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Organic Chemistry 2 > Exam 1 > Flashcards

Exam 1 Flashcards

1
Q

Spectroscopy

A

The measurement of light (or electromagnetic radiation)

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2
Q

λ

A

Wavelength

One complete wave cycle

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3
Q

ν

A

Frequency

The number of cycles per time period

ν = c / λ

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4
Q

ν ~

A

Wavenumber

ν ~ = 1 / λ

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5
Q

E

A

Energy

E = hν = hc / λ

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6
Q

UV, X Rays, Gamma Rays +

A

Ionization

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7
Q

Visible, near UV

A

Electronic Transitions

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8
Q

Infrared (IR)

A

Molecular Bond Vibration

** IR Spectroscopy **

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9
Q

Microwave

A

Rotational Molecule Motion

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10
Q

Radio

A

Nuclear Spin Transitions

** NMR Spectroscopy **

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11
Q

What is the function of IR Spectroscopy?

A

To identify functional groups on an unknown molecule by looking at differences in molecular vibrations.

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12
Q

What determines the X and Y position s of peaks in IR spec?

A

X: Determined by Beer’s Law ( A = ɛ c l) and Bond Polarity (more polar bonds are more deshielded = higher ppm) that change the frequency

Y: Determined by the mass of atoms in the bond (heavier atoms = higher ppm) and the bond strength (stronger bonds are more shielded = lower ppm)

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13
Q

Affect of Resonance on IR Spec Peaks

A

Broaden peaks

Lowers frequency = higher ppm

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14
Q

Affect of Hydrogen Bonding on IR Spec Peaks

A

Broadens peaks

(ex: O-H peak)

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15
Q

What happens with symmetrical molecules in IR Spectrums?

A

Not seen because a dipole moment is needed to see a peak in IR

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16
Q

Shielding

A

More electron density = less interaction with magnetic field = lower ppm = upfield = more shielded

Less electron density = more interaction with magnetic field = higher ppm = downfield = less shielded (deshielded)

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17
Q

What is required for a bond to be IR active?

A
  1. Applied radiation E needs to match the energy of a bond stretching or bending.
  2. Needs to result in a change in molecular polarity.
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18
Q

What is Beer’s Law?

A

A = ϵ c l

A = abosrption (intensity of peak on Y-axis)

ϵ = molar absorptivity constant (change in polarity)

c = concentration

l = path length (thicker sample = more absorption)

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19
Q

IR Template

A
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20
Q

Ester / Ether

C - O Stretch

A

1200 - 100

Strong peaks

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21
Q

Aromatic Ring

C = C Stretch

A

1700 - 1500

Variable peak with Overtones to the Left

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22
Q

Alkene

C = C Stretch

A

1680 - 1620

Variable Peaks

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23
Q

Ester

C = O Stretch

A

1750 - 1735 cm-1

~1735 cm -1

Strong Peak

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24
Q

Aldehyde

C = O Stretch

A

1740 - 1690 cm-1

~1725 cm-1

Strong Peak

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25
Q

Ketone

C = O Stretch

A

1750 - 1680 cm-1

~1710 cm-1

Strong Peak

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26
Q

Carboxlyic Acid

C = O Stretch

A

1780 - 1710 cm-1

~1710 cm-1

Strong Peak

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27
Q

Amide

C = O Stretch

A

1690 - 1630 cm-1

~1650 cm-1

Strong Peak

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28
Q

Alkyne

R - C ≡ C Stretch

A

2260 - 2100

Variable Stretch

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29
Q

Nitrile

R - C ≡ N Stretch

A

2260 - 2220

Medium Peak

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30
Q

Alkyl sp3

C - H Stretch

A

2950 - 2850

Jagged Strong Peaks

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31
Q

Alken sp2

C - H Stretch

A

3100 - 3010

Medium Peaks

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32
Q

Carboxlyic Acid

O - H Stretch

A

Centered ~3000

Broad, Strong/Variable Peak

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33
Q

Alcohol

O - H Stretch

A

3550 - 3200

Very Broad, Strong Peak

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34
Q

Amine

R - N - H Stretch

A

3500 - 3300

Medium Peak

** One Peak w/One Terminal H, Two Peaks w/Two Terminal H’s **

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35
Q

Amide

N - H Stretch

A

3700 - 3500

Medium Peaks

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36
Q

Alkyne

R - C ≡ H Stretch

A

~3300

Variable Peak

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37
Q

What information is provided by mass spectrometry?

A
  1. Molecular weight of compound
  2. High resolution MS = gives molecular formula
  3. Some bond connectivity information (mroe from ‘H NMR)
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38
Q

What is being measured in MS?

A

The mass of ions (generally cations)

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39
Q

Cation Generation

A

Electron Impact (EI) Ionization

** Creates the radical cation represented in M+ Peak **

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40
Q

Easiest Electrons to Ionize

A
  1. Lone Pair electrons
  2. Pi electrons (triple/double bonds)
  3. Sigma electrons (single bonds)
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41
Q

What does Fragmentation Depend On?

A
  1. Carbocation Stability: the compound will fragment at the spot that creates the most stable C+ firstmost

** Substitution, Induction Effect, and Hyperconjugation affect C+ stability **

  1. Radical Stability: when mutliple stable C+ are possible, then the more stable radical will be made secondarily
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42
Q

Carbocation Stability

A
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43
Q

Why is MS often Coupled with GC?

A

GCMS provides rapid indication of a reaction

(GC = separation into individual components)

44
Q

What is the molecular ion peak?

A

The peak furthest to the right that correlates to the molecular weight of your compound

** The unfragmented radical cation **

45
Q

What is the base peak?

A

The largest peak in the spectrum that represents the most stable carbocation

46
Q

Alcohol fragmentation

A
  1. Alpha fragmentation (forming a carbonyl)
  2. Kick off β-Hydrogen (protons shift to create water that will leave)
47
Q

What is the difference between Low Resolution (LR) and High Resolution (HR) mass spectrometry?

A

LR: nominal (integer) masses

HR: Masses go to the fourth decimal place, provides the molecular formula

48
Q

Br Isotope Pattern

(Mass Spec)

A

M+ ≈ M+2

49
Q

Cl Isotope Pattern

(Mass Spec)

A

M+2 ≈ 1/3 M+

50
Q

I Isotope Pattern

(Mass Spec)

A

Peak at 127

M+ 127 away from next peak

51
Q

S Isotope Pattern

(Mass Spec)

A

M+2 > M+1

** Very subtle because both peaks are VERY small **

52
Q

N Isotope Pattern

(Mass Spec)

A

A compound with an odd number of Nitrogens with have an odd nominal mass, so M+ will be on an odd number

53
Q

What information is provided by ‘H NMR?

A
  1. The number of hydrogen environments in a structure (the number of peaks)
  2. The number of hydrogens in each environment (integration of area under the peaks)
  3. Functionality in compound (chemical shift)

** 4. Provides connectivity information (spin-spin splitting - how many hydrogens are interacting)

  1. Provides stereochemistry information (coupling constants differentiate between cis and trans)
54
Q

What information is provided by 13C NMR?

A
  1. The number of carbon environments (the number of peaks)
  2. Functionality in compound (chemical shift)
  3. The number of hydrogens attached to each carbon (** when using off-resonance or DEPT 13C NMR **)
55
Q

Which nuclei have a nuclear spin?

A
  1. Odd number of protons (odd atomic number)

OR

  1. Odd number of protons and neutrons (odd mass number)
56
Q

How can you increase NMR peak resolution?

A

You can increase the radio frequency or teh external magnetic field to increase the amount of nuclei with an α spin (lower energy, bigger gap = more resolution).

57
Q

What challenges delayed the development of 13C NMR?

A
  1. Isotope Abundance: there’s only 1/100 carbons measurable (H: 100% abundance 1H; C: 99% abundance 12C and 1% abundance 13C)
  2. Low gyromagentic ratio = low energy gap = low NMR resolution (γ 13C = 1/4 γ 1H using ΔE = γ (h/2π) B0)
58
Q

What are the method for acquiring 13C NMR data?

A
  1. Continuous Wave (CW): fixed magnetic field and nuclei that are gradually scanned from low frequency to high frequency (** slow **)
  2. Foureier Transform (FT): a pulse of radiation that excites all the nuclei at once, causing them to precess. Then the relaxation to ground state is measured (by a computer program), creaing a FID that’s translated into meaningful data (** fast **)
59
Q

What is spin-spin splitting?

A

Protons within 2-3 bonds of each other can effect the chemical shifts of the neighboring environments

Ex: If Ha has an α spin, the chemical shift of Hb is slightly increased. If Ha has a β spin, the chemical shift of Hb is slightly decreased - creating 1:2:1 ratio (triplet peak)

60
Q

What do EWG and EDG do to chemical shifts?

A

EWG take electron density away from the protons, deshielding them more than usual, so their chemical shifts move upfield (gets bigger)

**Ex: Aromatic rings with EWG move from one peak at ~7.5 to two peaks between 8 - 7.5

EDG give electron density to the protons, shielding them more than usual, so their chemical shifts move downfield (gets smaller)

**Ex: Aromatic rings with EDG move from one peak at ~7.5 to two peaks between 6-7.5

** The effects of these groups drop off quickly with distance = strongly changes α hydrogens and softly changes β hydrogens **

61
Q

CH3 Alkane Chemical Shift

(NMR)

62
Q

CH2 Alkane Chemical Shift

(NMR)

A

1.3

(0.5 - 1.5)

63
Q

CH Alkane Chemical Shift

(NMR)

A

1.4

(1 - 1.9)

64
Q

Methyl Ketone + Any Protons α to a Carbonyl

(NMR)

A

2.1

(1.5 - 2.5)

65
Q

C ≡ C - H (Hydrogen Bond) Chemical Shift

(NMR)

A

2.5

(2 - 3)

66
Q

Aldehyde Chemical Shift

(NMR)

67
Q

Ester (ROOCR) Chemical Shift

(NMR)

68
Q

R-CH2-X (X = halogen) Chemical Shift

(NMR)

69
Q

Ether (RO-CH2-R) Chemical Shift

(NMR)

70
Q

ROOC-CH2-R Ester Chemical Shift

(NMR)

71
Q

Vinyl C = C - H Chemical Shift

(NMR)

A

5 - 6

** Vinyl = directly next to double bond, on double bonded carbon **

72
Q

Allylic C = C - CH3 Chemical Shift

(NMR)

A

1.7

** Allylic = on carbon bonded to the double bond carbon, one bond away **

73
Q

Aromatic Ph - H Chemical Shift

(NMR)

A

7 - 8

(~7.5)

74
Q

Benzylic Ph - CH3 Chemical Shift

(NMR)

A

2 - 2.5

(~2.3)

75
Q

Carboxylic Acid (HOOCR) Chemical Shift

(NMR)

76
Q

Why do esters and vinyl hydrogens absorb further downfield?

A

Their resonance structures place carbocations on carbons containing the hydrogen you inspect, which strongly deshields it = more downfield.

77
Q

Why do hydrogens bound to arenes and alkenes absorbe further downfield than alkoxy groups?

A

They have an induced field created by their pi bonds that strengthen the external magnetic field, creating larger chemical shifts as the energy gap between α and β spinning nuclei increases.

78
Q

Which energy values are directly proportional?

A

Energy and frequency

79
Q

Which energy values are inversely proportional?

A

Frequency, wavenumber, and wavelength are inversely proportional.

80
Q

How does the 13C NMR chemical shift scale compare to the ‘H NMR scale?

A

The 13C NMR scale is 15-20x larger than the ‘H scale, so:

(15-20) * ‘H scale = 13C scale

81
Q

What happens when more than one EWG acts on a given H environment?

A

Look at each piece, then add the smallest piece + the difference between the smallest and next piece for each piece

82
Q

What is proton decoupled 13C NMR spectroscopy?

A

NMR with one unsplit peak for each type of carbon environment.

83
Q

What is spin-spin splitting?

A

Protons within 3 bonds of each other can effect the chemical shift of the neighboring proton environment.

Ex: If Ha has an α-spin, it will slightly increase the chemical shift of Hb. If it has a β-spin, it will slightly cecrease the chemical shift to create a doublet.

84
Q

Relationship between 13C and ‘H NMR

A

γ 13C = 1/4 γ 1H

85
Q

What is a chemical shift?

A

The distance downfield (left) a peak appears relative to TMS (tertramethyl silane)

86
Q

Given that a NMR has a frequency of 480 Hz, how many Hz downfield is a peak found at 4.3 ppm?

87
Q

Number of Peaks

A

N + 1

(N = number of hydrogens surrounding proton environment)

88
Q

What does Resonance do to Chemical Shifts?

A

It distributes the + charge across mutliple carbons, so it causes strong deshielding

89
Q

Remember About O-H and N-H

A

O-H and N-H hydrogens do not participate in coupling, so do not include in N+1 rule

90
Q

What are coupling constants?

A

The difference in the chemical shift when protons have α or β spin. It is the gap between the split peaks.

91
Q

Vicinal C-H Coupling Constant

(free rotation)

A

Jab = 7 Hz

92
Q

Cis H - C = C - H Coupling Constant

(vicinal)

A

Jab = 10 Hz

93
Q

Trans H - C = C - H Coupling Constant

(vicinal)

A

Jab = 15 Hz

94
Q

Geminal C = C - H Coupling Constant

A

Jab = 2 Hz

95
Q

What information is provided by coupling constants?

A

They help differentiate between cis and trans alkenes

96
Q

What if the coupling constants are similar?

A

Peaks closer than 0.5 Hz together will appear as one peak with low resolution.

** Need a difference of 0.5 Hz or larger between coupling constants to see splitting of a peak clearly **

97
Q

Diastereotopic

A

Hydrogen environments are different, making the molecules diastereomers

** Chiral centers indicate distereomers **

98
Q

Enanteotopic

A

The hydrogen environments are the same, so the molecules are mirror images of each other or enantiomers

99
Q

Homeotopic

A

The hydrogen environments are the same, so the molecules are the same was well

100
Q

Do we see one or two peaks for axial and equitorial hydrogens on ‘H NMR?

A

At room temperature, we see one peak because it’s flipping too quickly to distinguish each chair conformation

At -78°C, we see two peaks because the ring conversion slows enough that the NMR resolution (1/100 sec.) can pick up each chair conformation

** Remember fan blade analogy for explanation **

101
Q

How can ROH and RNH2 peaks be verified?

A

You can use proton transfers with D2O to see if previous OH and NH peaks will relocate to 4.7

102
Q

Under what circumstances are peaks in a 13C NMR spectrum split?

A

** Off-Resonance 13C NMR **

The protons directly bonded to the carbon with split the corresponding carbon peak

103
Q

What is DEPT 13C Spectroscopy?

A

Gives the same information as normal and off-resonance 13C spectroscopy, but in a cleaner fashion. Distinguishes between methine, methylene, and methyl carbons

** DEPT-90 only shows methine (-CH) pointing up **

** DEPT-135 shows methine (-CH) and methyl (-CH3) pointing up, and methylene (-CH2) pointing down **

104
Q

What is COSY NMR?

A

2D NMR that has ‘H NMR on the X and Y axis, which tells you which protons are being split by which protons.

** Used to make more accurate proton assignments within the structure **

105
Q

What is HECTOR NMR?

A

2D NMR with 13C NMR on the X axis and ‘H NMR on the Y axis, that tells you which protons are attached to which carbons

106
Q

MRI

A

Magnetic resonance imaging

Essentially glorified ‘H NMR spectrometer that uses people as the sample, realtively noninvasive because it only uses radio waves

Organic Chemistry 2 (4 decks)

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