \mathrm{CH}_{3} \mathrm{CHO} \end{equation}

Short answer

a. C2H4: Two carbon atoms connected by a double bond with two hydrogens bonded to each carbon. b. CH3NH2: A central carbon bonded to three hydrogens and to a nitrogen, which has two hydrogens and a lone pair. c. HCHO: Carbon in the center with two single bonds to hydrogen and a double bond to oxygen. d. CH3COOH: Left carbon bonded to three hydrogens and right carbon; right carbon with a double bond to oxygen, single bond to an -OH group. e. CH3CHO: Left carbon bonded to three hydrogens, right carbon with a double bond to an oxygen and a single bond to a hydrogen; carbons connected with a single bond.

Step-by-step solution

Drawing Lewis Structure for C2H4

Start with a skeletal structure by connecting the two carbon atoms with a double bond because each carbon wants to form four bonds. Add hydrogens around each carbon to satisfy the remaining valencies, with each carbon forming two single bonds with two hydrogen atoms.

Drawing Lewis Structure for CH3NH2

Begin with the central carbon atom bonded to three hydrogen atoms and the nitrogen atom. Nitrogen forms three bonds and has one lone pair. Connect the nitrogen to the carbon with a single bond and add two hydrogens to it.

Drawing Lewis Structure for HCHO (Formaldehyde)

The central atom here is carbon. Carbon will form a double bond with oxygen to complete oxygen's octet, and two single bonds with two hydrogen atoms to complete carbon's four bonds.

Drawing Lewis Structure for CH3COOH (Acetic Acid)

Carbon atoms will be the central atoms. The left carbon (part of the methyl group) has three hydrogens. The right carbon (the carboxyl group) is double-bonded to an oxygen and single bonded to an -OH group. The left carbon is bonded to the right carbon with a single bond.

Drawing Lewis Structure for CH3CHO (Acetaldehyde)

The structure has two carbon atoms. The left carbon (methyl group) is bonded to three hydrogens. The right carbon (the aldehyde group) is double-bonded to an oxygen and single-bonded to a hydrogen. Connect the two carbons with a single bond.

Key concepts

Understanding the Condensed Structural Formula in Organic Compounds

The condensed structural formula is a method of describing the molecules of an organic compound in a simplified manner. Unlike the full structural formula that shows every single bond and atom in a molecule, the condensed formula consolidates adjacent atoms of the same element. It's more efficient and less cumbersome, especially when dealing with larger, more complex molecules.

For instance, acetic acid is represented as CH3COOH in its condensed form. The 'CH3' denotes a methyl group, a common fragment in organic chemistry consisting of one carbon atom bonded to three hydrogen atoms. The 'COOH' signifies the carboxylic acid group, where a carbon atom is double-bonded to an oxygen atom (carbonyl group) and also bonded to a hydroxyl group (-OH).

The usefulness of this formula lies in its ability to quickly convey a molecule's structure without drawing every single bond, which is particularly helpful when working with complex structures or when you need to write down a structure quickly during class or an exam.

The Role of Valence Electrons in Bonding

Valence electrons are the electrons in the outermost shell of an atom that are involved in chemical bonding. When drawing Lewis structures for organic compounds, valence electrons are of utmost importance because they determine how atoms will connect and form molecules.

Every element has a characteristic number of valence electrons; for example, carbon always has four valence electrons, hydrogen has one, and oxygen has six. Carbon, with its four valence electrons, is very versatile. It can form four single bonds, as seen in methane (CH4), or it can participate in double or triple bonds, exemplifying its ability to create a wide variety of molecular shapes and structures.

To visualize this, imagine a carbon atom with its four valence electrons as 'hands' that can grasp other atoms to form bonds. This ability of carbon atoms to easily link with other elements, including themselves, is the backbone of organic chemistry and the myriad of organic compounds that exist.

Determining Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. Understanding molecular geometry is crucial because it affects the molecule's physical properties and reactivity. The Lewis structure serves as the foundation for predicting molecular geometry through concepts like the VSEPR (Valence Shell Electron Pair Repulsion) theory, which states that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion.

For example, in the molecule ethene (C2H4), the carbon atoms form a double bond with each other and two single bonds with hydrogen atoms. This configuration results in a flat, planar geometry. Knowing the geometry helps in understanding how a molecule can interact with others, its boiling and melting points, and its potential biological activity.

While Lewis structures give us a two-dimensional projection, it's always essential for students to think beyond this and envision the molecules in three dimensions to appreciate the true nature of the compound's structure.