The module on the right provides examples of homotopic and heterotopic ligand pairs for analysis. These are displayed as three-dimensional structures in which the pairs are labeled A and B. The structures may be moved about and examined from various points of view. By using this resource the reader should be able to classify the nature of the relationship as homotopic, enantiotopic or diastereotopic.
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Example 13
Example 14
Example 15

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  enantiotopic
  diastereotopic
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General Summary of Isomerism and Molecular Descriptors

Methods of Describing Molecules with Increasing Refinement

  1. Composition
    The number and kinds of atoms that make up a molecule. This information is supplied by a molecular formula.

  2. Constitution
    The bonding pattern of the atoms of a molecule (ie. which atoms are connected to which other atoms and by what kind of bonds). Different bonding constitutions are interconverted only by breaking and reforming covalent bonds. This information is supplied by a structural formula, and is implicit in the IUPAC name.

  3. Configuration
    The permanent spatial relationship of the atoms of a molecule to each other. Different configurations are interconverted only by breaking and reforming covalent bonds. This information is given in a stereo-formula, and is also provided by a prefix to the IUPAC name (eg. cis & trans).

  4. Conformation
    The variable spatial orientation of the atoms of a molecule to each other that occurs by rotation or twisting of bonds. Different conformations are interconverted without breaking covalent bonds. This information is supplied by conformational formulas, and also by nomenclature terms (eg. gauche & anti).

Relationship of Constitutional and Stereoisomers

Relationships of Stereoisomers


Summary of Factors Influencing Alkyl Halide Reactions

The following table summarizes the expected outcome of alkyl halide reactions with nucleophiles. It is assumed that the alkyl halides have one or more beta-hydrogens, making elimination possible; and that low dielectric solvents (e.g. acetone, ethanol, tetrahydrofuran & ethyl acetate) are used. When a high dielectric solvent would significantly influence the reaction this is noted in red. Note that halogens bonded to sp2 or sp hybridized carbon atoms do not normally undergo substitution or elimination reactions with nucleophilic reagents.

Nucleophile

Anionic Nucleophiles
( Weak Bases: I–, Br–, SCN–, N3–,
CH3CO2– , RS–, CN– etc. )

pKa's   from -9 to 10 (left to right)

Anionic Nucleophiles
( Strong Bases: HO–, RO– )

pKa's   > 15

Neutral Nucleophiles
( H2O, ROH, RSH, R3N )

pKa's  ranging from -2 to 11

Alkyl Group

Primary
RCH2– 		Rapid SN2 substitution. The rate may be reduced by substitution of β-carbons, as in the case of neopentyl. Rapid SN2 substitution. E2 elimination may also occur.  e.g.
ClCH2CH2Cl + KOH ——> CH2=CHCl 		SN2 substitution. (N ≈ S >>O)

Secondary
R2CH– 		SN2 substitution and / or E2 elimination (depending on the basicity of the nucleophile). Bases weaker than acetate (pKa = 4.8) give less elimination. The rate of substitution may be reduced by branching at the β-carbons, and this will increase elimination. E2 elimination will dominate. SN2 substitution. (N ≈ S >>O)
In high dielectric ionizing solvents, such as water, dimethyl sulfoxide & acetonitrile, SN1 and E1 products may be formed slowly.

Tertiary
R3C– 		E2 elimination will dominate with most nucleophiles (even if they are weak bases). No SN2 substitution due to steric hindrance. In high dielectric ionizing solvents, such as water, dimethyl sulfoxide & acetonitrile, SN1 and E1 products may be expected. E2 elimination will dominate. No SN2 substitution will occur. In high dielectric ionizing solvents SN1 and E1 products may be formed. E2 elimination with nitrogen nucleophiles (they are bases). No SN2 substitution. In high dielectric ionizing solvents SN1 and E1 products may be formed.

Allyl
H2C=CHCH2–		 Rapid SN2 substitution for 1º and 2º-halides. For 3º-halides a very slow SN2 substitution or, if the nucleophile is moderately basic, E2 elimination. In high dielectric ionizing solvents, such as water, dimethyl sulfoxide & acetonitrile, SN1 and E1 products may be observed. Rapid SN2 substitution for 1º halides. E2 elimination will compete with substitution in 2º-halides, and dominate in the case of 3º-halides. In high dielectric ionizing solvents SN1 and E1 products may be formed. Nitrogen and sulfur nucleophiles will give SN2 substitution in the case of 1º and 2º-halides. 3º-halides will probably give E2 elimination with nitrogen nucleophiles (they are bases). In high dielectric ionizing solvents SN1 and E1 products may be formed. Water hydrolysis will be favorable for 2º & 3º-halides.

Benzyl
C6H5CH2– 		Rapid SN2 substitution for 1º and 2º-halides. For 3º-halides a very slow SN2 substitution or, if the nucleophile is moderately basic, E2 elimination. In high dielectric ionizing solvents, such as water, dimethyl sulfoxide & acetonitrile, SN1 and E1 products may be observed. Rapid SN2 substitution for 1º halides (note there are no β hydrogens). E2 elimination will compete with substitution in 2º-halides, and dominate in the case of 3º-halides. In high dielectric ionizing solvents SN1 and E1 products may be formed. Nitrogen and sulfur nucleophiles will give SN2 substitution in the case of 1º and 2º-halides. 3º-halides will probably give E2 elimination with nitrogen nucleophiles (they are bases). In high dielectric ionizing solvents SN1 and E1 products may be formed. Water hydrolysis will be favorable for 2º & 3º-halides.