Question

The molecular ion for compounds containing only \(\mathrm{C}, \mathrm{H}\), and \(\mathrm{O}\) always has an even mass-to-charge value. Why is this so? What can you say about the mass-to-charge ratio of ions that arise from fragmentation of one bond in the molecular ion? From fragmentation of two bonds in the molecular ion?

Short answer

Answer: Molecular ions containing only carbon, hydrogen, and oxygen atoms have an even mass-to-charge value because carbon and oxygen have even masses, and an even number of hydrogen atoms with odd masses result in an even total mass. Fragmentation can change the mass-to-charge ratio to an odd value if a hydrogen atom is involved in the broken bond. When breaking two bonds, the mass-to-charge values can remain even if both broken bonds involve hydrogen atoms or if both broken bonds are between carbon and/or oxygen atoms.

Step-by-step solution

Mass of Carbon, Hydrogen, and Oxygen Atoms

Let's consider the mass of the most common isotopes of carbon \(\mathrm{C}\), hydrogen \(\mathrm{H}\), and oxygen \(\mathrm{O}\) atoms: - Carbon-12: 12 mass units (even) - Hydrogen-1: 1 mass unit (odd) - Oxygen-16: 16 mass units (even) The mass-to-charge ratio of a molecular ion containing only these common isotopes can be calculated by summing the mass of each atom in the molecule and dividing it by the ion's charge.

Explanation for Even Mass-to-Charge Ratio

A molecule containing only carbon, hydrogen, and oxygen atoms will have an even total mass if the number of hydrogen atoms in the molecule is even. This is because the sum of an even number of odd mass hydrogen atoms (1 mass unit each) results in an even mass. Since carbon and oxygen both have an even mass, adding these to the sum will maintain the even mass. The mass-to-charge ratio will remain even as the charge of the molecular ion is always positive and, under normal circumstances, the charge is usually 1+ (from the H atom).

Fragmentation: Breaking One Bond

When breaking one bond in the molecular ion, there can be two scenarios: 1. The broken bond is between two carbon or oxygen atoms, or between carbon and oxygen. In this case, both fragments will have an even mass, keeping their mass-to-charge ratios even. 2. The broken bond is between a hydrogen atom and another element (carbon or oxygen). In this case, one fragment will lose a hydrogen atom and have an odd mass. This fragment's mass-to-charge ratio becomes odd. The other fragment's mass-to-charge ratio remains even.

Fragmentation: Breaking Two Bonds

When breaking two bonds in the molecular ion, there can be three scenarios: 1. Both broken bonds are between carbon and/or oxygen atoms. Both fragments keep their even mass-to-charge ratios. 2. One of the broken bonds is between carbon and/or oxygen atoms, while the other involves a hydrogen atom. One fragment will have an odd mass and odd mass-to-charge ratio, while the other fragment keeps an even mass-to-charge ratio. 3. Both broken bonds involve hydrogen atoms or a single atom loses two hydrogen atoms. Both fragmented ions have an odd mass, making their mass-to-charge values even. In summary, molecular ions containing only carbon, hydrogen, and oxygen atoms have an even mass-to-charge value because of the even mass of carbon and oxygen atoms and the presence of an even number of odd mass hydrogen atoms. After fragmentation, the mass-to-charge ratio may shift to an odd value if a hydrogen atom is involved in the broken bond.

Key concepts

Mass Spectrometry

Mass spectrometry is an analytic technique that measures the mass-to-charge ratio (m/z) of ions to identify and quantify molecules within a sample. It plays a pivotal role in organic chemistry for determining the molecular structure of organic compounds.

The process begins by ionizing the compound, usually by electron impact or chemical ionization. Once ionized, the molecular ions are propelled into a mass analyzer by an electric or magnetic field, which separates the ions based on their m/z ratios. The ions reach the detector, which records the number of ions at each m/z value, resulting in a mass spectrum. This spectrum displays the relative abundance of detected ions against their m/z ratios, which provides critical information about the molecular weight and structure of the compound.

Furthermore, mass spectrometry can handle complex mixtures without the need for prior separation, and it’s sensitive enough to detect trace amounts of substances, making it a powerful tool for both qualitative and quantitative analysis.

Molecular Ion Fragmentation

Molecular ion fragmentation is a crucial aspect of mass spectrometry that helps in understanding the structure of an organic molecule. After ionization, some molecular ions may break apart or fragment, resulting in smaller ions of different masses.

The pattern of this fragmentation provides valuable clues about the connections between different atoms within the molecule. For example, if a bond between a carbon and a hydrogen atom breaks, the resulting fragments will display a difference in mass that reflects the loss of the hydrogen atom. This can be a primary indication of the molecule's branching or the presence of specific functional groups.

Understanding the fragmentation pattern is key to interpreting mass spectra correctly. Each type of bond and functional group within a molecule has a tendency to fragment in a predictable manner, which skilled chemists can read much like a map, piecing together the molecular structure.

Isotope Masses

Isotope masses are important in mass spectrometry because isotopes of an element have slightly different masses. Since mass spectrometry is based on detecting the mass of ions, the presence of isotopes can influence the mass spectrum of a compound.

Most elements consist of a mixture of isotopes. For instance, carbon is predominantly composed of carbon-12, but it also contains carbon-13, which is a heavier isotope. This affects the analysis since a small peak at one mass unit higher than the main carbon peak usually indicates the presence of carbon-13 in the molecule.

These subtle differences in isotope masses enable chemists to determine not only the molecular weight but also provide insight into the elemental composition of the molecule. For organic molecules containing elements like chlorine or bromine, which have two major isotopes with significantly different masses, this can help confirm the presence of these elements within the molecule.