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
What is the difference between conformational isomers and configurational isomers?
Conformation > each pair can be interconverted by rotation of a single bonds
Configurational > each pair can not be interconverted by rotation of single bonds
What affects rotation of a molecule?
The presences of multiple bond restricts rotation. A molecule with no double bonds with be free to rotate at low energies, and molecule with a double bond will only be able to rotes at a impractically high energy level. Resonance forms also restrict rotation.
What are the different forms of conformational isomers?
- staggered (all ‘overlapping’ attached atoms are in-between each other all visible)
- eclipse (all attached atoms are overlapping each other can only see the ones attached to the first carbon)
What is the energy difference between eclipsed and staggered conformational isomers?
Eclipsed formation is unfavourable due to increased electron repulsion between the attached atoms increasing it in energy. Staggered in favourable as minimises electron repulsion, it also empty any C-H antibonding electrons.
What are the types of the conformations isomers in rings?
- chair conformation (one end up ward sand one downwards) it is the favoured conformation
- boat conformation (molecule twisted up on both ends)
What does axial and equatorial mean?
Axial > When the hydrogens are vertically up or down
Equatorial > when the hydrogens stick out sideways alternates slightly up or slightly down
The equatorial position is favoured as there is less interaction between the groups minimising repulsion. This main as many groups and the largest groups as possible go into equatorial position.
What is the definition of resonance?
This is the blending of two or more lewis structures with the delocalisation of pi electrons through a conjugated double bond to get a compound that is lower in energy. This means that all the bonds have some double bond character.P orbital have to overlap to make it possible. (not possible with cumulative double bonds)
What the main characteristics on a pi-bonding MO diagram?
- only ψ1 (lowest energy) and ψ2 are occupied
- ψ1 shows pi bonding between C2 and C3
- C2-C3 bonding weakened by node in ψ2 so must have partial double bond character
- pi bonding with no nodes are full double bonds
What does the number of nodes tells us about the MOs?
If the MOs are net bonding or anti bonding
What does the resonance forms tell us about a molecule?
- stabilises charges
- more resonance forms the more stable
- predicts partial double bond character
What is a Bronsted acid?
A Bronsted acid is a substance that yields (donates) H+ ions. The amount of dissociation controls the strength of the acid.
What are the two factors that affect the strength of an acid?
- strength of the H-A bonds (the weaker the more dissociation and stronger acid and lower pKa)
- Stability of the conjugate base (more stable the stronger the acid and lower the pKa)
What affects the stability of the conjugate base?
- resonance effects (delocalisation stabilises the formal charges)
- electronegativity and inductive effects
How does inductive effect affect the stability of the conjugate base?
- inductively electrons donating groups are more electron releasing than H. This stabilises the cations and destabilise anions. This will destabilise the conjugate base and make the acid weaker
- inductively electron withdrawing atoms more electron attraction than H. These destabilise cations and stabilise anions. This thus stabilises the cognates base and strengthens the acid
How does resonance forms control strength of acids?
They stabilise both anions and cations are so would expect the resonance form to stabilise the conjugate base and strengthen the acid
What is meant by pKaH?
The pKAH of the compound A is the pKa of the protonated form.
If the value is high is means the acid is weak and A must be a strong base.
Definition of pKa?
pKa is a number that describes the acidity of a particular molecule. It measures the strength of an acid by how tightly a proton is held by a Bronsted acid. The lower the value of pKa, the stronger the acid and the greater its ability to donate its protons.
Definition of Ka?
The acid dissociation constant, denoted by Ka, is the equilibrium constant of an acid’s dissociation reaction. This equilibrium constant is a numerical representation of an acid’s strength in a solution
