exam 1 (Orgo 2) Flashcards
1/2 amine/amide N-H stretches (IR)
primary: 2 peaks
secondary: 1 peak
overtone (IR)
secondary absorptions that are generally twice the frequency
(carbonyl (C=O))
aldehyde peak
2700-2830 cm-1 small doublet
higher bond energy
higher frequency (higher energy), higher cm-1
bond energy trend increase
single, double, triple
more EN
s character
stronger bond, higher frequency
DoU
(2+2C-H+N-X)/2
alcohol fragmentation
alpha + dehydration
amine fragmentation
alpha
ketone + aldehyde fragmentation
alpha + McLafferty (4 C!)
alpha mech
……………..⏺+……………………………….
C -❌- C - OH…………..➡️……..C = OH +
…………….- NH2………..➡️…….C = NH2 +
…………….= O……………➡️…….C ≡ O +
dehydration mech
…………❌……………………
C - C - OH…..➡️…..C=C
|………………………………..
H ❌…………………………..
McLafferty
H is 4 C away
……………………..⏺+
………………………O
………↙️⬅️⬅️⬅️||
……H………………C
……|⤵️……….↱/..\
R - C - C -❌- C…..R/H
⬇️
……..⏺+
……..O-H
……..|
……..C
…..//..\
… C….R/H
+ R-C=C
-15
*CH3
-18
H2O
-29
CH2CH3 or HCO
-35/37
Cl
-43
C3H7
/\
-79/81
Br
cm-1, m/z, ppm
IR, mass spec, NMR
M:M+2 = 1:1 (mass spec)
Br
M:M+2 = 3:1 (mass spec)
Cl
IR: aldehyde > ketone
less e- donation, so more double bond character, making for a higher frequency
angle strain in rings (IR)
more angle strain (less Cs), more energy, higher wavelength
mass spec instrument range
can range from 60-1200 MHz, but usually 300-400 MHz
anti-parallel spin state (NMR)
higher in energy, against B0, resonance
as B0 increases in strength
applied resonance frequency strength increases (to bring the nuclei into resonance)
induced field
Every nuclei in existence is surrounded by e-. The orbiting and circulating e- generate their own, tiny magnetic field. It shields the nuclei, making it require less radiofrequency (UPFIELD)
downfield
deshielded, left, more radiofrequency
feels more of the applied field
upfield
shielded, right, less radiofrequency
feels less of the applied field
delta scale
used to standardize observed frequencies for NMR
remember resonance frequency is dependent on the strength of the applied B0
The x-axis of an NMR spectrum is called the delta scale. It shows the position of resonance of each nucleus relative to a standard (TMS) and has units of ppm.
resonance frequency
the frequency that brings the NMR-active nuclei into resonance, dependent on nuclei itself and B0.
Directly proportional to B0.
applied field
B0
field anisotropy
the orientation of nuclei in the magnetic field determines the chemical shift
⬆️……….➡️……..🔄 …….⬅️
⬆️……⬆️….⬇️…………⬇️….⬆️
⬆️……….⬅️……..🔄……..➡️
B0
⬇️ shielding / ⬆️ deshielding
🔄 pi e- circulating
field anisotropy - double bond
Pi bonds have electron current above and below the plane. The direction of the induced magnetic field by the pi-electrons is ADDED to the applied field through the hydrogen, therefore the hydrogen is DESHIELDED
⬆️……….➡️……..🔄 ………⬅️
⬆️……⬆️….⬇️…..=…….⬇️….⬆️
⬆️……….⬅️……..🔄……….➡️
B0
⬇️ shielding / ⬆️ deshielding
🔄 pi e- circulating
field anisotropy - triple bond
the triple bond is like a cylinder so the pi-electrons completely circulate. Now the induced field is OPPOSING the applied field through the proton, making the proton SHIELDED from applied field
⬆️……….➡️……..🔄 …….⬅️
⬆️……⬆️….⬇️…|||…⬇️….⬆️
⬆️……….⬅️……..🔄……..➡️
B0
⬇️ shielding / ⬆️ deshielding
🔄 pi e- circulating
field anisotropy - aromatics (rings)
aromatics have a lot of pi-electron density known as ring current
induced field is ADDED to the applied field (even more now) through the protons, they are DESHIELDED
⬆️……….➡️……..🔄 ………⬅️
⬆️……⬆️….⬇️…..♨……⬇️….⬆️
⬆️……….⬅️………🔄……….➡️
B0
⬇️ shielding / ⬆️ deshielding
🔄 pi e- circulating
dihedral angles
J values descend from it
the angle between two bonds originating from different atoms in a Newman projection
greater dihedral angles = greater coupling constant
cis/trans
Z/E
diastereotopic protons
chemically non equivalent protons that each produce distinct chemical shifts.
Z/E and cis/trans
if there’s a stereocenter (SC) in a molecule, any CH2 group’s protons will also be diastereotopic…
- the effect is felt most strongly when the CH2 is directly next to the SC
- the proton signal may or may not be resolved on the NMR spectrum
A stereocenter is an atom, typically carbon, that has four attachments that are different from each other.
axial and equatorial Hs interconversion in NMR is…
…very fast at room temperature, it is faster than the NMR time scale can detect, so they average to a SINGLET
acidic protons (O-H, N-H) exchange…
…faster than the NMR timescale.
This is why alcohols and amines are typically contaminated with H2O or acidic or basic impurities. This can lead to proton exchange, which is very fast and results in an averaged uncoupled signal
13 C NMR
same principles as 1H NMR!
NO SPLITTING due to low abundace of 13C
R4C singlet
R3C-H doublet
R2CH2 triplet
RCH3 quartet
**a proton decoupled 13C NMR tells you how many different carbons a molecule has!!
shielding and symmetry are same
integration in 13C NMR…
means nothing, so no correlation between # of C and peak height
