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Organic Chemistry 1


 
 

Isomerism and Stereochemistry

  1. Isomers: Molecules that have the same chemical formula, but are structurally different in some way.

    1. Constitutional isomers: Isomers that have atoms connected to each other in different ways.

    2. Constitutional isomers

    3. Stereoisomers: Isomers whose atoms have the same connectivity, but are arranged differently in space relative to each other.

  2. Geometrical isomers: Stereoisomers whose substituents are arranged differently relative to each other around a double bond or ring.

    1. Cis-trans isomers: Geometrical isomers in which two identical substituents (or two pairs of two identical substituents) are bonded differently to carbon atoms participating in a double bond or ring bond.

      1. Cis isomer: Isomer in which the identical substituents are on the same side of the double bond or ring.

      2. Trans isomer: Isomer in which the identical substituents are on opposite sides of the double bond or ring.

      3. Cis-trans isomers

    2. Sequence Rules: Method for specifying the arrangement of substituents around a carbon-carbon double bond or a carbon-carbon bond in a ring.

      1. Rank the four substituents in order of the atomic numbers of the atoms bonded directly to the double bonded carbons, from highest (1) to lowest (4).

      2. If two substituents have the same ranking after Step 1, look at the next atoms in their substituent chains. For both substituents, list the atoms that are two bonds away from the chiral center in order of atomic number, from highest to lowest. Assign the lower number to the substituent that has the atom with the higher atomic number in the first position in its list; if this is the same for both substituents, continue down the list until a difference is found and assign a ranking in the same manner.

      3. If a substituent has a double (or triple) bond in its chain, it is counted as two (or three) bonds to the same atom for the purposes of making lists in Step 2.

      4. If multiple substituents are still the same after Step 2, move along the substituent chains until the first point of difference is found, and then rank the substituents as in Step 2.

      5. If the higher-priority substituents ("1" and "2") are on the same side of the double bond, then the substituents are arranged in Z configuration; if the higher-priority substituents are on opposite sides of the double bond, then they are arranged in E configuration.

    Sequence rules

    Specifying the stereochemistry of isomers of C3H5OCl

  3. Optical isomers: Stereoisomers that have all their atomsor groups arranged in the same way relative to each other but cannot be superimposed over each other. Optical isomers are chiral molecules.

    1. Chirality: Inability of a molecule to be superimposed on its mirror image so that all its atoms are in the same positions as the atoms in the mirror image. A molecules that does not have chirality is therefore achiral.

    2. Chiral center: Carbon atom bonded to four different substituents. Chiral molecules usually contain chiral centers, but the presence of a chiral center does not necessary makes a molecules chiral.

    3. Enantiomers: Optical isomers that are mirror images of each other.

    4. Diastereomers: Optical isomers that are not mirror images each other.

    5. Determining chirality: A molecule is chiral only if it does not have a plane of symmetry (i.e., if no plane exists that can divide that molecule into two identical halves).

    6. Cahn-Ingold-Prelog rules: Method for specifying the arrangement of four different substituents around a chiral center.

      1. Steps 1–4 are the same as Steps 1–4 of the Sequence Rules (see above).

      2. Draw the chiral center so that the bond to the substituent ranked "4" points into the plane of the paper the three remaining substituents will be pointing out of the plane of the paper.

      3. Draw a curved arrow from the substituent ranked "1" to the substituent ranked "2" to the substituent ranked "3." If this arrow is clockwise, the chiral center has R configuration; if it is counterclockwise, the chiral center has S configuration.

      4. Cahn-Ingold-Prelog rules

        Use of Cahn-Ingold-Prelog rules to specify the stereochemistry of optical isomers of C2H5OCl

    7. Conformational isomers: Stereoisomers that can be interconverted by the rotation of atoms about a σ-bond.

    8. Conformational isomers