Problem 101 - Chemistry The Molecular Science

Question

Gamma hydroxybutyric acid, GHB, infamous as a "date rape" drug, is used illicitly because of its effects on the nervous system. The condensed molecular formula for \(\mathrm{GHB}\) is \(\mathrm{HO}\left(\mathrm{CH}_{2}\right)_{3} \mathrm{COOH} .\) (a) Write the Lewis structure for GHB. (b) Identify the hybridization of the carbon atom in the \(\mathrm{CH}_{2}\) groups and of the terminal carbon. (c) Is hydrogen bonding possible in GHB? If so, write Lewis structures to illustrate the hydrogen bonding. (d) Which carbon atoms are involved in sigma bonds? In pi bonds? (e) Which oxygen atom is involved in sigma bonds? In pi bonds?

Step-by-Step Solution

Answer

GHB forms hydrogen bonds; sigma bonds are throughout; pi is in the COOH group.

Step 1: Lewis Structure of GHB

To draw the Lewis structure, we need to lay out the atoms and their connections as given in the formula HO(CH₂)₃COOH, and then fill in the necessary electrons to satisfy the molecular structure rules. The structure consists of four carbon atoms connected in a chain, with hydroxyl (OH) groups at the ends and a carboxyl (COOH) group also attached.

Step 2: Hybridization of Carbon Atoms

The three CH₂ groups in GHB involve carbon atoms with sp³ hybridization because each carbon is singly bonded to four other atoms (two hydrogens and two carbons in the chain). The terminal carbon in the COOH group, forming double bonds with oxygen, is sp² hybridized.

Step 3: Checking for Hydrogen Bonding

Hydrogen bonding in GHB is possible due to the presence of the -OH (hydroxyl) group and the -COOH (carboxyl) group. The electronegative oxygen atoms can attract hydrogen atoms from nearby -OH groups, facilitating hydrogen bonding.

Step 4: Identifying Sigma and Pi Bonds in Carbon

All carbon atoms in GHB form sigma (σ) bonds. Pi (π) bonds are present in the COOH group where the terminal carbon forms a double bond with the oxygen atom, accounting for one pi bond.

Step 5: Identifying Sigma and Pi Bonds in Oxygen

In the COOH group, the oxygen involved in the C=O double bond forms one sigma and one pi bond. The other oxygen in the COOH forms a sigma bond with a hydrogen atom and a sigma bond with the carbon.

Key Concepts

Molecular Formula

The molecular formula of a compound is crucial because it tells you the types and numbers of atoms present. For Gamma Hydroxybutyric Acid (GHB), the molecular formula is \[\text{HO(CH}_2\text{)}_3\text{COOH} \]This formula indicates:

• One hydroxyl group (OH)
• A chain of three CH₂ groups
• A carboxyl group (COOH) at one end

The arrangement is vital to understand the structure and functionality of GHB. Each segment provides insight into its ability to bond and interact, especially concerning chemical properties like acidity or the ability to partake in hydrogen bonding.

Hybridization

Hybridization is a concept that describes how atomic orbitals mix to form new hybrid orbitals, suitable for the pairing of electrons to create chemical bonds. In GHB:

• The carbon atoms in the \(\text{CH}_2\) groups are sp³ hybridized. This means each carbon atom in these groups forms four sigma bonds, allowing them to connect to two hydrogen atoms and two other carbons in the chain.
• The terminal carbon in the \(\text{COOH}\) group is sp² hybridized. Here, sp² indicates the capacity to form three sigma bonds and one pi bond due to the double bond with an oxygen atom, which exhibits more reactive characteristics.

Recognizing hybridization provides insight into the molecule's behavior and its potential reactions.

Hydrogen Bonding

Hydrogen bonding in GHB occurs due to specific electronegative interactions. To understand this:

• Hydrogen bonds form when hydrogen atoms, covalently bonded to electronegative atoms like oxygen or nitrogen, experience attraction to another electronegative atom nearby.
• In GHB, the presence of both \(-OH\) (hydroxyl) and \(-COOH\) (carboxyl) groups creates the perfect setup for hydrogen bonding. The highly electronegative oxygen in these groups can attract hydrogens from neighboring molecules.

Hydrogen bonding significantly affects properties like boiling and melting points, solubility, and intermolecular interactions, making it vital for understanding GHB's chemical behavior.

Sigma and Pi Bonds

In chemistry, sigma and pi bonds are types of covalent bonds, differing in formation and electron sharing:

• Sigma (σ) bonds are the strongest type of covalent bond, formed by the head-on overlapping of orbitals. In GHB, all carbon atoms engage in sigma bonding, which is the primary interaction holding the molecular structure together. This includes each carbon-hydrogen and carbon-carbon linkage.
• Pi (π) bonds arise from the side-by-side overlap of p-orbitals and provide additional bonding in double and triple bonds. In GHB, pi bonds occur in the \(\text{COOH}\) group where the terminal carbon-carries pi bonds with oxygen due to the C=O double bond.

Understanding the nature of sigma and pi bonds in GHB helps in predicting its reactivity and physical properties, such as rigidness and electrical conductivity.