Introduction to organic reactivity Flashcards
When to atomic orbital combine to form molecular orbitals?
- AOs of similar size
- AOs of similar energies
- AOs od appropriate symmetry
What is hybridisation?
It is the concept of mixing two atomic orbitals to give rise to a new type of hybridised orbitals resulting in more stable compounds.
you don’t hybridise
- halogens hydrogens
What is SP3 hybridisation?
p3 hybridization can explain the tetrahedral structure of molecules. In it, the 2s orbitals and all three of the 2p orbitals hybridize to form four sp3 orbitals. This minimises electron repulsion.
What is SP2 hybridisation?
sp2 hybridization can explain the trigonal planar structure of molecules. In it, the 2s orbitals and two of the 2p orbitals hybridize to form three sp orbitals. With one of the 2p orbital existing at a higher energy.
What is SP hybridisation?
sp Hybridization can explain the linear structure in molecules. In it, the 2s orbital and one of the 2p orbitals hybridize to form two sp orbitals with 2 of the p orbital occupying a slightly higher energy
What does a carbonyls atomic orbitals look like?
The electronegativity difference means that the pi and sigma bonds are distorted towards the oxygen (has more electron density), and the two anti-bonding orbitals distorted towards the carbon.
How to identify hybridisation state?
- count the number of atoms connect to the atom (not bonds)
- count number of lone pairs
- add them together
4 = sp3
3= sp2
2= sp
exception are if there a lone pairs next to a pi system
What is a constitutional isomer?
The same molecular formula but different structural formula
What is a stereoisomer (conformational)?
This have the same structural formula but different arrangement of atoms in space there are two types
- enantiomers
- diastereoisomers
What is an enantiomer?
Non-superimposable (cannot be rotated to form the original molecule) mirror images of each other. (e.g. complex ions in optical isomerism)
What is a diastereoisomers?
diastereomers are not mirror images of each other and non-superimposable.
- Diastereomers can have different physical properties and reactivity
- They have different melting points and boiling points and different densities.
- They have two or more stereocenters.
- they are chiral
Whats the definition of chirality?
Structures that cannot be superimposed on their mirror image.
If an molecule it chiral is will have
- no plane of symmetry.
- 1 stereogenic centre
what is achiral?
They can be rotated and superimposed into each other.
If a molecule is achiral is will have a plane of symmetry.
What is a stereogenic centre?
A carbon with 4 different atoms/groups attached to it.
What is a racemic mixture?
A mixture of two entantiomers in equal proportions, a reaction with achiral starting material and reagents will always produces a racemic mixture.
What id ematiomerically pure?
A single put enantiomer son a chiral compound, only formed in nature.
How do you adding absolute configuration (R and S nomenclature)?
- assign priority groups using atomic numbers of the atoms directly attached to stereogenic centres
- rotate the molecule so the lowest priority group is facing backwards (normally H, lined wedged)
- draw an arrow from the highest priory group through the second, third
if the arrow is anticlockwise = S
clockwise = R
What is optical rotation?
Chiral compounds are optically active, they rotate plane polarised light. Enantiomers rotate the plane of polarised light by equal and opposite angles.
How do you calculate the angle specific rotation of a enantiomer?
[a]20/D = [a]/cl
[a]= specific rotation
20 = temp run at
D = wavelength of light used
c= conc (g cm^-3)
l = length (dm)
How can you determine the number of isomers form the number of stereogenic centres?
In general there will be 2^n possible isomers.
n = number of stereogenic centres
What is resolution?
This is the process of separating enantiomers, as all physiochemical properties are identical its quite difficult. It is done by temporary conversion to diastereoisomers by derivatisation with an enantiomerically pure reagent (exits are only 1 form) or chiral chromatography.
(R + S) + R –> (RR + RS) –> (RR) + (RS) –> (R) + (S)
What is absolute stereochemistry?
R and S stereochemistry centres when there ins,roe than 2 centres its called relative stereochemistry.
What is relative stereochemistry?
The stereochemical relationship of one atom or group to another atom of group within a molecule.
- commonly cis and tans/ E/Z for rings and syn and anti for open chains
How to you apply E/Z isomerism to a molecule?
- Rank artoms directly attached to the carbons in the double bond by atomic number
- find highest priority
Z > will have the highest priority groups attached to each carbon on the same side
E > highest priority groups attached to opposite sides
