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MCAT Organic Chemistry
Flashcards based on Kaplan book readings from the Organic Chemistry section as well as additional supplemental information
16
Organic Chemistry
Undergraduate 4
07/24/2019

Additional Organic Chemistry Flashcards

 


 

Cards

Term
Major Functional Groups and Nomenclature
Definition
  • Nomenclature
    1. identify the longest carbon chain containing the highest-order functional group
    2. number the chain
    3. name the substituents
    4. assign a number to each substituent
  • Naming substituents and functional groups

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Term
Structural(Constitutional) Isomers
Definition
  • isomers of this type are the least similar of isomers
    • only share molecular formula and weight
    • don't share connectivity
    • vary in physical and chemical properties

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Term
Conformational Isomers and types of confirmations
Definition
  • subdivision of stereoisomers
    • share same connectivity, but differ in how these atoms are arranged in space
  • conformational are most similar of all isomers and are in fact the same molecule aside from natural rotation about single (σ) bonds
  • described best using newman projections which show angles of atoms and groups in relevance to each other
  • best thought of in terms of straight-chain and cyclic conformations
    • straight-chain: staggered/anti, gauche, (totally) eclipsed
    • cyclic: referred to be conformational shapes; chair, envelope, boat, puckered, etc.

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Term
Straight-Chain Conformations
Definition
  • way of thinking about conformational isomers as applied to straight chains (as opposed to cyclic molecules)
  • looks at angles separating different atoms/functional groups from one another
    • staggered - no overlap of atoms along line of sight
      • anti - particular type of staggered conformation in which largest functional groups are 180° from one another; most stable
      • gauche - particular type of staggered conformation in which largest functional groups are w/in 60° of one another; less stable
    • eclipsed - conformation in which groups or atoms are overlapping
      • eclipsed - largest groups are 120° apart (but are overlapping w/ other groups/atoms)
      • totally eclipsed - largest groups are 0° apart (totally overlapping); least stable

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  • conformational interconversion barriers are small and easily overcome at room temperature
Term
Cyclic Conformations and Cyclohexane
Definition
  • way of thinking about conformational isomers as applied to cyclic molecules (as opposed to straight-chain)
  • described by particular terms relating to overall shape
  • stability of conformation is based on ring strain(based on three types of strain)
    • angle strain - when bond angles deviate from ideal values through compression or stretching
    • torsional strain - when cyclic molecules must assume conformations that have eclipsed or gauche interactions
    • non-bonded strain (van der Waals repulsion) - non-adjacent atoms or groups compete for same space
  • cycloalkanes attempt to adopt non-planar conformations to alleviate strain
  • cyclohexane's most stable form is known as the chair conformation
    • atoms surrounding main ring are referred to by their orientation with the plane of the ring
      • axial groups/atoms are perpendicular to plane of ring and stick up or down
      • equatorial groups/atoms are parallel to plane of ring and stick out
    • bulkiest groups surrounding cyclohexane favor the equatorial position to reduce flagpole interactions arising from van der waals repulsion
    • chair flip - when cyclohexane goes from one chair form to another
      • must pass through half-chair conformation (high energy)
      • at end of flip, all axial groups become equatorial and vice versa

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Term
Configurational Isomers
Definition
  • subdivision of stereoisomers (other being conformational)
  • these isomers are less similar as they must undergo a break and reformation in covalent bonds to change from one form to another
  • determined based on their ability to be superimposed over one another and whether or not they are mirror images
    • enantiomers - non-superimposable mirror images
    • diastereomers - non-superimposable isomers that are not mirror images

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Term
Optical Isomers
Definition
  • optical isomers are molecules in which spatial arrangement of groups affects the rotation of plane-polarized light passing through
  • rely on chirality
    • if mirror image of molecule/center cannot be superimposed on original molecule, then it is chiral
    • only chiral molecules can be considered optically active
  • mixtures of chiral molecules of opposite types (enantiomers) in equal ratios create optically inactive mixture termed racemic
    • (+) - compound that rotates plane of polarized light to right - dextrorotatory (d-)
    • (-) - compound that rotates plane of polarized light to left - levorotatory (l-)
    • can only be determined experimentally and not by looking at the molecule's structure
Term
Enantiomers
Definition
  • subset of configurational isomers that are nonsuperimposable mirror images of one another
  • must have opposite configurations at every chiral center
  • have identical chemical and physical properties
    • only differ in optical activity and reactions in chiral environments
      • optical activity
        • (+) vs. (-) designation
      • reactions in chiral environments
        • if racemic mixture reacted w/ (+), (+,+) and (-,+) are formed
        • if racemic mixture reacted w/ (-), (-,-) and (+,-) are formed

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Term
Diastereomers
Definition
  • subset of configurational isomers that are non-superimposable and non-mirror images of each other
  • can only occur if molecule has two or more chiral centers
    • for any molecule w/ n chiral centers, 2n possible diastereomers(and stereoisomers) exist
  • have different chemical and physical properties
    • can behave in chemically similar ways due to presence of same functional groups
    • consistently different in physical properties due to differing spatial arrangements
  • diastereomers are optically active, but knowing the way that one diastereomer rotates polarized light, gives no indication as to how another might
  • by pulling individual enantiomers from pairs and comparing them to enantiomers from other pairs, we can identify diastereomers
    • I and II are not diastereomers of one another, but I and III or I and IV are

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Term
Meso Compounds
Definition
  • a molecule containing one or more chiral centers that is not optically active due to the presence of an internal plane of symmetry
  • molecular equivalents of racemic mixtures - chiral centers cancel out

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Term
Relative Configuration vs. Absolute Confirmation of Stereoisomers
Definition
  • configuration of stereoisomer - spatial arrangement of atoms or groups in molecules
  • relative configuration - configuration of chiral molecule in relation to another chiral molecule
    • often through chemical conversion
    • used to determine whether molecules are enantiomers, diastereomers or same molecule
    • require comparison of molecules
  • absolute confirmation - describes exact spatial arrangement of atoms or groups independent of other molecules
    • can be described using E/Z, cis/trans, and R/S
Term
E/Z vs. Cis/Trans
Definition
  • Cis/Trans system
    • used to describe geometric isomers that are a particular types of diastereomers
    • substituents must differ in position around immovable bond
    • really only applies to double bonds or cyclic molecules w/ one substituent on each side of the bond
    • indicates different molecules that cannot interconvert w/out breaking of a pi bond
    • cis - same side
    • trans - opposite sides

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  • E/Z system
    • similarly used to describe geometric isomers, but can include molecules w/ more than one substituent on either side of the immovable bond
    • E (entgegen or opposite) - two highest-priority substituents are on opposite sides of bond
    • Z (zusammen or together) - two highest-priority substituents on same side of bond

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Term
R/S Forms
Definition
  • used for chiral centers in molecules
  • R (rectus - right) and S (sinister - left)
  • To determine
    1. assign priority to groups surrounding center
    2. arrange molecule in space with lowest priority group facing back
    3. connect 1→2, 2→3, and 3→1 using arrows to create circle
    4. determine directionality of circle (clockwise or counterclockwise)
      • Clockwise means that center must receive R  designation
      • Counterclockwise means that center must receive S designation
    5. Name molecule by putting (R) and (S) in parentheses and separating from rest of name by hyphen
      • (R,S)-1,3-etc.
Term
Molecular Orbital Theory
Definition
  • formed when two atomic orbitals combined
  • obtained mathematically by adding or subtracting the wave functions of the atomic orbitals
    • if signs of wave functions are same, lower-energy bonding orbital produced
    • if signs of wave functions are opposite, high-energy antibonding orbital produced

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  • when formed by head-to-head or tail-to-tail overlap, resulting bond is called σ bond (single bond)
  • when two p-orbitals line up in parallel, their electron clouds can overlap to form π bond
    • are extremely weak bonds(in comparison to σ) and don't form unless σ bond is present
    •  1 π bond w/ a σ creates double bond
    • 2 π bonds w/ a σ create triple bond

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Term
Theory of Orbital Hybridization
Definition
  • accounts for why a molecules like methane which takes 2 electrons from 2s orbital, one from the px-orbital, and one from the py-orbital (different energies) still forms 4 equivalent bonds
  • these orbitals are formed by mixing different types of orbitals to produce a new orbital with hybrid s and p character
  • sp3
    • tetrahedral geometry
    • no unhybridized p-orbitals to form π bonds
    • accomplished by promoting one 2s electron into 2pz-orbital to produce four valence orbitals containing one electron each
    • 25% s character and 75% p character

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  • sp2
    • trigonal planar geometry
    • one unhybridized p-orbital can be used to form one π bond
    • one s-orbital is mixed w/ two p-orbitals and 3 sp2 orbitals are formed
    • bonds are 120° apart
    • 33% s and 67% p character
  • sp
    • linear geometry
    • two unhybridized p-orbitals can be used to form two π bonds
    • one s-orbital is mixed w/ one p-orbital to form two sp orbitaals
    • 50% s and 50% p character
    • oriented 180° apart
    • can be triple bond or two single bonds
Term
Resonance
Definition
  • delocalization of electrons that occurs in molecules w/ conjugated bonds
    • imparts increased stability
    • can also give a bond partial double-bond character like in proteins (making it immovable)
  • conjugation - alternating single and double/triple bonds
    • allows for alignment of a number of unhybridized p-orbitals along backbone of molecule
    • π electrons can delocalize throughout this p-orbital system
  • actual structures of molecules with resonance forms is something between all possible resonance forms
    • some forms are favored based on stability giving molecules more character of one form
      • lack of formal charges
      • formation of full octets on highly electronegative atoms (O or N)
      • stabilization of + and - charges through induction and aromaticity
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