Topic 7: Isomers, aromatic compounds, alcohols & amines Flashcards
Describe hybridization of molecular orbitals and types of hybrid orbitals
In carbon atom, s and p orbitals are very close in energy level
-> they overlap and combine to become hybrid orbitals
- 1 s and 3 p orbitals: 4 sp3 hybrid orbitals
-> found in alkanes with only single bonds - 1 s and 2 p orbitals: 3 sp2 hybrid orbitals
-> found in alkenes, forming double bond (1 sigma bond and 1 pi bond from the leftover p orbital between C) - 1 s and 1 p orbital: 2 sp hybrid orbitals -> found in alkynes, forming triple bond ( 1 sigma bond and 2 pi bond from the 2 leftover p orbitals)
Describe properties of alkanes (intercellular forces, boiling point, reactivity)
- Dispersion forces in between molecules -> weak forces
-> the larger the molecule, the stronger the bonds, the higher boiling and melting point - Branching like methyl, ethyl lowers the boiling point
+ these branching groups make the shape more compact
-> less surface area and less interactions
-> lower boiling point - Chemically inactive, but can be used as fuels for large amount of energy
Describe 2 types of newman projections
- Staggered: H atoms are as far away from each other as possible -> lowest energy level
- Eclipsed: H atoms are close to each other -> higher energy level
Identify and classify types of isomers
- Isomers: molecules with same molecular formula; either constitutional or stereoisomers
- Constitutional isomers: differ in sequence or arrangement of atoms
- Stereoisomers: differ in position of atoms in space; either conformational or configurational
- Conformational isomers: the atoms differ in positions by rotation around a C-C bond
- Configurational isomers: requires breaking and forming new bonds to change the positions of atoms/groups; either diastereoisomers or enantiomers
- Diastereoisomers: not mirror images
- Enantiomers: non-superimposable/chiral mirror images
Describe the properties of constitutional isomers
The physical and chemical properties of constitutional isomers can be very different, esp. when different functional groups are present.
Explain the rotation and breaking bonds in conformational and configurational isomers
- Conformational isomers: rotation about the C-C single bond - alkanes
-> inseparable as they keep rotating easily at room temperature - Configurational isomers:
+ rotation about the C=C double bond requires the outer pi bond to be broken -> lots of energy input
-> not usually happen
-> have to break and make bonds to interconvert the isomers
Describe stereogenic centers and the number and types of isomers can arise from “n” center
- Stereogenic/chiral centers: C atoms with 4 different substituents
- “n” chiral centers can give rise to 2^n isomers, either diastereoisomers or enantiomers
Explain how to assign priority in diastereoisomers and names of 2 cases
For each C atom in the C=C double bond, identify the group the with higher atomic number -> higher priority
Go along the chain until the 1st point of difference when there are identical groups
- If the priority groups of the 2 C are on the same side with respect to the double bond -> Z- or cis-
- If the priority groups of the 2 C are on the opposite side with respect to the double bond -> E- or trans-
Describe the properties of chiral objects
- Have at least 1 stereogenic/chiral centers
- No plane of symmetry
- Non-superimposable on its mirror image`
Describe how optical activity can be used to identify enantiomers
Unpolarised light oscillates in various directions with respect to the direction of light.
Light after shone through a polarising filter become polarised and oscillate in 1 plane only.
- Polarised light can be shone through sample of enantiomers which differ in how much they can rotate the plane of light.
- The amount or degree of rotation vary depending on the enentiomers.
+ Clock-wise rotation: (+) or R
+ Anticlockwise rotation: (-) or S
