Question

Notice in the following structures (a)-(d) that the bottom carbon and its substituents are written as \(\mathrm{CH}_{2} \mathrm{OH}\) in every case. How does the \(\mathrm{C}\) in this group differ in each case from the \(\mathrm{C}\) atoms above it? Why must the locations of the \(\mathrm{H}\) atoms and \(-\mathrm{OH}\) groups attached to the carbons between this one and the carbonyl group be shown?

Short answer

The \( ext{C} \) in \( ext{CH}_2 ext{OH} \) is a primary alcohol carbon; showing positions of H and -OH is crucial for stereochemistry.

Step-by-step solution

Identify the Functional Group in CH2OH

In each structure, the group labeled as \( ext{CH}_2 ext{OH} \) contains a carbon atom bonded to two hydrogen atoms and one hydroxyl group (-OH). This group is known as a primary alcohol, where the carbon bonded to the hydroxyl group is a primary carbon, meaning it is connected to only one other carbon atom.

Differentiate the Carbon Atom in CH2OH

The carbon in the \( ext{CH}_2 ext{OH} \) group is different from the carbons above it because it is sp³ hybridized and part of an alcohol group, making it a primary carbon. Conversely, other carbon atoms in the structure may have different hybridizations or be part of different functional groups, such as ketones, aldehydes, or other structural groups impacting their chemical behavior.

Explain the Importance of Hydrogen and Hydroxyl Positioning

The specific locations of hydrogen atoms and -OH groups are important for explaining the molecule's stereochemistry and its reactivity. The orientation of these groups affects physical properties, chemical reactivity, and biological activity. For example, in sugars, the configuration around each asymmetric carbon impacts its classification as a glucose or galactose and its folding pattern in higher structures.

Key concepts

Functional Groups

In organic chemistry, a functional group refers to specific atoms or clusters of atoms within a molecule that are responsible for the characteristic chemical reactions of that molecule. One of the simplest functional groups is the hydroxyl group (-OH), which classifies a molecule as an alcohol. In the primary alcohol \(CH_2OH\), we find a carbon atom attached to two hydrogen atoms and a hydroxyl group.
The presence of the hydroxyl group significantly influences the properties of the molecule:

• Increased polarity due to the electronegative oxygen atom in -OH
• Ability to form hydrogen bonds
• Influenced melting and boiling points

The term "primary" specifies that the carbon connected to the hydroxyl group is bonded to only one other carbon. This results in characteristic reactivity patterns as primary alcohols can be oxidized to form aldehydes first and then carboxylic acids.

Hybridization

The concept of hybridization explains the mix of atomic orbitals to form new, identical hybrid orbitals, suitable for the pairing of electron pairs to form chemical bonds.
In the case of a carbon atom in a methylene group like \(CH_2OH\), it undergoes \(sp^3\) hybridization. This means:

• The original one s and three p orbitals mix and rearrange to form four hybrid orbitals.
• Each hybrid orbital has 25% s character and 75% p character.
• The four \(sp^3\) hybrid orbitals spread into a tetrahedral shape, having bond angles of approximately 109.5 degrees.

Such hybridization is crucial as it dictates the molecule's shape and influences how it interacts with other molecules. In alcohols, this arrangement supports hydrogen bonding, enhancing solubility and reactivity patterns.

Stereochemistry

Stereochemistry is the study of the three-dimensional arrangement of atoms within molecules. In molecules, the positioning of atoms can significantly influence physical and chemical properties.
For example, even if two molecules have the same chemical formula, their difference in atom arrangement can yield vastly different substances. Take glucose and galactose; both are sugars, but they differ in stereochemistry.
The \(CH_2OH\) group can affect the stereochemistry of a molecule, particularly when it modifies the spatial orientation of the hydroxyl group relative to the carbon backbone. The specific arrangement of atoms or groups in space, termed chiral centers, can impact:

• Reaction pathways
• Biological activity
• Chemical behavior

Understanding stereochemistry is essential in explaining how molecules like sugars fold, function biologically, and interact with other molecules in biological systems.