Chapter 10: Problem 24
Assume that you are in a laboratory carrying out the catalytic hydrogenation of cyclohexene to cyclohexane. How could you use mass spectrometry to determine when the reaction is finished?
Short Answer
Expert verified
Use mass spectrometry to observe when the peak for cyclohexene (m/z = 82) disappears, leaving only cyclohexane (m/z = 84) peak.
Step by step solution
01
Understand the Reaction
Catalytic hydrogenation of cyclohexene to cyclohexane involves the addition of hydrogen (H2) across the double bond in cyclohexene, converting it into cyclohexane.
02
Identify Compounds by Mass
Determine the molar mass of cyclohexene and cyclohexane. Cyclohexene has a molar mass of about 82 g/mol, and cyclohexane has a molar mass of about 84 g/mol. This difference is due to the addition of hydrogen.
03
Mass Spectrometry Setup
Prepare the samples for mass spectrometry analysis. Inject the sample into the mass spectrometer to obtain the mass spectra before, during, and after the reaction.
04
Analyze Mass Spectrum Peaks
In the mass spectrometer output, look for peaks corresponding to molecular ions of cyclohexene (m/z = 82) and cyclohexane (m/z = 84). The peak at m/z = 82 will decrease and the peak at m/z = 84 will increase as cyclohexene is converted to cyclohexane.
05
Determining the Reaction Endpoint
The reaction is considered complete when the peak at m/z = 82, representing cyclohexene, is no longer present, indicating it has been fully converted to cyclohexane. Consequently, only the peak at m/z = 84, representing cyclohexane, should be visible.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Catalytic Hydrogenation
Catalytic hydrogenation is a chemical reaction where hydrogen molecules are added to another molecule, typically one containing a double bond. This process requires a catalyst, usually a metal like palladium, platinum, or nickel, to help the hydrogen molecules bond with the compound. In the context of converting cyclohexene to cyclohexane, the catalyst facilitates the addition of hydrogen to the carbon-carbon double bond present in cyclohexene, thereby transforming it into the fully saturated cyclohexane.
A catalyst works by providing an alternative path for the reaction that lowers the energy barrier needed for the reaction to occur. This makes the reaction more efficient and faster. In laboratory settings, careful selection of the catalyst type and conditions (like temperature and pressure) helps in optimizing the hydrogenation process.
Hydrogenation is widely used in various industries, including food (for hydrogenating vegetable oils), pharmaceuticals, and petrochemicals. In these fields, understanding the precise reaction conditions is crucial to obtain desired products efficiently and safely.
A catalyst works by providing an alternative path for the reaction that lowers the energy barrier needed for the reaction to occur. This makes the reaction more efficient and faster. In laboratory settings, careful selection of the catalyst type and conditions (like temperature and pressure) helps in optimizing the hydrogenation process.
Hydrogenation is widely used in various industries, including food (for hydrogenating vegetable oils), pharmaceuticals, and petrochemicals. In these fields, understanding the precise reaction conditions is crucial to obtain desired products efficiently and safely.
Cyclohexene to Cyclohexane Conversion
The conversion of cyclohexene to cyclohexane is a typical example of a hydrogenation reaction where a double bond in cyclohexene is saturated through the addition of hydrogen, turning it into cyclohexane. This reaction is characterized by a gain of two hydrogen atoms and a loss of a double bond.
The molecular formula for cyclohexene is C6H10, and when it is hydrogenated, it gains two hydrogen atoms to become cyclohexane, with a formula of C6H12. This molecular transformation results in a slight increase in molecular mass from 82 g/mol to 84 g/mol, which is significant for detecting the reaction's progress using analytical techniques like mass spectrometry.
This conversion not only demonstrates the principle of hydrogenation but also allows students and professionals to observe how double bonds in organic compounds can be converted to single bonds, effectively altering the chemical properties and reactivity of the molecule involved.
The molecular formula for cyclohexene is C6H10, and when it is hydrogenated, it gains two hydrogen atoms to become cyclohexane, with a formula of C6H12. This molecular transformation results in a slight increase in molecular mass from 82 g/mol to 84 g/mol, which is significant for detecting the reaction's progress using analytical techniques like mass spectrometry.
This conversion not only demonstrates the principle of hydrogenation but also allows students and professionals to observe how double bonds in organic compounds can be converted to single bonds, effectively altering the chemical properties and reactivity of the molecule involved.
Molecular Ion Peaks
Molecular ion peaks are essential features in mass spectrometry that help identify and quantify chemical compounds in a sample. These peaks represent ions formed by the removal of an electron from a molecule, therefore showing the molecular weight of the analyte. The significance of these peaks in the catalytic hydrogenation of cyclohexene to cyclohexane is immense.
In mass spectrometry, the molecular ion peak for cyclohexene appears at m/z = 82, while for cyclohexane, it appears at m/z = 84. By monitoring these peaks during the hydrogenation process, it is possible to determine when cyclohexene is fully converted to cyclohexane. As the reaction progresses, the intensity of the m/z = 82 peak diminishes, while the m/z = 84 peak becomes more pronounced.
Thus, the disappearance of the cyclohexene peak and the predominance of the cyclohexane peak serve as indicators that the reaction has reached completion. This ability to monitor changes in molecular ion peaks is invaluable in chemical analysis, confirming the formation of products and ensuring the reaction process has been successful.
In mass spectrometry, the molecular ion peak for cyclohexene appears at m/z = 82, while for cyclohexane, it appears at m/z = 84. By monitoring these peaks during the hydrogenation process, it is possible to determine when cyclohexene is fully converted to cyclohexane. As the reaction progresses, the intensity of the m/z = 82 peak diminishes, while the m/z = 84 peak becomes more pronounced.
Thus, the disappearance of the cyclohexene peak and the predominance of the cyclohexane peak serve as indicators that the reaction has reached completion. This ability to monitor changes in molecular ion peaks is invaluable in chemical analysis, confirming the formation of products and ensuring the reaction process has been successful.