Organic Chemistry- Spectroscopy Flashcards
What procedure would be the most efficient to identify an unknown compound?
Spectroscopy
What is spectroscopy?
Measures the energy difference between the possible states of a molecular system by determining the frequencies of electromagnetic radiation absorbed by the molecules.
The different levels of the frequencies absorbered by the molecule are associated with different types of molecular motion like what?
Molecular rotation, vibration of bonds, electron absorption and nuclear spin
Different types of spectroscopy measure different types of _________?
Molecular properties.
Spectroscopty allows us to identify _______ and _______.
The presence of specific funtional groups and The onnectivity (backbone) of a molecule.
In medical context, what is spectroscopy important in?
Magnetic resonance imaging (MRI)
MRI scanners actually measure what and then do what?
H-NMR spectra of water molecules in different environments in the body. They then convert these signals into greyscale, allowing excellent visualization of the body, especially soft tissue.
What is the big advantage of laboratory spectroscopy?
Only a small quantity of a sample is needed. Also the sample may be reused after a test is performed.
What is the downside of spectroscopy?
It’s difficult to do without special equipment.
What does infrared spectroscopy measure?
Molecular vibrations, which can be seen as bond stretching, bending, or combinations of different vibrational modes.
How do you record the Infrared spectrum?
Infrared light is passed through a sample, and the absorbance is measured.
By determining the bonds in a molecule, what can be understood?
Hope to infer the functional groups in the molecule.
What is the infrared light range?
Wavelength of 700nm to 1mm
What is the useful absorptions for spectroscopy?
Wavelengths of 2500 to 25,000nm
On IR spectrum, we use an analog of frequency called what?
Wavenumber
What is the standard range of wavenumber?
2500 to 25,000nm is 4000 to 400cm^-1
When light of the standard range of wavnumber is reached what happens?
The molecules eneter excited vibrational states.
What are the types of vibrations that can occur in IR spectroscopy?
Twisting and folding
In Infrared spectroscopy, more complex vibration pattern are caused by what?
The motion of the molecule as a whole.
In IR spectroscopy, in what range can more complex vibration patterns be seen? What is it called?
1500 to 400cm^-1
Called the Fingerprint region
Why is it called the fingerprint region in IR spectroscopy?
The specific absorbance pattern is characteristic of each individual molecule
For an absorption to be recorded, the vibration must result in a change in what?
The bond dipole moment.
Which type of molecules do not exhibit absorption?
Molecules that do not experience a change in dipole moment, such as those composed of atoms with the same electronegativity or molcules that are symmetrical.
What are examples of molecules that do not exhibit absorption?
O2
Br2
Triple bond in acetylene (C2H2)
What groups do you need to memorize the absorptions?
Hydroxyl group
What does the absorption of the O-H group look like?
Absorbs with a broad (wide) peak at around one of two frequencies: 3300cm^-1 for alcohols, and 3000cm^-1 for carboxylic acids.
Why is the frequency for O-H group absorption lower for carboxylic acids?
The carbonyl of a carboxylic acid pulls some of the electron density out of the O-H bond, whifting the absorption to a lower wavelength.
What does the absorption for the carbonyl look like?
It absorbs around 1700cm^-1 with a sharp deep peak.
What are some common characteristics of absorption?
The bond between any atom and hydrogen always has a relatively high absorption frequency and how as we add more bonds between carbon atoms, the absorption frequency increases.
What does the N-H bonds absorption frequency look like?
It’s in the same region as O-H bonds around (33oocm^-1) but have a sharp peak intead of a broad one.
For the MCAT, all information comes from the frequencies between What?
1400 and 4000cm^-1
How are IR spectrum plotted on a graph?
As percent transmittance, the amount of light that passes through the sample and reaches the detector vs wavenumber.
Will you even have to interpret ultra violet spectroscopy on the MCAT?
No, not data, but it may be discussed.
How are UV spectra obtained?
By passing ultraviolet light through a sample that is usually dissolved in an innert, nonabsorbing solvent, and recording the absorbance.
The absorbance of UV spectra are plotted against what?
Wavelength
How is absorbance caused in UV spectrum?
b electronic transitions between orbitals.
What is the biggest information we get from a ultra violet spectroscopy?
The wavelength of maximum absorbance
What does the wavelength of maximum abosrbance tell us?
The extent of conjugation within conjugated systems
The more conjugated the compound, the lower _______ and the greater the _______.
Lower the energy
The greater the wavelength of maximum absorbance
Why does UV spectroscopy work?
Molecules with pi-electrons or nonbonding electrons can be excited by ultraviolet light.
Molecules with a lower energy gab between HOMO and LUMO are more easily what?
Excited and can absorb longer wavelengths (lower frequencies) with lower energy.
What does HOMO and LUMO stand for?
HOMO: Highest occupied molecular orbital
LUMO: Lowest unoccupied molecular orbital
What are conjugated molecules?
Molecules wtih unhybridized p-orbitals
What type of molecules can be excited by ultraviolet light?
Conjugated molecules
What does conjugation do to wavelengths?
Shifts the aborption spectrium, resulting in higher maximum wavelengths (lower frequencies)
What is an example of a molecule that can be seen with UV spectrum?
Benezene has three broad absorbances, which mark the energy level transitions; these are found at 180, 200, and 255nm wavelengths
Larger conjugated molecules may what, in UV spectroscopy?
Aborb light in the visible range, leading to color
What is UV spectroscopy somteimes called? Why?
UV-Vis spectroscopy
Because the technique for UV spectroscopy cal also be used at visible wavelengths
What is the most important spectrscopic technique on the MCAT?
Nuclear magnetic resonance (NMR) spectroscopy
NMR spectroscopy is based on what?
The fact that certain atomic nuclei have magnetic moments that are oriented at random. When such nuclei are placed in a magnetic field, their magnetic moments tend to align either with or against the direction of this applied field
What type of nuclei are in the alpha-state?
Nuclei with magnetic moments that are aligned with the field
The alpha-state is what?
Lower energy
What nuclei are in the beta-state?
The nuclei can then be irradiated with radiofreuqncy pulses that match the energy gap between the two states
What is the beta-state?
Higher energy
The absorption of the radiation leads to what?
Excitation at different frequencies.
Excitement at different frequencies is dependent on what?
Dependent on an atom’s magnetic environment. In addition, the nuclear magnetic moments of atoms are affected by nearby atoms that also possess magnetic moments.
What is MRI?
Magnetic resonance imaging (MRI) is a noninvasive diagnostic tool that uses proton NMR
What happens in an MRI?
Multiple cross-sectional scans of the patient’s body are taken, and the various chemical shifts of abosrbing protons are translated into specific shades of grey. This produces a picture that shows the relative density of specific types of protons.
A dark area on a T1-weighted MRI tends to correspond with what?
Water
A light area on a T1-weighted MRI tends to correspond to what?
Fattier tissue
Diagnosticians compare what to detect what’s in a MRI?
Normal MRI comparison help to detect abnormalities in the scanned region.
How is a typical NMR spectrum plotted?
Frequency vs. Absorption of energy
What is the standardized method of plotting NMR spectrum?
Use an arbitrary variable called chemical shift. The chemical shift is plotted on the x-axis, and it increases toward the left (referred to as downfield). To make sure that we know just how far downfield compounds are , we use tetramethylsilane (TMS) as the calibration standard to mark 0 ppm
What are the units of chemical shift?
Units of parts per million (ppm) of spectrometer frequency
NMR is most commonly used to study what?
1H nuclei (protons)
What type of atoms can be studied using NMR?
Any atom possessing a nuclear spin (with an odd atomic number, odd mass number, or both)
What are examples of atoms that can be studies using NMR?
13C 19F 17O 31P 59Co
In NMR, most hydrogen nuclei come into resonance where?
0-10 ppm downfield from TMS
Each distinct set of nuclei give rise to what?
A separate peak
If multiple protons are chemically equivalent, what happens?
Having the same magnetic environment will lead to the same peak
When graphing NMR spectrum, the height of each peak is dependent on what?
Is proprotional to the number of protons it contains
What is the area under the peaks called in NMR plotting?
Integration
Analyzing the integration of each peak tells you what?
The ratio of the points corresponding eactly to the ratio of protons that produced each peak.
What happens to the hydrogens that are attached to other electronegative molecules?
These atoms pull electron density away from the surrounding atoms, ths deshielding the proton from the magnetic field.
The more the proton’s electron density is pulled away, the less ______ resulting in what?
The less it can shield itslef from the applied magnetic field, resulting in a reading further downfield.
What would help shield a hydrogen’s nuclei and give it more of a position upfield?
If there was an electron-donating groups, such as the silicon atom in TMS, it would help shield the H nuclei
What does TMS stand for?
Tetramethylsilane
Why is TMS used as the reference?
Everything else in proton NMR will be more shielded thatn TMS
What does it mean when a compound contains protons that are within 3 bonds of each other?
A compound in which there are hydrogens on two adajacent atoms
What happens when protons are in such close proximity to each other, that are not magnetically identical?
Spin-spin couping (splitting) occurs
What occurs to the magnetic environment when prtons are in such proximity?
The magnetic environment of Ha can be affected by Hb, and vice versa. Thus, at any given time, Ha can experience two different magnetic environments because Hb can be in either the alpha- or beta-state. The different states of Hb incfleunces the nucleus of Ha, causing slight upfield and downfield shifts.
What does it mean when an obsorption is a doublet?
There is an appoximately a 50% change that Hb will be in either of the two states, so the resulting absorption is a doublet
What is a doublet?
Two peaks of identical intensity, equally spaced around the true chemical shift of Ha
What will the two protons in such close proximity appear as? Why?
Ha and Hb will both appear as doublets because each one is coupled with one other hydrogen
How do we determine the number of peaks present?
Use the N + 1 rule: if a proton has n protons that are three bonds away, it will be split into n + 1 peaks
What protons should you not include if they are 3 bonds away?
Protons attached to oxygen or nitrogen
What is coupling constant, J? What are units?
The magnitude of splittting, measured in hertz
In 1,1-dibromo-2-chloro-ethane, the Ha nucleus is affected by two nearby Hb nuclei, and can be in one of what four different states?
alpha alpha
alpha beta
beta alpha
beta beta
How many unique frequencies will 1,1-dibromo-d-chloro-ethane have? Why?
Three because alpha beta and beta alpha are the same
In 1,1-dibromo-2-chloro-ethane, Ha will apear as what?
Three peaks (a triplet) centered on the true chemical shift, with an area ratio of 1:2:1
In 1,1-dibromo-2-chloro-ethane, Hb will apear as what? Why?
A doublet because both hydrogens are attached to the same carbon, which can freely rotate, they will be magnetically identical.
What will the integration for 1,1-dibromo-2-chloro-ethane of Hb look like?
It will be large than the triplet of Ha
What is multiplet?
Peaks that have more than four shifts
What are the area ratios for the number of adjacent hydrogen and total number of peaks?
Hydrogens # of peaks Area ratio
0 1 1
1 2 1:1
2 3 1:2:1
3 4 1:3:3:1
4 5 1:4:6:4:1
5 6 1:5:10:10:5:1
6 7 1:6:15:20:15:6:1
7 8 1:7:21: 35:35:21:7:1
What are the chemical shift ranges of deshielded aldehyde?
9-10 ppm
What is the chemical shift range of deshielded carboxylic acid?
10.5-12 ppm
What is the chemical shift range of the hydrogen of a aromatic ring?
6.0-8.5ppm
What is the chemical shift range of hydrogens on sp3-hybridized carbons?
4.6-6.0 ppm
What is the chemical shift range of hydrogens on sp-hybridized carbons?
2.0-3.0 ppm
What do the electronegative groups do to the protons again?
They pull electron density away from the protons. The more electron density that is pulled away from the protons, the more deshielded it will be and the further downfield the proton will appear.
