Introduction to Reactions Flashcards
Unit 2, Lectures 9-17 - Mark Elliot
Curly arrows principles
Shows the movement of electron pairs
Starts from an electron pair (lone pair or bond) and moves toward the atom/bond they are being transferred to.
Starts at bond if breaking bonds.
Starts at a lone pair if making bonds.
Nucleophilic substitution
Electron-rich species are attracted to areas of low electron density and so replace a leaving group on an sp3 hybridised carbon. Leaving groups tend to be halides.
SN1
Unimolecular nucleopilic substitution: The leaving group breaks away from the carbon atom first, forming a carbocation intermediate, then the nucleophile attacks the carbocation.
SN2
Bimolecular nucleophilic substitution: The nucleophilic group attacks and the leaving leaves simultaneously. The reaction is complete in one step, with no intermediates. The nucleophile must attack from the opposite side of the molecule to the leaving group.
When is SN1 preferred?
When the carbocation intermediate would be tertiary, as they are more stable, and you can’t form pentavalent carbons.
Weak nucleophiles are more likely to undergo SN1 as the rate-determining step is the formation of the carbo cation.
Polar protic solvents: H2O, ethanol
Good leaving groups: I-, Br-, Cl-, H2O.
When is SN2 preferred?
Favoured by methyl or primary carbons due to less steric hindrance.
Strong nucleophiles
Polar aprotic solvents: Acetone, DMF
Good leaving group: 1-, Br-, Cl-, H2O.
Addition reactions
Forming saturated compounds from unsaturated ones, e.g., alkenes, carbonyls.
Homolytic bond cleavage
When a bond is broken equally so that each atom keeps an electron. Referred to in bond dissociation energies.
Heterolytic bond cleavage
When one atom in a bond cleavage gets both the electrons.
SN1 reaction rate
Rate = k[RX]
Where [RX] is the concentration of the substrate. The 1 in SN1 refers to the first-order nature of the reaction.
SN2 reaction rate
In example of CH3I and OH-:
Rate = k[CH3I][OH-]
The 2 refers to the second-order nature of the reaction, as changing the concentration of either species will affect the rate of reaction.
Elimination reactions
When an unsaturated compound is formed from saturated compounds. Products are usually alkenes or alkynes with side products of water or halide ions.
E1: Unimolecular elimination
The leaving group departs first, then a bas removes an H from a neighbouring C.
E2: Bimolecular elimination
A concerted mechanism where the base removes the H from the adjacent C as the leaving group leaves.
E1cB: Unimolecular conjugate base elimination
Typically occurs when there is an insufficient leaving group (OH-, Br-, I-) and the reaction requires a conjugate base, which deprotonates the adjacent carbon before the leaving group leaves, temporarily forming a carbanion.
How to break a bond
Break through homolytic fission
Heterolytic fission
Put enough electrons to fill both the bonding and anti-bonding orbitals so that they cancel each other out.
How using anti-bonding orbitals can break bonds.
If you excite electrons into antibonding orbitals by providing energy (UV light, heat), it can continuously destabilise a bond to the point of breaking it.
Frontier orbitals
Made up of the LUMO (the orbital where excited electrons would move to) and the HOMO (the highest energy orbital that still has electrons in it)
Predicting reactions
If there were two halide groups in a compound, in order to predict which group would undergo substitution first, you would have to look at the molecular orbital energy diagrams for each halide-carbon bond. The bond that has the lowest energy HOMO would leave first.
Bases acting as nucleophiles
When they attack carbons, they are seen as nucleophiles as they have higher electron density
Acids acting as electrophiles
Lewis acids are described as electron acceptors, so when attracted to areas of high electron density, they are seen as electrophiles.
SN2 reaction profile diagram
Negative quadratic shape, with an asymptote to the x axis at the end. The stationary point indicates the transition point of the reaction, and the distance between the y intercept and the stationary point indicates the activation energy.
Implications of Ea
The higher the activation energy, the fewer molecules that have sufficient energy at any given time, and the slower the reaction rate will be.
SN1 reaction profile
Shaped like a negative quartic graph, with two transition state peaks, the first showing the activation energy. The trough between the two peaks represents the intermediate formed.
Transition state
The unstable configuration where bonds are simultaneously forming and breaking. A short-lived, high-energy arrangement of atoms that can either form products or revert back to the reactants.
Exothermic vs endothermic reaction profiles
Exothermic reaction profiles have reactants at a higher energy than the products, and the transition state has the highest energy overall. In endothermic, however, reactants have a lower energy than the product, though the transition state still holds the highest energy.
