Chem 261

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Chem 261
2012-03-02 10:52:13
2nd Definitions

Second Def.
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  1. Activation energy (Delt G++)
    The difference in energy levels between ground state and transition state. The amount of activation energy required by a reaction determines the rate at which the reaction proceeds. The majority of organic reactions have activation energies of 10-25 kcal/mol.
  2. Allylic
    Used to refer to the position next to a double bond. For example, H2C=CHCH2Br is an allylic bromide, and an allylic radical is a conjugated, resonance-stabilized species in which the unpaired electron is in a p orbital next to a double bond (C=C-C*<->*C-C=C).
  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 opposite sides of the molecule. (See Syn stereochemistry.)
  4. Bond dissociation energy
    The amount of energy needed to break a bond homolytically to produce two radical fragments.
  5. 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.
  6. Dehydrohalogenation
    A reaction that involves loss of HX from the starting material. Alkyl halides undergo dehydrogenation to yield alkenes on treatment with strong base.
  7. Dehydration
    A reaction that involves loss of water from the starting material. Most alcholols can be dehydrated to yield alkenes, but aldol condensation products (beta-hydroxy ketones) dehydrate particularly readily.
  8. Electrophile
    An "electron-lover," or substance that accepts an electron pair from a nucleophile in a polar bond-forming reaction.
  9. Endothermic
    A term used to describe reactions taht absorb heat and therefore have positive enthalpy changes. Inreaction energy diagrams, the products of endothermic reactions have higher energy levels than the starting materials do.
  10. Entgegen (E)
    A term used to describe the stereochemistry of a carbon carbon double bond. The two groups on each carbon are assigned first priorities according to the Cahn-Ingold-Prelog sequence rules, and the two carbons are then compared. If the high-priority groups on each carbon are on opposite sides of the double bond, the bond has E geometry. (See Zusammen)
  11. Enthalapy change
    (delta H)
    The heat of reaction. The enthalpy change that occurs during a reaction is a measure of the difference in total bond energy between reactants and products.
  12. Entropy change
    (delta S)
    The change in amount of disorder. The entropy change that occurs during a reaction is a measure of the difference in disorder between reactants and products
  13. Exothermic
    A term used to describe reactions that release heat and that therefore have negative enthalpy changes. On reaction energy diagrams, the products of exothermic reactions have energy levels lower than those starting materials
  14. Geometric isomers
    An old term for cis-trans isomers
  15. Gibbs free-energy change
    (delta G)
    The total amount of free-energy change, both enthalpy and entropy, that occurs during a reaction. The Gibbs gree energy change for a reaction is given by the formula

    delta G = delta H - T delta S
  16. 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.
  17. Hammond postulate
    A postulate stating that we can get a picture of what a given transition state looks like by looking at the structure of the nearest stable species. Exothermic reactions have transition states that resemble starting material, whereas endothermic reactions have transition states that resemble products.
  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. Hydrogenation
    Addition of hydrogen to a double or triple bond to yield the saturated product
  22. 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.
  23. Asymmetric center or carbon
    An atom (usually carbon) that is bonded to four different groups and is therefore chiral
  24. Chiral Center
    An atom (usually carbon) that is bonded to four different groups and is therefore assymetric
  25. Stereogenic center or carbon
    Is any atom at which the interchange of two groups gives a stereoisomer. Note: asymmetric carbon actoms and carbon atoms in double bonds are Stereogenic centers of Stereogenic carbon atoms
  26. 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, one right-handed and one left-handed. The most common (though not the only) cause of chirality in a molecule is the presence of a carbon atom that is bonded to four different subsituents (see Achiral)
  27. Enantiomers
    Stereoisomers of a chiral substance that have a mirror-image relationship. Enantiomers must have opposite configurations at all stereogenic centers in the molecule
  28. 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
  29. Optical isomers
    Enantiomers. Optical isomers are isomers that have a mirror-image relationship
  30. Dextrorotatory
    A word used to describe an optically active substane 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)
  31. Levorotatory
    Used to describe an optically active substance that rotates the plane of polarization of plane-polarized light in a left-handed (counter-clockwise) direction.
  32. Specific rotation
    The specific rotation of a chiral compound is a physical constant where the path length of the sample solution is expressed in decimeters and the concentration of the sample is expressed in g/mL
  33. Racemic mixture
    A mixture consisting of equal parts (+) and (-) enatiomers of a chiral substance. Even though the individual molecules are chiral, racemic mixtures are optically inactive.
  34. 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 S I reaction of a chiral alkyl halide.
  35. Naming Stereoisomers
    A method is required to name and describe the arrangement of atoms in three dimensions that occurs in enantiomers. The three dimensional arrangement of the substituents about the asymmetric carbon atom is referred to as configuration
  36. Configuration
    The three-dimensional arrangement of atoms bonded to a stereogenic center relative to the stereochemistry of other stereogenic centers in the same molecule.
  37. Absolute configuration
    The actual three-dimensional structure of a chiral molecule. Absolute configuration are specified verbally by the Cahn-Ingold-Prelog R,S convention and are represented on paper by Fischer projections.
  38. R,S convention
    A method for defining the absolute configuration around stereogenic centers. The Cahn-Ingold-Prelog priority or sequence rules are used to assign relative priorities to the four substituents on the stereogenic center, and the center is oriented such that the group of lowest (fourth) priority faces directly away from the viewer. If the three remaining substituents 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).
  39. Diastereomer
    A term that indicates the relationship between nonmirror-image stereoisomers. Diastereomers are stereoisomers that have the same configuration at one or more stereogenic centers, but differ at other stereogenic centers.
  40. Meso compound
    A meso compound is one that contains stereogenic centers but is nevertheless achiral by virtue of a symmetry plane. For example, (2R3S)-tartaric acid has two chiral carbon atoms but is achiral because of a symmetry plane between carbons 2 and 3.
  41. 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 plane of the page, and the vertical arms of the cross represent bonds going back into the plane of the page.
  42. Intermediate
    A species that is formed during the course of a multistep reaction but is not the final product. Intermediates are more stable than transition states but may or may not be stable enough to isolate.
  43. Markovnikov's rule
    A guide for determining the regiochemistry (orientation) of electrophilic addition reactions. In the addition of HX to an alkene, the hydrogen atom becomes bonded to the alkene carbon that has fewer alkyl substituents. A modern statement of this same rule is that electroplilic addition reactions proceed via the most stable carbocation intermediate.
  44. Mechanism
    A complete description of how a reaction occurs. A mechanism must account for all starting material and all products, and must describe the details of each individual step in the overall reaction process.
  45. Molecular formula
    An expression of the total numbers of each kind of atom present in a molecule. The molecular formula must be a whole-number multiple of the empirical formula.
  46. 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)
  47. Polar reaction
    A reaction in which bonds are made when a nucleophile donates two electron to an electrophile and in which bonds are broken when one fragement leaves with both electrons from the bond. Polar reactions are the most common class of reactions. (see Heterogenic bond formation; Heterolytic bond breakage)
  48. Rate-limiting step
    The slowest step in a multistep reaction sequence. The rate-limiting step acts as a kind of bottlenect in multistip reactions and is observed by kinetics measurements.
  49. Reaction energy diagram
    A pictorial representation of the course of a reaction, in which potential energy is plotted as a function of reaction progress. Starting materials, transition states, intermediates, and final products are all represented, and their appropriate energy levels are indicated.
  50. Regiochemistry
    A term describing the orientation of a reaction that occurs on an unsymmertrical substrate. Markovnikov's rule, for example, predicts the regiochemistry of electrophilic addition reactions.
  51. Regiospecific
    A term describing a reaction that occurs with a Specific regiochemistry to give a single product rather than a mixture of products.
  52. 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 geometery and R,S configurational assignments can be made. (see Entgegen; R,S convention; Zusammen)
  53. Sterochemistry
    The branch of chemistry concerned with the three-dimensional arrangement of atoms in molecules.
  54. Stereoisomers
    Isomers that have their atoms connected in the same order but have different three-dimensional arrangemens. The term stereoisomer includes both enantiomers and diastereomers but does not include constitutional isomers.
  55. Stereospecific
    A term indicating that only a single stereoismer is produced in a given reaction, rather than a mixture.
  56. Tranisition state
    An imaginary activated complex between reagents, representing the highest energy point on a reaction curve. Transition states are unstable complexes that can't be isolated.
  57. Unsaturated
    An unsaturated molecule is one that has multiple bonds and can undergo addition reactions. Alkenes and alkynes, for example, are unsaturated (see saturated)
  58. Vicinal
    A term used to refer to a 1,2-disubstitution pattern. For example, 1,2-dibromoethane is vicinal dibromide.
  59. 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.
  60. 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 Entgegan; Sequence rules)