23. The concept of isomerism, types of structural isomerism, nomenclature of organic compounds Flashcards
What are isomers?
Compounds with same formula but different structure
3 forms of isomerisms
- Constitution → Order / sequence of atoms: which atom is connected to which other one by covalent bond
- Configuration: Spacial structure of atoms that are directly connected to each other (that are connected to one specific C)
- Conformation: Spacial structure of atoms that are directly not connected to each other
What is constitution?
Order / sequence of atoms: which atom is connected to which other one by covalent bond
What are the 3 types of constitutional isomerism?
- Branching/ backbone isomers
- Positional isomers
- Tautomers
What is keto-enol tautomerism
A type of constitutional isomerism that refers to a chemical equilibrium between the keto form (carbonyl structure containing α-hydrogen) and the enol form (a double bond adjacent to an alcohol, -C=C-OH) of a compound
What is keto form?
(carbonyl structure containing α-hydrogen
What is an enol form?
a double bond adjacent to an alcohol, -C=C-OH)
What is configurational isomerism?
Spacial structure of atoms that are directly connected to each other (that are connected to one specific C)
→ 2 types: optical and geometric isomers
What is stereoisomerism? (optical)
asymmetric compound with no symmetry axis and no plan of symmetry
→ these compounds mirror each other
=> A.K.A chiral compounds
How does Fischer-projection work?
- Most oxidized C is on the top
- C-chain is vertical
- C-C bonds trend backwards, where as OH and H trends forwards
What are the 2 types of chiral compounds?
L- and D-
How does Cahn-Ingold-Prelog (CIP-) projection work?
- 4 groups are sorted by atomic number
- Last group - put to the back
- counterclockwise
- remaining 3 groups - in one plan - in front of chiral C atom
- clockwise
Cahn-Ingold-Prelog (CIP-) projection
How does ranking of groups work?
- According atomic number
- if 2 or more of them are identical → the rule applied to next atom
What are prochiral compounds? (stereoisomerism)
Compounds containing C-atom with 3 different groups
=> They are also achiral
What are enantiomers?
- A type of configurational isomer
- Compounds that are mirror image to each other
- Identical achiral property
- Rotate polarized light in opposite direction
- different when they react with other chiral compounds
What is racemic mixture (racemate)?
Mixture that contains equal amounts (5-%-50%) of 2 enantiomers
What are diastereomers?
Stereoisomers that are not perfect mirror of each other
What are epimeres?
Diastereomeres that differ in the configuration of exactly 1 chiral center
What type of stereoisomers is this?
Enantiomers
Is this a chiral or an achiral compound?
Achiral compound
→ b/c they are symmetrical
What are geometric isomers?
Stereoisomerism that results from restricted rotation about chemical bonds
- Is sometimes possible when a ring is present
- Is sometimes possible when a double bond is present
How are the different groups sorted when presenting in cis-trans isomers?
By atomic number
What are trans-isomers?
Isomers with 2 identical groups closed to each other
What is conformational isomerism?
Spacial structure of atoms that are directly not connected to each other
2 types of H atoms in conformational isomer
- Axial H-atoms: are above / below the level of the ring
- Equatorial H-atoms: are in the level of the ring
2 types of conformation of cyclohexane
Boat and chair conformation
Which conformation type of cyclohexane is more favorable? Why?
- Chair conformation → more stable → is more favorable
- Boat conformation is less favorable → H-atoms are close to each other → unstable
2 types of conformation based on Newman-projection
Staggered and eclipsed conformation
Describe staggered conformation
- Geometric isomer
- Staggered conformation is the arrangement of atoms or groups of atoms in a molecule resulting in a 60o dihedral angle
Describe eclipsed conformation
- Geometric isomer
-
Two atoms and/or groups whose dihedral angle is 0o. In other words, their bonds to the axis of rotation are aligned
- → less STABLE
What are the 4 characteristics of conformational isomer (Newman’s projection)?
- Anti
- Syn
- Periplanar
- Clinal
What does anti mean in conformational isomer (Newman’s projection)?
The substituents have the maximal distance from each other → energetically favorable
What does syn mean in conformational isomer (Newman’s projection)?
The substituents have the minimum distance from each other → require more energy
What does periplanar mean?
the covalent bonds are parallel to each
What does clinal mean?
covalent bonds are not in the same plane
What type of conformation is this?
Anti, periplanar
What type of conformation is this?
Syn, clinal
What type of conformation is this?
Anti, clinal
What type of conformation is this?
Syn, periplanar
What are the 2 type of conformation of nucleoside?
Anti
Syn
Describe the anti form of nucleoside?
Base is rotated away from the ribose
Describe the syn form of nucleoside?
Base is in proximity, right above the ribose
!!! Pyrimidines cannot be in this conformation because oxygen will be too close to each other
