135B- organic spectroscopy Flashcards
DBE equation
nitrogen could also be boron
DBE above 4 generally looking at aromatic substance
DBE for this
1
equation for wavenumber
length is usually in cm-1
E=hf equation units
equation for c speed of light
this is in ms-1, but if you want cm-1 use 3x10^10
which electrons are higher in E/ HOMO
lone pairs as they are non bonding
Why do we only usually see electrons promoted to π* not σ*
π* is lower in energy as worse overlap so it is easier to be promoted to something of lower E
This will have two absorption bands as there is a choice of 2 electrons to be promoted. Which promotion is higher in E
pi–> pi*
n–>pi* lone pair is higher E than the pi electron so there is a lower jump in energy
On a UV spec where will the higher energy photons be on the frequency axis
higher energy is closer to 0
lower wavelength= higher frequency higher frequency is higher energy E=hf
which has lower HOMO/LUMO gap
red/ blue peak is lower than red E=hf
Why is graphite black but diamond white (HOMO/LUMO gap)
in graphite carbons are sp2 hybridised and have a lone electron in p orbitals/ delocalised so has lots of conjugation. Very small HOMO/LUMO gap so can absorb all frequencies/wavelengths of light and is black
diamond is white as has high HOMO/LUMO gap
What makes a smaller HOMO/LUMO gap
conjugation
will a higher conjugation molecule have a lower/higher molar extinction co-efficient
higher as smaller HOMO/LUMO gap so can absorb more frequencies of light
what colour would this be
colourless/ absorbing nothing in the visible light region
units for molar coefficient ε
M-1 cm-1
what is the Beer-Lambert Law (ε)
A = ε·c·l
c= M
l= cm
what is CD
difference in absorbance between left and right-handed circularly polarized light
enantiomer CD spectral should be mirror images
what is c speed of light in cm-1 instead of m
what are the equations for working out absorbance and transmittance
How to plot a graph using Beer-Lambert Law A = ε·c·l
What are needed for fast/ slow vibrations
what does a higher wavenumber mean in terms of energy
higher wavenumber= high energy
What is the only type of vibration where the bond length changes
stretching
Why will molecules never not vibrate
we can never get to 0K
axis for IR spec (transmittance)
wavenumber from highest to lowest
and transmission
axis for UV spec (absorbance)
wavelength (nm) vs absorbance
Where is the fingerprint region
below 1500 cm-1
C=O stretch on IR
1700 cm-1
ester is 1740
C-O stretch on IR
1100 cm-1
C-H stretch on IR
3000 cm-1
O-H stretch on IR
3300 cm-1
CO2 stretch on IR
2350 cm-1
Why is the acid OH stretch broader than the alcohol stretch
- acid OH vibrating at slightly lower E (lower WN)
- forming hydrogen bonds slightly stretching O-H in HOH bond making it weaker than the other one. The one that’s H bonding is going to give you broad/ the one that wouldn’t would give steep (if O would only form 1H bond (one H not both)
How will an IR spec be skewed if the same is wet
will have an OH stretch
Why do arene C-H bonds and alkyl C-H bonds have different wavenumbers
In arene C is sp2; bonds are slightly stronger
In alkyl C is sp3 so bonds are weaker
How do you tell the difference in IR spec between an aldehyde and its conjugate ketone (ketone and aldehyde between A and B graphs)
Aldehyde= A
Ketone= B
only an aldehyde will have the alkl and arene peak/ the two fingers at about 3000
aldehyde will have the twin peaks and ketones will just have the one
explain why a ketone needs more energy to vibrate the C=O bond than an ester
- C-O bond is polarised in the ester as O is more electronegative/ draws e- towards/ C becomes δ+
- In C=O the electrons usually lie more towards the O, but because C-O is drawing electrons away from C, the e- in C=O are pulled more towards an equal sharing
- So needs less energy to vibrate
Which plate of the detector will get the heavier fragment
higher mass= less travel so will be the first plate
What type of solutions would you see the sodium cation
buffers
What are the tell tale signs on a spectra that the compound contains a bromine atom
Two lines 2 mass units apart due to two isotopes in a 1:1 ratio
what would a diatomic bromine mass spec look like
which peak is the most stable species
the biggest peak- so the 43 fragment
What would M-15 have lost
a methyl group
What is the M+1 peak
M+. peak is due to the C13 isotope presence
What does the difference in 1da vs 0.5 da mean
peaks are closer together as Z increases
so z=1 at 1da difference and z=2 at 0.5 da difference
boron useful to know
Why would we use high resolution mass spec when investigated a complex compound
can measure to many decimal places to distinguish between groups and some fragments/molecules can have the same mass number
What are the tells of chlorine in a mass spec
two isotopes of 35 and 37. 35 is more abundant
What is the difference in the peaks seen between ESI+ and ESI- spectroscopy
boron mass spec
two isotopes 10 and 11. 10 is 23 and 11 is 100
What effects how far the peaks move along the scale on a NMR diagram
how much electron density there is around a carbon/ this is related to wether they are connected to an electron withdrawing group and deshielded
If the carbon has an electronwithdrawing group next to it will it be more/less deshielded and therefore will the peak be higher/lower on an NMR scale
if the carbon is more positive it is more deshielded and therefore higher on the scale
which carbon resonates at a higher frequency
1- higher on the scale as it is more positive and deshielded
what makes a carbon more/less shielded
more electrons around the carbon= more shielded/ has a δ- charge
less electrons around the carbon= less shielded
Why does CH3F resonate at a higher frequency than the other compounds
F Is the most electronegative out of the others. Electron withdrawing from C making it δ+, therefore will resonate at higher frequency than the carbons in the other molecules (most positive C)
why does t-BuOH only have 2 peaks but butanol has 4
carbon closest to O resonates at higher F
1- t-BuOH only has two carbon environments/ oxygen has same relationship to other carbons that are- same magnetic environment so only give one signal for the 3 carbons
2- butanol- 1 is the least shielded and shielding increases across carbons so each have their own peak
Why does CH3Li have the lowest chemical shift
C is more electronegative than Li so pulls electrons towards its giving it a δ-. Meaning C is more shielded and therefore resonates at lower frequency
C-O can also be other electron withdrawing groups
Rank the carbons from least to most shielded/ resonating at higher to lower frequency
- C=O Is closest to the strongest electron withdrawing group
- Purple is closest to weaker electron withdrawing group
- Pink is the adjacent carbon to the C=O (stronger withdrawing)
- Green is adjacent to C-O (weaker withdrawing)
Will sp, sp2 or sp3 carbons resonate at higher frequencies
sp and sp2 carbons will have higher frequencies/higher shifts
How many carbon environments does this molecule have
the last carbon on the benzene ring will always have its own environment
will carbons will similar resonate have their own peaks
no- the peak will just be a thicker line
Rank the carbons from highest chemical shift to lowest/ predict NMR spectra
Rank the carbons from highest chemical shift to lowest/ predict NMR spectra
A most shift- is attached to an electronegative group/ least shielded as has the least amount of electrons around it
Rank the carbons from highest chemical shift to lowest/ predict NMR spectra
the last carbon on the benzene ring is the least shielded after the one that is next to the NO2 (?)
Rank the carbons from highest chemical shift to lowest
1 will have the highest chemical shift as it is the least shielded
- methyl is always last as it is sp3 hybridised
Rank the carbons from highest chemical shift to lowest
What is the solvent used for C-13 DEPT spectra
CDCl3
How is a C-13 DEPT spectra different from a normal one
and in C13 spectra the3 protons are decoupled so there’s no C-H splitting- all peaks are singlets
Which is the C-13 DEPT spectra and which is the C-13 one
DEPT is the one on the bottom
What is the difference between APT (attached proton test) spectra and DEPT spectra
They are the same except you do see quandary carbons in APT which you don’t in DEPT
quandary carbon- carbon attached to 4 other carbons
In HNMR which side of the spectrum will shielded protons be and which side are desheilded ones
What makes a proton more deshielded/ resonate with a higher frequency
being close to electronegative atoms/ withdrawing groups
What are the most desheilded protons and why
aldehyde, because you can draw a resonance structure to put the positive charge on the carbon
desheilded= less electrons around it
Which protons have the most electron density around them and which have the least
left-least
right-most
The H-NMR spec for acetic acid has 2 peaks. Which H has the highest chemical shift and which peak will be the highest
red has highest shift as it is the H closest to the O
green highest integral height as there are greater ratio of hydrogens with that environment
-you need to reference that 6mm represents 1 proton
What does the integral (blue lines) represent on a H-NMR spec
the number of protons e.g. green integral would represent 4 protons and the red would be 1
rank the protons from least to most shielded
purple- least shielded
then pink, then green
HNMR- why do we have doublets
protons influence each other, we allow protons to couple,
-the applied MF on A will be influenced by the minute MF that X has (X has magnetic field due to electrons being around it which have their own spin and MF) and visa versa
- the two magnetic fields will interact
diagram- top line is where the protons resonate at without influence/ second line-A will resonate at higher F when the two magnetic fields are aligned
HNMR-why is the distance between the two green peaks the same distance as between the two red ones (bottom spectrum) and what is the distance called
the influence of HA on HX is the same as the influence of HX on HA
-distance called the coupling constant (J) measured in Hz) and tells you have strong the influence on each proton is due to the other
*****THIS CALCULATION DOESNT WORK
The coupling constant/splitting value (distance between the two green/ two red peaks is called J. How do you calculate J and what would its value be if the splitting distance was 0.2ppm in a 400MHz field
What is the HNMR spec of an ethyl group
quartet and a triplet
HNMR- Why will the CH3 peak be a triplet
the three protons are influenced by the neighbouring 2 protons. If both protons in the CH2 are aligned with the externally applied MF, the CH3 peak will be pushed to a higher chemical shift
- If both are opposite aligned will be pushed lower
-the two lines should be roughly equal in height because influence should be the same
-the peak in the middle is where the CH3 would be if there wasn’t any influence
HNMR- Why is there a ratio of 1:2:1 of the triplet
-one alignment of the magnetic fields between CH2 and CH3 push the peak to the right, and one pushes to the left
-two sets of influences and because of that the peak in the middle is double the intensity of the other ones
HNMR- Why will the CH2 peak be a quartet
the two protons are influenced by the neighbouring 3.
-[two outside peaks] when all 3 proton magnetic fields are aligned with the applied one/ when all 3 protons are have opposite MF
-[peaks in the middle] you have 2 possibilities, where 2 up and 1 down, or 1 up and 2 down and there are 3 different arrangements of each
- so will have the same size peak in a ratio of 3:3
what is the rule relating the number of peaks on an HNMR spec and the number of neighbouring hydrogens on the H environment you’re measuring
no. peaks = neighbouring H + 1
HNMR- This relates the number of peaks to number of neighbouring protons- describe the pattern to the triangle
the number in the middle is the sum of the numbers on either side
154 ppm into A
1.54 A (/100)
HNMR- What is the usual J value for alkyl chains and what can be implied about a molecule if the value is less than this. An alkyl J value implies it has flexibility
there is rigidity or the bond could be shorter in the molecule for some unknown reason
what changes the value of the coupling constant J
angle between the two hydrogens
HNMR- Why do Z and E isomers have different coupling constant J values even though they are the same hydrogens (both attached to sp2 carbons around a double bond)
J is dependant on the angle between the two hydrogens and the angle is different between the two structures
HNMR- What is the coupling constant for terminal double bonds, Z alkenes and E alkenes
E alkene (on the end) is more like 14-18 for X-X not XY
HNMR- Why does an E alkene have a 14-18 (higher range) of J values when it has two X groups (groups that are the same) either side compared to when they have different groups (Y and X)
Y and X will influence the chemical shift because they will add or withdraw electron density with different abilities (as X and Y are different), therefore changing the geometry and chemical shift and electron density of the molecule
What is the Benzene resonance in ppm and why is it a singlet in HNMR
7.3 ppm, all the hydrogens are the same and no reason for multiplicity
cannot have splitting (HA influence on HX) if the two protons are identical
In HNMR Other double bonds are around 4.5 to 6 ppm but benzene is at 7.3ppm. This means its hydrogens have less electron density around them, why is this
higher ppm= the protons must be more desheilded
-carbon is sp2 hybridised so has one p orbital with electron which forms the ring
-each e- has a spin and so forms a ring current which generates a magnetic field which will influence the applied magnetic field
exam- each H is deshielded by local magnetic field due to spinning of π electrons and resonate at high frequencies
are protons in a benzene ring or Z/ E alkene more desheilded
benzene protons are more desheilded
In CNMR why are the carbons in a benzene ring more desheilded than the carbons in a different double bond (E/Z) and will therefore resonate at a higher frequency
-carbon is sp2 hybridised so has one p orbital with electron which forms the ring
-each e- has a spin and so forms a ring current which generates a magnetic field which will interact with the applied magnetic field
exam- each C is deshielded by local magnetic field due to spinning of π electrons and resonate at high frequencies
In HNMR- benzene resonates at 6.3 ppm
If there was an electron donating group or electron withdrawing on the ring, where would the new peak shift to
electron donating- slightly lower ppm
electron withdrawing- higher ppm (more desheilded)
HNMR- Why do the hydrogens on the bottom diagram come out at a higher chemical shift than the top one, and how do you explain this with diagrams
top- electron donating group so hydrogens are more sheilded
bottom- electron withdrawing group means less electrons around the H and therefore more desheilded
is OMe electron withdrawing or donating
donating
Which configuration is ortho, meta and para
Is this the H-NMR spec for para, meta or ortho
para
Is this the H-NMR spec for para, meta or ortho
meta
Is this the H-NMR spec for para, meta or ortho
ortho
doublet, triplet, triplet, doublet
Why will these protons be different on an HNMR spec and how many peaks overall will there be.
-the peaks for the outlines H’s.
one is by the Y and one is by the X, both will have different electron withdrawing/donating abilities
4 peaks- the H’s one removed from both X and Y are also different
Which hydrogens will be doublets and which will be triplets
orange and blue- doublets
greens and yellow- triplets (coupled to two protons)
note- triplets depend on coupling constant; green is coupled to blue and yellow. If the coupling constants are very close then it will be a triplet. If they are different it will be a doublet
will the H’s be doublet, singlet, triplet
blue-singlet (no H neighbours)
yellow and green-doublet
red-triplet (if coupling constant of nearby protons is similar)
Why are the blue H’s the same and the yellow H’s the same. What HNMR graph will this show
because of the line of symmetry through X and Y. Two doublets
Why is this a J3 coupling
coupling of hydrogens 3 bonds away (blue to yellow is 3 bonds difference)
short range coupling
what is a W/ J4 coupling of hydrogens on an inorganic molecule and how can the protons interact if they are not next to each other
hydrogens are 4 bonds away
the lobes of the C-H bonds are coming into the centre of the benzene ring so you can have magnetisation transfer between this proton and that proton
large range coupling
C-H bond is sp2s
sp2 orbital from carbon and s orbital from H
What is true about the two hydrogens coupling constants because they are so far away
they will be very small
What is a normal coupling constant for J3 hydrogens
6,8,10 Hz
What is a normal coupling constant for J4/ W hydrogens
1.5 Hz
HNMR- Which colour is the W coupling constant and which is the J3
-this diagram is called a doublet of doublets
green- J3
blue- W (1.5 Hz)
the fact the normal peaks are split into smaller peaks (smaller doublet) is due to W proton interaction
Why is the splitting of the CH3 peak in a 1:2:1 ratio due to coupling constants in this HNMR spec
1/ CH3 group is influenced by the pink and purple H
2/the first split is due to the pink H, by the CC Ja-x
3/the purple H will split the new CH3 singles again by the same amount (same coupling constant)
4/ 2 in the middle because 2 lines from the splitting diagram
Why is the splitting of the CH2 in a 1:3:3:1 ratio for this HNMR diagram
all splits are by the same amount as all have the same coupling constant
These diagrams are both doublet of doublets. What is the difference between the two HNMR
diagram 1- normal splitting (due to J3 hydrogens next to each other), then second smaller splitting (1.5Hz CC from a W/J4 splitting)
-one coupling constant is large for one pair of hydrogens and the other coupling constant is smaller for the other coupling of hydrogens
diagram 2- both couples of hydrogens have similar coupling constants
What’s the difference between a doublet of doublets and a triplet (in both senarios something is being influenced by 2 protons) in HNMR
doublet of doublet- one H coupling constant is different from the other
triplet- both H’s have the same coupling constant
HNMR- this is a doublet of triplets. The red line is the H being split. How many hydrogens it it interacting with/ being split by and what are their environments
being split by 3 protons. On one side, 1 proton is in an environment, and on the other side 2 protons are in a different one
There are 2 identical protons in the blue environment
How many proton environments does this have
3
Which signal is which HNMR
yellow- OH as H is the most desheilded
blue- triplet because split by 2 identical protons (their coupling constant is the same. Stops this from being a doublet of doublets)
red- doublet as only influenced by blue proton
pink- singlet
What halogen is NMR active
F, has a half spin like a protons so protons will couple with fluorine
Br, Cl are not NMR active
How will the highlighted peaks be different on a HNMR spec for each molecule
7.15- will be singlet as no neighbouring interactions
6.98- will be doublet [flourine is NMR active and so interacts as a proton]
This is a hypothetical diagram for the yellow highlighted proton. This has J3 and W coupling. What is it J3 coupled with and what is it W coupled with.
-Which lines on the graph correlate to which coupling. [the W coupling hydrogens have different CC’s]
J3=flourine. Is closest so will have the largest coupling constant
W coupling- with the blue and orange hydrogens
- the orange H has a lower CC than the blue one
What hydrogen coupling does the green proton have
J3 coupling with brown and pink- making it a triplet
W coupling with flourine- making it a messy triplet
If pink has a W coupling constant with mustard, mustard will also have a W coupling with pink
- W couplings arnt unidirectional
do they easy ones for this website chances are you’ll do well in the exam http://nmr-challenge.uochb.cas.cz/
what are the two bromine isotopes Br= 79.9
79 and 81
A UV spectra peak at 264nm usually means what is true about a compound
aromatic
where are peaks on an HNMR spec that show he compound is aromatic
7-8 ppm
What is benzene mass number on a mass spec
78
Which peak corrosponds to what carbon environment
red- most desuhielded
yellow- most shielded
blue- is the second most deshielded
where do aldehydes and ketones fall on this scale
above 190ppm about 200ppm. Anything below this is usually an ester
what functional group could these labeled peaks be
C=C
What does a DBE of 5 mean when we know the molecule is an aromatic
benzene is DBE=4, so there must be another double bond in the molecule still (perhaps in a side chain)
