Orgo 12.13-12.15

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  1. __ gives info about the __ in a molecule, but it tells little about the size of the molecule or what __ are present. To determine a structure, we need a __ and __. __ were once obtained by careful analysis of the __, and a molecular weight was determined by __ or some other difficult technique. These are long and tedious processes, and they require a large amount of pure material. Many important compounds are available only in small quantities, and may be impure.
    • infrared spectroscopy
    • functional groups
    • heteroatoms
    • molecular weight
    • molecular formula
    • molecular formulas
    • elemental composition
    • freezing-point depression
  2. __ provides the molecular weight and valuable info about the molecular formula, using a very small sample. __ can provide an accurate molecular formula, even for an impure sample. The mass spectrum also provides structural info that can confirm a structure derived from __ and __.
    • mass spectrometry
    • mass spectrometry
    • NMR
    • IR spectroscopy
  3. __ is fundamentally different from __. __ involves the absorption (or emission) of light over a range of wavelengths. __ does not use light at all. In the mass spectrometer, a sample is struck by __, breaking the molecules apart. The masses of the fragments are measured, and this info is used to reconstruct the molecule. The process is similar to analyzing a vase by shooting it with a rifle, then weighing all the pieces.
    • mass spectrometry
    • spectroscopy
    • mass spectrometry
    • high-energy electrons
  4. A __ ionizes molecules in a high vacuum, sorts the ions according to their masses, and records the abundance of ions of each mass. A __ is the graph plotted by the mass spectrometer, with the masses plotted as the x axis and the relative number of ions of each mass on the y axis. Several methods are used to ionize samples and then to separate ions according to their masses. The most common technique for forming the ions is __ and the best for separating the ions is __.
    • mass spectrometer
    • mass spectrum
    • electron impact ionization
    • magnetic deflection
  5. In the __, the sample is bombarded by a beam of electrons. When an electron strikes a neutral molecule, it may ionize that molecule by knocking out an additional electron. When a molecule loses one electron, it then has a __ and __.The ion is therefore a __.
    • ion source
    • positive charge and one unpaired electron
    • radical cation
  6. Most carbocations have a __ with __ in its valence shell. The carbon atom has __ around it, and they bond it to __. This unusual cation is represented by the formula [CH4]+, with the + indicating the positive charge and the ⋅ indicating the __.
    • three-bonded C atom
    • 6 paired electrons
    • seven electrons
    • four other atoms
    • unpaired electron
  7. In addition to ionizing a molecule, the impact of an energetic electron may break it apart. This __ gives a characteristic mixture of ions. The __ corresponding to the mass of the original molecule is called the __, abbreviated __. The ions of smaller molecular weights are called __. Bombardment of ethane molecules by energetic electrons prdouces the molecular ion and several fragments. Both charged and uncharged fragments are formed, but only what? 
    • fragmentation
    • radical cation
    • molecular ion
    • M+
    • fragments
    • the positively charged fragments are detected by the mass spectrometer.
  8. Once __ and __ have formed a mixture of ions, these ions are separated and detected. The most common type of mass spectrometer separates ions by __. 

    After ionization, the positively charged ions are attracted to a __, which has a narrow slit to allow some of the ions to pass through. The ion beam enters an evacuated flight tube, with a curved portion positiioned between the poles of a large magnet. When a charged particle passes through a magnetic field, a __ bends its path. The path of a heavier ion bends less than the path of a lighter ion.
    • ionization
    • fragmentation
    • magnetic deflection
    • negatively charged accelerator plate
    • transverse force
  9. The exact radius of curvature of an ion's path depends on its __, symbolized by __. In this expression, __ is the __ and __ is its __ of the electronic charge. The vast majority of ions have a charge of __. 

    At the end of the flight tube is another slit, followed by an __ connected to an __. At any given magnetic field only ions of one particular mass are bent exactly the right amount to pass through the slit and enter the detector. The detector signal is proportional to the number of ions striking it. By varying the magnetic field, the __ scans through all the possible ion masses and produces a graph of the number of ions of each mass.
    • mass-to-charge ratio
    • m/z (or m/e)
    • mass of the ion
    • z
    • chartge in units 
    • +1
    • ion detector
    • amplifier
    • s[ectrpeter
  10. The __ usually plots the spectrum as a graph on a computer screen. In the printed __, all the masses are rounded to the nearest whole-number mass unit. The peaks are assigned abundances as percentages of the strongest peak, called the __. Notice that the __. It is simply the strongest peak, making it easy for other peaks to be expressed as percentages.
    • mass spectrometer
    • mass spectrum
    • base peak
    • base peak does not necessarily correspond to the mass of the molecular ion
  11. A molecular ion peak (also called a __) is observed in most mass spectra, meaning that a detectable number of molecular ions (M+) reach the detector without __. These molecualr ions are usually the particles of highest mass in the spectrum and (for compounds not containing __) the molecular ion usually has an even-numbered mass. The value of __ for hte molecular ion immediately gives the molecular weight of the compound. If no molecular ion peak is observed in the standard mass spectrum, the operator can use a gentler __. The energy of the electron beam can be decreased from the typical 70 electron volts to 20-25 eV, where much less __.
    • parent peak
    • fragmenting
    • nitrogen
    • m/z
    • ionization
    • fragmentation
  12. __ is combined with __ for routine analysis of mixtures of compounds, such as reaction mixtures or environmental samples. The __ uses a heated capillary column coated on the inside with silicon rubber to separate the components of teh mixture. A small amount is injuected into a heated injector, where a gentle flow of helium sweeps it into the column. As the sample passes through the column, the more volatile components move through the column faster than the less volatile components. The separated components leave the column at different times, passing through a transfer line into the ion source of the mass spectrometer, where the molecules are ionized and allowed to fragment.
    • mass spectrometry
    • gas chromatography
    • gas chromatograph
  13. Most __ systems use a __ to separate the ions. In a high vacuum, the ions pass down the length of four rods, which have varying voltages applied to them. The varying electric fields cause the ions to follow __, and only one mass reaches the detector at any instant. By scanning the voltages, a wide range of masses can be measured in less than 1 second. In this way, many __ are taken and stored on a computer disk as the components of the sample pass from the chromatograph column into the mass spectrometer. This powerful __ allows many components of a mixture to be separated by the __ and later identified by their __.
    • gas chromatograph-mass spectrometer
    • quadrupole mass filter
    • complex orbits
    • mass spectra
    • GC-MS combination
    • gas chromatograph
    • mass spectra
  14. Although __ usually show the particle masses rounded to the nearlest whole number, the masses are not really integral. The 12C nucleus is defined to have a mass of exactly __, and all other nuclei have masses based on this standard. 

    Determination of a molecular formula is possible using a __, one that uses extra stages of __ or __ to form a very precise beam and to detect particule masses to an accuracy of about 1 part in 20000. A mass determined to several significant figures using an __ is called an __. Although it is not really exact, it is much more accurate than the usual integral mass numbers. Comparing the exact mass with masses calculated by molecular formula makes it possible to identify the correct formula.
    • mass spectra
    • 12 atomic mass units
    • high-resolution mass spectrometer (HRMS)
    • electrostatic or magnetic focusing 
    • HRMS
    • exact mass
  15. Whether or not a __ is available, molecular ion peaks often provide info about the molecular formula. Most elements do not consist of a single isotope, but contain heavier isotopes in varying amounts. these heavier isotopes give rise to small peaks at higher mass numbers than the major __. A peak that is one mass unit heavier than the M+ peak is called the __; two units heavier, the __; and so on.
    • high-resolution mass spectrometer
    • M+ molecular ion peak
    • M +1 peak
    • M+2 peak
  16. In practice, there are __ at every __. These background peaks are often similar in intensity to the __, preventing an accurate measurement of the __ peak. __ is much more reliable. 

    Some elements are recognizable from __, however. A typical compound with no __, __, or __ has a small __ and an even smaller (or no visible) __. If a compound contains sulfur, the __ is larger than the __; about 4% of the M+ peak. If chlorine is present, the __ (containing 37Cl) is about a third as large as the M+ peak (containing 35Cl). If bromine is present, the M+ and M+2 ions have about equal abundances; the molecular ion appears as a doublet separated by two mass units, with one mass corresponding to 79Br and one to 81Br.
    • background peaks
    • mass number
    • M+1 peak x2
    • High=resolution mass spectrometry

    • molecular ion peaks
    • sulfur, chlorine or bromine
    • M+1 peak
    • M+2 peak
    • M+2 peak
    • M+1 peak
    • M+2 peak
  17. Iodine is recognized by the presence of the __ at __. This clue is combined with a characteristic 127-unit gap in the spectrum corresponding to loss of the iodine radical. __ gives an odd molecular weight, and usually gives some major even-numbered fragments. Stable compounds containing only C, H, and O have even molecular weights, and most of their major fragments are odd-numbered.
    • iodonium ion (I+)
    • m/z 127
    • Nitrogen (or an odd number of nitrogen atoms)
  18. In addition to the molecular formula, the __ provides structural info. An __with a typical energy of 70 eV has far more energy than needed to __ a molecule. The impact forms the __, and it often breaks a bond to give a __ and a __. The resultiing __ is observed by the __, but the __ is not accelerated or detected. We can infer the mass of the uncharged radical from the amount of mass lost from the molecular ion to give the observed __.
    • mass spectrum
    • electron
    • ionize
    • radical cation
    • cation 
    • radical
    • cation
    • mass spectrometer
    • uncharged radical
    • cation fragment
  19. This __ does not occur randonly; it tends to form the most stable fragments. By knowing what stable fragments result from different kinds of compounds, we can recognize structural features and use the mass spectrum to confirm a proposed structure.
    bond breaking
  20. The mass spectrum of __ shows several characteristics typical of __. Like other compounds not containing __, the molecular ion has a __, and most of the fragments are __. The __ corresponds to loss of an __, giving an __ and a __. The __ is not detected, because it is not charged and is not accelerated or deflected.
    • hexane
    • straight-chain alkanes
    • nitrogen
    • even numbered mass
    • odd-numbered
    • base peak (m/z 57)
    • ethyl group
    • ethyl radical
    • butyl cation
    • neutral ethyl radical
  21. A similar fragmentation gives an __ and a __. In this case, the __ is detected. __ gives a __ and a __. 

    Cleavage to give a __ and a __ is weak because the __ is less stable than a __. Cleavage to give a __ and a __ is not visible because the methyl cation is less stable than a substituted cation. The stability of the cation is apparently more important than the __of the radical, since a weak peak appears corresponding to loss of a methyl radical, but we see no cleavage to give a methyl cation.
    • ethyl cation
    • butyl radical
    • ethyl fragment (m/z 29)
    • symmetric cleavage of hexane
    • propyl cation
    • propyl radical

    • pentyl cation (m/z 71) 
    • methyl radical
    • methyl radical
    • stability
  22. __ and __ help to explain the mass spectra of branched alkanes as well. __ of a branched alkane commonly occurs at a branch carbon atom to give the most highly substituted cation and radical. Both __ give __, but the second gives a primary radical instead of a methyl radical. Therefore, the second fragmentation accounts for the base (largest) peak, while the first accounts for another large peak at __. Other fragmentations (to give primary cations) account for the weaker peaks.
    • cation and radical stabilities
    • fragmentation
    • fragmnetations
    • secondary cations
    • m/z 71
  23. Fragmentation in the mass spectrometer gives __ whenever possible. The most common fragmentation of alkenes is cleavage of an __ to give a __. 

    Compounds containing __ tend to fragment at the carbon (called a __) next to the __. Such a cleavage forms a __.
    • resonance-stabilized cations
    • allylic bond
    • resonance-stabilized allylic cation
    • aromatic rings
    • benzylic carbon
    • aromatic ring
    • benzylic cation
  24. ketones and aldehydes: __

    ethers: __

    amines: __
    • loss of alkyl groups to give acylium ions
    • alpha cleavage or loss of an alkyl group
    • alpha cleavage to give stabilized cations
  25. __ are often seen corresponding to loss of small, stable molecules. Loss of a small molecule is usually indicated by a __ with an __, corresponding to loss of an even mass number. A __ may lose water, CO, CO2, and even ethene, or other alkenes. The most common example is the loss of water from alcohols, which occurs so readily that the molecular ion is often weak or absent. The peak corresponding to loss of water (the M-18 peak) is usually strong, however.
    • mass spectral peaks
    • fragment peak
    • even mass number
    • radical cation
  26. In addition to losing water, alcohols commonly fragment next to the __ to give a __. This fragmentation is called an __ because it breaks the bond next to the carbon bearing the __.
    • carbinol carbon atom
    • resonance-stabilized carbocation
    • alpha cleavage
    • hydroxyl group
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Orgo 12.13-12.15
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