Chem Final Review Chapter 5

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  1. Stereochemistry definition
    study of the 3D structure of molecuels

    same bonding sequence, but they differ in the orientation of their atoms in space
  2. Objects with left and right handed forms are __. How do when this property exists?

    Nonsuperimposable mirror-image molecules are called __. A __ compound always has an __.
    • chiral
    • if hte mirror image is different from the actual object (nonsuperimposable)

    • enantiomers
    • chiral compound
    • enantiomer
  3. Most common feature of chirality? This feature is the most common ex. of a __, the IUPAC term for any atom holding a set of ligands in a spatial arrangement that is not superimposable on its mirror image.
    • carbon bonded to four diferent atoms, creating an asymmetric carbon atom or chiral carbon atom 
    • chirality center
  4. Chirality centers belong to an even broader group called __. A __ is any atom at which the interchange of two groups gives a stereoisomer.
    stereocenter x2
  5. If there is an internal plane of symmetry, what is it?

    Any asymmetric carbon has two possible mirror-image spatial arrangements, which we call __.
    achiral, even if it may have chiral centers

  6. Cahn-Ingold-Prelog convention
    assigning R (right) or S (left) to its 3d structure

    • 1) assign priorities to each group
    • 2) using models, put the fourth group away from you and view the molecule with the first, second, and third groups radiating twoard you. Draw from 1 to 3
  7. Rotation of the plane of polarized light is __, and substances that rotate the plane of polarized light are said to be __. 

    Properties of enantiomers?

    Directions of light?
    • optical activity
    • optically active
    • have identical physical properties, except for the direction they rotate the plane of polarized light

    • dextrorotatory (+ or d): toward the right
    • levorotatory (- or l): toward the left
  8. Formula for specific rotation
    observed rotation/ c (g/mL) * length in dm
  9. A solution of equal amounts of two enantiomers, so that the mixture is optically inactive, is called a __. Sometimes, its called a __, __, or a __. 

    When will this kind of mixture be created?
    • racemic mixture
    • racemate
    • +/-
    • d,l

    when optically inactive reactants are used
  10. the __ is defined as the ratio of its rotation to the rotation of a pure enantiomer. 

    optical purity

    (observed rotation)/ (rotation of pure enantiomer) x 100%
  11. The __ is a similar method for expressing the relative amounts of enantiomers in a mixture. To comput ethis, we calculate the excess of the predominant enantiomer as a percentage of hte entire mixture. For a chemically pure compound, teh calculation of this generally gives the same result as the calculation of optical purity and we use the terms interchangeably.
  12. When can allene be chiral?
    when we add molecules to it, causing its mirror to be different
  13. Rotating a Fischer projection by 180 degrees does what to it?
    keeps it the same
  14. Stereoisomers that are not mirror images of one another= __ 

    The biggest example of diastereomers? Other examples?

    cis-trans isomers

    moelucels with two or more chirality centers
  15. What is the 2^n rule?
    a compound with n asymmetric carbon atoms might have as many as 2^n stereoisomers
  16. Compounds that are achiral even though they have asymmetric carbon atoms are called __.
    meso compounds
  17. Absolute configuration:
    the detailed stereochemical picture of a molecule, including how the atoms are arranged in space. Alternatively, the R or S config at each chiral center
  18. Relative conf
    the experimentally determined relationship between the configs of two molecues
  19. Properties of diastereomers
    different physical properties
  20. Alkyl halide
    vinyl halide
    aryl halide
    • 1) has a lalogen bonded to one of the sp3 carbon atoms 
    • 2) halogen atom bonded to one of teh sp2 hybrid carbon atoms of an alkene
    • 3) halogen atom bonded to one of the sp2 carbons of an aromatic ring
  21. Geminal dihalide
    vicinal dihalide
    • 1) halogen atoms bonded to the same carbon atom
    • 1) two halogens bonded to adjacent carbon atoms
  22. What are the effects that affect dipole moments?
    electronegativity: I < B < Cl < F

    bond lengths: CF < CCl < CBr < CI

    These oppose each other to lead to an overall result of: CI < CBr < CF < CCl
  23. Two types of intermolecular forces influence the BP of alkyl halides. The __ is the strongest intermolecular attraction in alkyl halides. __ also affect it. 

    What is the trend in BP?
    • London force
    • dipole dipole

    Trend: higher MW have higher BP because heavier and greater surface area
  24. Density trend
    • Alkyl fluorides and chlorieds with just one Cl are less dense than water
    • two or more Cl are denser, as well as alkyl Br and Alkyl I
  25. free rad bromination of alkenes can be carried out in a selective manner. Why?
    because the bromine will abstract a proton from the allylic position and the most subbed will be alkene is the most stable
  26. Allylic Bromination steps
    Initiation: Br absorbs light, forming rads

    First Prop: A bromine rad abstracts an allylic H

    Second Prop: Either radical carbon can react with Br
  27. What is the reaction with NBS?
    NBS + HBr--> Br2 and that other product with the H
  28. In many eliminations, a molecule of HX is lost from the alkyl halide to give an __. These eliminations are called __ because a hydrogen halide has been removed from the alkyl halide.
    • alkene
    • dehydrohalogenations
  29. In a __, a nucleoophile replaces a leaving group from a carbon atom, using its lone pairs of electrons to form a new bond to the arbon atom.

    In a __, both the halideion and another substituent are lost. A new pi bond is formed.
    • nucleophilic substitution
    • an elimination
  30. In the elimination, the reagent reacts as a __. 

    SN2 reactions
    - how many steps?
    - what type of reaction?
    - middle structure?
     - order of reactivity?

    • - one
    • - concerted reaction
    • - transition state
    • - CH3X> primary> secondary (doesn't occur on tertiary)
  31. What can we generalize about bases? Give an example.
    A base is always a stronger nucleophile than its conjugate acid- methoxide ion versus methanol ion
  32. Differentiate between base and nucleophile.
    if the new bond is to a proton, it has reacted as a base. If the new bond is to a carbon, it has reacted as a nucleophile
  33. Nucleophilicity trend
    decreases from L to R in the periodic table, following the increase in EN from left to right, but increases down the periodic table
  34. Bulky groups on nucleophile?
    will hinder back side attack and prevent substitution--> elimination
  35. Good leaving groups should be __.

    Rates for Sn2
    weak bases (conjugate bases of strong acids)

    CH3X > prim > sec >> tert
  36. Stereochemistry of Sn2
    stereospecific: inversion of configuation
  37. Sn1
    - nucleoophile?
    - steps?
    - reactivity?
    - stereochemistry?
    • - very weak solvent
    • Steps:
    • 1) formation of carbocation (rate limiting)
    • 2) nucleophilic attack on the carbocation
    • 3) loss of proton to solvent ( if the nucleophile is water or an alcohol

    - 3> 2> 1> Ch3X

    - because its carbocation is planar, it can approach from both faces, causing a racemic solution (front side: retention of config/ back side: inversion)
  38. Types of rearrangement
    • hydride shift
    • methyl shift
  39. Sn1 vs. Sn2

    - nucl
    - substrate
    - solvent
    - stereochem
    - rearrangement
    • Sn1: nucleophile strength doesn't matter
    • Sn2: strong nuc required

    • Sn1 substrates: 3>2 (1 and Ch3X are unlikely)
    • Sn2: Ch3X> 1> 2 (3 unlikely)

    • Sn1: requires a good solvent
    • Sn2: may go faster in less polar solvent

    • Sn1: inversion and retention
    • Sn2: complete inversion

    • Sn1: rearrangement
    • Sn2: none
  40. E1 reaction
    - reactivity
    - steps
    - competes?
    • - 3> 2>>1
    • Steps:
    • 1) unimolecular ionization to give a carbocation (rate-limiting)
    • 2) Deprotonation by a weak base gives the alkene

    competes with Sn1
  41. Zaitsev's rule
    in elim rxns, the most subbed alkene usually predominates
  42. E2
    - base
    - reactivity
    - Steps
    - products
    - stereochemistry
    • - can be bulky
    • - 3>2>1
    • - Steps:
    • -- single step; base abstracts proton on carbon neext to leaving group and the leaving group leaves, forming an alkene
    • - can form multiple products based on substituted
    • - Stereospecific: they must be anti to each other
  43. E1 vs E2

    - base
    - solvent
    - reactivity
    - orientation
    - Stereochemistry
    - rearrangement
    • E1: base strength is unimportant 
    • E2: strong base

    • E1: requires a good ionizign solvent
    • E2: solvent polarity is not so important

    E1, E2: 3> 2> 1

    E1, E2: Zaitsev

    • E1: no particular geometry required for slow step
    • E2: coplanar arrangement (usually anti) required for the transition state

    • E1: rearrangement common
    • E2: no rearrangements
Card Set:
Chem Final Review Chapter 5
2013-12-07 03:36:03
CHM 201

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