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Kirsty 3.1 > MAAC: Organic Spectroscopy > Flashcards

MAAC: Organic Spectroscopy Flashcards

1
Q

There are several spectorscopical techniques used to detemermine structure of molecules.

They all reply on the interaction of various regions of EM spectrum with molecules.

What are examples of these regions and corresponding techniques?

A

1.ULTARVIOLET / VISIBLE REGION (200–800nm)

  • interaction cause excitation of electrons
  • analytical techniques UV / Vis spectroscopy

2. INFRARED REGION (2000–25000nm)

  • causes vibration of bonds between atoms
  • analytical technique Infrared Spectroscopy

3. RADIOWAVES (10 – 103M – FM/AM region)

  • affects the spin of certain atomic nuclei
  • analytical technique – NMR spectroscopy
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2
Q

What are examples of analytical tools and what they can tell us about the structure of the molecule?

A
  • UV/Vis spectroscopy - Is there a conjugated pi system present?
  • Infrared spectroscopy - What functional groups are present?
  • Mass spectroscopy - Size and formula
  • NMR - What Carbon-hydrogen framework is present?
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3
Q

What is the order of EMR

What properties does EMR display?

A

EMR is energy that displays both particle and wave properties

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4
Q

What is a particle of EMR called?

How is the energy of this particle calculated?

A
  • A particle of electromagnetic radiation is called a photon.
  • The relationship between the energy (E) of a photon and the frequency (n) of the photon is:

E = hv

  • Where energy is measured in joules (J)
  • Frequency is measured in Herts (Hz) or cm-1
  • h is a constant called Planck’s constant = (6.626x10-34 J s)
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5
Q

What does the wave component of EMR consist of?

A
  • Electromagnetic radiation consists of waves of 2 vectors at right angles to each other: Eletric and magnetic vector
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6
Q

What parameters can be used to describe waves?

A
  • Frequency - number of waves produced in 1s (Hz or cm-1)
  • Wavelength - distance between the same point of 2 sucessive waves (cm)
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7
Q
  1. What is the speed of light?
  2. What equation is used to calculate this?
  3. What is the relationship between E, v, Wavelength?
  4. What alternative parameter can be used instead of wavelength and why?
  5. What is the equation for this?
A
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8
Q

What happens in UV/ Vis spectroscopy?

A
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9
Q

What are types of possible chromophores?

A
  1. Polyenes
  2. Aromatics
  3. Conjugated carbonyl compounds
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10
Q

What are polyenes and what does there spectra look like?

A
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11
Q

What do conjugated carbonyl compounds look like?

What electron transisitions are involved?

A
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12
Q

What does the spectra for a conjugated carbonyl compound look like?

A
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13
Q

What do aromatic spectras look like?

A
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14
Q

How do substituents effect the wavelength absorbed off molecules?

A
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15
Q

Structure Elucidation – Secondary Technique:

What does it mean when absorption peak present and not present?

A
  • No absorption peak → no conjugated π – system
  • If an absorption peak is present – then a long a indicates a long or extended chromophore.
  • If an absorption peak is present – then a short a indicates a short chromophore.
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16
Q

What is the beer lamber law equation?

What do values of E tell you about the structure/ transitions of a molecule?

A
17
Q

What are the limitations of UV spectroscopy?

A
  • Cannot prove the identity of a compound but only indicate the structure.
  • Compounds with different overall structures but the same chromophore will give the same UV characteristics.
18
Q

Pharmaceutical applications of IR spectroscopy

A
  • Qualitative fingerprint check for identity of raw materials used in manufacture and for identifying drugs
  • Identify functional groups in structure
  • Can analyse semi-solids and solids e.g. creams and tablets
19
Q
  1. What type of spectrospcopy is IR?
  2. What does IR do?
  3. How is data plotted?
  4. What is the difference in E in UV and IR?
A
20
Q

What types of stetching/ bending can occur from IR absorption? What bonds/ molecules are capable of this?

A
21
Q

Some bonds in molecules have multiple ways they can vibrate/ bend/ stretch.

  1. Why is this?
  2. What does this look like in terms of spectra?
A
22
Q

What does the absorption frequency of a bond depend on?

A
  • Mass of atoms attached to bond
  • Strength of bond
23
Q

How does the mass of atoms and stength of bond affetc frequencu absorbed?

A
24
Q

Grid of bond strength and frequency absorbed

A
25
Q

View image

A
26
Q

Bond IR frequency Bond strength (kJ mol-1)

C-H 2900-3200 440

N-H 3300-3400 450

O-H 3500-3600 500

What is the order of bond strength and which bond requires most E for vibration?

A
  • O-H bond requires more energy to make stretch than N-H
  • O-H bond stronger than N-H, stronger than C-H
27
Q

Look at

A
28
Q

What samples states of samples can be analysed using IR - how are each carried out?

A
  • Liquid films
    • Drop of liquid spread between 2 IR plates (not suitable for volatiles)
  • Solutions
    • Must select appropriate solvent based on solubility of sample
    • Background correction using blank solvent
  • Solids
  • Ground solid to a paste and add mulling agent (nujol – an oil)
  • Paste is then placed between 2 IR plates
  • Or, mix sample with dry KBr in a mortar then subject to high pressure in an evacuated die to produce transparent disc
  • Gases
    • Since densities are much less than liquids, need longer pathlengths (usually 10 cm or more)
29
Q

Describe the process of analysising a solid.

A
  • Make a solution in a solvent that does not have a complex spectrum (solvent also absorbs à own peaks). Typical solvents are chloroform and carbon tetrachloride. Use a NaCl cell.
  • Mix the ground solid with liquid paraffin or nujol à paste known as a ‘mull’.
  • Sandwich the paste between 2 NaCl plates.
  • Mix with dry KBr in a mortar and compress into a disc. No interference due to KBr, but difficult to make good discs.
  • However, new spectrometers do not need the press and the
  • KBr/sample can be mixed and scanned in a pan.
  • Some instruments do not need the KBr – just the solid.
  • Attenuated total reflection (ATR) can analyse samples directly in the solid or liquid state.
30
Q

How does ATR work?

A

ATR uses a property of total internal reflection resulting in an evanescent wave. A beam of infrared light is passed through the ATR crystal, made of germanium, KRS-5, zinc selenide, or diamond while silicon is ideal for use in the Far-IR region.

In the case of a liquid sample, pouring a shallow amount over the surface of the crystal is sufficient.

In the case of a solid sample, it is pressed into direct contact with the crystal to remove trapped air.

31
Q

Draw backs of solid preperation?

A
  • NaCl dissolves in water, therefore, solvent and sample needs to be DRY! Also the sweat from your hands dissolves these cells!!
  • Sandwich the paste between 2 NaCl plates. Also get absorption due to paraffin / nujol, but it is quick to run and modern

instruments can abstract background nujol / paraffin.

32
Q

How do you prepare a liquid sample?

A

Place 1 drop between two NaCl plates. ‘THIN FILM’ method.

Or place a drop of liquid directly onto the ATR.

Solutions of a known concentration can also be prepared in a solution cell with a solvent such as dichloromethane

33
Q

Discuss analysis or IR and BRitish pharmacopeia

A
  • In order to prove a drug is what it should be, the BP has IR spectra of standard compounds
  • The IR spectra of an unknown must match EXACTLY with that from the BP
    • Includes all peaks and relative intensities
  • The fact that functional groups give absorptions at particular frequencies can be used to determine the purity of a compound.
  • Contamination from solvent residues or by-products will show absorptions not observed in a pure compound.
  • Possible to distinguish the same compound marketed by a number of different manufacturers
    • Spectra will be slightly different

Kirsty 3.1 (47 decks)

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