Question

Describe the geometry around each of the three central atoms in the \(\mathrm{CH}_{3} \mathrm{COOH}\) molecule.

Short answer

CH₃ carbon: tetrahedral; CO carbon: trigonal planar; OH oxygen: bent.

Step-by-step solution

Determine the Lewis Structure

Draw the Lewis structure for acetic acid, \(\mathrm{CH_{3}COOH}\). Ensure all valence electrons are accounted for and all atoms satisfy the octet rule. Start by connecting the atoms: \(\mathrm{CH_{3}}\) connected to \(\mathrm{C=O}\) and \(\mathrm{OH}\). This gives us CH₃ attached to C double bonded to O and single bonded to OH.

Identify the Central Atoms

The central atoms are the ones directly bonded to more than one other atom within the molecule. In \(\mathrm{CH_{3}COOH}\), the three central atoms are the first carbon (in \(\mathrm{CH_3}\)), the second carbon (in \(\mathrm{CO}\)), and the oxygen in the \(\mathrm{OH}\) group.

Determine the Geometry Around Each Central Atom

For the first carbon in \(\mathrm{CH_3}\), the geometry is tetrahedral due to the four bonding pairs (3 hydrogens and 1 carbon). For the second carbon in \(\mathrm{C=O}\), the geometry is trigonal planar, as there are three effective pairs around the carbon (one \(\mathrm{C{-}C}\) bond and one \(\mathrm{C=O}\) double bond). For oxygen in the \(\mathrm{OH}\) group, the geometry is bent due to two bonding pairs and two lone pairs.

Key concepts

Lewis Structure

The Lewis Structure is a key concept in chemistry that helps visualize the arrangement of atoms, their connections, and the distribution of electrons in a molecule. To draw a Lewis Structure, follow these steps:

• Count the total number of valence electrons for all atoms in the molecule.
• Connect atoms using single bonds.
• Distribute the remaining electrons to satisfy the octet rule, which often means eight electrons around an atom.
• For the \(\mathrm{CH_3COOH}\) structure, think of it as combining the parts: \(\mathrm{CH_3}\ \mathrm{C=O}\ \mathrm{OH}\).

This results in a clear representation where the \(\mathrm{CH_3}\) is connected to the carbon which is double-bonded to an oxygen and single-bonded to \(\mathrm{OH}\). This structure ensures that the electrons are appropriately distributed.

Tetrahedral Geometry

Tetrahedral Geometry arises when a central atom is bonded to four other atoms, maximizing distance and minimizing repulsion between electron pairs. In the context of \(\mathrm{CH_3COOH}\), the first carbon in \(\mathrm{CH_3}\) group is surrounded by three hydrogen atoms and one carbon atom.

• This results in a tetrahedral shape.
• The bond angles are approximately \(109.5°\), providing a symmetrical distribution of atoms around the central carbon.

This geometry is common in organic compounds where carbon atoms frequently form four sigma bonds. It's the basic structure for alkanes and simple alcohols.

Trigonal Planar Geometry

Trigonal Planar Geometry occurs when three atoms are bonded to a central atom, forming a shape that lies in a single plane. In \(\mathrm{CH_3COOH}\), the second carbon (also known as the carbonyl carbon) displays this geometry.

• Here, the carbon is engaged in a double bond with oxygen and single bonds with another carbon and oxygen.
• The bond angles in a trigonal planar arrangement are \(120°\).
• This helps in evenly spreading out the attached atoms around the central atom.

This type of geometry is typical for molecules with a \(\mathrm{C=O}\) double bond and is crucial in understanding the reactivity and interactions in carbonyl-containing compounds.

Bent Geometry

Bent Geometry arises when a central atom is bonded to two other atoms and has one or more lone pairs of electrons. In \(\mathrm{CH_3COOH}\), oxygen in the \(\mathrm{OH}\) group showcases this geometry.

• The oxygen atom has two lone pairs and two bonds.
• This results in a bent shape with bond angles less than \(109.5°\) (typically around \(104.5°\)).

Lone pairs exert more repulsive force than bonding pairs, which pushes the bonded hydrogen and oxygen slightly closer together. This bent structure is characteristic of water and similar molecules with central atoms having lone electron pairs.

Octet Rule

The Octet Rule is a fundamental concept in chemistry that states atoms tend to form bonds until they have eight electrons in their valence shell, resembling the electron configuration of noble gases. This rule helps predict how atoms will bond and the overall structure of the molecule.

• Atoms achieve this configuration through sharing (covalent bonds), gaining, or losing electrons.
• For instance, in \(\mathrm{CH_3COOH}\), each carbon and oxygen atom aims to complete its octet.
• The hydrogen atoms are exceptions, as they only aim for two electrons to complete their outer shell.

While the rule works well for many compounds, there are exceptions, especially with molecules involving transition metals or when elements form compounds with less or more than eight electrons.