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Kaplan O-Chem
MCAT O-Chem
| Term | Definition |
|---|---|
| Shielding | The phenomenon of atoms pushing electron density toward surrounding atoms; in NMR spectroscopy, pulls a group further upfield on the spectrum. Deshielding is the opposite—the pulling of electron density by surrounding atoms; in NMR, pulls a group further downfield on the spectrum. |
| Nuclear Magnetic Resonance Spectroscopy (NMR) | A technique that measures the alignment of magnetic moments from certain molecular nuclei with an external magnetic field; can be used to determine the connectivity and functional groups in a molecule. |
| Lowest Unoccupied Molecular Orbital (LUMO) | The lowest-energy molecular orbital that does not contain electrons; in UV spectroscopy, electrons are excited from the HOMO to the LUMO. |
| Highest Occupied Molecular Orbital (HOMO) | The highest-energy molecular orbital containing electrons; in UV spectroscopy, electrons are excited from the HOMO to the LUMO. |
| Coupling | In NMR spectroscopy, a phenomenon that occurs when there are protons in such close proximity to each other that their magnetic moments affect each other's appearance in the NMR spectrum by subdividing their peaks into subpeaks; also called splitting. |
| Chemical Shift (/δ/) | An arbitrary variable used to plot NMR spectra; measured in parts per million (ppm). |
| Wavenumber | An analog of frequency used for infrared spectra instead of wavelength. |
| Ultraviolet Spectroscopy (UV) | A technique that measures absorbance of ultraviolet light of various wavelengths passing through a sample. |
| Spectroscopy | Laboratory technique that relies on measurement of the energy differences between the possible states of a molecular system by determining the frequencies of electromagnetic radiation (light) absorbed by the molecules or their response to a magnetic field. |
| Infrared Spectroscopy (IR) | A technique that measures molecular vibrations at different frequencies, from which specific bonds can be determined; functional groups can be inferred based on this information. |
| Fingerprint Region | In an IR spectrum, the region of 1400 to 400 cm⁻¹ where more complex vibration patterns, caused by the motion of the molecule as a whole, can be seen; it is characteristic of each individual molecule. |
| Stationary Phase | The phase over which the mobile phase runs during chromatography; usually a solid material. Also called the adsorbent. |
| Retardation Factor (/R/_f) | A ratio used in thin-layer chromatography to identify a compound; calculated as how far the compound traveled relative to how far the solvent front traveled. |
| Mobile Phase | A liquid (or a gas in gas chromatography) that is run through the stationary phase in chromatography. |
| Immiscible | Describes two solvents that will not mix with or dissolve each other. |
| High-Performance Liquid Chromatography (HPLC) | A form of chromatography in which a small sample is put into a column that can be manipulated with sophisticated solvent gradients to allow very refined separation and characterization; formerly called high-pressure liquid chromatography. |
| Gas Chromatography | A type of chromatography used to separate vaporizable compounds; the stationary phase is a crushed metal or polymer and the mobile phase is a nonreactive gas. |
| Extraction | A method used to separate a component in a mixture by exploiting its solubility properties. Two solvents are usually used (one aqueous and one organic), and the component of interest will be soluble in one phase, while the impurities will be soluble in the other. |
| Distillation | A separation technique used to separate liquids with different boiling points. The mixture is heated slowly, and as the liquid with the lower boiling point changes to its gaseous form, it passes through a condenser, where it cools back into its liquid form. |
| Chromatography | A separation technique that uses the retention time of a compound in the mobile phase as it travels through the stationary phase to separate compounds with different chemical properties. |
| Ketone | A compound that has a nonterminal carbonyl group. Ketones are named by replacing the -e in the corresponding alkane with -one. |
| Carboxylic Acid | A compound that has a -COOH terminal group. Carboxylic acids are named by replacing the -e in the corresponding alkane with -oic acid |
| Carbonyl | A functional group consisting of a double bonded carbon and oxygen. |
| Amide | A compound that has a carbonyl group bonded to nitrogen. Amides are named by dropping the -oic acid in the corresponding carboxylic acid and adding -amide. Substituents attached to nitrogen are listed as prefixes and are preceded by N-. |
| Aldehyde | A compound that has a carbonyl as a terminal group. Aldehydes are named by replacing the -e in the corresponding alkane with -al. Formaldehyde, acetaldehyde, and propionaldehyde are common names used for the simplest aldehydes. |
| α-Carbon | The carbon adjacent to a carbonyl; in amino acids, the chiral stereocenter in all amino acids except glycine. |
| Anhydride | A functional group containing two carbonyls separated by an oxygen atom (RCOOCOR); often the condensation dimer of a carboxylic acid. |
| Alkane | Compounds consisting of only carbons and hydrogens bonded with σ bonds. As chain length increases, boiling point, melting point, and density increase. However, chain branching decreases both boiling point and density. |
| Phosphodiester Bond | A type of bond that links the sugar moieties of adjacent nucleotides in DNA. |
| Inorganic Phosphate (Pᵢ) | Derived from phosphoric acid, the molecule that forms high-energy bonds for energy storage in nucleotide triphosphates like ATP; also used for enzyme regulation. |
| Gabriel (Malonic-Ester) Synthesis | A method of synthesizing amino acids that uses potassium phthalimide and diethyl bromomalonate followed by an alkyl halide; two substitution reactions are followed by hydrolysis and decarboxylation. |
| Strecker Synthesis | A method of synthesizing amino acids that uses condensation between an aldehyde and hydrogen cyanide, followed by hydrolysis. |
| Thin Layer Chromatography (TLC) | A type of chromatography that uses silica gel or alumina on a card as the medium for the stationary phase. |
| Specific Rotation ([/α/]) | A standardized measure of a compound's ability to rotate plane-polarized light. |
| Newman Projection | A method of visualizing a compound in which the line of sight is down a carbon-carbon bond axis. |
| Meso-compound | A stereoisomer with an internal plane of symmetry. Meso compounds are optically inactive. |
| Equatorial | Describes groups on a cyclic molecule that are in the plane of the molecule. Contrast with axial. |
| Cis-Trans Isomers | A type of diastereomer with different arrangements of substituents about an immovable bond. |
| Chiral | A molecule or carbon atom bonded to four different groups and without a plane of symmetry; thus, it is not superimposable upon its mirror image and has an enantiomer. Contrast with achiral. |
| Axial | Describes groups on a cyclic molecule that are perpendicular to the plane of the molecule, pointing straight up or down. Contrast with equatorial. |
| Chiral | A molecule that either does not contain a chiral center or contains chiral centers and a plane of symmetry; as such, it has a superimposable mirror image. Contrast with chiral. |
| Torsional Strain | Increased energy that results when molecules assume eclipsed or gauche staggered conformations. |
| Structural Isomers | Molecules that have the same molecular formulas but different connectivity; also called constitutional isomers. |
| Steroisomers | Compounds with the same molecular formula and connectivity but different arrangements in space. Stereoisomers include cis-trans isomers, diastereomers, enantiomers, conformational isomers, and meso compounds. |
| Ring Strain | Energy created in a cyclic molecule by angle strain, torsional strain, and nonbonded strain; determines whether a ring is stable enough to stay intact. |
| Relative Configuration | The spatial arrangement of groups in a chiral molecule compared to another chiral molecule. |
| Racemic Mixture | A mixture that contains equal amounts of (+) and (-) enantiomers. Racemic mixtures are not optically active. |
| Optical Isomers | Configurational isomers that have different spatial arrangements of substituents, which affects the rotation of plane-polarized light. |
| Nonbonded Strain | Increased energy that results when nonadjacent atoms or groups compete for the same space; also called steric strain. |
| Enantiomers | Nonsuperimposable stereoisomers that are mirror images of each other. Enantiomers differ in configuration at every chiral center but share the same physical and chemical properties in a nonstereospecific (achiral) environment. Their optical activities are exactly the opposite of each other. |
| Diasteromers | Stereoisomers that are not mirror images of each other. Diastereomers differ in their configurations at least one chiral center and share the same configuration at least one chiral center. They differ in physical and chemical properties. |
| Constitutional Isomers | Molecules that have the same molecular formulas but different connectivity; also called structural isomers. |
| Conformational Isomers | Stereoisomers that differ by rotation about one or more single bonds, usually represented using Newman projections. |
| Configurational Isomers | Isomers that can only interconvert by breaking bonds; include enantiomers, diastereomers, and cis-trans isomers. |
| Chemical Properties | Characteristics of compounds that change chemical composition during a reaction; determines how a molecule will react with other molecules. Contrast with physical properties. |
| Lactone | A cyclic ester; named according to the number of carbon atoms other than the carbonyl carbon and for the straight-chain form of the compound. |
| Decarboxylation | The complete loss of a carboxyl group as carbon dioxide. |
| Lactam | A cyclic amide; named according to the number of carbon atoms other than the carbonyl carbon. |
| Ester | A compound that has a -COOR group. Esters are named as alkyl or aryl alkanoates. |
| Condensation Reaction | A reaction that combines two molecules into one, with the loss of a small molecule. |
| Carboxylic Acid Derivative | A compound that can be created from a carboxylic acid by nucleophilic acyl substitution; includes anhydrides, esters, and amides. |
| Transesterfication | The process that transforms one ester to another when an alcohol acts as a nucleophile and displaces the alkoxy group on an ester. |
| Relative Configuration | The spatial arrangement of groups in a chiral molecule compared to another chiral molecule. |
| Steric Hinderance | The prevention of a reaction at a particular location in a molecule by substituent groups around the reactive site. |
| Induction | The pull of electron density across σ bonds. |
| Conjugation | Alternating single and multiple bonds that create a system of parallel unhybridized p-orbitals; thus, electrons can be shared between these orbitals, forming electron clouds above and below the plane of the molecule and stabilizing it. |
| Spin Quantum Number (/mₛ/) | Describes the intrinsic spin of the two electrons in an orbital by arbitrarily assigning one of the electrons a spin of +1/2 and the other a spin of −1/2 . |
| Sigma (/σ/) Bond | The bonding molecular orbital formed by head-to-head or tail-to-tail overlap of atomic orbitals; all single bonds are σ bonds. |
| Principal Quantum Number (/n/) | Describes the shell in which an electron is found; values range from 1 to ∞. |
| Pi (/π/) Bond | The bonding molecular orbital formed when two parallel p-orbitals share electrons; exists as electron clouds above and below the σ bond between the two nuclei. |
| Magnetic Quantum Number (/mₗ/) | Describes the orbital in which an electron is found; possible values range from - to +. |
| Azimuthal Quantum Number (/I/) | Describes the subshell in which an electron is found; possible values range from 0 to n - 1, with /l/ = 0 representing the s subshell, l = 1 representing p, l = 2 representing d, and l = 3 representing f. |
| Oxidation State | An indication of the hypothetical charge that an atom would have if all of its bonds were completely ionic. |
| Nucleophile | A species that tends to donate electrons to another atom. With the same attacking atom (OH⁻, CH₃O⁻) in aprotic solvents, nucleophile strength correlates to basicity. In protic solvents and in situations where the attacking atom is different (OH⁻, SH⁻), nucleophile strength correlates to smaller size. |
| Leaving Group | Atoms or groups that can dissociate from the parent chain to form a stable species after accepting electron pairs. Weak bases tend to be good leaving groups |
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