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Queen's University
CHEM 281
John Carran

STEREOCHEMISTRY ISOMERS Constitutional Stereoisomer Isomer Enantiomer Diasteriomer isomer  different compounds that have the same molecular formula constitutional isomer: different connectivity eg. butane & 2-methypropane have the molecular formula C H 4 10 stereoisomer: same connectivity of atoms, but a different arrangement of atoms in space  enantiomer – molecules that are non-superposable mirror images eg. 2-butanol can be represented both ways  diasteriomer – molecules that are not mirror images eg. cis and trans isomers trans – highest priority groups acis – highest priority groups are on opposite sides of the double bondthe same side of the double bond -a chiral molecule cannot be superposed on it’s mirror image  this can have biological implications  chiral center: when a carbon molecule is bonded to 4 different substituents  stereogenic center: 3  tetrahedral: sp hybridized carbon (4 individual single bonds)  chirality centers 2  trigonal planar: sp hybridized carbon (1 single bond and 2 double bonds)  cis and trans molecules Naming Stereoisomers: -for chiral centers, the R/S configuration is used  label each substituent from highest to lowest priority  orient the lowest priority group so that it is “into” the page (usually hydrogen)  determine if the remaining three groups follow a clockwise or counterclockwise pattern  clockwise = R  counterclockwise = S counterclockwise pattern, this a molecule is an (S) enantiomer -for achiral centers (double bonds), the E/Z configuration is used  determine the two highest priority substituents  both substituents are on the same side = Z  substituents are on opposite sides = E Optical Activity  enantiomers exhibit the same physical properties  they rotate plane polarized light in equal, but opposite directions  clockwise = (+)  counterclockwise = (-) *there is no correlation between R/S and +/-, this can only be determined experimentally ( )  racemic mixtures have an equal amount of R and S enantiomer  %EE = 0, they do not rotate plane polarized light  diasteriomers have different physical properties  if the compound is not RR/SS or RS/SR, it is a diasteriomer  meso compounds have at least 2 chiral centers  a plane of symmetry is present in the molecule  these are not optically active, they do not rotate plane polarized light  cyclic compounds may or may not rotate plane polarized light, depending on their configuration  1,2-disubstituted/1,3-disubstituted:  if there is a plane of symmetry > cis stereochemistry  there is no rotation of plane polarized light  if there is no plane of symmetry > trans stereochemistry  there is rotation of plane polarized light  1,4-disubstituted:  there is always a plane of symmetry  there is no rotation of plane polarized light Fisher Projections -horizontal lines represent bonds that are “out of” the page -vertical lines represent bonds that are “into” the page Configuration of Chiral Center -if a reaction does not involve the chiral center, the original configuration is still maintained  it is still possible for R/S to change, if the reaction results in a change of priority relative configuration: molecules share three groups in common and can be superposed absolute configuration: R/S designation (R)-2-butanol (S)-2-butanol Separating Racemic Mixtures 1. React racemic mixtures with pure chiral molecule ( ) ( ) ( ) ( ) *sal1 = R from carboxylic acid and R from amine *sal2 = S from carboxylic acid and R from amine 2. Diasteriomer salts are formed, which can then be separated based on their differing physical properties (crystallization, chromatography etc.) 3. React salts to get the original enantiomers back ( ) ( ) ( ) ( ) *H O is used as a catalyst and competes against the amine to break the bond 3 Nucleophiles, Electrophiles & Leaving Groups Nucleophiles -any atom with a long pair of electrons or that is electron rich  can be neutral or negatively charged  more nucleophilic if it can easily donate electrons (eg. Lewis base) -general trends in nucleophilicity:  correlates with polarizability when comparing within a group  atoms are larger as you move down the group  weaker hold on electrons, more easily removed *trend is generally true in protic and aprotic solvents, with the exception of halogens Te > Se > S > O  strength of halides as nucleophiles is dependent on the solvent *halide nucleophilicity can reverse, depending on the solvent  polar protic  nucleophilicity relates to polarizability  hydrogen bonding interferes with nucleophilic activity, by bonding to the nucleophile instead o smaller atoms (O) form stronger hydrogen bonds, making them less nucleophilic  polar aprotic  nucleophilicity relates to basicity  there is no hydrogen bonding present to “tie up” the nucleophile o HF is a weaker acid than HI, making it a stronger base (high pK ) and a better nucleophile a  correlates with basicity when comparing across a row  high pK a weak acid C > N > O > F  correlates with basicity when compared with the same nucleophilic atom in a series  increasing basicity = increasing nucleophilicity  high pK a strong base = weak acid = high pK a  a negatively charged nucleophile will be more reactive than the corresponding conjugate acid  electron density is higher  BULLET shaped nucleophiles are an exception eg. pK CN = ~9.4  this is relatively low a  shape minimizes steric hindrance, which is ideal for S 2N Electrophiles -any atom with a bond to a more electronegative atom (making it partially positive) or with a weak bond to a leaving group  nucleophiles are attracted to electron deficient atoms Leaving Groups -the molecule that breaks away from the electrophile in a reaction  this atom leaves with both the electrons in the electron pair  this is known as a heterolytic bond cleavage  hetero- because both electrons are taken -generally have a low pK a  low pK a strong acid = weak base  weak bases stabilize e , they do not want to give them up -orbital mismatch helps determine the ability of the leaving group  greater mismatch in orbital size = weaker bond  it is more likely that the bond will break and the leaving group will be able to depart I > Br > Cl > F -hydroxyl groups (OH-) are generally not good leaving groups  OH- can be turned into a good leaving group through protonation or sulfonate esters  protonation requires acidic conditions  sulfonate ester reactions must be done in pyridine  mesyl chloride  tosyl chloride  triflic anhydride *addition of the sulfonate does not change the stereochemistry of the cabon  only inverts if/when a S 2 mNchanism is followed *successful due to resonance stabilization -epoxides should be poor leaving groups, due to the presence of O -  due to ring strain, the molecule is very unstable  has angles of 60° vs. the desired 109.5°  ring can be broken very easily, allowing O to leave Substitution Reactions S 2 Mechanism N substituted, nucleophilic, bimolecular -bimolecular means that the rate of reaction depends on both of the
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