Chem 245 Flashcards

Qualitative / Quantitative Elemental Analysis

Qualitative EA: what type of atoms are present?

Quantitative EA: how much of each type of atom is/are present?

The combination of both provides the empirical formula

Empirical vs Molecular Formula

Empirical Formula: represents the simplest whole number ratios of elements present in a molecule

Molecular Formula: Chemical formula representing the exact number and types of each element in a single compound

• both don’t provide any structural or connectivity analysis

Calculating Empirical Formula

1. Write out balanced combustion equation

   1. C_xH_yO₂ + H₂O (excess) —> xCO₂ + y/2•H₂O

2. Calculate the mmol of the atomic elements present in the unknown sample

   1. 1:1 ratio for CO₂ and C, and 1:2 ratio for H₂O and H

3. Calculate the mass (in mg) of the atomic elements present in the unknown sample

4. Calculate the weight% of the atomic elements present in the unknown sample

5. Check to see if the weight % sums to 100%. If they don’t, the difference is the weight % of O

6. Assume you have 100g of material. The % now equals grams of each element. Calculate the moles of each element

7. Calculate the empirical formula by dividing through by the lowest mole value. Round to the nearest whole number

Nitrogen Rule

If the amu value (or M.W) is an odd number, you either have

• one nitrogen

• an odd number of nitrogen atoms (1,3,5,7…)

If the amu value (or M.W) is an even, you either have

• 0 nitrogen atoms

• an even number of nitrogen atoms (0,2,4,6…)

Index of Hydrogen Deficiency (IHD)

• look at how many H atoms are expected, then look at how many are actually present

• In saturated alkanes, the molecular formula should be C_nH_2n+2

• A unit of unsaturation is added for each multiple bond in a molecule or ring

• IHD of 4 or more, think aromatic ring

Heteroatom

Anything that’s not carbon or hydrogen

Index of Hydrogen Deficiency (IHD): What to add/remove for each heteroatom

If element included is in:

• group 5 (N-Bi), add one H to the molecular formula

• group 6 (O-Te), no change is required

• group 7 (F-I), subtract one H

Rule of 13

• Mass spec (HRMS) provides the exact mass (in amu)

• We translate n and r into C_nH_n+r and

  U= (n-r+2)/2

  • U= units of unsaturation

Gas Chromatography Mass Spectrometry (GC-MS)

• compounds vaporized into gas, carried through long columns using inert He gas (mobile phase)

  • inert: doesn’t react

• compounds interact differently with the column, resulting compounds eluting at various times (retention times)

• once these compounds elute, they undergo analysis using MS

Key information:

• Product formation: a new spot formed

• Reaction completion: loss of all starting material

• Side reactions: New spot thats not product or starting material

• reaction monitoring: how long does a reaction take? How may equivalents are required?

General Schematic of an MS

Sample Inlet

Pressure is atmospheric or vacuum depending on the next step

• depends on type of ionization

The sample:

• must be volatile at high temperatures

  • can’t put transition metals, their M.W is so heavy that it won’t transition from liquid to gas

• temperature must not decompose sample

Ionization Chamber

• the sample is introduced into the ionization chamber, at which point it is ionized (electron is dislodged, might be lone pair)

• the method of ionization differs depending on the requirements and/or the compound of interest

• ionization creates the molecular ion, which is unstable and can decompose

  • the most intense peak furthest to the right

• decomposition creates daughter or fragment ions. These create a fingerprint of the compound

  • uncharged or neutral (don’t see them)

M=neutral molecule

M^+•=molecular ion (radical cation)

e^(-)=electron

x=daughter or fragment ions

Electron Impact Mass Spectrometry (EI-MS)

• large potential difference in accelerating plates (1-10 kV)

• focusing slit provides a uniform beam of positive ions

• ionization potential for most organic molecules (8-15 eV)

• cheap and reliable method

Electrospray Ionization (ESI)

• for larger biomolecules

• solvent evaporates out of the droplets, decreasing in size, increasing the positive character

• as charges get closer, the droplets get smaller

• results in Coulombic explosions

• softer ionization technique (less harsh conditions)

• more reliable ionization (per molecular ion)

Mass Analyzers and Detectors

• only cations are detected in mass spectrometry

  • neutral ions are not detected

  • anions can be detected depending on the ionization mode used

• ions are separated based on their mass-to-charge ratio (m/z)

• z=1

• we don’t see larger ions because they don’t get affected by the negative plate and end up hitting the wall

Production of a Spectrum

• molecular ion peak can be the base peak

• molecular ions and/or fragment ions travel through the mass analyzer, and collide with the detector

• detector amplifies the signal

• computers transform the signal into a spectrum

• anything that’s not a molecular ion peak is a daughter fragment

• Aromatic ring at 77m/z

• M-15=loss of a methyl group

Fragmentation

• molecular ions are unstable, and fragment to provide daughter/fragment ions

• properly trained chemists can tease out a wealth of information from these fragmentation patterns

• electrons in lone pairs > electrons in π-bonds > electrons in sigma bonds

HRMS and Exact Mass

• HRMS provides an accurate measurement of mass (+0.00005%)

Isotopic Patterns in Mass Spec.

• one amu above molecular ion peak = M+1

  • two amu above the molecular ion peak = M+2

• what causes smaller peaks: isotopes

  • not generally important

  • Cl and Br have significant isotopic peaks and are important in analysis

M+2 Peaks with Cl and Br

• M+2 peak is important and diagnostic for Cl and Br

• there is significant isotopic abundance of these heavier isotopes of both Cl and Br

• the relative intensities between M and M+2 tell us whether Cl or Br is present

  • if M and M+2 are ~ 1:1 = Bromine

  • if M and M+2 are ~ 3:1 = Chlorine

Spectroscopy

The study of the interaction between matter and electromagnetic radiation

• electromagnetic radiation behaves like a wave and like a particle

• particle component termed ‘photons’

• Photon: a small, massless particle that contains a small wavepacket of EM radiation/light

IR Absorption Figure

Fourier Transform IR (FTIR) Figure

Sample Preparation: Problem, Glass and Plastics Absorb strongly in the IR region

Molecular Vibrations

• the quantum mechanical energy levels observed in IR spectroscopy, which we perceive as heat

• absorption of energy changes vibrational state

• covalent bonds naturally oscillate in a way that resembles an oscillating spring

• As a covalent bond oscillates, the dipole moment of a bond (molecule) will change as well

• This change in dipole moment causes an EM field to be produced

  • the weaker the dipole moment, the weaker the field

  • the stronger the dipole moment, the stronger the field

• molecules only absorb select frequencies. These frequencies match the natural vibrational frequencies

How can electromagnetic (EM) radiation interact with matter?

As atomic particles exhibit both wave and particle properties, EM radiation interacts with matter in 2 ways:

• collision

• coupling: wave property of the radiation matches the wave property of the particle, the waves couple to the next quantum energy level

Vibrational Modes: Stretching Vibrations

Vibrational Modes: Bending Vibrations

Bond Properties

• the force (F) needed to extend or compress a spring by some distance (X) is proportional to that distance

• F=kX, k is a force constant that is representative of the stiffness of the spring

• chemical bonds are like springs

• the stronger the bond, the greater the force constant (k)

• bonds between atoms of larger masses (µ) vibrate at lower frequencies

Bond Strength

sp CH > sp² CH > sp³ CH

Absorptions of CH and CC bonds

• frequency of absorption is a function of hybridization (tells us bond strength)

CH stretching

• less than 3000 cm^-1 usually indicates saturated sp³ CH

• in between 3000 cm^-1 to 3150 cm^-1 usually indicates sp or sp² (vinyl or aromatic) CH

• stronger bond = higher absorption

• exceptions to the rules:

  • Aldehydic: two bands 2900-2750 cm^-1

  • Cyclopropyl: ~3100cm^-1, strong absorption because of ring strain

Alkenes

Characteristic regions:

• sp² CH stretch ( >3000cm^-1)

• sp² CH oop (1000-650cm^-1)

• C=C stretch (1660-1600cm^-1)

Conjugation effects: have lower wavenumbers

OH stretch

• concentrated ~3300 cm^-1 (broad)

• ~3600cm^-1 (sharp) : dilute

• intramolecular hydrogen bonding will weaken the OH stretching vibration

  • more concentrated=more H-bonding=blob peak

Amines

• N-H stretching located in the same region as OH stretching

• substitution of the amine can be inferred from the observed absoprtions

• don’t see 3º amine, it has no H

Nitrile

• characteristic medium intensity absorption in the triple bond region

• C≡N: ~2250 cm^-1

• CN stretch: 1000-1350cm^-1

• Pitfalls:

  • C≡C bond: located near C≡N

  • conjugation

Carbonyl compounds

Characteristic absorption from 1650-1850cm^-1

• depends on structure the carbonyl is bound to

Factor 1: EWG and ⍺-haloeffect

• groups which remove e^- from the C=O will manifest as a higher frequency of absorption

Factor 2: Conjugation (resonance)

• carbonyls that are conjugated will show lower frequency of absorption when compared with an unconjugated molecule

Factor 3: H-bonding

• predominant in carboxylic stretch

• weakens the C=O bond stretch (lowering force constant (k))

  • gives lower absorption

Aldehydes

Characteristic regions

• 1725-1740 cm^-1

• pair of weak bands in CH (only seen when not obscured by CH region)

  • 2800-2860cm^-1

  • 2700-2760cm^-1

Esters

• need to be careful with our conjugation arguments

• if the carbon chain on the ether O can have resonance (ending up with a negative charge on the carbon) it will pull e ^- density and strengthen the absorption

Deuterated Solvents

• deuterated solvents are used prepare NMR samples for analysis

• the ¹H nuclei are NMR active. ²H (or D) are not NMR active

Nuclear Spin

• any atomic nuclei with either an odd mass or odd atomic number, or both will possess a quantized atomic spin

• the number of spin states allowed is defined:

  • 2nI + 1

    • n = nuclear spin of interest

    • I = physical constant

  • so, if we are interested in knowing the number of possible spin states for ¹H (nuclear spin quantum number = 1/2)

Nuclear Spin in an Applied Field

• when a charged particle (the nucleus) is spinning, it creates a magnetic field (µ)

• spin states are equivalent in energy in an applied magnetic field

Magnetic Fields

⍺ and β Spin States

• If the normally disordered magnetic moments are exposed to an external magnetic field, they will align into two forms

  • alpha (⍺) – aligned with magnetic field (β₀)

  • beta (β) – opposing β₀

• when an external magnetic field is applied (β₀), the degenerate spin states split into two states of unequal energy

Energy Absorption

• energy absorption is a quantized phenomenon

  • E_(absorbed) = (E_{-1/2 state} - E _{+1/2 state}) = hv

• the energy gap between each state increases as the applied magnetic field strength increases

Larmor Frequency

• in the presence of an applied magnetic field, a spinning nucleus begins to wobble

• a nucleus that precesses about its own axis, which also possesses angular frequency

• this is called the Larmor frequency (ω)

Chemical Shift (∂)

• electrons swirl around the central nucleus

• in an applied magnetic field, valence electrons are caused to circulate

• this generates a counter magnetic field which opposes the applied field

• lower energy difference between spin states

• lower energy requires to flip spin states

¹³C NMR – Chemical Shift

• the more substituted a carbon is, the more deshielded it becomes

• increasing electronegativity of nearby groups, the more deshielded

• increasing the number of nearby electronegative groups, the more deshielded

• decreasing distance between the carbon nuclei and nearby electronegative groups, the more deshielded

Chemical Shift – Anisotropy

Anisotropy: non uniform application of an electric field

• presence of EWG will remove electron density

• in the presence of an applied magnetic field, the electrons in π bonds begin circulate, causing an induced magnetic field

• areas are more shielded and more deshielded than expected

• e.g. Benzene: carbons are near the induced magnetic field with the applied magnetic field

  • causes a large deshielding event

Function of TMS

• NMR active nuclei will couple with nearby NMR active atoms

• to avoid this we use TMS (tetramethylsilane)

• TMS serves as a reference compound

¹³C NMR – Number of Signals

• ¹²C is the most abundant isotope of carbon (not NMR active)

• ¹³C has a natural abundance of ~1.1%

• our starting point for spectral interpretation

• the number of signals is determined by the number of non-equivalent carbon atoms present in the molecule

  • equivalent carbon nuclei are in the same chemical and magnetic environment

Downfield vs Upfield

DEPT NMR

• the result is a ¹³C NMR spectrum were signals display different phases depending on the number of hydrogen atoms attached

DEPT 135

Positive: CH₃, CH

Negative: CH₂

Importance: odd H’s up, even H’s down

DEPT 90

Positive: CH

Negative: N/A

Importance: only shows methine (only CH Carbons)

DEPT 45

Positive: CH₃, CH₂, CH

Negative: N/A

Importance: Quaternary carbons do not appear

¹H NMR

• chemical shift: shielded or deshielded

• number of signals: how many non-equivalent protons

• integrals/integration: how many protons are present

• spin-spin splitting: are they seeing other protons

• J-Coupling constants: distance between peaks (in Hz) of a particular signal?

• spectral window from 0 to 10ppm

Integrals & Integration

• method to quantify the relative number of equivalent protons

• divide each integration value by the smallest one to get the ratio

• the user sets the ratio (multiply the ratio if needed)

Spin-Spin Coupling

More complicated patterns exist when coupling constants of neighboring nuclei are different

• multiplet= (mult.)

Signal Splitting

• equivalent protons cannot split each other

• to split each other, H’s need to be 2-3 bonds away from one another

• n+1

• H’s part of a tert-butyl group show up as singlet

Pascal’s Triangle

• splitting patterns follow similarly to Pascal’s Triangle

• the number of signals a nuclei will exhibit depends on the number of active nuclei it is surrounded by (2-3 bonds away)

J-Coupling Constants

• protons influence other protons creating different environements

  • affects the splitting pattern

  • affects the size of the spliting

• our primary concern would be ³J_HH for most protons and ²J_HH for alkenes

• dependent on the MHz of the instrument used

  • difference in ppm x MHz of the instrument = J_coupling

Exchangeable Protons

• X-H protons will exchange via hydrogen bonding

• X-H protons do not split

• often appear as broadened peaks

Alkenes

The protons can’t rotate due to the double bond, and the splitting pattern is like a 1:1 doublet splitting pattern with some space in between

• is more deshielded when part of the alkene