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MCAT O Chem (VL)

QuestionAnswer
Degree of unsaturation [(2N+2)-X]/2 N --> carbons H --> H or monovalent atoms (F, Br) N --> counts as 1C and 1H If = 0, then saturated
Bond dissociation energy Energy required for homolytic break (creating radicals), as opposed to heterolytic break (creating ions)
Bond dissociation energy relationship with length and hybridization More s character = Short length = higher bond dissociation energy
Constitutional isomers Same molecular formula but different connectivity
Conformational isomers Same molecular formula and connectivity but different rotation about a single sigma bond. They are the exact same molecule. Staggered vs eclipse conformation.
Staggered vs eclipse conformation Conformational isomers. Staggered don't overlap, eclipsed does.
Newman projection Big circle is back carbon
Gauche vs anti conformation Gauche gives you relative minimum (staggered but large groups are near by). Anti gives you absolute minimum. Staggered and large groups are at opposite sides.
When could the gauche conformation be more stable than anti When it allows hydrogen bonding between the large groups (provided the groups are not too large)
Cahn-Ingold-Prelog rules Absolute conformation rule for sterecenters. Number from greatest to lowest atomic number, if isotopes then go by atomic weight R is clockwise, S is counter-clock
Fischer projection Vertical lines go towards the back, horizontal go to front
Enantiomers mirror image that aren't superimposable. Will have opposite absolute conformations
Enantiomers and plane polarized light A pair of enantiomers will rotate plane polarized light with equal magnitude but in opposite directions.
Optically active Compound that rotates polarized light. If cloclwise then dextrorotatory (d) (+). If counterclockwise then levorotatory (l) (-). Racemic mixture (50-50 mix) has a specific rotation of 0 degrees
Number of possible stereoisomers 2^n n=# of chiral centers
Diastereomers Steroisomers that aren't enantiomers. So they are nonsuperimposable nonmirro images. One of the chiral centers has the same absolute configuration while the other changes.
Specific rotation for diastereomers They differ. No specific relationship between the specific rotations for the diastereomers. Also they have different melting points, bo, solubilities, dipole moments....
Enantiomer similarities Tend to have similar mp, bp, solubility, dipole... etc. Unlike diastereomers.
Epimers THey are diastereomer where only one chiral center was inverted (Epimeric carbon)
Magnitude and sign of specific rotation Can only be known through experimentation.
Anomers Epimers that form a ring structure
Meso compounds Compound with two chiral centers (all have same atoms attached), but it has innternal symetry (S and R, or R and S). Therefore instead of 2^n possibilities it's only 2^n -1 They are optically inactive.
Mose compounds are optically..... Inactive
Geometric isomers Diasteromers that differ in orientation of substituents around a ring or double bond. Two high priority n same side is cis, other is trans
Two factors influencing bp and mp of hydrocarbons Branching decreases the vanderwaal forces so it has lower mp and bp. Molecular weight also because the greater then MW the more surface for forces to interact and the higher mp and bp.
Three types of reaction intermediates Carbocations, alkyl radicals or carbanions.
Carbocations sp2 with empty p orbital.
Alkyl radicals Since they are electron deficient they are kind of like carbocations. sp2 with unpaired electron in p orbital
Carbanion sp2 with lone pair in p or sp3 with unpair in sp3
Two ways to stabilize organic intermediates Inductive effect s(sigma bonds) or resonance effects (delocalization of pi bonds)
Carbocations and alkyl radicals are more stable with 30, 20, 10 and methyl (more stable to least)
Conjugated system 3 or more atoms that bear p orbital
Organic compound acidity Strong acids > sulfonic acids > carboxylic acids > phenols > alcohols and water > carbonyl compounds > sp > sp2 > sp3 (PAGE 87)
Ringstrain triangle and square. unstable.
Hydrogenation reactions for cycloalkanes Works best for cyclopropanes and cyclobutane because they want to break free from the ring. Cyclopentane... hexane.. and up don't have ring strain so they don't react.
Free radical halogenation (termination and inhibition and racemization) of alkanes Page 101. One halogen radical form can give rise to a bunch of others. Racemization with top or bottom addition of halogen. Termination when radicals end (X + X) (R + R) (R + X) Inhibition by molecular O because R. + O=O ---> R-O-O.
Selectivity Selectivity = reactivity/site available
Br vs Cl selectivity Br are much more selective than Cl for bromination and chlorination of alkanes. Most selective... I > Br > Cl > F
Which is a nucleophile, lewis base or lewis acid? Lewis base.
Nucleophiles Unshared pair of electrons or pi bonds and frequently a negative or partially negatibr vharge
Nucleophilicity trend Increases as negative charge increases. Increases down a periodic table and left. Down a group is larger atoms and more polarizable (ability to distorn an atom around it). to the left because of less electronegativity
SN2 basics Depends on 2 (conc. of nucleophile and electrophile. 1 step. Nucleophilic attack from the back. Better in aprotic solvent (acetone, DMF, DMSO) b/c H bonding solvents will solvate nucleophile. Bulky electrophiles inhibit. Cause stereochemical inversion.
What forms in an SN1 A carbocation!
Two distinct steps in SN1 Halogen takes itself out, so carbocation remains, then nucleophilic attacks from either side. Racemic mixture.
Unimolecular reaction rate depends on only one variable. Ex. SN1
SN1 basics Two step. Only dependent on electrophile concentration. Reaction rate also depends on how stable the carbocation will be. Protic solvents (water or alcohol) because stabilizaes cation and solvate the leaving group. Also solvolysis (solvent attacks carbcat
Ether and acid First the acid protonates ether (positive sign on O, making it a good leaving group). The base becomes the nucleophile and the R group the electrophile with al alcohol as the leaving group. Can go SN1 or SN2 once protonation occurs.
Thiols R-SH
Amine NR3
Know imine vs amide vs. amine Page 20
Alkyl vs aryl amines Aryl amines have the nitrogen bound to sp2 carbon and is part of an aromatic ring
Amines aren't chiral because.... They can rapidly covert from one side to another.
Alkylation of amines primary, secondary amines and others can do SN2 against alkyl halide replacing its H with the alkane. This repeats until tert amine which doesn't have lone e- pair
E1 elimination Protic solvent prefered. Two steps. Dependent on substrate (just one, the thing being eliminated)
Substrate reactibity for SN1, SN2, E1 and E2 SN2 prefers less substituted because of steric hinderance. SN1 prefers more substituted for stability of carbocation. E1 prefers more substituted because of carbocation stability. E2 prefers more substituted because they are more likely to eliminate (?)
Dehydrations of alcohol? E1? E2? It's E1. Acid protonates OH of alcohol, H2O leaves, carbocation remains, conjugate base picks up H and double bond forms. Watch out for hydride and methide shifts.
E2 Aprotic base. Watch out for bulky bases because they'll only pick up the most outer hydrogens. Depends on amount of base and substrate. Pick E2 over E1 by using stronger base.
Methods for creating alkyl halides Free-radical halogenation Alcohol + acid Substitution with other halides Alcohol + phosphorus halides (PBr3, PCl3) Alcohol + thionyl chloride (SOCl2)
Mrkovnikov's rule Addition reactions. The most table carbocation intermediate is always formed, then it will go SN1 by halogen or do hydride shifts to gain more stability and then undergo SN1.
Hydration of alkenes Acid-catalyzed (water is the nucleophile, but H3O provides the H for hydration) Oxymercuration-Demercuration
Oxymercuration-Demercuration is used for To form a markovnikov alcohol (hydration of alkenes). Cationic 3 membered cyclic intermediate forms. No carbon skeleton rearrangement happens. Water opens ring at most substituted carbon.
Markonikov alcohol means... The OH group is in the most sustituted Carbon. Anti markonikov means the opposite
Hydroboration alkenes antimarkovnikov alcohols, where OH and H added are syn (same side of double bond). Uses BH3 and oxidation with H2O2
Antimarkonikov halogen alkanes synthesis HBr in presence of peroxyde. Syn addition
Dihalogenations of alkenes Addition of X2. First Br-Br molecule splits and one of the Br forms a 3 membered cyclic molecule with Br at the tip, then the Br comes close to another Br2 molecule induces dipole and it breaks open the ring. Stereochemistry is anti.
Alkene + H2O + Br2 In the presence of H2O, OH and Br are added anti.
Alkene + peroxy acid Epoxide intermediate is formed. Then two OH added tans to where the double bond was (trans diols)
Peroxy acid Page. 152
Epoxide Pg. 153
KMnO4 + alkene cis-diols formation. Organometallic intermediate helps OH to form cis.
Hydrogenation of alkenes and alkynes H2 in presence of metal catalyst (Ni, Pd, Pt). Syn addition. Need 2 equiv of H2 for alkynes
Alkyne hydration to stop at alkene in cis mode Use CaCO3 or BaSO4 to stop at alkene level and get cis. These reagents are known as "Lindlar catalyst"
Alkyne hydration to stop at alkene in trans mode Use Na and liquid ammonia (NH3)
Ozonolysis O3 reacts with pi bond to make cyclic intermediate "ozonide", hydrolyzed to yield aldehyde and ketone.
Criteria for aromatic compounds 1. Cyclic 2. Cyclic delocalized p orbital system must be flat and planar 3. Delocalized p orbital must have Huckel number of pi electrons
Huckel numbers 4n + 2 = #pi electrons n can only be a whole number
electrophilic aromatic substitution Only with very electrophilic reagents
Ring activating bs ring deactivating groups Groups on aromatic compounds that make subspitution easier (activating/electron giving) or harder (deactivating/electron withdrawing).
Ortho/para -NR3, -OH (very strong activator) -OR, -NH(C=O)R, -O(C=O)R, -R (moderate activator) -Cl, Br, I (mild deactivators)
Meta -NR3, -NO2, C(triple bond)N (very strong deactivator) -SO3H, (C=O)R, (C=O)OH, (C=O)OR, (C=O)NH2, and -NH3 (moderate-mild deactivators(
Ortho/para directing groups have Lone pairs of electrons on the atom of attachment to the aromatic ring
Oxidants Chromic acid, chromate salts, dichromate salts, permanganate, chromium trioxide (all these are aq oxidants), and pyridinium chlorochromate (PCC) (anhydrous oxidant)
Enolate ion Resonance stabilized carbanion. Negatively chargeda nd nucleophilic.
Keto-Enol Tautomerism page 179
Organometallic reagents Perform nucleophillic additions to carbonyl carbons. Basic structure is R- M+. Function as either strong bases or nucleophiles. Ex. Grignard and lithium reagents
Grignar reagent Organomettalic reagent. Made by action of alkyl halide on magnesium.
Created by: valen1014 on 2013-05-08



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