User:Benjah-bmm27/degree/1/PJW

Structure and reactivity, PJW

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Structure

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Levels of structure

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Connectivity is the domain of regiochemistry, whereas configuration and conformation come under stereochemistry.

Table of types of structure and isomerism

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The different levels of structure and the corresponding types of isomerism
Level of structure Type of isomerism Definition of isomers
Chemical formula Isomerism Isomers have the same chemical formula but are otherwise different somehow.
Connectivity Structural isomerism
(constitutional isomerism)
Structural isomers have the same chemical formula but different connectivity.
Structural isomerism is a subset of isomerism.
Spatial arrangement Stereoisomerism Stereoisomers have the same connectivity but a different spatial arrangement (a different 3D shape).
Stereoisomerism is a subset of isomerism.
Chirality Enantiomerism Enantiomers have the same connectivity and spatial arrangement except they have opposite chirality, i.e. opposite absolute configuration. Enantiomers are non-superposable mirror images of each other
Enantiomerism is a subset of stereoisomerism
Configuration Diastereomerism Diastereomers have the same connectivity but a different (but not opposite) configuration and are thus not mirror images of each other. Diastereomers are stereoisomers that are not enantiomers
Diastereomerism is a subset of stereoisomerism
Configuration Cis–trans isomerism Cis–trans isomers have the same connectivity but a different spatial arrangement, are not mirror images of each other, and are not conformers of each other. Not being conformers means cis–trans isomers cannot be superposed by interconverted by rotations about formally single bonds alone. This usually requires the presence of a bond about which rotation is restricted, such as a double bond or a bond in a ring.
Cis–trans isomerism is a subset of diastereomerism
Conformation Conformational isomerism Conformers have the same connectivity and configuration but a different spatial arrangement and can be interconverted by rotations about formally single bonds. They can be mirror images of each other, but this is not a requirement.
Conformational isomerism is a subset of diastereomerism

IUPAC Gold Book definitions

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  • connectivity: "In a chemical context, the information content of a line formula, but omitting any indication of bond multiplicity."
  • constitution: "The description of the identity and connectivity (and corresponding bond multiplicities) of the atoms in a molecular entity (omitting any distinction arising from their spatial arrangement)."
  • superposability: "The ability to bring two particular stereochemical formulae (or models) into coincidence (or to be exactly superposable in space, and for the corresponding molecular entities or objects to become exact replicas of each other) by no more than translation and rigid rotation."
  • projection formula
  • absolute configuration
  • relative configuration
  • configuration (stereochemical): "...the arrangements of atoms of a molecular entity in space that distinguishes stereoisomers, the isomerism between which is not due to conformation differences."
  • conformer
  • rotamer
  • polytopal rearrangement

Relationship between structure, bonding, and electrons

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Orbitals and axial chirality

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Reactivity

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Acids vs. bases

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Electrophiles vs. nucleophiles

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Hard and soft

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Summary of the chemical properties of hard and soft species
Property Hard Soft
Oxidation state high low or zero
Polarizability low high
Electronegativity high low
HOMO of the Nu low-lying high-lying
LUMO of the E+ high-lying low-lying
  • Hard electrophiles react best with hard nucleophiles, as they have:
  • Soft electrophiles react best with soft nucleophiles, as they have:
    • large atomic or ionic radii
    • low (or zero) oxidation states
    • high polarizabilities
    • low electronegativities
    • high-lying HOMOs (nucleophiles) and low-lying LUMOs (electrophiles)
  • HSAB theory is also used in inorganic chemistry: Level 2 transition metal chemistry (hard and soft metals and ligands)

Substitution reactions

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