MCAT Organic Chemistry Flashcards
A destabilizing interaction between substituents that occupy axial positions on the same side of a cyclohexane ring
1,3-diaxial interaction
Epimers formed by ring closure
Anomers
The most stable conformation of cyclohexanes
chair conformation
Any compounds that have the same molecular formula and the same connectivity but that differ from one another by rotation about a sigma bond
Conformational isomers
Any compounds with the same molecular formula but whose atoms have different connectivity
Constitutional isomers
Molecules that are mirror images and non-superimposable
Enantiomers
Diasteromers that differ in configuration at only one of many chiral centers
Epimers
Diastereomers that differ in orientation of sibstituents around a ring or double bond
Geometric isomers
A molecule that contains chiral centers and an internal plane of symmetry
Meso
Any compound with the same molecular formula and connectivity that differ only in the spatial arrangement of atoms, are known as stereoisomers. Note that if the compounds only differ by rotation around a sigma bond they are not stereoisomers but confirmational isomers
Stereoisomers
A purification method based on a difference in boiling points
Distillation
A separation technique that relies on relative solubilities of the two solvents used
Extraction
A method that detects the vibration of covalent bonds and can differentiate their frequencies, which is related to the type of bond
Infrared spectroscopy
A rapid technique used to separate compounds based on their polarity
Thin-layer chromatography (TLC)
Forces between temporary dipoles formed in nonpolar molecules because of a temporary asymmetric electron distribution
van der Waals Forces
not forming a homogeneous mixture when added together.
Immiscible
Why is it best to use a 5% sodium bicarbonate solvent when trying to extract carboxylic acids?
A solvent of dilute sodium hydroxide could do it, but it would also convert phenols into their anionic salts
What would you use to extract phenols in an extraction technique?
I would use 10% sodium hydroxide to convert phenols into their corresponding anionic salts
In thin layer chromatography, compounds are separated based on differing___
polarities
In TLC, the more polar components of the mixture interact more with the polar stationary phase, and travel at a __
slower rate
In TLC, the less polar components have a greater affinity for the solvent than the stationary phase, and travel with the mobile solvent __
at a faster rate than the more polar components
What is the Rf value in TLC
It is the distance traveled by an individual component divided by the distance traveled by the mobile solvent
What is TLC good for doing?
TLC is a good technique for separating very small amounts of material in order to assess how many compounds make up a micture
When would you use column (flash) chromatography over thin-layer chromatography?
It is best to use flash chromatography to isolate bulk compounds
In flash chromatography, the more polar compound will___
spend more time adsorbed on the polar solid phase, travel slowly down the column than nonpolar compounds
Compounds in flash chromatography can be expected to leave the column ___
in order of polarity (least polar compounds first, then most polar compounds)
When would you use ion exchange chromatography?
When the materials being separated have varying charge states
____is frequently used in the separation of mixtures of proteins
Ion exchange chromatography
In a cation exchange resin, ion exchange chromatography, proteins with pI values greater than the pH of the mobile phase ___
will be positively charged and elute slowly compared to those with pI values below the solution pH
In an anion exchange resin, proteins with pI values above the pH of the solution___
will elute first
Why is the speed and efficiency if High Performance Liquid Chromatography more than column chromatography
The mobile phase is forced through the stationary phase at very high pressure, both the speed and efficiency of the separation is increased, it is an improvement over column chromatography
In HPLC, _____ elute first
More polar compounds elute first in HPLC as they have a higher affinity for the mobile phase
Why do nonpolar compounds elute slower from HPLC?
They interact longer with the nonpolar stationary phase, they elute last
___is a techinque used to separate bulk materials based on molecular size
Size Exclusion Chromatography
In size exlusion chromatography which sort of molecules elute faster and why?
Size exlusion chromatography uses chemically inert porous polymer beads, that allow permeation of small molecules through them, larger molecules move down by the interparticle pathway, smaller particles go down by the intraparticle pathway. The larger particles have a more direct pathway and elute faster
___ is used for the separation of full proteins from smaller peptide chains
Size exclusion chromatography, HOWEVER the technique is not effective when separating two compounds that are similar in size
___ is most commonly used to purify proteins or nucleic acids from complex biochemical mixtures like cell lysates, growth media, or blood, rather than a reaction mixture
Affinity chromatography
___ is a separation technique that takes place between the mobile gas phase and a stationary liquid phase
Gas chromatography
The separation of compounds in a mixture using gas chromatography is based on their __
different volatilities
In Gas chromatography, if the interactions of the substrates with the column are similar, the more volatile components emerge from the column__
the more volatile components emerge from the column first, and the less volatile components emerge from the column later
Branching tends to __
inhibit van der Waals forces by reducing the surface area available for intermolecular interactions
___is the process of raising the temperature of a liquid until it can overcome the intermolecular forces that hold it together in the liquid phase
Distillations
___is performed when trace impurities need to be removed fro ma relatively pure compound, OR when a mixture of compounds with significantly different boiling points need to be separated
Simple distillation
Fractional distillation is used when __
the difference in boiling points of the components in the liquid mixture are not that large
How does absorption spectoscopy work?
Molecules are exposed to various forms of light, exciting the molecules to a higher energy state, then when energy is released as their relax back to ground state the measured energy can reveal structural features of the molecules in the sample
___allows researchers to determine the mass of compounds in a sample
Mass spectrometry
What occurs in the mass spectrometer?
Molecules are ionized in a high vacuum by bombarding them with high energy electrons, then they enter region of spectrometer where they are acted on by a magnetic field that causes a change in flight path based off the mass of the ion.
On a mass spectrometer, bromine will appear as ___
79 amu or 81 amu nearly equal in height
On a mass spectrometer chlorine will occur in two isotopes at ___
35 amu (75% abundance) and 37 amu (25% abundance)
Mass spectra for chlorinated molecules will have __
a peak 2 amu heavier than the main peak and about one-third its height
What kind of molecules are ultraviolet/visible spectroscopy used for?
It is very useful in monitoring complexes of transition metals in the closely spaced d-orbitals
-MAINLY : it is used to study highly conjugated organic systems
Describe how conjugation affects ultraviolet and visible spectroscopy
The wavelength of maximum absorption for any compound is directly related to the extent of cinjugation in the molecule, the more extensive the conjugated system is, the LONGER the wavelength of maximum absorption will be
The color a compound maximally absorbs is __
complementary to the color it will appear to our eyes
Example: IF i absorb BLUE i appear ORANGE
If i absorb RED I appear GREEN
If I absorb Yellow I appear VIOLET
On IR spectroscopy a very strong and sharp peak at 1700 cm-1 is a
Carbonyl group
C=O
On IR spectroscopy, Alkenes are centered around _____
1650 cm-1
If you don’t see a carbonyl group in IR spectroscope, you can eliminate a bunch of functional groups such as
aldehydes, ketones, carboxyic acids, acid chlorides, esters, amides, anhydrides
A triple bond on IR spectroscopy will appear between
2260-2100 cm-1
Why is there broadness in the O-H stretch on IR spectroscopy?
The broadness is due to the hydrogen bonding
The O-H stretch on IR spectroscopy can be found around ___
3600-3200 cm-1
The C-H stretch for sp3 carbons on IR spectroscopy is
3000-2850 cm-1
The C-H stretch for sp2 carbons on IR spectroscopy is __
3150-3000 cm-1
The C-H stretch for sp carbons on IR spectroscopy is __
3300 cm-1
In all types of NMR spectroscopy light from the __ of the electromagnetic spectrum is used to induce energy absorption
Radio frequency range
Equivalent hydrogens in H NMR are those that__
have identical electronic environments. They are considered equivalent if they can be interchanged by a free rotation or a symmetry operation (mirror plane or rotational axis)
A set of protons near an electronegative group is to be ___ and will appear ___ in the H NMR spectrum
deshielded and will appear DOWNFIELD (to the LEFT)
3 factors involed in proton shielding are
- electronegativity of the neighboring atoms
- hybridization
- acidity and hydrogen bonding
The greater the s-character, the more ___
deshielded the set of protons is and will appear DOWNFIELD (left)
An aromatic proton on an H NMR will appear around
6.5-8 ppm
an alkene proton will appear on an H NMR around
5-6 ppm
