MCAT - Organic Chemistry Flashcards
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Coordinate Covalent Bond
One nucleus donates both of the electrons in the bond.
Electrostatic Forces
The attractive force between electrons and the nuclei that is responsible for all molecular bonds.
Hybridization
sp - 180° - linear
sp2 - 120° - trigonal planar
sp3 - 109.5° - tetrahedral, pyramidal, or bent
dsp3 - 90°, 120° - trigonal-bipyramidal, seesaw, t-shaped, linear
d2sp3 - 90° - octahedral, square pyramidal, square planar
When one or more shape is possible, it is determined by the number and position of the lone pairs of electrons.
Instantaneous Dipole Moment
Exists in an otherwise nonpolar molecule. Occurs because the electrons in a bond move about the orbital and at any moment may not be distributed exactly between the two bonding atoms, even when the atoms are identical.
London Dispersion Forces
Occur between two instantaneous dipoles; the weakest dipole-dipole force (vs. hydrogen bonds which are the strongest dipole-dipole forces).
Zwitterion
A neutral molecule with a positive and negative electrical charge at different locations within the molecule at a pH of 7.
Ex: amino acids -> amine group deprotonates the carboxylic acid group
Isomers
Molecules that have the same molecular formula but are different compounds.
Conformational Isomers (Conformers)
Not true isomers - different spatial orientations of the same molecule. At low temperatures, the anti-conformation is the most common.
Structural Isomers
Have the same molecular formula but different bond-to-bond connectivity; simplest form of isomer.
Stereoisomers
Two molecules with the same molecular formula and same bond-to-bond connectivity that are not the same compound. Like conformational isomers but contain at least one *chiral/stereo center (C bonded to 4 different substituents).
2 types:
- enantiomers
- diastereomers
Enantiomers
Stereoisomers that have opposite absolute configurations at EACH chiral center (mirror images). Cannot be separated by physical means.
When equal in concentration -> racemic mix
Resolution = the separation of enantiomers.
Same physical and chemical characteristics except in 2 cases:
- Reactions with other chiral compounds
- Reactions with polarized light
Diastereomers
Stereoisomers that are not mirror images (are not the same compound). Can be separated by physical means (crystallization).
Geometric isomer: special type of diastereomer, have different physical properties.
- Cis: has dipole moment - strong intermolecular forces so has high boiling point; lower symmetry does not form crystals as easily so has lower melting point; has steric hindrance so has higher heats of combustion
- Trans: no dipole moment - lower boiling point, higher melting point, lower heats of combustion
–> better to use E (opposite) and Z (same)
Epimer
Diastereomers that differ at only one chiral center.
Anomers
Two diastereomers formed from a ring closure at an epimeric C.
Anomeric carbon: the chiral carbon of the anomer.
Ex: glucose - anomeric C determines if it’s alpha-glucose or beta-glucose.
Meso Compounds
Optically inactive (achiral) molecule with two chiral centers; have a plane of symmetry between their centers which divides the molecule into halves that are mirror images of each other.
Absolute Configuration
The only way to absolutely describe a chiral molecule.
R = rectus: right (clockwise)
S = sinister: left (counterclockwise)
*for when H (or other 4th priority substituent) is on DASHES.
Mirror images of chiral molecules always have opposite absolute configurations.
*note: retention of configuration does not mean that absolute configuration is retained; it means that there is no inversion.
stereoisomers of a chiral molecule = 2^n where n = # of chiral Cs
Abs config does NOT indicate direction in which a compound rotates plane-polarized light.
Relative Configuration
Two molecules have the same relative configuration about a C if they only differ by one substituent and all other substituents are oriented identically around the same C.
–> In SN2 reactions, it is the relative configuration that is inverted.
Polarimeter
Screens out photons from a light source to get only photons of a certain orientation of electric field.
Plane-Polarized Light
The resulting photons filtered by a polarimeter that all have the same orientation of electric field.
Observed Rotation
Gives the direction and degree to which a compound rotates the electric field in plane-polarized light.
Optically Active
When a compound does not contain any mirror images (only one stereoisomer present) so when plane-polarized light is projected through the compound, the orientation of the electric field is rotated.
+ or D: compound that rotates plane-polarized light clockwise
- or L: compound that rotates plane-polarized light counterclockwise
vs. racemic mix -> many electric field orientations = optically inactive.
Maximum # of optically active isomers in a compound = 2^n where n = # of chiral centers.
Simple Rotation
A standardized form of observed rotation arrived at through calculations using observational rotation and experimental parameters.
Specific rotation = observed rotation after the following experimental factors have been adjusted:
- length of polarimeter
- concentration of solution
- temperature
- type of wavelength of light used
Physical Properties of Alkanes
Increase MW (molec weight) = increase BP (boiling pt); increase MP (melting pt).
Increase branching = decrease BP; increase MP (crystal solids).
Alkanes have the lowest density (think: oil spill -> alkanes float on water).
The first four alkanes are gases at room temperature.
Similar properties in alkenes and alkynes (although alkynes are slightly more polar, slightly more soluble in H2O).
Ring Strain
Some ring structures put a strain on the C-C bonds because they bend them away from the normal 109.5° angle of the sp3 C and cause crowding.
–> ring strain is zero for cyclohexane and increases as the number of Cs increases and decreases from there (up until 9, after which it is zero again as Cs are added).
Combustion
A (radical) reaction when alkanes are mixed with oxygen and energy (heat) is added.
CH4 + 2O2 –(flame)–> CO2 + 2H2O + Heat
-> exothermic, so heat released is the measure of stability.
Heat of Combustion
The change in enthalpy of a combustion reaction. Combustion of isomeric hydrocarbons (same molec formula) requires equal amounts of O2 and produces equal amounts of CO2 and H2O so the heat of combustion is used to compare relative stabilities of isomers.
–> higher heat = more energy = less stable molecule
For cycloalkanes, comparisons can be made of different ring sizes on a “per CH2” basis.
Ex: molar heat of combustion for cyclohexane is almost twice as much as cyclopropane but “per CH2” group heat is much higher for cyclopropane because of ring strain.
Enthalpy (H)
A measure of the total energy of a thermodynamic system (a thermodynamic potential). It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.
–> a state function and an extensive quantity. Measured in Joules.
ΔH is positive in endothermic rxns
ΔH is negative in exothermic rxns
-> for an exothermic rxn at constant pressure, the system’s change in enthalpy equals the energy released in the rxn, including the energy retained in the system and lost through expansion against its surroundings.
–> ΔH = Q, where Q is the energy added to the system through heat.
Halogenation (Radical Rxns)
Alkanes react with halogens (F, Cl, Br, but not I) in the presence of heat or light to form a free radical.
Chain rxn:
- Initiation: the halogen starts as a diatomic molecule. It is homolytically cleaved by heat or UV light, resulting in free radicals.
- Propagation: the halogen free radical removes an H from the alkane resulting in an alkyl radical. The alkyl radical may now react with a diatomic halogen creating an alkyl halide and a new halogen radical. Most products are formed here, can continue indefinitely.
- Termination: either 2 radicals bond or a radical bonds to the wall of the container to end the chain rxn or propagation.
-> exothermic process.
Alkyl radical stability is the same as carbocation stability: 3° > 2° > 1° > methyl.
Alkyl radicals exhibit trigonal planar geometry.
Halogens
Order of reactivity: F, Cl, Br, I (I will not react with alkanes in halogenation).
Order of selectivity: I, Br, Cl, F
–> how selective a radical halogen is when choosing a position on an alkane.
Four Main Reaction Types
- Addition: A + B –> C
- Elimination: A –> B + C, one reactant splitting into two products.
- Substitution: A-B + C-D –> A-C + B-D
- Rearrangement: A –> B, a reactant undergoing bond reorganization to give an entirely new molecule.
Index of Hydrogen Deficiency (AKA Degrees of Unsaturation)
DU = [(2n - 2) - x]/2
n = # carbons x = # hydrogens
Synthesis of Alkenes
Elimination: one or two functional groups are removed to form double bond
- Dehydration of an alcohol (E1)
- Dehydrohalogenation (E1 or E2 - strong bulky base for deprotonation)
Zaitsev’s (Saytzeff’s) Rule
The major product of elimination will be the most substituted alkene.
Reactions of Alkenes
- Catalytic hydrogenation: adds Hs, makes alkane
- Ozonolysis: oxidation (splits double bond and makes carbonyls) using O3
- *Electrophilic addition: attraction of electrophiles (often hydrogen halides) to the double bond.
- Hydration: addition of H2O in acidic conditions at low T (reverse of dehydration - at high T)
- Halogenation: X2 adds anti to either C in the double bond: first X (electrophile) will add to least-substituted C per Markov (with H2O, hydroxide will add second to the most-substituted C instead of other X).
Markovnikov’s Rule
The hydrogen will add to the least substituted carbon of the double bond.
-> hydrogen halides (HX) and hydration (H2O) addition to alkenes.
Oxymercuration/Demercuration
A two step process that follows Markovnikov’s rule;
- A partially dissociated mercury-containing reagent +Hg(OAc) (as The electrophile) forms a 3-membered ring with the C=C and then adds water in an *anti-addition (from opposite sides of the double bond) to make the organomercurial alcohol.
- A reducing agent and base is added for the formation of the alcohol through demercuration.
Anti-Markovnikov Addition
In the presence of peroxides (ROOR), Bromine from HBr, not the hydrogen, will add to the least-substituted carbon (does not apply with other halogens - those will still add Markovnikov).
–> hydroboration (BH3) to make alcohols (from the peroxide), syn-addition.
Benzene
Undergoes substitution, NOT addition. A flat molecule stabilized by resonance. An EWG (electron-withdrawing group).
-> ortho, meta, para
EWGs (like N+O2, N+R3, CCl3, ketones, aldehydes, esters, SO3H, CN) are meta-directors.
EDGs (electron-donating groups like O-, OH, NR2, OR, R) are ortho/para-directors.
- Halogens are exceptions -> EWGs but ortho/para-directors.
Huckel’s Rule
If a compound has planar, monocyclic rings with 4n+2 pi electrons (n being any integer, including 0), it is by definition an aromatic compound.
Substitution Reactions
SN1 and SN2 - numbers represent the order of the rate law (unimolecular and bimolecular) and NOT the number of steps.
-> SN1 has 2 steps.
1. LG leaves.
2. Nucleophile attacks.
First step is formation of the carbocation and is the RDS (slowest) -> concentration of the substrate (electrophile) is all that matters (not the nucleophile).
- Has carbocation intermediate so more stable (3°) is better.
- Carbocation intermediate is planar so nucleophile attacks from either side -> gives racemic mix.
- Often has competing elimination (E1) if the nucleophile also acts as a base.
- > SN2 has 1 step.
1. Nucleophile attacks the intact substrate from behind the LG and knocks LG off -> rate is dependent on the concentration of the nucleophile and the electrophile. - Doesn’t like steric hindrance so 1° is better (with strong base and hindrance in electrophile or nucleophile, will get E2 instead).
- Inversion of configuration (stereochemistry - not necessarily absolute configuration).
SN1 vs. SN2
The nucleophile and the 5 Ss:
Nucleophile: SN2 requires a strong nucleophile while nucleophile strength doesn’t affect SN1.
- Substrate: SN2 rxns don’t occur with sterically hindered substrate. SN2 requires a methyl, 1°, or 2° substrate while SN1 requires a 2° or 3° substrate.
- Solvent: a highly polar solvent increases the rxn rate of SN1 by stabilizing the carbocation, but slows down SN2 rxns by stabilizing the nucleophile.
- Speed: the speed of an SN2 rxn depends upon [substrate] and [nucleophile], while the speed of an SN1 depends only on [substrate].
- Stereochemistry: SN2 inverts stereochemistry about chiral center, while SN1 creates a racemic mixture.
- Skeleton: SN1 may be accompanied by carbon skeleton rearrangement, but SN2 never rearranges the carbon skeleton.
