Question

Carboxylic acids often give a strong fragment ion at \(m / z(\mathrm{M}-17)\). What is the likely structure of this cation? Show by drawing contributing structures that it stabilized by resonance.

Short answer

Answer: The likely structure of the cation formed in the fragmentation of carboxylic acids at \(m/z(\mathrm{M}-17)\) is \(\ce{R-CO+}\), where the OH group from the carboxylic acid has been removed. This cation is stabilized by resonance between two contributing structures, which distribute the positive charge between the carbon and oxygen atoms.

Step-by-step solution

Determine the mass difference of the fragment ion

Since the mass difference is 17 units, we can deduce that the missing atoms in the cation must have a combined mass of 17 atomic mass units (amu). Since we are dealing with carboxylic acids (which contain carbon, hydrogen and oxygen atoms), the most likely combination for the 17 amu loss is the OH group (16 amu for O + 1 amu for H = 17 amu)

Determine the structure of our cation

As we have determined that the missing fragment is likely an OH group, we can now figure out the structure of the cation resulting from the loss of this group. Starting from a standard carboxylic acid structure, which looks like \(\ce{R-COOH}\), we remove the OH group to find the cation, which has the structure \(\ce{R-CO+}\).

Draw contributing resonance structures

We can now draw the resonance structures for our cation (\(\ce{R-CO+}\)). Since there are three participating atoms (C, O and the positive charge), we can have resonance structures with the double bond between the C atom and O atom, and the positive charge moving between the C and the O atom. Resonance Structure 1: O O+ // // R - C R - C Resonance Structure 2: O O // // R - C+ R - C The cation is stabilized by resonance between these two structures, as it distributes the positive charge between the carbon and oxygen atoms.

Key concepts

Mass-to-Charge Ratio (m/z)

The mass-to-charge ratio, often abbreviated as m/z, is a crucial concept in mass spectrometry. When you encounter carboxylic acids in a mass spectrometry exercise, you might find that they often produce a unique fragment ion with a certain m/z value. This ratio represents the mass of an ion divided by its charge. For most organic molecules, the charge is typically +1, making the m/z value numerically equal to the ion's mass measured in atomic mass units (amu).

Understanding m/z can tell you a lot about the structure of the molecule based on the fragments observed. For example, seeing a strong fragment ion at m/z (M-17) suggests the loss of an OH group, which weighs 17 amu—crucial insight for chemists trying to determine structural components of a molecule.

Resonance Structures

Resonance structures are a way of representing the delocalization of electrons in molecules where more than one valid Lewis structure can be drawn. Carboxylic acids are a classic example where resonance plays a pivotal role in stability. In resonance structures, double or triple bonds and lone pairs are shuffled, but atom positions remain unchanged.

When you’re drawing them for an ion like the carboxylic acid fragment, it's like playing with molecular legos, where the pieces are electrons, and you move them around to see different stability scenarios. It demonstrates that the actual molecule is a hybrid of these structures, leading to enhanced stability as the charge is distributed over multiple atoms. For the R-CO+ ion, resonance helps spread out the positive charge between the carbon and oxygen, reducing stress on any one atom.

Atomic Mass Units (amu)

In mass spectrometry and chemistry overall, we measure the mass of atoms and molecules in atomic mass units (amu). One amu is defined as one-twelfth the mass of a carbon-12 atom. It's a minuscule unit of mass that provides a convenient way to discuss and compare the weights of different atoms and molecules on an equal scale.

When looking at mass fragmentation patterns, especially in exercises involving carboxylic acids, you'll see that the molecular weights (or m/z values in mass spectrometry terms) are often given in amu. This unit is fundamental in solving exercises because it quantifies the lost or gained fragments during ionization.

Fragmentation Patterns in Mass Spectrometry

Fragmentation patterns in mass spectrometry are like the molecular fingerprints for substances. Each molecule, based on its structure, will break apart in a distinct pattern under the ionizing conditions of a mass spectrometer. These patterns are identified by the m/z values of the fragments and are instrumental in deducing the structure of the molecule.

In the case of a carboxylic acid molecule, a clear pattern is the loss of an OH group—a common fragmentation. This notable detachment forms an ion with a mass 17 units less than the parent molecule. Understanding these patterns allows chemists to piece together the molecular puzzle, identifying the structure just by looking at the spikes on a mass spectrum chart—it's detective work at the molecular level!