Question

Chloral \(\left(\mathrm{Cl}_{3} \mathrm{C}-\mathrm{CH}=\mathrm{O}\right)\) forms a monohydrate, chloral hydrate, the sleep-inducing depressant called "knockout drops" in old movies. (a) Write two possible structures for chloral hydrate, one involving hydrogen bonding and one that is a Lewis adduct. (b) What spectroscopic method could be used to identify the real structure? Explain.

Short answer

Draw hydrogen bonding and Lewis adduct structures for chloral hydrate. Use NMR spectroscopy to identify the correct structure.

Step-by-step solution

- Understand the Problem

Identify that we are given chloral \(\text{Cl}_{3}\text{C}-\text{CH}=\text{O}\) and we need to find the structures of its monohydrate, chloral hydrate, focusing on hydrogen bonding and Lewis adduct formats.

- Draw the Structure Involving Hydrogen Bonding

Hydrogen bonding consists of an electrostatic attraction between a hydrogen atom, bonded to a highly electronegative atom such as oxygen, and another electronegative atom. Draw the chloral hydrate structure showing the interactions between the hydrogen of water and the oxygen of the chloral's carbonyl group. The structure is: \(\text{Cl}_{3}\text{C}-\text{CH}(OH)_{2}\) where one of the OH groups forms a hydrogen bond with the \(\text{O}\) in the hydrate form.

- Draw the Structure Involving a Lewis Adduct

A Lewis adduct is formed when a molecule with a lone pair electrons (Lewis base) donates that pair to the molecule with an empty orbital (Lewis acid). Draw chloral hydrate where the lone pair of one of the oxygen atoms of the hydrate form bonds with the carbonyl carbon atom. The structure looks like this: \(\text{Cl}_{3}\text{C}-\text{CH}(OH)_{2}\) with a coordination between the carbon and one of the oxygen atoms forming a complex.

- Determine the Spectroscopic Method for Identification

To identify which structure of chloral hydrate is correct, use \(\text{NMR spectroscopy}\). \(\text{NMR (Nuclear Magnetic Resonance)}\) spectroscopy can identify hydrogen bonding patterns and the electronic environments around the nuclei, which will differ between the hydrogen bonded and Lewis adduct structures.

- Explain why \(\text{NMR spectroscopy}\) is Appropriate

Explain that \(\text{NMR spectroscopy}\) is appropriate because it detects differences in the electronic environment of hydrogen atoms, which would show up differently depending on whether the hydrates are forming hydrogen bonds or Lewis adducts.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special type of electrostatic attraction. It occurs between a hydrogen atom that's bonded to a highly electronegative atom, like oxygen, nitrogen, or fluorine, and another electronegative atom. In the case of chloral hydrate, the structure \(\text{Cl}_{3}\text{C}-\text{CH}(OH)_{2}\) can exhibit hydrogen bonding. One of the hydrogen atoms from the hydroxyl group (OH) in \(\text{CH}(OH)_{2}\) forms a hydrogen bond with the oxygen atom of the carbonyl group (C=O) from chloral. This hydrogen bond stabilizes the structure by attracting the hydrogen towards the oxygen. This bonding type is essential in many biological molecules and gives water its unique properties.

Lewis Adducts

A Lewis adduct occurs when a Lewis base donates a pair of electrons to a Lewis acid, leading to the formation of a new bond. In chloral hydrate, the structure \(\text{Cl}_{3}\text{C}-\text{CH}(OH)_{2}\) can form a Lewis adduct. Here, one of the oxygen atoms in the hydrate form acts as a Lewis base, donating its lone pair of electrons to the carbonyl carbon atom of the chloral, a Lewis acid. This interaction creates a complex where the electron pair is shared, creating a new bond between the oxygen and carbon atoms. Lewis adducts are prevalent in many chemical reactions and are crucial in understanding molecular interactions.

NMR Spectroscopy

NMR (Nuclear Magnetic Resonance) spectroscopy is a powerful analytical tool used to determine the structure of a molecule. In the context of identifying the correct structure of chloral hydrate, NMR spectroscopy can provide detailed information. It works by placing the sample in a strong magnetic field and then exposing it to radiofrequency radiation. The nuclei of the atoms in the sample can absorb and re-emit this radiation at specific frequencies dependent on their local electronic environment. For chloral hydrate, hydrogen bonding and Lewis adduct structures will show distinct differences in their NMR spectra. Specifically, the electronic environment around the hydrogen atoms will differ, which can be detected using NMR. This makes it possible to distinguish between the two possible structures.

Chemical Structure Determination

Chemical structure determination involves identifying the molecular structure of a compound. For chloral hydrate, two plausible structures are based on either hydrogen bonding or Lewis adduct formation. By utilizing spectroscopic methods such as NMR, one can collect data on the electronic environments and bonding patterns within the molecule. Hydrogen bonding in the hydrate form would create specific shifts in the NMR spectrum, differing from those observed if a Lewis adduct is formed. Thus, analyzing the NMR data allows scientists to definitively determine the accurate structure of chloral hydrate. Structure determination is not only essential for academic understanding but also for practical applications in medicinal chemistry and pharmacology.