Princeton Review - Mcat Ochem

Front 1

isomers

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have the same atomic content and chemical formula, but differ in the way their atoms are bonded, connected, and arranged.

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types of isomers: (2 things)

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structural isomers, stereoisomers

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types of stereoisomers: (2 things)

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enantiomers, diastereomers

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types of diastereomers: (2 things)

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geometric (cis-trans) isomers, configurational isomers

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structural isomers

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two entirely different substances that happen to have identical chemical formulas

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stereoisomers

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atoms are similarly bonded and connected, differing from each other only in the way the atoms are oriented in space

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chiral center

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any central atom bonded to four different atoms or groups. Labeled by asterisk.

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chiral molecule

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cannot be superimposed on the molecule that represents its mirror image.

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enantiomers

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made up of two molecules that represent mirror images of each other but cannot be superimposed on one another.

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behavior of enantiomers

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behave as the same molecule: physical properties identical --> separation difficult

Optically active in opposite directions of equal magnitude.

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dextrorotatory enantiomer

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rotates light in positive (+) direction

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levorotatory enantiomer

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rotates light in negative (-) direction

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racemic mixture

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a mixture of enantiomers in which R and S (and therefore + and -) are present in equal concentrations.

Shows no rotation of light.

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maximum number of stereoisomers for a molecule w/ n chiral centers

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2^n

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physical properties of diastereomers

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different physical properties. unpredictably different optical activity.

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epimers

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a subclass of diastereomers in which only one stereocenter has two substituents switched

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meso compound

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not chiral --> not optically active. plane of symmetry within the molecule.

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geometric diastereomers arise b/c of:

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hindered rotation around double bonds

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aliphatic

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alkanes, alkenes, and alkines

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alkyl group

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a functional group derived from an alkane --> branched hydrocarbon

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alkane solubility

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insoluble in water b/c nonpolar

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nonpolar molecules are held together primarily by ___________________

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van Der Waal's forces

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van Der Waal's forces increase w/:

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increasing molecular surface area

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when a nonpolar substance undergoes transition from one phase to another, it must overcome _________________

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van Der Waal's forces

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___________ tends to reduce molecular surface area

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branching

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higher molecular weight increases ____________ of a liquid

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density

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halogenation reaction

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RH + X2 --> RX + HX

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halogenation involves a ___________ intermediate

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halogen free radical

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three steps of halogen chain reaction

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1) initiation
2) propagation
3) termination

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regioselective

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halogen only adds to one type of carbon in the compound.

occurs when bromine is the halogen.

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the major product of a halogenation rxn is determined by...

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which of the radicals produced is the most stable

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the energy required to remove a hydrogen from a tertiary carbon is ______ than that required for abstraction from a primary carbon

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less

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order of carbon free radicals from greatest to least stability/preference:

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3 > 2 > 1 degree

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chloronation reactions

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1:2:3 degree radicals form w/ 1:4:5 degree selectivity --> large mixture of products w/ no clear product formed in great excess

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combustion reaction

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alkanes undergo oxidation in the presence of oxygen.

CH4 + 2O2 --> CO2 +2H2O + heat

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pyrolysis

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"cracking" of an alkane

alkane + heat --> H2 + smaller alkane + alkenes

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numbering of alkenes begins...

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at the end closest to the double bond

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nucleophile

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donates electrons. Lewis base.

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electrophilic addition

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a process in which the carbon-carbon double bond, acting as a nucleophile, donates electrons to an electrophile

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electrophile

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"seeks" electrons. Lewis acid.

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where does halogen usually bind in electrophilic addition?

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electronegative atom binds to carbon containing lesser number of hydrogens

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Markovnikov's rule

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in electrophilic addition, electronegative atom binds to more substituted side of double bond

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hydride shift

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a hydrogen atom shifts position to form a more stable 3 degree carbocation

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methyl shift

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a methyl group shifts position so that a more stable 3 degree carbocation is formed --> rearrangement

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when HX adds to a double bond in the presence of peroxides...

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anti-Markovnikov addition occurs b/c carbon free radical instead of carbocation intermediate

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if in aqueous solution, when will equilibrium favor hydration?

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when acid is relatively dilute and temperature is low

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if in aqueous solution, when will equilibrium favor dehydration?

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when acid is relatively concentrated and temperature is relatively high

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hydroboration-oxidation

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adds the -OH group anti-Markovnikov. syn addition (H and -OH added to same side)

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addition of halogens occurs by a _____________ intermediate

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cyclic halonium ion

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if halogenation of an alkene is conducted not in a carbon tetrachloride solution but in aqueous solution, the solvent molecules produced a _____________

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halo alcohol (halohydrin) via syn addition

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Huckel number

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4n + 2

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aromatic compounds may have _________ (3 things) in the ring in addition to C

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N, O, or S

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benzene tends to undergo __________ rather than ___________

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substitution rather than addition, b/c it preserves the ring

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reactants for halogenation of benzene

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Cl2/AlCl3 or Br2/FrBr3

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products of benzene halogenation

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PhH + Cl2/AlCl3 --> PhCl + HCl

PhH + Br2/FrBr3 --> Ph-Br + HBr

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reactants for nitration of benzene

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HNO3/H2SO4

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products of benzene nitration

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PhH + HNO3/H2SO4 --> Ph-NO2 + H2O

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reactants of alkylation of benzene

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RCl/AlCl3

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products of benzene alkylation

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PhH + RCl/AlCl3 --> Ph-R + HCl

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inductive effects

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a single substitution of a benzene ring raises or lowers the likelihood that the ring will undergo a second substitution

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ring activating group

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have a lone pair of electrons on the atom directly attached to the ring.

donates electrons to the ring. increases the likelihood of additional substitution.

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ring deactivating group

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have a + or partial + charge on the atom directly attached to the ring.

a group that withdraws electrons from the ring. decreases likelihood of additional substitution.

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meta directors

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all electron-withdrawing groups except the halogens

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ortho and para directors

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all electron-donating groups plus the halogens

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nucleophilic substitution

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a reaction btwn an aliphatic compound and a nucleophilic reagent.

R-W + Z --> R-Z + W

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the key to electrophilicity is...

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the presence of an unshared electron pair

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SN2 reaction

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occurs in one step. product inverts, giving it opposite configuration if chiral.

rate = k[substrate][nucleophile]

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SN1 reaction

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Two steps --> leaving group leaves, then nucleophile attacks.

rate = k[substrate]

carbocation intermediate achiral --> racemic product

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reaction is favored by circumstance that:

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1) stabilizes transition state, intermediate, and product
2) destabilizes or impairs formation of reactant

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relative reactivity of SN2

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CH3 > 1 > 2 > 3 degree because of steric hindrance

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good leaving groups in SN1 reaction

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are electron hogs. bad nucleophiles.
Best leaving groups are weak bases.

in halogens, leaving ability enhanced as you move down periodic table: I- > Br- > Cl- > F-

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best leaving groups in SN2 reactions

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good leaving groups delocalize charge

weak bases best leaving groups

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____________ base tends to be a stronger nucleophile

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stronger

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____________ base is the better nucleophile, ___________ base is the better leaving group

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stronger, weaker

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protic solvents

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solvents that contain -OH or -NH groups. slow the SN2 reaction by forming hydrogen bonds around the nucleophile

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polar aprotic solvents

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raise the rate of SN2 reactions by destabilizing the reactant. solvate metal cations while leaving nucleophilic anions "naked", giving anions greater nucleophilicity

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SN1 reactions are favored by __________ solvents and disfavored by ________ solvents b/c...

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polar protic, nonpolar b/c polar solvents stick to the carbocation's positive charge, adding to its stability

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whether the SN1 or SN2 is favored is dependent on the ____________

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substrate

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E1 elimination

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1) alkyl halide looses a halide to form carbocation intermediate
2) weak base takes proton from intermediate to form alkene

rate = k[substrate]

3 > 2 > 1 degree

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E2 elimination

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occurs in one step

rate = k[alkyl halide][base]

abstracted H and leaving group must be anti to each other

3 > 2 > 1 degree

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___________ base and or ___________ of base favors E2

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stronger, greater concentration

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tertiary substrates favor elimination or substitution

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favor elimination over substitution

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factors that favor E2 over SN2:

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high concentration of strong base, heat, solvent of low polarity

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phenol

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a benzene attached to a hydroxyl group

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alcohols are ___________ acids and __________ bases

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weak, weak

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high Ka corresponds to __________ pKa, both of which correspond to ____________ acidity

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low, higher

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alkoxide ion

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a hydroxyl group w/out its proton

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oxonium ion

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a hydroxyl group that has acquired an extra proton

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acidity ____________ as alcohols get larger and aquire greater substitution by alkyl groups

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decreases