ochem exam 3
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chelsiejoe on November 16, 2011
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62 terms
Terms | Definitions |
|---|---|
 dipole dispersion forces, pulls molecules together | Van der waals |
| What has a higher VDW radius than methyl? | iodine |
| Haloalkane BP | polarizability of the unshared electrons of the halogen atom |
| Density of haloalkanes | greater than that of other hydrocarbons of similar MW bc of the halogen's larger mass-to-volume ratio |
| Alkane to haloalkane? | free radical halogenation (Cl2 or Br2 + light) replaces in the order--allylic>3>2>1>methyl |
| regioselectivity of halogenation | greater for bromination than chlorination (due to Hammond's postulate) |
| BDEs | bond dissociation enthalpies--of homolytic bond cleavage for a given type of bond (BDEs bonds broken-BDEs bonds made=overall energetics of rxn) Exothermic rxns have negative values overall |
| the three parts of radical chain mechanism | chain initiation (radicals formed from nonradical compounds), chain propagation (a radical and a molecule react to given a new radical and new molecule), termination (radicals destroyed; add one of each together) |
| Hammond's postulate | transition state of an exothermic rxn occurs early and looks more like the reactants; thus changes in reactants have a larger effect on the rate endothermic occurs later and looks more like the products *for both chlorination and bromination of alkanes, the RDS is the extraction of a H to form an alkyl radical (endo for bromination/exo for chlorination) |
| allylic substitution | adding a halogen to a carbon next to a double bond *proceeds by a radical chain mechanism **bc of delocalization of unp e density through resonance, the allyl radical is more stable than the tert butyl radical |
| chlorination/bromination of alkanes | 1. happens with HEAT OR LIGHT through radical chain mechanism 2. take a hydrogen from the carbon bonded to the most carbon atoms and replace with the halogen |
| alkene to haloalkene (allylic bromination) | 1. high temps (heat initiates radical mech) using the halogens themselves 2. then bromination using NBS is initiated by light 3. also use Ch2Cl2 |
| alkenes to bromoalkane (under radical conditions) | 1. adds H AND Br to LESS sub'd carbon of double bond (breaks double bond) 2. radical chain mechanism 3. so you need HBr + peroxides 4. *ADDS BOTH BY NON MARKOV |
| Sn2 stereochem | INVERSION AT CHIRAL CENTER (turns R to S and vice versa) |
| Sn1 stereochem | results in RACEMIZATION (50% R/50% S) |
| haloalkane structure | haloalkanes that can form more stable carbocations react faster in an Sn1 mechanism 1. shifts can occur |
| What prevents Sn2? | steric hindrance on the backside of the C-Lv (bulk around the rxn site) |
| leaving group ability | the more stable the anion produced upon rxn, the greater Lv group ability |
| protic solvents | H bond DONORS (most contain -OH groups) |
| aprotic solvents | cannot donate an H (diethyl ether, DMS, DMF) |
| polar solvents | interact strongly with ions and polar molecules |
| NP solvents | do not act strongly with ions and polar molecules |
| What measures solvent polarity? | dielectric constant |
| solvolysis | Nu sub. rxn where the solvent is the nucleophile |
| What solvents accelerate sn1? | polar protic--by stabilizing the charged carbocation intermediate *formic acid (HCOOH), ethanol, methanol, acetic acid (CH3COOH), water |
| What solvents accelerate sn2? | polar aprotic--they do not react strongly with the nucleophile *DCM, THF, DMF, MeCN, DMSO |
| good Nu | generally anions |
| moderate Nu | neutral with one or more available lone pairs |
| poor Nu | polar protic solvents |
| Nucleophilicity | the stronger the interaction of a nucleophile with solvent, the lower the nucleophilicity |
| What Nu are better for sn2? | small Nu with very little steric hindrance |
| primary haloalk react through sn2... | due to an absence of steric hindrance and lack of carbocation stability |
| secondary haloalk react through sn2... | in aprotic solvents with good Nu |
| secondary haloalk react through sn1... | in protic solvents with poor Nu *E1 is less when sn1 occurs |
| tertiary haloalk--sn1 | bc the steric hindrance makes sn2 impossible, plus the attached alkyl groups stabilize the carbocation |
| What is b elimination? | removal of atoms or groups of atoms from adjacent carbons |
| haloalkANE to alkENE | dehydrohalogenation--b elimination (E1 or E2) that involves loss of an H and a halogen from adjacent carbon atoms (makes a double bond) |
| Zaitsev's elimination | b elimination to give the more stable alkene |
| E1 | Lv departs to give carbocat, then a proton is taken off an adjacent carbon atom (BY A BASE) to create the product alkene *only depends on [X] |
| E2 | the halogen departs at the same time as the H atom is removed (BY BASE) to create product alkene *depends on [X] AND [OH] |
| stereoselectivity of E2 | lowest energy transition state is the one in which the Lv group and the H atom that depart are oriented ANTI and COPLANAR (this determines whether E or Z alkenes are produced) *for cyclohexane derivatives, both Lv and H must be axial |
| regioselectivity of E1 and E2 | both follow Zaisev's as long as Lv and H can be oriented anti coplanar *the more stable alkene is generally the more sub'd alkene |
| How to decide between substitution or elimination | look at: structure of haloalkane, choice of solvent, and the relative base strength of the nucleophile |
| What class of haloalkanes do not react through e1/sn1? | primary *SN2 is favored for all Nucleophiles EXCEPT VERY STRONG BASES--where E2 would dominate (H2N- or sterically hindered tert butoxide) |
| secondary haloalkanes through E2 | 1. if Nu is strong base (CA w/pKa above 11) such as hydroxide, alkoxide, acetylides, and H2N- |
| secondary haloalkanes through sn2 | weak bases (CAs with pKas lower than 11) that are good or moderate nucleophiles |
| secondary haloalkanes through e1/sn1 | poor nucleophiles (that are polar protic solvents) react through a combo of e1 and sn1 and the exact ratio is hard to predict |
| tertiary haloalkanes through sn2 | NO!!!! |
| tertiary H.A. through e2 | if the Nu is a strong base (CA w/pka above 11) *(-OH, alkoxide, acetylide, H2N-) |
| tertiary H.A. through sn1/e1 | other Nu in a polar protic solvent gives a combo of sn1/e1 |
| Why are alcohol BP higher than other hydrocarbons of similar MW? | because of intermolecular association through hydrogen bonding, increased dispersion forces with increase of MW |
| Why are alcohols more soluble in water? | bc they react with water by H bonding |
| Alcohols--acids or bases? | Both. they can act as weak acids (H donors) or weak bases (H acceptors) |
| In the presence of strong acid -OH... | can be protonated and made into the better Lv group (-OH2) *this is a common mechanistic scheme in alcohol rxns |
| alcohols to metal alkoxides | Li, Na, K....liberates H2 |
| tertiary alcohols to H.A. | HCl, HBr, HI *carbocat int |
| primary/secondary alcohols to H.A. | PBr3, SOCl2, SOBr2 *sn2 with inversion when replacing alcohol with halogen *alcohols react with sulfonyl chlorides to give alkyl sulfonates (the sulfonate group is a good leaving group analogoues to a halogen atom) |
| ALCOHOLS to alKENEs | treated with strong acid leads to dehydration, which is elimination of water from adjacent carbon atoms to give an alkene *Zaitsev's |
| primary alcohols to CARBOXYLIC ACIDS | H2CrO4--first converted to aldehyde, then aldehyde hydrate which is further oxidized to the carboxylic acid |
| primary alcohols to ALDEHYDES | PCC with no water, prevents further oxidation |
| secondary alcohols to KETONES | PCC or H2CrO4 |
| tertiary alcohols | are NOT oxidized! |
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