Chapter 9 - Isomerism Flashcards
What is constitutional (structural) isomerism?
It is a type of isomerism exhibited by substances with the same molecular formula but different structural formula.
What is stereoisomerism?
It is a type of isomerism exhibited by substances in which atoms have the same connectivity but different arrangements in space. (stereoisomers)
What are the 3 types of constitutional isomerism? Briefly describe.
1) Functional group isomerism (different functional groups)
2) Positional Isomerism (Different positions of the same functional group)
3) Chain Isomerism (Different skeletal chain)
What are 2 types of stereoisomerism? Briefly describe.
1) Cis-trans isomerism (Caused by restriction of rotation of a covalent bond due to double bonds/ring system)
2) Enantiomerism (Present in molecules that can exist as two non-superimposable mirror images)
What is positional isomerism? How do their properties differ?
Positional isomers are molecules with the same molecular formula and same functional group, but differ in the position of that functional group.
They have similar chemical properties but different physical properties.
What is chain isomerism? How do their properties differ?
Chain isomers are molecules with the same molecular formula and same functional group, but different carbon skeletons.
They have similar chemical properties but different physical properties.
What is functional group isomerism? How do their properties differ?
Functional group isomers are molecules that have the same molecular formula but different functional groups.
They have very different physical and chemical properties.
Why are the rotations of a covalent bond restricted in the presence of double bonds?
For rotation to occur where a double bond is present, the pi bond must break and reform. Thus, the energy barrier to double bond rotation is at least as great as the energy needed to break the pi bond, which is far beyond the energy available at normal temperatures.
Why are the rotations of a covalent bond restricted in the presence of ring structures?
Rotations around the single bonds in a ring is prevented by linkages in the ring. Due to the ring structure, the C-C bonds that make up the ring cannot rotate without breaking the other C-C bonds. Thus, the atoms/group of atoms bonded to each carbon are fixed in space with respect to each other. (Atoms on one side of the plane will stay that side)
How do we differentiate between cis isomers and trans isomers?
The isomer where the same groups are on the same side of the double bond is known as the cis isomer, while the isomer where the same groups are on the opposite side of the double bond is called the trans isomer.
What are 3 definitive criteria for a molecule to exhibit cis-trans isomerism?
1) The molecule has a covalent bond with restricted rotation (double bond/ring structure)
2) The groups bonded to each of the atoms on each end of the bond with restricted rotation cannot be the same.
3) In the case of cycloalkenes, only cycloalkenes with 8 or more carbon atoms can exist as cis-trans isomers as those with 7 carbon atoms and below exist only as the cis-isomer as the rings are not large enough to accommodate the trans double bond.
Can cis-trans isomerism still occur if all 4 groups are different?
Yes, as they satisfy the criteria for cis-trans isomerism. (Lecture notes page 43)
What is the definitive criterion for a molecule to exhibit enantiomerism?
The non-superimposability of mirror images (to be chiral). The mirror image is the enantiomer of the compound.
What is a chiral centre?
It is any atom with a tetrahedral geometry around it that is bonded to 4 DIFFERENT groups.
Why is there no optical activity observed when plane-polarised light passes through a sample with achiral molecules?
Because of the symmetry of the molecules and the large number and random distribution of the molecules in a sample, each of the rotations are cancelled out, leading to no net rotation of the plane-polarised light and hence no optical activity is observed.
