Question

One way to determine the number of acidic hydrogens in a molecule is to treat the compound with \(\mathrm{NaOD}\) in \(\mathrm{D}_{2} \mathrm{O},\) isolate the product, and determine its molecular weight by mass spectrometry. For example, if cyclohexanone is treated with \(\mathrm{NaOD}\) in \(\mathrm{D}_{2} \mathrm{O},\) the product has \(\mathrm{MW}=\) 102\. Explain how this method works.

Short answer

Acidic hydrogens in cyclohexanone are replaced by deuterium, increasing the mass from 98 to 102 amu.

Step-by-step solution

Understanding Deuterium Exchange

The first part of this method involves replacing acidic hydrogens in the molecule with deuterium atoms. In \(\mathrm{D}_2\mathrm{O}\), deuterium (D) replaces hydrogen (H) in the molecule. This is particularly effective with acidic hydrogens because they can readily exchange with deuterium in the presence of \(\mathrm{NaOD}\). For cyclohexanone, it specifically targets hydrogens that are next to the carbonyl group due to tautomerism.

Identify Sites of Exchange

Identify the acidic hydrogen atoms which can be exchanged for deuterium. In cyclohexanone, these hydrogens are on the carbons adjacent to the carbonyl group because of their acidity, facilitating the exchange due to keto-enol tautomerism.

Calculating the New Molecular Weight

Consider how many hydrogens have been replaced by deuterium: in cyclohexanone, typically two acidic hydrogens (alpha hydrogens adjacent to the carbonyl) are replaced. Each hydrogen replaced by deuterium adds 1 amu (atomic mass unit) to the molecular weight since deuterium is heavier than hydrogen by exactly this difference.

Verify with Example Calculations

The initial mass of cyclohexanone is \(C_6H_{10}O\) which equals \(98\) amu. The replacement of two hydrogens with deuterium adds 2 amu (1 amu per deuterium atom), giving a new molecular weight of \(102\) amu.

Key concepts

Acidic Hydrogens

Acidic hydrogens are unique because they can easily be replaced in a chemical reaction. These hydrogens are often found near electronegative elements or groups, such as oxygen in a carbonyl group.

Cyclohexanone, for example, has hydrogens adjacent to the carbonyl group, which are more acidic. This is because the carbonyl group can stabilize the conjugate base formed when the hydrogen is removed.

This property is why these hydrogens are termed "acidic" – they can act like acids in reactions, readily participating in exchanges with other atoms like deuterium.

Mass Spectrometry

Mass spectrometry is a powerful tool used to determine the molecular weight of a compound. This technique measures the mass-to-charge ratio of molecules, helping scientists analyze the composition of a sample.

In this process, molecules are ionized and then sorted based on their mass. When cyclohexanone undergoes deuterium exchange, mass spectrometry can precisely measure the increase in molecular weight.

By comparing the molecular weight before and after deuterium exchange, the number of acidic hydrogens replaced can be determined.

Cyclohexanone

Cyclohexanone is a six-membered ring-containing ketone. Its structure features a carbonyl group, making it an excellent candidate for studying deuterium exchange reactions.

The carbonyl group's presence influences the acidity of hydrogen atoms adjacent to it.

In the case of cyclohexanone, when treated with \(\mathrm{NaOD}\) in \(\mathrm{D}_2\mathrm{O}\), the hydrogens adjacent to the carbonyl group are likely to be replaced by deuterium. This leads to a measurable change in molecular weight due to the heavier mass of deuterium compared to hydrogen.

Keto-Enol Tautomerism

Keto-enol tautomerism is a chemical equilibrium between a keto form and an enol form. In cyclohexanone, this tautomerism enables the exchange of acidic hydrogens with deuterium.

The process involves temporary conversion to the enol form, where the keto form's carbonyl oxygen can form a double bond with an adjacent carbon, while the alpha hydrogen moves to the oxygen, forming an -OH group.

This movement allows deuterium to replace the acidic hydrogen easily during the reaction, facilitating the deuterium exchange in cyclohexanone.