Topic 7: reaction mechanisms Flashcards
(23 cards)
Define reaction mechanism
- Overall description = all reactants + products
- Which bonds broken + formed
- In which order it happens
- Relative rates of steps
Give the 2 ways bonds break
1) Homolytic cleavage = symmetrical breaking
- Forms 2 radicals
2) Heteroytic cleavage = asymmetrical breaking
- Forms 2 ions
Explain the arrow conventions
- Half-headed = 1 e-
- Full-headed = 2 e-
- Begins = e- rich site = negative
- Move towards = e- poor site = positive
Give the 2 types of bonding
1) Homogenic bond = 2 free radicals join
2) Heterogenic bond = 2 ions join
Key points of free radical reactions
- Homolytic bond breaking
- Homogenic bond formations
- Less common
Key points of polar reactions
- Heterolytic bond breaking
- Heterogenic bond formation
- More common
Describe free radical reactions
- Free radical = odd number e- = missing octet = very reactive
TYPES:
1) FR substitution
2) FR addition
Describe free radical addition
- Free radical = X* = seeking e- for stability
- Pi bond = double bond breaks
- X* = connets to C + other C→C*
- New free radical formed = reaction continues = until runs out
- E.g. ethylene polymerization
Describe free radical substitution
- Cl-Cl → 2 Cl* free radicals
- Bond between H-CH3 breaks
- Cl* = connects to H = Cl-H + CH3 becomes FR → CH3*
- New free radical formed = reaction continues
- Bond between another Cl-Cl breaks
- CH3* = connect to Cl = CH3-Cl + other Cl→Cl*
- E.g. chlorination of methane
Describe chlorination of methane
- Important industrial reaction = produce organic solvents
STEPS:
1) Initiation
2) Propagation
3) Termination
Describe initiation
Cl-Cl → Cl* + Cl*
- Stable molecules = broken to 2 free radicals
- Via = UV/heat
- Slowest step in whole process
Describe propagation
CH4 + Cl→CH3 + H-Cl
CH3* + Cl-Cl→CH3Cl + Cl*
- Free radical = attacks stable molecule
- Produce another free radical + stable molecule
- Stable molecule = loses X = new FR
- FR + X = new stable molecule
- Faster = FR are reactive
- Is a chain reaction
- Most product formed during this step
- Step stops = limiting reactant runs out
Describe termination
1) Cl* + Cl* → Cl-Cl
2) CH3* + Cl* → CH3Cl
3) CH3* + CH3* → CH3CH3
- 2 free radicals react together
- Rare step = FR too reactive to build up connection = very little product formed via step
Describe electrophilic addition HX mechanism
2HC=CH2 + H-X → 3HC-C⁺H2 + X⁻→ 3HC-CH2Br
- C=C breaks + H-X breaks
- 1 C gets H = C-H
- 1 C = C+
- X = X⁻
- C⁺ + X⁻ = C-Br
- Intermediate = carbocation
Define carbocation
- sp2 hybridized
- Trigonal planar geometry
- Empty 2p orbital
Explain the reaction energy diagram for electrophilic addition
- 1st AE = higher than 2nd
- 1st step = slow = breaking pi bond = need energy
- 2nd step = fast = -ve particle attacking +ve particle = coulomb’s law large electrostatic attraction
- Process = exergonic = products below reactants = spontaneous
Describe Markovnikov’s rule for electrophilic addition
- HX = H atom will be captured by C with most H attached
Describe the SN2 reaction mechanism
- Substitution Nucleophilic Bimolecular
- Direct collision between 2 molecules
- Rate Law → Rate = k[RX][Nu:-]
- 2nd order reaction
- RX = alkyl halide
- Nu:- = nucleophile = base
- L = leaving group = gets kicked off main group
- Transition state = trigonal bipyramidal geometry
Explain steric hinderance in SN2 reactions
- Nucleophile = able to attack central C = need enough space
- If C attached to large groups = little space for attacks
- SN2 = doesn’t work well for 2’/3’ alkyl halides
- Obstruction = slow reaction
Explain the energy diagram for SN2
- 1-step reaction = 1 AE = 1 transition state
- Nucleophile attacks halide = transition state = partial chemical bonds
- Nu-C bond forms + C-X bond breaks = final product
- Exergonic reaction
Describe SN1 reaction
- Substitution Nucleophilic Unimolecular
- Mechanism for tertiary alkyl halide = cannot undergo substitution via SN2
- Rate Law → Rate = k[RX]
- 1st order reaction
- Doesn’t depend on nucleophile concentration
- Nothing to attack = bond between C-X breaks itself = using polar solvent = slow step
- Solvent = solvate X- when it detaches
- C = C+
- Then nucleophile attacks C+ = fast step
Describe carbocation stability
- More stable = faster step 1
- Methyl → tertiary = increased
- Methyl groups = donate e- density to C+ = reduces total +ve charge
- Less +ve = lower energy + AE = carbocation more stable = faster
Explain the energy diagram for SN1
- Similar to electrophilic
- 2 steps
- 1st = slow
- 2nd = faster
- Both steps = form intermediate carbocation
