H3 Chemistry 9813 · Study focus: H3 Chemistry: Apply the Hammond postulate to reaction mechanisms
H3 Chemistry: Apply the Hammond postulate to reaction mechanisms
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Apply the Hammond postulate to reaction mechanisms
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Locate the elementary step
Apply the Hammond postulate to one elementary step at a time, not to an overall equation with hidden intermediates.
On a reaction-coordinate profile, stable species are minima and transition states are maxima; compare a maximum with the adjacent minima.
Nearest in energy, not nearest on paper
A transition state resembles the stable species nearest to it in energy.
A strongly endothermic step has a late, product-like transition state; a strongly exothermic step has an early, reactant-like transition state.
When carbocation formation is endothermic, the transition state already has substantial C–leaving-group cleavage and developing positive charge.
A substituent that stabilises that carbocation can therefore also stabilise the product-like transition state and lower the activation barrier.
Text alternative: Text alternative states the relative energies of both minima, locates each maximum, and describes the partial bond and charge at each transition state.
Connect energetics to structure
When carbocation formation is endothermic, the transition state already has substantial C–leaving-group cleavage and developing positive charge.
A substituent that stabilises that carbocation can therefore also stabilise the product-like transition state and lower the activation barrier.
Ionisation of tert-butyl bromide
Consider the endothermic elementary cleavage (CH3)3C–Br → (CH3)3C+ + Br−.
The ionisation transition state is late and carbocation-like because the carbocation products of this step lie higher than the reactant.
- Sketch this elementary-step profile and annotate the extent of C–Br cleavage and positive-charge development at the maximum.
Open the feedback checkpoint after attempting
- Credit an uphill product minimum, a single intervening maximum, and a late transition-state sketch with a substantially broken C–Br bond and developing tertiary carbocation character.
Read an exothermic capture step
The reaction of a carbocation with CN− is strongly downhill when the new C–C bond forms.
Hammond reasoning predicts an early, reactant-like transition state with relatively little new bond formation.
- Place the transition state for (CH3)3C+ + CN− → (CH3)3C–CN on an exothermic profile and describe its structure.
Open the feedback checkpoint after attempting
- The maximum should lie closer in energy to the reactants; the transition state should retain carbocation/nucleophile character and show only limited C–CN bond formation.
Compare substituent sensitivity
A product-like ionisation transition state responds strongly to substituents that stabilise developing carbocation charge.
An early transition state in a highly exothermic capture step has less developed product bonding and normally shows less product-like substituent sensitivity.
- Compare how replacing methyl by hydrogen near the reacting carbon would affect the barriers of endothermic ionisation and exothermic nucleophile capture.
Open the feedback checkpoint after attempting
- The larger effect is expected for ionisation because its late transition state has more carbocation character; do not claim that Hammond alone supplies a numerical barrier.
Repair the time-order shortcut
The nearest stable species means nearest in energy, not whichever species is drawn immediately before or after the transition state.
Hammond predicts qualitative resemblance; it does not say that a transition state is an isolable intermediate or an exact copy of a reactant or product.
- Correct the statement: ‘Every transition state is product-like because products occur after it.’
Open the feedback checkpoint after attempting
- State that exothermic steps tend to have reactant-like transition states and endothermic steps product-like transition states because energy proximity governs resemblance.
Assess, delay and continue
Complete the twelve Hammond diagnostics before opening the unseen profile; retain the distinction between a transition state and an intermediate.
After the different delayed profile, compare how nucleophile identity and concentration affect substitution rate.
- Attempt the stored Hammond assessment, mark both the energy comparison and the structural resemblance, then schedule the different re-test.
Open the feedback checkpoint after attempting
- Continue when the profile direction and transition-state structure agree, then test nucleophile effects with controlled rate data.