Reaction Mechanisms Flashcards
(23 cards)
How do you deal with a complex molecule?
Identify:
- Functional groups (amide, acid, aromatic ring etc)
- Hybridisation
- Nucleophiles & electrophiles (lone pairs & partial charges)
- Comparison between atoms of same element (2 of the same groups but in different parts of the molecule)
What is meant by remote groups?
Groups are far from each other so will have predictable properties
What is meant by proximal groups?
Groups that are close together so will modify the chemistry of each other
E.g. 2 methyl groups near an acid group will hinder the acid and affect its reactivity
What is heterolytic cleavage?
Will form 2 ionic species
Normally polarised bonds & ionic reactions
What is homolytic cleavage?
Will form radical species
Requires energy (e.g. heat or light) and forms weak bonds that are susceptible (e.g. halogens or peroxides)
What do curly arrows show?
Movement of electrons
Double headed = direction of flow of a pair of electrons
Single headed = direction of flow of a single electron
Half head = movement of one electron (used for radicals)
How do arrows show the movement of electrons?
- Tail shows source & head points to destination
- Direction of arrow is from electron rich to electron poor (negative to positive)
- Must balance
- Obeys octet rule
Draw Lewis structure if needed
Octet rule
Each atom has 8 electrons in its valence shell
4 types of reaction mechanisms
- Substitution
- Addition
- Elimination
- Rearrangement
Substitution reactions
One group places another, can be
- Nucleophilic at sp3 carbon
- Nucleophilic at sp2 carbon
- Electrophilic on an aromatic system
Addition reactions
Adds a group to the molecule
Two molecules become one
- adding group to an electron rich double bond = electrophilic addition
- adding group to an electron poor double bond = Nucleophilic addition
Elimination reactions
Removes a group and forms a double bond
One molecule becomes two
- Adjacent carbon atoms lose H and a leaving group
- Proton can be lost from O or N
Rearrangement reactions
Molecule undergoes rearrangement of its constituent parts
Reactivity order
- RO-
- ROH
- Pi bonds
- Sigma bonds
What does a high charge density indicate?
It’s highly reactive
What does electron density depend on?
- Full charges are most reactive
- Dipoles are significant
- Delocalisation results in stability
- Inductive and resonance effects
Trend of electronegativity
Increases as you go along the group and decreases as you go down
Properties of inductive effects
- Occur through sigma bonds
- Polarised towards more electronegative atom
- Drops off with distance
Order of electronegative atoms
- F
- O
- Cl
- Br
Relative stability of negative inductive effects
The closer the electronegative atoms to the negatively charged atom, the more stable the molecule
As it takes away the charge = charge is distributed
Inductive effects of alkyl substitution
Tertiary cation is the most stable and primary is the least stable
More alkyl substituents means the electron density is donated to the positive charge
This is a positive inductive effect
Resonance effects through pi bonds
Shared electron density due to overlap of P orbitals
Known as:
- resonance contributions/forms/structures
- canonical forms
Key feature of resonance
When resonance is possible, the molecule is more stable as electron density is donated to stabilise the negative charge
This can either make the species more reactive or less reactive —> the more resonance structures possible = the more stable
