Introduction to organic reactivity

Question 1

When to atomic orbital combine to form molecular orbitals?

Answer 1

• AOs of similar size
• AOs of similar energies
• AOs od appropriate symmetry

Question 2

What is hybridisation?

Answer 2

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

Question 3

What is SP3 hybridisation?

Answer 3

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.

Question 4

What is SP2 hybridisation?

Answer 4

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.

Question 5

What is SP hybridisation?

Answer 5

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

Question 6

What does a carbonyls atomic orbitals look like?

Answer 6

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.

Question 7

How to identify hybridisation state?

Answer 7

1. count the number of atoms connect to the atom (not bonds)
2. count number of lone pairs
3. add them together

4 = sp3
3= sp2
2= sp

exception are if there a lone pairs next to a pi system

Question 8

What is a constitutional isomer?

Answer 8

The same molecular formula but different structural formula

Question 9

What is a stereoisomer (conformational)?

Answer 9

This have the same structural formula but different arrangement of atoms in space there are two types
- enantiomers
- diastereoisomers

Question 10

What is an enantiomer?

Answer 10

Non-superimposable (cannot be rotated to form the original molecule) mirror images of each other. (e.g. complex ions in optical isomerism)

Question 11

What is a diastereoisomers?

Answer 11

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

Question 12

Whats the definition of chirality?

Answer 12

Structures that cannot be superimposed on their mirror image.
If an molecule it chiral is will have
- no plane of symmetry.
- 1 stereogenic centre

Question 13

what is achiral?

Answer 13

They can be rotated and superimposed into each other.
If a molecule is achiral is will have a plane of symmetry.

Question 14

What is a stereogenic centre?

Answer 14

A carbon with 4 different atoms/groups attached to it.

Question 15

What is a racemic mixture?

Answer 15

A mixture of two entantiomers in equal proportions, a reaction with achiral starting material and reagents will always produces a racemic mixture.

Question 16

What id ematiomerically pure?

Answer 16

A single put enantiomer son a chiral compound, only formed in nature.

Question 17

How do you adding absolute configuration (R and S nomenclature)?

Answer 17

1. assign priority groups using atomic numbers of the atoms directly attached to stereogenic centres
2. rotate the molecule so the lowest priority group is facing backwards (normally H, lined wedged)
3. draw an arrow from the highest priory group through the second, third
   if the arrow is anticlockwise = S
   clockwise = R

Question 18

What is optical rotation?

Answer 18

Chiral compounds are optically active, they rotate plane polarised light. Enantiomers rotate the plane of polarised light by equal and opposite angles.

Question 19

How do you calculate the angle specific rotation of a enantiomer?

Answer 19

[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)

Question 20

How can you determine the number of isomers form the number of stereogenic centres?

Answer 20

In general there will be 2^n possible isomers.
n = number of stereogenic centres

Question 21

What is resolution?

Answer 21

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)

Question 22

What is absolute stereochemistry?

Answer 22

R and S stereochemistry centres when there ins,roe than 2 centres its called relative stereochemistry.

Question 23

What is relative stereochemistry?

Answer 23

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

Question 24

How to you apply E/Z isomerism to a molecule?

Answer 24

1. Rank artoms directly attached to the carbons in the double bond by atomic number
2. 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

Question 25

What is the difference between conformational isomers and configurational isomers?

Answer 25

Conformation > each pair can be interconverted by rotation of a single bonds
Configurational > each pair can not be interconverted by rotation of single bonds

Question 26

What affects rotation of a molecule?

Answer 26

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.

Question 27

What are the different forms of conformational isomers?

Answer 27

• 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)

Question 28

What is the energy difference between eclipsed and staggered conformational isomers?

Answer 28

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.

Question 29

What are the types of the conformations isomers in rings?

Answer 29

• chair conformation (one end up ward sand one downwards) it is the favoured conformation
• boat conformation (molecule twisted up on both ends)

Question 30

What does axial and equatorial mean?

Answer 30

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.

Question 31

What is the definition of resonance?

Answer 31

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)

Question 32

What the main characteristics on a pi-bonding MO diagram?

Answer 32

• 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

Question 33

What does the number of nodes tells us about the MOs?

Answer 33

If the MOs are net bonding or anti bonding

Question 34

What does the resonance forms tell us about a molecule?

Answer 34

• stabilises charges
• more resonance forms the more stable
• predicts partial double bond character

Question 35

What is a Bronsted acid?

Answer 35

A Bronsted acid is a substance that yields (donates) H+ ions. The amount of dissociation controls the strength of the acid.

Question 36

What are the two factors that affect the strength of an acid?

Answer 36

• 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)

Question 37

What affects the stability of the conjugate base?

Answer 37

• resonance effects (delocalisation stabilises the formal charges)
• electronegativity and inductive effects

Question 38

How does inductive effect affect the stability of the conjugate base?

Answer 38

• 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

Question 39

How does resonance forms control strength of acids?

Answer 39

They stabilise both anions and cations are so would expect the resonance form to stabilise the conjugate base and strengthen the acid

Question 40

What is meant by pKaH?

Answer 40

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.

Question 41

Definition of pKa?

Answer 41

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.

Question 42

Definition of Ka?

Answer 42

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