Spectroscopy

Question 1

What are the 2 main regions you should examine on an IR spectrum?

Answer 1

swords between 1630-1850 (for carbonyls) and tongues between 3200-3400 (for OH groups of alcohols and COOH)

Question 2

Which fx group gives the big hairy beard shape on an IR spectrum?

Answer 2

COOH

Question 3

Which fx group gives the mitten shape on an IR spectrum?

Answer 3

alcohol

Question 4

where on the IR spectrum should you look in order to quickly determine if any C=C are present?

Answer 4

3000

peaks just above 3000 are alkenes
peaks just below 3000 are alkanes

Question 5

on an IR spectrum, what might a peak between 2050-2200 mean?

Answer 5

presence of a triple bond (like CN or C triple bond C)

Question 6

what the lowest part of the IR spectra where you could see a C=O

Answer 6

C=O peaks rarely exist below 1650 on the IR spec

Question 7

on an IR spectrum, what’s the difference in appearance between an alcohol and a COOH

Answer 7

alcohol looks like a mitten, but COOH is a much broader beard or tongue shape that reaches below the 3000 region and damn near dominates the entire left side of the spectrum

Question 8

which carbonyl shows up on the IR spec between 1690 - 1740?

Answer 8

aldehyde

Question 9

which carbonyl shows up on the IR spec between 1680 - 1750?

Answer 9

ketone

Question 10

which carbonyl shows up on the IR spec between 1735 - 1750

Answer 10

ester

Question 11

what structure’s carbonyl shows up on the IR spec between 1710 - 1780?

Answer 11

COOH

Question 12

which carbonyl shows up on the IR spec between 1630-1690?

Answer 12

amine

Question 13

which carbonyl shows up on the IR spec between 1740 - 1775?

Answer 13

anhydride

Question 14

which ‘ghost town’ region of the IR spec is likely to show a peak for triple bonds?

Answer 14

2000-2200

Question 15

what types of structures’ triple bonds are commonly shown in IR spec in the 2000-2200 region?

Answer 15

triple bonded carbons like alkyne or nitrile

Question 16

what is the point of an IR spec?

Answer 16

to help you figure out a structure’s functional groups

Question 17

If you have any amines, amides, or terminal alkyne C-H, where are they likely to show on the IR spec?

Answer 17

3200 ish

Question 18

is a vinylic carbon in a double bond or next to a double bond?

Answer 18

in a double bond

Question 19

where does the peak for a vinylic carbon bound to a H appear on an IR spec?

Answer 19

3100 ish

Question 20

where does the peak for a triple bonded/alkyne carbon bound to a H appear on an IR spec?

Answer 20

3200 ish

Question 21

where does the peak for an amine bound to a H appear on an IR spec?

Answer 21

3400 ish

Question 22

where does the peak for an O bound to a H appear on an IR spec?

Answer 22

3500 ish

Question 23

How might a C=C bond be presented on an IR spec?

Answer 23

a weak, thin peak (especially compared to the broader, stronger peak of a C=O) in the carbonyl region?

Question 24

where does the peak for regular C-H bonds appear on an IR spec?

Answer 24

between 2800 - 3000

Question 25

where/how do alkanes fragment during mass spectrometry?

Answer 25

between the C-C bonds

Question 26

what are the 2 ways alcohols can fragment during mass spectrometry?

Answer 26

alpha cleavage | dehydration

Question 27

what happens to an alcohol when it is fragmented via alpha cleavage during mass spectrometry?

Answer 27

bond between the alpha carbon and the rest of the chain breaks, and then the alpha carbon forms the cation with a resonance stabilized pi bond with the alcohol's O the radical is also produced

Question 28

what happens to an alcohol when it is fragmented via dehydration during mass spectrometry?

Answer 28

an H and an OH are lost from the molecular ion/unstable intermediate and a pi bond forms between their respective carbons in order to fill the void H2O is also produced

Question 29

what is the way amines can fragment during mass spectrometry?

Answer 29

alpha cleavage like how alcohols do

Question 30

what is the way that ketones and aldehydes can fragment during mass spectrometry?

Answer 30

McLafferty rearrangment

Question 31

Does CNMR spec detect all unique carbons or just the unique carbons with hydrogens attached?

Answer 31

all the UNIQUE carbons

Question 32

what does it mean to call two carbon environments homotopic?

Answer 32

that they are interchangeable via rotation if you rotate the molecule 180 degrees while your eyes were closed, you wouldn't be able to tell the molecule moved at all.

Question 33

what does it mean to call two carbon environments enatiotopic?

Answer 33

that they are interchangeable via reflection if you rotate the molecule 180 degrees, you'll get the mirror image of the molecule

Question 34

what does it mean to call two carbon environments diastereotopic?

Answer 34

that they are not interchangeable via rotation NOR reflection this usually happens when the molecule has chiral centers

Question 35

why does pentane have 3 unique carbon environments instead of 2?

Answer 35

Because the carbon in the middle/the point of axis (C3) doesn't have another carbon to be mirror imaged with. It counts as it's own environment.

Question 36

when figuring out the impact of an electronegative atom on the chemical shift of a proton (in HNMR) which position is most impactful: proton alpha to electronegative atom, proton beta to electronegative atom, or proton gamma to electronegative atom,

Answer 36

impact goes alpha > beta > gamma

Question 37

why does C-Cl have a bigger wavenumber on IR spec than C-O?

Answer 37

C-Cl is lighter/has lower atomic mass and will be further downfield where the higher wavenumbers are

Question 38

what does a primary amine look like on IR?

Answer 38

2 Rick lips (around 3350 and 3450)

Question 39

what does a secondary amine look like on IR?

Answer 39

1 Rick chin around 3400

Question 40

where do the Rick lips of a primary amine show up on the IR spec?

Answer 40

around 3350 and 3450

Question 41

where does the Rick chin of a secondary amine show up on the IR spec?

Answer 41

around 3400

Question 42

in IR, why are some signals stronger/more intense than others?

Answer 42

Some bonds absorb the IR radiation more efficiently than others (the stronger the dipole moment, the stronger the signal)

Question 43

why does a conjugated ketone have a lower IR signal than a regular ketone?

Answer 43

The conjugated ketone's resonance weakens its carbonyl's double bond (by giving it some single bond character) Weaker bond = weaker signal

Question 44

What impact do electronegative atoms' alpha position have on the chemical shift of protons in HNMR?

Answer 44

whatever the table says

Question 45

What impact do electronegative atoms' beta position have on the chemical shift of protons in HNMR?

Answer 45

about 1/5 of the alpha position

Question 46

What impact do electronegative atoms' gamma position have on the chemical shift of protons in HNMR?

Answer 46

virtually no impact on the shift

Question 47

In pronton NMR, do all of the unique proton environments make signals?

Answer 47

Yes? Even if the molecule has symmetry

Question 48

on HNMR, what are the two ways integration (total # of Hs in a signal) can be shown?

Answer 48

but the number of spikes or by the given number (i.e. 6H or 3H)

Question 49

what structure occurs between 1500-1600 on the IR spec?

Answer 49

aromatic compound

Question 50

how many peaks are present on the IR of an aromatic compound?

Answer 50

between 1 and 3, depending on the compound