chapter 12

Question 1

what is electromagnetic radiation?

Answer 1

• it is light, but visible light forms but a small portion of the electromagnetic spectrum

Question 2

what is infrared radiation?

Answer 2

• a form of electromagnetic radiation with a frequency just below that of visible light

Question 3

what is infrared spectroscopy (IR)?

Answer 3

• a technique that uses infrared light to vibrate covalent bonds in molecules and measures the vibrations to collect information about functional groups
  
  • the bonds that appear on an IR spectrum must have a dipole moment, such as the polar OH bond in water. In contrast, the Br-Br bond in Br₂ does not have a dipole and thus does not appear on an IR spectrum
    
    • the y-axis displays percent transmittance (the proportion of IR light transmitted through the sample), which is the reciprocal of absorbance. regions that have absorbed light will appear as valleys, not peaks, along the graph
    • the x-axis displays the frequency of light in a unit or measurement known as wavenumbers (cm^-1), with frequency increasing from right to left along the x-axis

Question 4

IR spectrum frequencies of select bonds

Answer 4

bond | wavenumber (cm^-1) | shape if applicable

• C=N, 1550-1650, -
• C=C, 1600-1680, -
• C=O, 1650-1780, sharp
• C triple bond C, 2100-2260, -
• C triple bond N, 2220-2260, -
• O-H, 3200-3600, broad
• N-H, 3300-3500, -

Question 5

unlike IR, which identifies the bonds in a molecule, nuclear magnetic resonance (NMR) spectroscopy aims to?

Answer 5

• characterize a molecules atoms
  
  • places a sample in a magentic field. Of the sample has a nuclear spin due to an odd number of protons or neutrons, then it will be affected by the magnetic field. The atomic nuclei will allign with the field (a lower energy state) or against the field (a higher energy state). the frequency of transition between these states, or resonance, is measured by NMR.
    
    • H (proton) NMR

Question 6

what are resonance frequencies?

Answer 6

• chemical shifts ranging from zero on the far right to positive on the far left
  
  • a shift of zero is arbitrarily assigned to the peak corresponding to tetramethylsilane (TMS) as a reference point

Question 7

what occurs in proton NMR?

Answer 7

• each hydrogen atom (or set of equivalent atoms) produces a single peak on teh spectrum
  
  • the peak area (or integration) directly correlates to the number of protons represented by that peak
  • hydrogen atoms being shielded, or blocked, from experiencing the full extent of the magnetic field have peaks closer to 0ppm

Question 8

proton NMR chemical shifts:

Answer 8

functional group | shift (ppm)

• CH_3, 0.9
• CH₂OH, 1.0-5.5
• C triple bond CH, 2-3
• CH triple bond CH, 4.5-6
• Ar (aromatic ring)-H, 6-8.4
• -CHO, 9.5
• COOH, 10.5-12

Question 9

to predict splitting patternsm follow the?

Answer 9

• n + 1 rule
  
  • states that any peak will be split into a number of smaller peaks equal to the number of adjacent hydrogen atoms plus 1

Question 10

what is UV-Vis spectroscopy?

Answer 10

• a technique that analyzes the absorbance of visible or UV light by organic compound
  
  • since frequency and wavelength are inversely proportional, UV light also posesses a shorter wavelength than visible light but a higher frequency
  • On a UV-Vis spectrum, the absorbance, or the proportion of light abosrbed by the sample, is plotted against the wavelength of this light. conjugate compounds with alternating single and double bonds tend to readily absorb UV-Vis light due to the delocalization of pi electrons

Question 11

what is mass spectrometry?

Answer 11

• useful for isentifying unknown organic compounds
  
  • in this technique, a sample is injected into a mass spectrometer, which exposes the molecules to a beam of electrons. these high energy electrons cause the sample to ionize or become charged. removal of one electron from the parent molecule creates the M⁺ cation, which has the same molecular weight as the uncharged parent molecule. sample molecules also fragment into a variety of smaller charged specoes. the mass and charge of the fragments are measured and plotted on a mass spectrum
    
    • The peak of the highest m/z value is usually the M⁺ peak which lets us know the molecular weight of the unknown molecule.

Question 12

how can you isolate the single product that you actually wish to analyze?

Answer 12

• using separation techniques

Question 13

what is the use of distilation

Answer 13

• separating one liquid from another by utilizing the difference between the two liquids boiling points

Question 14

what is simple distilation vs fractional distilation?

Answer 14

• simple distilation is used wehn the liquids in question have very dissimilar boiling points (at least 25 degrees C apart)
• fractional distilation is utilized when the compounds to be separated have boiling points seperated by less then 25 degrees apart

Question 15

what is vacuum distillation?

Answer 15

• boiling point occurs at the temp at which the vapor pressure is eqaul to the atmospheric pressure of the liquid in question
  
  • used when we cannoy raise the temperature (which raises the vapor pressure) so we decrease the atmospheric pressure instead to meet the vapor pressure. this can be accomplished by attaching a vacuum to the distillation apparatus. Lowering the ambient pressure of the surroundings results in a decreased b.p. for all compounds involved, allowing us to then conduct a typical distilation procedure

Question 16

compounds with disparate solubility characteristics are most often separated using?

Answer 16

• liquid-liquid extraction
  
  • uses a separatory funnel and take advantage of acid-base characteristics
  • aqueous = polar
  • organic = nonpolar layer
    
    • add strong base or acid if separating an acid and base
    • use a strong base to separate 2 acids

Question 17

what is recrystallization?

Answer 17

• the desired solid product is placed in a liquid solvent and heated, causing it to dissolve. upon dissolution, the solvent-product mixture is cooled and the solid once again forms as a precipitate
  
  • an ideal recrystalization solvent is one in which our desired product is highly soluble at hight temps. at the same time, however, our product should be relatively insoluble in this solveny under low-temp conditions, or it will never precipitate back out of the solution when the mixture is cooled
    
    • once we are satisfied with our solid product, we need to isolate it form the liquid solvent from which it is help using filtration
      
      • separates a solif product from unwanted fluids so we set up a filter that the liquid can pass through