how to solve problems

mass spec

• this tells us the mass of the compound, number of carbons, number of hydrogens and other atoms
• find the base peak and the molecular ion peak
  • take special notice to these peaks:
    • Br:
    • Cl:
• use the rule of 13 to try and solve for initial ratio
• solve for units of saturation

IR

1. look for a carbonyl group (1880-1640)
   • amide (the C-N bond causes resonance, lowering the frequency)
     • 1700-1640
     • NH2: 3350-3180
     • NH: 3330
     • NH bend: 1640 - 1550
   • carboxylic acid
     • 1710
     • OH: 3400-2400
     • CO stretch: 1320 - 1110
   • ketone
     • 1715
     • C-CO-C: 1300-1100
   • aldehyde
     • 1725
     • aliphatic CH: 2850, 2760
   • ester
     • 1735
     • C-O stretch, two bands, 1300-1100
   • acyl chloride
     • C-Cl: 730-550
   • anhydride
     • band 1: 1810
     • band 2: 1760
2. look for alcohol, amine, nitrile
   • alcohol: 3400-3300 (broad), 3650-3600 (sharp)
     • CO stretch: 1260 - 1000
   • amine 3500-3300, CN stretch (1350-1000):
     • primary (NH2): 2 bands (1640-1510), NH bend (800)
     • secondary (NH): 1 band 1500, NH bend (800)
     • tertiary (N): no band, just CN stretch (1350-1000)
3. triple bonds
   • CN: 2250
   • CC: weak/sharp near 2150 and acetyle CH near 3300
4. double bonds: 1650
5. nitro group
6. review CH groups
   • 3000ish sp3

H NMR

C NMR

questions

• empirical vs. molecular
• index of hydrogens
• rule of thirteen

tutorials

Elemental Analysis and IHD (review) (Tutorial 1)

Learning Objectives:

• empirical vs. molecular formula
  • calculate and distinguish
• identify components of elemental analysis equipment
• define Index of Hydrogen δ or IHD
• calculate IHD from:
  • chemical structure
  • molecular formulae
  • amu
• rule of 13
• provide possible structures using provided and calculated IHD

NMR:

• Identify regions of the electromagnetic spectrum and identify which characterization methods exploit each specific region (in the context of CHEM 245)
• Explain nuclear spin in the context of NMR
• Distinguish between alpha (α) and beta (β) spin states
• Predict which spin state (α or β) is the more populated
• Describe how irradiation produces an NMR signal (simply)
• Define chemical shift in terms of electronic and magnetic environments
• Identify and use the terms “upfield” and “downfield”
• Identify and use the terms “shielded” and “deshielded”

—

• historically: incinerated compound in presence of O₂
  • canisters caught the oxidized versions of the compound
• types of EA
• empirical vs. molecular formula
• problem types:
  • find empirical formula based on C_xH_yO_z sample submitted for analysis
    1. write out balanced combustion equation
    2. calculate mmol of atomic elements present in sample based on given H₂O and CO₂ (combustion products)
    3. find mass of atomic elements present in sample
    4. calculate percent weight of atomic elements present in sample
    5. check if precents sum to 100% (or if don’t there is oxygen in sample)
    6. assume 100g of sample, % is g of atomic elemental
    7. divide through by lowest mole to find empirical formula
  • find molecular formula given empirical formula: MF = XEF
  • $$IHD = \frac{\# \text{ H saturated compound} - \# \text{ H unsaturated compound}}{2}$$
    • group V: add one H
    • group VI: no change
    • group VII: subtract one H
    • problem types:
      • calculate IHD:
        • check for molecules from the mentioned groups. apply proper addition or subtraction of saturated hydrogens
    • rule of 13: high resolution mass spectroscopy (HRMS) gives us mass in amu
      • $$\frac{M}{13} = n + \frac{r}{13}$$
        • C_nH_n + r and $$IHD = U = \frac{n-r+2}{2}$$
        • steps to solve problem:
          1. use rule of 13 calculate base molecular formula and IHD by dividing the given amu by 13 and setting the fraction part as r
          2. adjust base molecular and IHD for addition of other molecules if required.
             • subtract from the base molecular formula
             • add to IHD/U

Rule of 13, Nuclear Magnetic Resonance (NMR) Spectroscopy

Fundamentals (Tutorial 2)

• spectroscopy: study of interaction between matter and EMR
• NMR: interaction of EMR and nucleus of atom
  • $¹³$C and $¹$H
• NMR active nuclei: odd mass or odd atomic number
  • quantized spin angular momentum m_s and magnetic momentum
  • nuclear spin quantum number I: $¹³$C: 1/2 and $¹$H: 1/2
  • allowed spin states: 2I + 1
  • in absence of magnetic field, all spin states are degenerate
• nuclear spin in an applied field
  • when charged particle spins, creates magnetic field
  • spin states no longer degenerate in applied magnetic field
    • aligned: lower energy
    • oppsing: higher energy
• alpha/beta spin states
  • alpha: aligned with field (lower energy)
  • beta: opposing the field (higher energy)
  • when external magnetic field applied, degenerate spin states split into two states, lower energy state is aligned with magnetic field and higher energy state
    • energy gap increases with increasing magnetic field strength

Chemical shift, ¹³C (Tutorial 3)

• how many α and β will there be?
  • boltzmann distribution
  • need nuclei (alpha or beta) to be in excess to see any signal
    • excess nuclei depends on how much sample and how strong the external magnetic field is
• larmor frequency: the frequency that a nuclei begins to precess it’s own axis
  • nuclei wobble in presence of applied magnetic field
  • larmor frequency is unique to every nuclei
• how does NMR occur?
  • nuclei aligned with an applied magnetic field absorb energy, flip and relax back down
  • we only see nuclei in excess
• what does NMR tell us
  • chemical shift: chemical environment
  • multiplicity: neighbouring NMR active nuclei
  • integration: # eq protons
  • coupling constants: type of interactions with other nuclei of same type
• what does sheilding/deshielded mean?
  • valence electrons circulate in applied magnetic field
  • the induced magnetic field from electrons opposes applied field
  • more electron density -> more sheilded (right)
  • less electron density -> more deshielded (left)
• $¹³$C NMR Table
  • 0 - 50 ppm: saturated carbon (no EWG)
  • 25 - 80 ppm: saturated carbon (EWG)
  • 60 - 80 ppm: alkyne
  • 100 - 180 ppm: unsaturated carbon sp², aromatic ring
  • 150 - up ppm: carbonyl ring, aldehyde, ketone
• tips for $¹³$C NMR
  • methyl groups sheild neighbouring Cs
  • the more substituted a carbon is, the more deshielded it becomes
  • ppm increases from electronegativity, increasing number EWG, decreasing distance between carbon and EWG
• why is alkyne more sheilded than alkene? anisotropy
  • EWG remove electron density
  • in presence of applied magnetic field, electrons in pi bonds circulate, creating induced magnetic field
  • areas are more shielded or more deshielded than expected
  • (less sheilded) sp2 with EWG/sp2 < sp < sp3 (more sheilded)

¹H, Integration, Spin-Spin Splitting (Tutorial 4)

1. Learning Objectives:

   • Identify the chemical shift (ppm) for various functional groups in 13C NMR
   • Identify a potential structure based on the number of signals
   • Identify a possible structure using a combination of spectroscopic

   techniques, including 13C NMR spectroscopy

   • Identify the sub-spectra of DEPT NMR spectroscopy and what

   connectivity information is present

   • Identify possible structure(s) using a combination of spectroscopic

   techniques, including DEPT NMR spectroscopy

   —

   • downfield == deshielded == low electron density
   • upfield == sheilded == high electron density
   • alkyne is more upsheild than alkene and benzene
   • most downfield is carbonyl, aldehyde, ketone
   • ¹²C is most adundant isotype of carbon, but is not NMR active
     • ¹³C: has 1.1% natural abundance
   • what is TMS: (CH$₃)₄$Si, reference to 0
   • what is proton-decoupled: element coupling
   • DEPT NMR: signals display different phases depending on number of hydrogens attached to carbon
     • DEPT-135: positive (CH3, CH), negative: (CH2)
     • DEPT-90: positive (CH)
     • DEPT-45: position (CH3, CH2, CH)
     • $¹³$C{$¹$H} NMR: all signals

¹H, J-coupling constants, multiplet skewing, exchangeable protons (Tutorial 4/5)

1. Learning Objectives

   • Identify a potential structure based on the number of signals, splitting

   patterns and/or J-coupling constants

   • Construct a target compound from 1H NMR spectrum
   • Calculate J values from 1H NMR spectrum
   • Correctly use short forms of NMR in presenting data in laboratory reports
   • Identify exchangeable protons
   • Explain why deuterated solvents are used in NMR spectroscopy
   • Identify deuterated solvents used in NMR spectroscopy

   —

   • what does ¹H NMR spectroscopy tell us: chemical shift, number of signals, integrals/integration, spin-spin splitting, J-coupling constants
   • integrals and integration: relative number of equivalent protons
     • area under each peak proportional to number of Hs generating that peak
   • j coupling constant: how much are you interesting with other protons?
   • spin-spin coupling: are you seeing other protons?

2. (Tutorial 5 start)

   • signals split via 2nl + 1, l = 1/2 for H
   • equivalent protons do not split each other
   • protons need to be 2-3 bonds away from each other to split each other (sigma)
   • H NMR signals split according to pascal’s triangle
   • report centre of signal for chemical shift
     • multiplet: report range of chemical shift
       • how does spin-spin splitting work?
         • protons have spin; neighbouring protons can sense the spin of other protons
         • in an external magnetic field B₀, spin of protons H_a and H_b will either:
           • both align with field: H_a will shift downfield from “uneffected” position because H_b deshields our signal
           • one align with field, other is against field: H_a will shift upfield from “uneffected” position because H_b shields our signal
             • probability based: half of H_A shifts left, other half shifts right, so we get doublet
   • for ethyl group (ethyl iodide): 3 Hs on ethyl can arrange as: 3 (1/2), 3 (-1/2), 1 (1/2) 2 (-1/2), 1 (-1/2) 2 (1/2)
     • this is why there is a splitting of 4, with height of signals corresponding to degeneracy

   

   • pascal triangle tells us relative height
   • CDCl₃: 1:1:1 triplet
     • don’t read D
     • only want H from sample and H from solvent
     • solubility
     • also other deuterated solvents
   • J-coupling constant: show how strongly nuelci affected by neighbours
     • J-constants of groups that split each other must be identical: (difference between splitted signals) between groups
       • J depends on MHz of instrument: ppm * MHz
     • useful for identifying sterochemistry, such as alkenes (trans or cis)

infrared spectroscopy

EM spectrum, FT-IR spectrometer, samples, molecular vibration modes

Bond properties, Bond strengths, Alkanes, Alkenes, Alkynes

Arenes, Alcohols, Amines, Carbonyl compounds, Chlorides

mass spectroscopy

Rule of 13, spectrometer, ionization techniques

spectra analysis, isotope identification

Characterization Problem Solving

synthesis

Nickel Complex

[1,1’-biphenyl]-4-carboxylic acid

Acetylcinnamic acid

4-Methoxybenzaldehyde

Tripheylmethanol