Problem 94 - Chemistry and Chemical Reactivity

Question

(a) Glycolaldehyde was featured in the story "Molecules in Space" (page 372 ). Indicate the unique bond angles in this molecule. (b) One molecule found in the 1995 Hale-Bopp comet is HC \(_{3}\) N. Suggest a structure for this molecule. (Hint: it is based on a chain of atoms.)

Step-by-Step Solution

Answer

(a) Bond angles in glycolaldehyde: ~109.5° and 120°. (b) HC₃N structure: H-C≡C-C≡N with 180° bond angles.

Step 1: Understanding Glycolaldehyde

Glycolaldehyde is the simplest sugar, consisting of two carbon atoms. Its chemical formula is C\(_2\)H\(_4\)O\(_2\), and the molecule consists of an aldehyde group (CHO) and a hydroxyl group (OH) attached to each carbon atom.

Step 2: Identify the Bond Angles in Glycolaldehyde

Glycolaldehyde has a tetrahedral geometry around its carbon atoms. The C-C-O bond angle is approximately 109.5° due to the tetrahedral arrangement. The C-H bonds also form angles of approximately 109.5°. The C=O bond in the aldehyde functional group has a bond angle of approximately 120° due to the sp\(^2\) hybridization of the carbon.

Step 3: Describe HC3N Structure

HC\(_3\)N is hydrogen cyanide with two additional carbon atoms. It's also known as cyanoacetylene. The simplest structure involves a linear arrangement of atoms: H-C≡C-C≡N. The molecule has a cumulative bonding of carbon-carbon triple and single bonds arranged in a linear fashion.

Step 4: Identify the Bond Angles in HC3N

Given the linear structure of HC\(_3\)N, the bond angles for H-C≡C and C≡C-C are 180° owing to the linear geometry around the triple-bonded carbons and cyanide group.

Key Concepts

Glycolaldehyde Structure

Understanding the structure of glycolaldehyde is important because it is the simplest form of a sugar molecule. Its molecular formula is C\(_2\)H\(_4\)O\(_2\), indicating two carbon atoms, four hydrogen atoms, and two oxygen atoms. In its structure, one carbon atom is connected to a hydroxyl group (-OH) which means it belongs to the alcohols. The other carbon atom is connected to an aldehyde group (-CHO), providing the characteristic feature of aldehydes.
This arrangement makes glycolaldehyde a small yet significant organic molecule, acting as a fundamental building block for larger sugars. Each group bonded to the carbon atoms affects the overall geometry and bond angles, which align with the molecule's spatial configuration.

Bond Angles

Bond angles in a molecule determine how the atoms are spatially oriented in a chemical structure. For glycolaldehyde, these bond angles are critical for understanding its three-dimensional shape. The molecule embraces a tetrahedral geometry around its carbon atoms, where:

• The C-C-O and C-H bond angles are approximately 109.5°, due to the tetrahedral arrangement that allows maximum separation of bonds.
• The aldehyde group introduces a C=O double bond with an angle around 120°, stemming from sp\(^2\) hybridization.

This variation in angles helps create the specific molecular shape, impacting how glycolaldehyde interacts with other molecules.
Knowing these specific angles is key for predicting the behavior of glycolaldehyde in chemical reactions.

Cyanoacetylene

Cyanoacetylene, also known scientifically as HC\(_3\)N, is an intriguing molecule due to its presence in space and simplicity in structure. It has a molecular formula of H-C≡C-C≡N, where hydrogen is bonded to a carbon chain consisting of triple bonded carbon atoms followed by a nitrogen atom.
This sequence is notable for its linear configuration, resulting in a distinct type of cumulative double bonding. Cyanoacetylene is especially interesting because its linear chain and electronic characteristics make it a participant in both chemical synthesis and astrobiological environments, including outer space phenomena such as comets like the Hale-Bopp comet.

Linear and Tetrahedral Geometry

The molecular geometries such as linear and tetrahedral are fundamental concepts in chemistry for visualizing how atoms are arranged in space. In a linear geometry, as seen in molecules like cyanoacetylene (HC\(_3\)N), atoms align in a straight line, leading to 180° bond angles.

• This alignment occurs due to the triple bonds between carbon atoms, resulting in a linear shape because all electron regions are in a single axis.

On the other hand, tetrahedral geometry, present in the glycolaldehyde molecule, involves atoms being spaced symmetrically around a central atom, forming a tetrahedron.

• This creates bond angles of approximately 109.5°, as seen around the carbon atoms in glycolaldehyde.

Understanding these geometric configurations aids in predicting molecular behavior and interactions, as they dictate how molecules fit together and respond to their environment.