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Chemistry (S2) > 2.6 Chemical Analysis > Flashcards

2.6 Chemical Analysis Flashcards

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1
Q

What did chemical analysis start of with?

A

Chemical tests - can show if a functional group or ion is present

Hard to identify the whole structure

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2
Q

How is elemental composition of a substance used in chemical analysis?

A

Tells us molecular formula but doesn’t tell us 3d arrangements of atoms structure

Substance is burned in oxygen and then the gas emitted is accurately weighed

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3
Q

What does Mass spectrometry tell us?

A

What the formula of the molecule is

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4
Q

What does chromatography show?

A

Purity and…
Retention time - can be used to identify sample using a standard

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5
Q

UV-VIS spectroscopy

A

Can quantify the amount of material present
Based on how different functional groups absorb UV/visible light from the spectrum

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6
Q

What other type of spectroscopy is infrared similar to? And what is the difference?

A

UV-VIS
Infrared - inspects how different bonds in a molecule absorb light as the molecule moves

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7
Q

What do lines on the IR spectrum show?

A

Different functional groups

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8
Q

What is scissoring movement in a water molecule?

A

When the hydrogens move closer to one another - both at the same time

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9
Q

What are the strongest bonds in IR spectrum?

A

Asymmetric tretches

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10
Q

What is a symmetric stretch

A

When the bonds are moving in the same direction at the same time (bonds are cheering)

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11
Q

What is asymmetric stretch?

A

When one bond gets longer whilst other gets shorter (seen in water, primary amines etc)

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12
Q

What is the IR spectrum below 1500 referred as?

A

Fingerprint region

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13
Q

What is the intensity of an IR absorption dependant on?

A

Whether there is a dipole or not

And number of the bonds

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14
Q

Where is an alkane seen on IR spectrum?

A

C-H bonds between 2800 and 3000

No other functional groups/peaks

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15
Q

What does a terminal alkyne IR look like?

A
  • normal C-H stretches 2800-3000
  • C-H bond at the end of terminal alkyne ~3300
  • C≡C seen at ~2200
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16
Q

What would be different in an internal alkyne IR to terminal one?

A

We wouldn’t see terminal C-H at ~ 3300
C≡C peak would be smaller due to increased symmetry

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17
Q

What is the characteristics of an Alcohol IR spectrum?

A

Very large broad O-H peak, above the C-H region @ ~3400

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18
Q

What is the characteristics of a primary amine?

A

Signal with 2 peaks above the C-H region
NH2 produces 2 stretches

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19
Q

How is a secondary amine IR spectra different to a primary one?

A

Only one signal/peak above the C-H region and a bit to the right (less polar)

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20
Q

What can be seen in the IR of a carboxylic acid?

A

Very strong C=O peak @ ~1700
Combined with O-H broad signal

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21
Q

What distinguishes a carboxylic acid IR spectrum from other carbonyls?

A

The OH stretch

Aldehydes and ketones dont have OH broad signal

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22
Q

What is the difference between peaks on an IR and NMR spectrum?

A

IR - peaks are upside down
NMR - peaks are right way up

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23
Q

What does the simplest form of NMR look at?

A

The invironment of protons (¹H) in an individual molecule

24
Q

What does the number of signals on an nmr spectrum tell us?

A

The different environments that hydrogen has within the molecule

25
Q

Why is hydrogen good to be looked at on nmr?

A

¹H isotope is very abundant - 99.98%
Makes it easy to get a good signal to see hydrogen

26
Q

Which type, apart from hydrogen nmr, is used to analyse structure?

A

¹³C - 1% abundant
nmr signal is weaker due to lower abundance
Longer time and stronger magnets required to produce good signal

27
Q

What 4 factors are used to identify a molecule using nmr?

A
  • Number of signals
  • Chemical shift
  • Integration
  • Coupling
28
Q

Which nucleus spin is easiest to work with?

A

1/2 as it is nmr active

29
Q

What value of spin do both an individual proton and neutron carry?

A

1/2

30
Q

What values of overall (proton and neutron combined) spins give a nmr active nuclei?

A

Half integers (1/2, 3/2….)
Combination of odd and even proton and neutron or vise versa

31
Q

What are the tow possible energies when nucli with an overall spin 1/2 are in presence of an external magnetic field

A

m = 1/2 (aligned with magnetic field)
m = -1/2 (against applied magnetic field) - higher in energy

32
Q

What happens when a magnetic field is applied during nmr?

A

Nucli are split into energy levels (half and minus half)

33
Q

Which state are nucli more abundant in when magnetic field is applied?

A

m = 1/2 (the lower energy state)

34
Q

What does the addition of a radio-frequency pulse do in nmr?

A

Flips the spin on some of the lower energy nucli so they are in higher energy state (m=-1/2)

35
Q

When can the radio frequency signal be no longer increased in nmr?

A

When all the low energy nucli’s spins are flipped to the higher energy level

This is referred to as sample saturation

36
Q

The system in higher energy level is more….

A

Unstable

37
Q

What do the nucli do as a result of the instability?

A

Interact with their surroundings in different forms to lose energy and go back to lower energy state

38
Q

What happens as the nucli go back to their original energy state?

A

They emit energy which is picked up a detector in the nmr machine and then processed through computer to generate a nmr spectrum

39
Q

What does the position of signals on the x axis of an nmr spectrum tell you?

A

The environment. Eg. electrons around the, orbitals electrons are in

40
Q

What does each line on a carbon spectrum indicate?

A

Each unique carbon environment

41
Q

What is the order of ¹H chemical shifts from upfield to downfield?

A
  • Hydrogens which aren’t near anything electronegative (0-2)
  • Then it’s the H on a carbon that is bonded to something electronegative (3-5)
  • H on double bonds or aromatic rings (6-8)
  • H of aldehydes (9-10)
42
Q

What is the order of ¹³C Chemical Shifts from upfield to downfield?

A
  1. Carbons just bonded to H and other C (0-30)
  2. C attached to electronegative atoms eg. O, N and halogens (40-80) goes further left with increasing electronegativity
  3. Sp2 hybridised carbons - double bonds and aromatic rings (100-160)
  4. Carbons in carboxylic acids (160-180)
  5. Carbons in aldehydes and ketons (200-220)
43
Q

Why does carbons have a larger effect on the magnetic field experienced by the nucleus?

A

Carbon has p orbitals for its electrons - wider range on x axis
(0-200+)

44
Q

What does the number of signals depend on?

A

Number of unique environments in the molecule

45
Q

What is rotation possible in?

A

Single/sigma bonds

46
Q

What does area under the curve indicate?

A

Integral - number of atoms producing that signal

47
Q

Describe Toluene

A

Symmetrical molecule
5 carbon environments
4 hydrogen environments

48
Q

What is coupling?

A

When nuclei interact with one another

49
Q

Learn coupling names

A

1-7
Singlet, doublet, triplet, quartet, quintet, sextet, spetet

50
Q

How is coupling constant calculated?

A

Distance between 2 lines (Hz)
Bigger coupling constant = bigger interaction

51
Q

What is the general formula to work out the number of coupling lines?

A

(N+1)
Where N = number of identical neighbours

52
Q

What 2 factors cause splitting?

A
  1. Interaction with neighbours magnetic field
  2. External magnetic field
53
Q

What is the distance between lines with 2, 3 and 4 bond coupling?

A

2 = 12-15Hz
3 = ~ 7Hz
4 = 1-3Hz

54
Q

2 identical neighbours =

A

Triplet (3 lines)

55
Q

Does ¹³C spectrometry have coupling?

A

ABSOLUTELY NOT

56
Q

What is the maximum number of bonds away a H can be to be a neighbour?

A

3 bonds

Chemistry (S2) (9 decks)

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