Test 2 Flashcards
stereo centers
an atom at which exchange of two groups produces a different stereoisomer
stereoisomers
isomers with same general connectivity, but different spatial arrangement of atoms (cis-, trans-)
types of stereoisomers
- enantiomers
- diastereomers
enantiomers
nonsuperimposable images (like hands and feet)
diastereomers
not related to an object as its mirror image
chiral object
not superimposable with its mirror image
achiral object
superimposable with its mirror image (ie, has mirror plane within the molecule or a center of symmetry)
center of symmetry
point at which identical groups or parts of an object are located equidistant in opposite directions from that point
R, S Nomenclature
Priority Rules
- atomic number
- atomic number of atom with first point of difference
- greater number of substituents, when substituents of same priority (eg CH2Cl > CH3)
- atoms participating in double/triple bonds considered to be bonded to an equivalent number of simple “phantom” atoms (eg, instead of carbon-carbon double bond; two single-bonded carbons bound to one other carbon)
Determining R or S Configuration
- assign priority to each group/substituent on the molecule
- orient molecule so lowest priority group facing away from viewer (at back)
- determine if priority proceeds clockwise (R) or counterclockwise (S)
two or more chiral centers
- maximum possible stereoisomers: 2n
- chiral centers reversed in entantiomer
- may also exist as diastereomers or meso compounds (contain two or more chiral centers, but achiral)
determine entantiomers of a molecule
- draw all possible entantiomers of the compound
- determine if any are identical (ie, have plane of symmetry)
physical properties of entantiomers
- all physical properties identical (melting/boiling points, color, etc)
- differ in how they rotate the plane of polarized light (so, entantiomers are optically active)
plane-polarized light
selectively transmitted light waves vibrating only in parallel planes (ordinary light consists of waves vibrating in all planes perpendicular to its plane of propagation)
polarimeter
used to show the ability of molecules to rotate a plane of polarized light
- sample tube with solvent placed in polarimeter
- analyzing filter adjusted until dark (0º)
- optically active sample rotates light (ie, some passes through)
- analyzing filter rotated again to restore darkness
observed rotation
- expressed as aº
- degrees rotated to restore darkness
- magnitude based on concentration, length of sample tube, temperature, solvent, wavelength of light used
specific rotation
- observed rotation at specific cell length and sample concentration
- [a]Tλ = (observed rotation)/(length in decimeters × concentration)
- length - standardized at one decimeter
- concentration - g/mL
- T - temperature (ºC)
- λ - wavelength of light (usually sodium D-line, λ = 589 nanometers)
- no absolute relationship between R, S and dextrorotatory/lavrorotatory
dextrorotatory vs levrorotatory
- dextrorotatory - filter turned right/clockwise (+)
- levrorotatory - filter turned left/counterclockwise (-)
racemic mixture
- 50/50 mixture of entantiomers
- sometimes indicated as ± or (d,l)
achiral molecules and plane-polarized light rotation
- some can rotate plane-polarized light, but light from the opposite direction also rotated in the exact opposite way and cancel
- thus, achiral molecules considered optically inactive
optical purity
- specific rotation of entantiomers divided by specific rotation of entantiomerically pure substance of same concentration, multiplied by 100
- ([a] sample)/([a] pure entantiomer) × 100
- entantiomeric excess (ee) - difference in number of moles of each entantiomer in a mixture (%R - %S); numerically identical to optical purity
steps to determine optical purity
- calculate entantiomeric excess/optical purity
- ([a] sample)/([a] pure entantiomer) × 100
- calculate ½(100 - ee) to determine the precise percentages of R-entantiomers and S-entantiomers in the racemic mixture
- add the percentage of the excess entantiomer to the original calculated entantiomeric excess for the total percentage of excess entantiomer in the compound
resolution
separation of racemic mixture into its salts
- form diastereomers by reacting racemic mixture with entantiomerically pure resolving agent, then separate based on physical properties and reconvert into entantiomers
- use enzymes (chiral, so produce single entantiomer products)
- chromatography - sample to be purified interacts with a solid material and different components of the sample separate based on their different relative interactions with the solid material. for resolution, a column is packed with chiral material with which entantiomers react differently.
alkene, alkyne, arene
- alkene (CnH2n for simple) - one or more double bond(s)
- alkyne (CnH2n-2 for simple) - one or more triple bond(s)
- arene - unsaturated, cyclic hydrocarbon
aryl group; phenyl group
- aryl group - created by removing an H from an arene (Ar-)
- phenyl group - aryl group substituent of benzene
structure of alkenes
- shapes - ~120º bond angles about double-bonded carbons (so coplanar; ie trigonal planar)
-
orbitals - carbon-carbon double bond consists of one sigma and one pi bond. sp2 orbitals form sigma bonds; unhybridized p orbitals overlap to form pi bond (must lie in plane for this to occur, so all atoms coplanar)
- pi-bond breaks when 90º dihedral angle between carbons becayse no overlap; requires much energy so no rotation about double bonds
- cis, trans isomerization - each carbon has a maximum of two different groups bound to it
alkene nomenclature
- number the longest carbon chain that contains the double bond in the direction that gives the carbon atoms of the double bond the lowest possible numbers
- for two or more double bonds, indicate the number with “-adien”, “-atrien”, etc
- indicate the location of the double bond by the number of its “first” (lowest number sequentially) carbon
- name branched/substituteted alkenes like alkanes
- number the carbon atoms, locate and name substituent groups, locate the double bond, and name the parent chain
common substituent names
- methylidene: CH2=; IUPAC methylene
- vinyl: CH2=CH-; IUPAC ethenyl
- allyl: CH2=CHCH2-; IUPAC 2-propenyl
designating isomeric configuration in alkenes
- cis,trans - carbon atoms of the main chain (cis - same side; trans - opposite side)
- E,Z - uses priority rules from R,S system
- Z - higher priority substituents on same side of double bond
- E - higher priority substituents on opposite side of double bond
cycloalkenes
- carbon atoms of double bond considered 1 and 2; other carbons numbered to give the substituent encountered first the smaller number
- configuration about double bond always cis until octene
polyenes
- contain two or more double bonds
- cis, trans isomerism possible about each double bond
- 2n stereoisomers possible for a compound with n double bonds
Addition of Hydrogen Halides (description, stereoselectivity, regioselectivity, rearrangement, racemic mixture, stereospecificity)
- results in alkane with hydrogen and halogen attached
- stereoselectivity N/A
- regioselective (follows Markovnikov’s Rule)
- rearranges to form tertiary cation if possible
- no stereoselectivity, so no racemic mixtures
- no stereospecificity
Acid-Catalyzed Hydration (description, stereoselectivity, regioselectivity, rearrangement, racemic mixture, stereospecificity)
- addition of water across double bond
- stereoselectivity N/A
- regioselective (follows Markovnikov’s Rule)
- rearranges to form tertiary cation if possible
- no stereoselectivity, so no racemic mixtures
- no stereospecificity
Bromonation/Chloronation (description, stereoselectivity, regioselectivity, rearrangement, racemic mixture, stereospecificity)
- addition of bromine or chlorine gas across double bond
- antistereoselective (bromines/chlorines always trans- one another; trans-diaxial on cyclohexene)
- regioselectivity N/A
- no rearrangment occurs
- racemic mixtures
- stereospecific (cis-compound produces meso halonium ion, entantiomeric products; trans-compound produces entantiomeric halonium ion and meso products)
Addition of HOBr or HOCl (description, stereoselectivity, regioselectivity, rearrangement, racemic mixture, stereospecificity)
- addition of hydroxyl and halide across double bond
- antistereoselective (hydroxyl and halide always trans- one another)
- regioselective - hydroxyl attaches to more substituted carbon of double bond
- no rearrangment
- racemic mixtures formed
- no stereospecificity
Oxymercuration-Reduction (description, stereoselectivity, regioselectivity, rearrangement, racemic mixture, stereospecificity)
- hydration of alkene with mercury (II) acetate in water followed by reduction of compound with sodium borohydride
- antistereoselective (hydrogen and hydroxyl always trans- one another)
- regioselective (follows Markovnikov’s Rule)
- no rearrangement (no real carbocation formed)
- racemic mixture
- not stereospecific
Hydroboration-Oxidation (description, stereoselectivity, regioselectivity, rearrangement, racemic mixture, stereospecificity)
- net hydration of alkene across carbon-carbon double bond, but hydrogen added to more substituted end of double bond
- syn-stereoselective (hydrogen and hydroxyl add to same face of molecule)
- regioselective (non-Markovnikov addition)
- no rearrangment
- racemic mixture
- not stereospecific
Osmonium Tetroxide (description, stereoselectivity, regioselectivity, rearrangement, racemic mixture, stereospecificity)
- alkene converted to glycol (1,2- diol)
- syn-stereoselective
- not regioselective
- no rearrangement
- racemic mixture
- stereospecific reaction (cis-isomer produces meso compound; trans-isomer produces entantiomers in a racemic mixture)
Ozonolysis (description, stereoselectivity, regioselectivity, rearrangement, racemic mixture, stereospecificity)
- cleaves double bond, forms two carbonyl (C=O) bonds in place of the original double bond
- no stereoselectivity
- no regioselectivity
- no rearrangement
- no racemic mixtures
- no stereospecificity
Catalytic Hydrogenation/Catalytic Reduction (description, stereoselectivity, regioselectivity, rearrangement, racemic mixture, stereospecificity)
- attaches one hydrogen to each carbon of the original double bond
- normally, syn-stereoselective (but some antistereoselectivity occurs in solution)
- no regioselectivity
- no rearrangement
- racemic mixture technically formed, but significantly more cis-isomer than trans-isomer
- not stereospecific
heat of hydrogenation
standard enthalpy of catalytic hydrogenation of double bond; measures bond stability, which depends on:
- the amount of substituents about the double bond (more substitutents, lower heat of hydrogenation)
- lower for trans-isomers due to steric interactions in cis-isomers
