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1A Chemistry > Shapes & Structures 1 > Flashcards

Shapes & Structures 1 Flashcards

1
Q

Formula for formal charge?

A

Formal charge = Nv - Ne where:

Nv = no. valence electrons associated with atom if hypothetically neutral, = group no. for groups 1-12, or group no. - 10 for groups 13-18

Ne = no. e- associated with atom (assuming e- are shared equally between bonding atoms): one per bonding pair, 2 per lone pair

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2
Q

why do EN atoms increase shift?

A

An electronegative atom makes a nucleus resonate at a higher frequency by deshielding it:

  • E- create a magnetic field opposing the applied one
  • EN atom attracts bonding e- → lower e- density around magnetic nucleus → weaker local magnetic field → stronger net magnetic field
  • EN atom deshields nucleus
  • Larger energy difference between spin states
  • Higher resonating frequency → higher shift
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3
Q

For 13C and proton NMR, the reference compound is tetramethylsilane (TMS). Why is this used?

A

Inert

Only one 13C / 1H environment → one peak

Si less EN than C → Cs have higher e- density → more shielded → smaller ΔE between spin states → lower resonating frequency → less shifted (positioned to the right), out the way

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4
Q

why is deuterium used as an NMR solvet?

A

Deuterium is used (e.g. CD2Cl2, deuterated chloroform) since it’s not spin-active, so doesn’t interfere with the solute spectrum.

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5
Q

Carbon NMR shift region for sp3?

A

0-50 ppm

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6
Q

Carbon NMR shift region for sp3 carbons with EN groups attached?

A

50-100 ppm

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7
Q

Carbon NMR shift region for triple bonded carbons?

A

Around 80 ppm

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8
Q

Carbon NMR shift region for sp2 carbons: i.e. double bonds, benzene?

A

100-150 ppm (benzene higher)

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9
Q

Carbon NMR shift region for acid derivatives: esters, acyl chlorides, amides, acid anhydrides?

A

160-170 ppm

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10
Q

Carbon NMR shift region for carbonyls: ketones, aldehydes?

A

around 180-200 ppm

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11
Q

which of aldehydes and ketones have higher carbon shift?

A

ketones

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12
Q

which broad categories do the following shift regions indicate?

  • 0-50 ppm
  • 50-100 ppm
  • 100-150 ppm
  • 150-200 ppm
A
  • sp3 carbons
  • sp3 carbons with EN groups attached, also double bonds
  • sp2 (trig planar), e.g. double bonds/benzene
  • sp2 with EN groups attached, e.g. ketones
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13
Q

word for

  • coupling over 2 bonds?
  • over 3 bonds?
A
  • 2 = geminal (twins)
  • 3 = vicinal
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14
Q

Attached proton test (APT):

Peaks from carbons with an __ number of protons attached point one way (same as deuterated solvent, since ?)

A

Peaks from carbons with an even number of protons attached point one way (same as deuterated solvent, since it has 0 Hs & 0 is even)

Peaks from carbons with an odd number of protons attached point other way

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15
Q

for a 400 MHz instrument, how many MHz represent 1 ppm?

A

400

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16
Q

Coupling constants – order these in terms of relative size:

  • Geminal, tetrahedral
  • Geminal, across double bond
  • Vicinal, tetrahedral, same environment
  • Vicinal, trans
A
  • Vicinal, trans: ~15 Hz
  • Geminal, tetrahedral ~13 Hz
  • Vicinal, cis: ~10 Hz
  • Vicinal, tetrahedral, same environment ~ 3 Hz
  • Geminal, across double bond 0-0.3 Hz
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17
Q

Coupling constants: give rough values for each:

  • Vicinal, trans
  • Geminal, tetrahedral
  • Vicinal, cis
  • Vicinal, tetrahedral, same environment
  • Geminal, across double bond
A
  • Vicinal, trans: ~15 Hz
  • Geminal, tetrahedral ~13 Hz
  • Vicinal, cis: ~10 Hz
  • Vicinal, tetrahedral, same environment ~ 3 Hz
  • Geminal, across double bond 0-0.3 Hz
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18
Q

roofing indicates what?

A

Protons in similar environments –> smaller shift difference in Hz between coupling nuclei → stronger roofing effect (until singlet seen)

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19
Q

Range of shifts of protons attached to carbon?

A

0-14 ppm

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20
Q

Shifts of protons attached to carbon:

What does proton shift = 0.5 ppm indicate?

A

on 3-membered ring

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21
Q

Shifts of protons attached to carbon:

What does proton shift = 1.0 ppm indicate?

A

methyl

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22
Q

Shifts of protons attached to carbon:

What does proton shift = 1.5 ppm indicate?

A

-CH2- (alkyl)

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23
Q

Shifts of protons attached to carbon:

What does proton shift = 2.5 ppm indicate?

A

Most attached to double/triple bond:

  • next to benzene
  • next to C=O
  • next to C=C
  • -N-CH2- (next to N in amine)
  • -C≡C-H (terminal alkyne)
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24
Q

Shifts of protons attached to carbon:

What does proton shift = 3.5 ppm indicate?

A
  • -CONH–CH2- (next to N in amide)
  • next to alcohol
  • next to ether
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25
Q

Shifts of protons attached to carbon:

What does proton shift = 4 ppm indicate?

A
  • -COO-CH2- (next to ester)
  • Cl-CH2- (next to chlorine)
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26
Q

Shifts of protons attached to carbon:

What does proton shift around 5.5 ppm indicate?

A

alkene / double bond

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27
Q

Shifts of protons attached to carbon:

What does proton shift around 7.5 ppm indicate?

A

on benzene

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28
Q

Shifts of protons attached to carbon:

What does proton shift = 8 ppm indicate?

A

-O-CO-H (formate/methanoate ester)

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29
Q

Shifts of protons attached to carbon:

What does proton shift = 10 ppm indicate?

A

-CO-H (aldehyde)

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30
Q

Proton shift for 3-membered ring?

A

0.5 ppm

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31
Q

Proton shift for

  • methyl?
  • alkyl?
A

Methyl 1 ppm

Alkyl 1.5 ppm

32
Q

Proton shift for

  • aklyl next to benzene?
  • on benzene?
A
  • next to = 2.5 ppm
  • on = 7.5, varies
33
Q

Proton shift for

  • aklyl next to double bond, -C=C-CH2-
  • on double bond?
A
  • next to = 2.5 ppm
  • on = 5.5, varies
34
Q

Proton shift for -N-CH2- (next to N in amine)?

35
Q

Proton shift for -C≡C-H (terminal alkyne)

36
Q

Proton shift for -CONH–CH2- (next to amide)

37
Q

proton shift for -O-CH2- (next to alcohol/ether)

38
Q

proton shift for -COO-CH2- (next to ester)

39
Q

proton shift for Cl-CH2- (attached to chlorine)

40
Q

proton shift for -O-CO-H (formate/methanoate ester)

41
Q

proton shift for -CO-H (aldehyde)

42
Q

Why do protons on aromatic rings have greater shifts (~7.5) than alkene protons (5.5), & why does the proton on a terminal alkyne have such a low shift (~2.5)?

A

Due to local magnetic fields set up by e- in π-systems

43
Q

why do shifts of exchangeable protons attached to electronegative elements vary so much?

A

Shifts may take a range of values, since they depend on amount of H bonding, which depends on:

  • Compound
  • Solvent
  • Sample concentration
  • Temperature
44
Q

how would you distinguish peaks of exchangeable protons attached to electronegative elements?

A
  • Broader than other signals
  • Exchangeable protons don’t couple (swapped protons may be spin up or down, so nearby protons experience an average spin state)
  • D2O shake test
    • Shake sample, prepared in deuterated solvent, with heavy water
    • All NH and OH protons are exchanged for deuterons from solvent → ND & OD; protons end up in HOD molecules
    • Broad peaks disappear since deuterium resonates at a different frequency to normal protons
45
Q

For exchangeable protons attached to electronegative elements, how could you remove broad signals & observe coupling?

A

prepare a dilute solution in a dry solvent

46
Q

proton shift in water?

A

1.5 ppm, broad

47
Q

proton shift in amines?

A

1-3.5 ppm, broad

48
Q

proton shift in alcohols?

A

1-3.5 ppm, broad

49
Q

proton shift in

  • Ar-NH2
  • Ar-OH
A
  • 3.5-5.5 ppm, broad
  • 5.5-9 ppm, broad
50
Q

proton shift in HOD?

A

4.5 ppm, broad

51
Q

proton shift in amides -CONH-

A

6-11 ppm, broad

52
Q

proton shift in carb acids?

A

confirm

broad, probs high around 10

53
Q

what does a broad proton peak around 1-3.5 ppm indicate?

A
  • amine
  • alcohol
  • water
54
Q

what does a broad proton peak around 6-11 ppm indicate?

A

amide

(maybe carb acid)

55
Q

why does deuterated chloroform have a quintet, in ratio 1:2:3:2:1?

56
Q

Show that a high wavenumber indicates high-energy vibration.

A

Wavenumber, ṽ, = 1/λ.

Proportional to frequency since:

f = c/λ = c(1/λ)

So f = cṽ

So f ∝ ṽ

ΔE = hf, so frequency of absorption ∝ change in energy of vibrational transition.

So a high wavenumber indicates high-energy vibration.

57
Q

give formula for reduced mass

show how it can be simplified if one atom is heavier

A

The reduced mass, μ, accounts for mass of both atoms:

μ = m1m2/(m1 + m2)

When m1 > m2, μ ~ m1m2/m1 = m2

i.e. vibrations of heavier atom are negligible compared to that of lighter atom

58
Q

what is Raman spec used for?

A

Used to determine frequencies for symmetrical molecules whose vibrations aren’t IR-active.

Analyses frequencies scattered by a sample (as opposed to absorbed).

For diatomic molecules:

  • Homonuclear diatomics are only Raman-active
  • Heteronuclear diatomics are both IR-active & Raman-active
59
Q

IR: name the species responsible for peaks in the following regions:

  • 0-1500 cm-1 (fingerprint)
  • 1500-2000
  • 2000-2500
  • 2500-4000
A
  • 0-1500 = X-Y single bonds plus pther vibrational modes
  • 1500-2000 = double bonds
  • 2000-2500 = triple bonds
  • 2500-4000 = X-H single bonds
60
Q

IR: What peaks are seen in the double bond region, 1500-2000 cm-1?

A

C=O, variable

C=C, weak

Benzene: several peaks, medium

NO2

61
Q

IR: range of C=O peaks?

A

1640-1820 cm-1

62
Q

IR: range of C=C peaks?

A

~1635-1690 cm-1, weak

63
Q

IR: range of benzene peaks?

A

several peaks ~1625-1450 cm-1, medium

64
Q

IR: peaks for NO2?

A

Symmetric stretch: ~1350 cm-1

Asymmetric stretch: ~1530 cm-1

65
Q

C≡C peak?

A

~2100-2250 cm-1, weak

66
Q

C≡N peak?

A

~2250 cm-1, strong

67
Q

OH peak – with and without H bonding?

A

H bonding: ~3300 cm-1, broad

No H bonding (due to steric bulk): ~3600, sharp

68
Q

Amine NH2 IR frequencies?

A

Symmetric ~ 3300 cm-1

Asymmetric ~3400 cm-

69
Q

IR: C-H peak range?

A

2900-3200 cm-1

70
Q

IR ethyne peak? (one case of C-H stretch which is useful)

A

~3300 cm-1, strong & sharp

71
Q

what could a sharp peak at 3300 cm-1 be?

A
  • ethyne (strong)
  • Amine NH
  • OH, no H bonding (sterics)
72
Q

what could a peak at 2250 cm-1 be? if strong, likely to be__, if weak __?

A

C≡C (weak)

C≡N (strong)

73
Q

IR peaks 1350-1530 could be?

A
  • Benzene: several peaks ~1625-1450 cm-1, medium
  • NO2 group:
    • Symmetric stretch: ~1350 cm-1

Asymmetric stretch: ~1530 cm-1

74
Q

IR peak at 1640 could be?

A

C=O (varied) or C=C (weaker)

75
Q

ketone IR frequency?

76
Q

compare the IR peaks of the C=O in

  • amides
  • acid chlorides
  • carboxylic acids/ esters
A

frequency: acid chloride > carb acid / ester > amide

  • acid chloride ~1790
  • ester ~ 1745
  • carb acid ~ 1730
  • amide ~ 1660
  • N donates e- density (lone pair is weakly e–-donating into pi-system → 2 e- shared over 3 atoms → weakens C=O pi bond)
  • Oxygen is intermediate between N & Cl: more donating than Cl since lone pair interacts, but more withdrawing than N since more EN
  • Cl withdraws e- density (lone pair doesn’t interact with carbonyl since wrong shell)
77
Q

how do the IR peaks of C=O on rings compare to normal ketone?

A

normal ketone ~1715. ring C=O higher peaks

Smaller ring → stretching frequency increased by 30-35 cm-1:

  • 5-membered ~ 1745
  • 4-membered ~ 1780
  • 3-membered ~ 1815

Since:

  • During a vibration, as bonds compress, C experiences resistance from adjacent Cs
  • Fewer-membered ring → smaller bond angle → bonds compressed more → more resistance → requires more energy → higher stretching frequency

1A Chemistry (17 decks)

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