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• Please attend all the lectures• In future notes will be placed on WebCT
• My office hours are 2-330pm Tuesdays and Thursdays• I will not re-teach during office hours• You must bring worked examples so that I can trouble shoot
your problems• There will be 3 of 4 midterms (70%) and one final exam (30%)
• Please read each chapter before the lecture or immediately after• Also answer the questions in the text. During revision answer the
questions at the end of the chapter• Do not look at the answers until you have made significant effort• It is easy to trick yourself into thinking you know the solution by
• In CHEM 2321 I showed you structures of products which results from specificreactions. Some examples are:
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• However how do we know? Did I just guess? • Well no, there are tools we can use to determine the structures of a molecule• These are mass spectrometry (MS), infrared (IR) spectroscopy, nuclear magnetic
resonance spectroscopy (NMR) and ultraviolet spectroscopy• When these techniques are used in combination we will see we can get a great deal
of structural information and use it to identify the molecule
• So why is it important to know the structure, why should we care?• In chemical research you will normally have a target molecule to make. However
you mostly likely will have to invent the route to make this molecule• This molecule may be a drug, catalyst or polymer for example, but how do you
know if you succeeded? Guess?• No: You’ll use the knowledge of the techniques just mentioned to find the structure
of the molecule(s) you just made• This brings me to the experiment cycle:• 1) Design an experiment to yield desired molecule• 2) Conduct the experiment• 3) Find out what happen!• This is where the techniques we will cover in the first two weeks or so will help
you!• 4) Plan your next move• 5) Design an experiment…and so on
• So what would we like to know about the molecule to draw a structure• 1) We’d like to know the mass (how big and what elements are in this thing!)• 2) What functional groups are present (is it an ester, amine etc..)• 3) Is there a conjugated system present (linked π-bonds, systems)• 4) How are these connected• Well each technique is like a piece of the jigsaw puzzle, which is the molecule’s
structure• So what technique tells us what•• Mass Spectrometry: Mass Spectrometry: What size and formula What size and formula •• Infrared Spectroscopy:Infrared Spectroscopy: What functional groups are thereWhat functional groups are there•• Ultraviolet spectroscopy: Ultraviolet spectroscopy: Is there a conjugated system presentIs there a conjugated system present•• Nuclear magnetic Resonance:Nuclear magnetic Resonance: What is the carbonWhat is the carbon--hydrogen frameworkhydrogen framework• Let’s begin with Mass Spectrometry………
• Mass Spectrometry is a way to measure the mass and therefore the MW of a molecule. From this we can get the molecular formula
• In addition as the molecule fragments (breaks apart) we can get additional information about the structure
• There are many types of mass spectrometers based on how they determine the mass, below depicts one type called magnetic sector, others are quadrapole, ion trap and time of flight (TOF). How ever all contain an ion source, mass analyzerand a detector
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• In order to determine a mass of a molecular fragment it must be an ion, positive or negative
• Due to the large amounts of energy transferred to the molecule it normally fragments into smaller pieces, some will be positively charged and some will be neutral
• The charged fragments can be controlled by the magnetic fields and are guided and focused so they can be detected. Neutral fragments cannot so they hit walls in the machine etc and are therefore not detected
• The spectrometry sorts the fragment but their mass to charge ration m/z since the charge is normally +1 this value is simply the mass of the fragment
• The data we get from a mass spectrometer is normally shown as a bar graph of m/z (X axis) and number of ions (intensity) on the y axis. The tallest peak is normally assign 100% intensity and is called the base peak
• A peak that corresponds to the unfragmented molecule ion is called the parent peak or molecular ion (M+)
• So what information can we get from these spectra• Well the molecular weight, we can use this to see if it matches the product we
expect from the reaction. Hexane (MW = 86) 1-hexene (84) 1-hexyne (MW = 82) can easily be distinguished
• With some high resolution mass spectrometers (0.0001 amu, atomic mass units) we can determine the chemical formula
• For example C5H12 and C4H8O both have the MW of 72. However C5H12 has an exact mass of 72.0939 amu and C4H8O has an exact mass of 72.0575 amu
• A high resolution machine can easily tell them apart• Note that exact mass refers to molecules with specific isotopic compositions.
Therefore normally you use the mass of the most abundant isotope 1.00783 for 1H and 12.00000 for 12C etc…
• If you cannot see your M+ with EI then you would normally use a softer (gives the molecule less energy therefore less fragmentation) ionization technique for example CI or ESI
• Knowing the molecular weight means we can narrow down the choices of molecular formula
• A molecule with a m/z of 110 could yield a formula of C8H14, C7H10O or C6H10N2
• A further point is atoms have isotopes therefore we can use the exact mass to narrow the molecular formula to one. We can see the effect of isotopes if we look at the mass spectrum of methane again
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Small peak at M+1 due to 13C or 2H incorporation as their natural abundance is 1.10% and 0.015% respectively
• Once we know the formula we can calculate the level of unsaturation and from this we know how many double bonds, rings etc… we need in the structure and we can begin to think of structures
• If all that MS gave was the mass, therefore the molecular formula, it would still be a great analytical method. However, how the molecule fragments also provides a great deal of information
• When we use EI each molecule has a unique fragmentation pattern like a finger print. The reason why the fragmentation patterns are so different is due to chemical structure so it is unlikely that two compounds with have identical patterns
• In fact the EI pattern is often used for identification (we’ve seen CSI). This works by a computer program comparing the spectrum against an know database for example National Institute of Standards and Technology (NIST) database
• When fragmentation occurs the radical cation in the case of EI breaks apart forming one cation fragment and one radical fragment
• These fragments can give you structural information about a molecule, unsurprisingly the way a molecule fragments, which part of the molecule becomes the cationic fragment and which the radical fragment, is govern by stability
• Let’s consider the fragmentation of 2,2-dimethylpropane when it fragments it does so, so that the positive charge is on the tert-butyl group
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• Due to the stability of this cation it is the base peak and has an m/z of 57 • So how can we use fragmentation to get structural info: well you just learned that
the presence of a strong peak at an m/z of 57 could be a tert-butyl group• Let’s look at some more; hexane shows signals at 86 (molecular ion), 71, 57, 43,
• Common fragmentation mechanisms: So we know that when we use EI we get a radical cation, most common forms are: [Detail mechanism on board: α-cleavage, C-Y cleavage and H-Y elimination]
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• Common fragmentation mechanisms of functional groups:• Alcohols: Fragment by α-cleavage and H-Y elimination; in this case dehydration
• The equation before basically says that the energy of a photon is directly proportional with its frequency (higher frequency higher energy). However inversely proportional with wavelength.
• Take home message high frequencies, small wavelengths mean high energy!• If we multiple the energy value by Avogadro’s number NA we arrive at numbers we
are more use to!
λλmolkJ
mhcNE A /1020.1
)(
4−×==
• E = Energy of mole photons at a specific wavelength• h = Plank’s constants (J•s)• ν = Frequency (Hz)• λ = Wavelength (m)• c = Speed of light
• Ok so now we have a rough idea of the energy of light at variouswavelength/frequencies but as organic chemists we are interested on how this light interacts with organic molecules
• Remember light is one way we can a energy to a molecule!• So when we expose an organic molecule to light (shine light on it), the molecule may
absorb light of a specific wavelength and transmit light of other wavelengths. When this ocurrs in the visable range we see different colours!
• We can measure this and we produce an absorption spectrum below shows and absorption spectrum when we shine infrared light on ethanol
• So what happens when a molecule absorbs the radiation? • Well this energy is distributed over the molecule in some way, this may lead to a
bond stretching and bending, or for an electron to jump into a higher energy orbital• As you will see the different frequencies of light do different things and all tell us
structural information.• To answer that all important question, what did I just make!• Let’s first consider what happens when we shine infrared on a molecule• The region of the Infrared (IR) spectrum most interesting for organic chemists ranges
from 3 ×10-6 to 3 ×10-5 m• With frequencies expressed as wave numbers
• Molecules have energy and this cause bonds to stretch and bend as well as other vibrations to occur some allowed vibrations are shown below
• Remember energy in molecules is quantized this means that a molecule can only stretch and bend at specific frequencies
• One way to picture this is shown below:
• When a molecule is irradiated with electromagnetic radiation energy is absorbed if the frequency of the radiation matches the frequency of the vibrating bond
• The result of the energy absorption results in the bonds stretching and compressing a little more that before absorption
• Because each frequency absorbed by a molecule corresponds to a specific molecular motion, we can identify these by measuring the IR spectrum
• With this information we can get structure information
• However as mentioned vibration energy is quantized, it must follow certain rules
• This means that the lowest energy level E0 = ½ hν the next E1 = 3/2 hν and so on. However this rule can sometimes be over come and leads to overtones
• However a molecule is not a harmonic oscillator it is an anharmonic oscillator• This is because a bond can come apart and cannot be compress beyond a certain
point. Note how the allowed transitions become closer together in the anharmonicoscillator as the energy goes up therefore calculations can over predict.
• For a diatomic the following equation has been derived,
• By obtaining the IR spectra we can see the molecule motion and the correlate this to functional groups. Therefore IR is used to see what functional groups are in a molecule
• The Full interpretation of and IR spectrum is difficult due to its complexity however the region between 4000 cm-1 and 1500 cm-1 has indicative bands of function groups
• Between 1500-400cm-1 is called the fingerprint-region and can be used to identify compounds
• So let’s have a look at some • C=O absorption of a ketone range 1680 - 1750 cm-1
• O-H absorption of a alcohol range 3400 - 3650 cm-1
• C=C absorption of an alkene ranges 1640 - 1680 cm-1
• Intensity of absorption depends on the dipole moment the higher the dipole moment the higher the absorption no dipole, no absorption!
• Other important shifts are in your handout and overleaf…..
• Alkanes: IR spectra of these are fairly uniform as only C-H and C-C bonds are present
• Alkenes: Interestingly it is possible to use IR to determine substitution patterns on C=C as for mono and di-substitute alkene there is a =C-H out of plane bend between 700-1000 cm-1
• The structure of a molecule is determined by spectroscopic methods• Two of these are Mass Spectrometry (MS) and Infrared Spectroscopy (IR)• In MS ions are first formed by collision with a high energy electron beam• The ion may fragment during this process, all ion are sorted according to their
mass-to-charge ratio m/z• The ionized molecule is called the molecular ion, M+, and its weight is that of the
molecule being analyzed• Structure clues can be obtained from the fragmentation patterns• Infrared spectroscopy (IR) involves the interaction of the molecule with
electromagnetic radiation, during this process certain frequencies are absorbed• Since every function group has a characteristic combination of bonds every
functional group has unique IR absorptions• For example C=O, C=C and C≡C-H all have unique bands• By running an IR it is possible to see what functional groups are present