Chem 261

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mct
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139140
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Chem 261
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2012-04-05 11:14:27
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Third Def
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Third Def
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  1. Abolute configuration
    The actual three-dimensional structre of a chiral molecule. Absolute configurations are specified verbally by the Cahn-Ingold-Prelog R,S convention are represented on paper by Fischer projections.
  2. Achiral
    Having a lack of handedness. A molecule is achiral if it has a plane of symmetry and is thus superimposable on its mirror image (see chiral)
  3. Anti stereochemistry
    Referring to opposite sides of a double bond or molecule. An anti addition reaction is one in which the two ends of the double bond are attacked from different sides. For example, addition of Br2 to cyclohexene yields trans-1,2-dibromocyclohexane, the product of anti addition. An anti elimination reaction is one in which the two groups leave from oppositie sides of the molecule (syy syn stereochemistry.)
  4. Chiral
    Having handedness. Chiral molecules are those that do not have a plane of symmetry and are therefore not superimposable on their mirror image. A chiral molecule thus exists in two forms, on right-handed and one left-handed. The most common (though no the only) cause of chirality in a molecule is the presence of a carbon atom that is bonded to four different substituents (see Achiral)
  5. Chiral center
    An atom (usually carbon) that is bonded to four different groups and is therefore chiral (compare and see stereogenic center).
  6. Cis-trans isomers
    Certain kinds of stereoisomers that differ in their stereochemistry about a double bond or on a ring. Cis-trans isomers are also called geometric isomers.
  7. Configuration
    The three-dimensional arrangment of atoms bonded to a stereogenic center relative to the stereochemistry of other stereogenic centers in the same molecule.
  8. Dehydration
    A reaction that involves loss of HX from the starting material. Most alcohols can dehydrated to yield alkenes, but aldol condensation procucts (beta-hydroxy ketones) dehydrate particularly readily.
  9. Dextrorotatory
    A word used to describe an optically active substance that rotates the plane of polarization of plane-polarized light in a right-handed (clockwise) direction. The direction of rotation is not related to the absolute configuration of the molecule (see levorotatory).
  10. Dehydrohalogenation
    A reaction that involves loss of HX from the starting material. Alkyl halides undergo dehydrohalogenation to yield alkenes on treatment with strong base.
  11. Diastereomer
    A term that indicates the relationship between nonmirror-image stereoisomers. Diastereomers are stereoisomers that have the same confiuration at one or more stereogenic ceters, but differ at other stereogenic centers.
  12. Electrophile
    An "electron-lover", or substance that accepts an electron pair from a nucleoplhile in a polar bond-forming reaction.
  13. Enantiomers
    Stereoisomers of a chiral substance that have a mirror-image relationship. Enantiomers must have opposite configurations at all stereogenic centers in the molecule.
  14. Entgegen (E)
    A term used to describe the stereochemistry of a carbon carbon bond. The two groups on each carbon are first assigned priorities according to the Cahn-Ingold-Prelog sequence rules, and the two carbons are then compared. If the high-priority groups oneach carbon are on opposite sides of the double bond, the bon has E geometry.
  15. Fischer projection
    A means of depicting the absolute configuration of chiral molecules on a flat page. A Fischer projection employs a cross to represent the stereogenic center; the horizontal arms of the cross represent bonds coming out of the page, and the vertical arms of the cross represent bonds going back into the plane of the page.
  16. Geometric isomers
    An old term for cis-trans isomers
  17. Halonium ion
    A species containing a positively charged, divalent halogen. Three-membered-ring bromonium ions are implicated as intermediates in the electrophilic addition of bromine to alkenes
  18. Heterolytic bond breakage
    • The kind of bond breaking that occurs in polar reactions when one fragment leaves with both of the bonding electrons, as in the equation
    • A:B -> A+ + B:-
  19. Homolytic bond breakage
    • The kind of bond breaking that occurs in radical reactions when each fragment leaves with one bonding electron according to the equation
    • A:B -> A* + B*
  20. Hydration
    Addition of water to a molecule, such as occurs when alkenes are treated with strong sulfuric acid
  21. Hydroboration
    Addition of borane (BH3) or an alkylborane to an alkene. The resultant trialkylborane products are useful synthetic intermediates that can be oxidized to yield alcohols.
  22. Hydrogenation
    Addition of hydrogen to a double or triple bond to tield the saturated product
  23. Hyperconjugation
    A weak stabilizing interaction that results from overlap of a p orbital with a neighboring sigma bond. Hyperconjugation is important in stabilizing carbocations and in stabilizing substituted alkenes.
  24. Initiator
    A substance with an easily broken bond that is used to initiate radical chain reactions. For example, radical chlorination of alkanes in initiated when light energy breaks the weak chlorine-chlorine bond to form chlorine radicals.
  25. Isomers
    Compounds that have the same molecular formula but have different structures
  26. Levorotatory
    Used to describe an optically active substance that rotates the plane of polarization of plane-polarized pight in a left-handed (counterclockwise) direction (see Dextrorotary)
  27. Meso Compound
    A meso compound is one that contains sterogenic centers but is neverhteless achiral by virtue of a symmetry plane. For example, (2R,3S)-butanediol has two chiral carbon atoms but is achiral because of a symmetry plane between carbons 2 and 3.
  28. Nucleophile
    A "nucleus-lover" or species that donates an electron pair to an electrophile in a polar bond-forming reaction. Nucleophiles are also Lewis bases (see Electrophile).
  29. Optical isomers
    Enantiomers. Optical isomers are isomers that have a mirror-image relationship.
  30. Polar reaction
    A reaction in which bonds are made when a nucleophile donates two electrons to an electrophile and in which bonds are broken when one fragment leaves with both electrons from the bond. Polar reactions are the most common class of reactions (see Heterogenic bond formation; Heterolytic bond breakage).
  31. R,S convention
    A method for defining the absolute configuration around stereogenic centers. The Cahn-Ingold-Prelog sequence rules are used to assign relative priorities to the four substituents on the stereogenic center, and the center is oriented such that the groups of lowest (fourth) priority faces directly away from the viewer. If the three remaining sustituents have a right-handed or clockwise relationship in going from first to second to third priority, then the stereogenic center is denoted R (rectus,right). If the three remaining substituents have a left-handed or counterclockwise relationship, the stereogenic center is denoted S (sinister, left) (see Sequence rules).
  32. Racemic mixture
    A mixture consisting of equal parts (+0 and (-) enantiomers of a chiral substance. Even though the individual molecules are chiral, racemic mixtures are optically inactive
  33. Racemization
    The process whereby one enantiomer of a chiral molecule becomes converted into a 50:50 mixture of enantiomers, thus losing its optical activity. For example, this might happen during an SN1 reaction of a chiral alkyl halide.
  34. Radical
    When used in organic nomenclature, the word radical refers to a part of a molecule that appears in its name-for example, the "phenyl" in phenyl acetate. Chemically, however, a radical is a species that has an odd number of electrons, such as the chlorine radical, Cl.
  35. Radical reaction
    A reaction in which bonds are made by donation of one electron from each of two reagents and in which bonds are broken when each fragment leaves with one electron. (see Homogenic bond formation; Homolytic bond breakage).
  36. Regiochemistry
    A term describing the orientation of a reaction that occurs on an unsymmetrical substrate. Markovnikov's rule, for example, predicts the regiochemistry of electrophilic addition reactions.
  37. Regiospecific
    A term describing a reaction that occurs with a specific regiochemistry to give a single product rather than a mixture of products.
  38. Resolution
    The process by which a racemic mixture is separated into its two pure enantiomers. For example, a racemic carboxylic acid might be converted by reaction with a chiral amine base into a diastereomeric mixture of salts, which could be seperated by fractional crystallization. Regeneration of the free acids would then yield the two pure enantiomeric acids.
  39. Sequence rules
    A series of rules devised by Cahn, Ingold, and Prelog for assigning relative priorities to substituent groups on a double-bond carbon atom or on a stereogenic center. Once priorities have been established, E,Z double-bond geometry and R,S configurational assignments can be made (see Entgegen; R,S convention; zusammen).
  40. Specific rotation [alpha)D
    The specific rotation of a chiral compound is a physical constant that is defined by the equation [alpha]D =observed rotation/(path length*Concentration) where the path length of the sampe is expressed in decimeter and the concentration of the sample solution is expressed in g/mL.
  41. Stereochemistry
    The branch of chemistry concerned with the three-dimensional arrangement of atomes in molecules.
  42. Stereogenic center
    Is any atom at which the interchange of two gropus gives a stereoisomer (see and compare with chiral center). NOTE asymmetric carbon atoms and carbon atoms in double bonds are Stereogenic centers or Stereogenic carbon atoms
  43. Stereoisomers
    Isomers that have their atoms connected in the same order but have different three-dimensional arrangements. The term steriosomer includes both enantiomers and diastereomers but does not include constituional isomers.
  44. Stereospecific
    A term indicating that only a single stereoisomer is produced in a given reaction, rather than a mixture.
  45. Syn stereochemistry
    A syn addition reaction is one in which the two ends of the double bond are attacked from the same side. For example, OsO4 induced hydroxulation of cyclohexane yields cis-1,2-cyclohexanediol, the product of syn addition. A syn elimination is one in which the two groups leave from the same side of the molecule.
  46. Tautomers
    Isomers that are rapidly interconverted. For example, enols and ketones are tautomers since they are rapidly interconverted on treatment with either acid or base catalysts.
  47. Vicinal
    A term used to refer to a 1,2-disubstitution pattern. For example, 1,2-dibromoethane is a vicinal dibromide.
  48. Vinylic
    A term that refers to a substituent at a double-bond carbon atom. For example, chloroethylene is a vinylic chloride, and enols are vinylic alcohols.
  49. Zusammen (Z)
    A term used to describe the stereochemistry of a carbon-carbon double bond. The two groups on each carbon are assigned priorities according to the Cahn-Ingold-Prelog sequence rules, and the two carbons are compared. If the high-priority groups on each carbon are on the same side of the double bond, the bond has Z geometry (see Entgegen; Sequence rules).

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