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A transition state is an energy maximum with partial bonds that lasts one vibration and can never be isolated; an intermediate is a minimum between steps with complete bonds; n steps give n transition states and n − 1 intermediates.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will count intermediates and transition states from a mechanism's steps, sort descriptions between the two, plot a two-step energy profile, and place an observed species on it.
You can write each step of a mechanism in curly arrows. Between the arrows, the molecules pass through structures that are not reactants or products; this lesson names the two kinds.
An energy profile plots energy along the path of a reaction. A transition state (‡) is a maximum on it. An intermediate is a minimum between two steps. A step is one hill: reactants of that step over one transition state to its products.
Every step of a mechanism climbs one hill. At its top is the transition state: the arrangement where old bonds are partly broken and new ones partly formed. It has no energy dip around it, so it lasts about one molecular vibration (around $10^{-13}$ s) and can never be isolated.
Between two steps the profile dips into a valley: an intermediate, a real species with complete bonds. It may last microseconds or less, but it is a genuine structure and can sometimes be detected — a carbocation, for example, has been observed in superacid solvents.
Counting follows: a mechanism with n steps has n transition states and n − 1 intermediates.
Another way: table
The two compared.
| Transition state | Intermediate | |
|---|---|---|
| on the profile | a maximum | a minimum between steps |
| bonds | partial | complete |
| lifetime | one vibration | short, but real |
| isolable? | never | sometimes |
Walk along a profile from left to right. Each rise and fall is a step; each peak is a transition state; each dip that is not the start or the end is an intermediate. A two-step reaction therefore shows two peaks and one dip. The heights matter too: the next lesson reads activation energies and relative stabilities off them, and the one after asks which peak controls the rate.
A transition state can be isolated in a bottle. It is a maximum with no lifetime.
Intermediates and transition states are the same. One is a valley, one a hilltop.
An intermediate is the product. It reacts further.
Every reaction has an intermediate. A one-step reaction has none.
Three steps means three hills.
One transition state per step.
Between the hills are two valleys.
Two intermediates.
So it has three transition states and two intermediates.
n and n − 1.
How many hills?
One.
How many intermediates?
None: one step goes straight over one transition state.
A mechanism has $4$ step(s). How many intermediates does it pass through?
Answer:
Match each description to a transition state or an intermediate.
| transition state | intermediate | |
|---|---|---|
| a maximum on the energy profile | ||
| a minimum between two steps | ||
| a carbocation after the C–Br bond has fully broken | ||
| a structure with one bond half formed and another half broken |
A student proposes to cool a reaction mixture quickly and collect its transition state in a bottle. Can it be done?
A two-step reaction's energies, in tens of kJ/mol relative to the reactants: reactants 0, first transition state $8$, intermediate $4$, second transition state $5$, products $0$. Plot them at stages 1 to 5 along the reaction.
Plot your answer on the grid:
In very strongly acidic, non-nucleophilic solvents, chemists have recorded the spectrum of the tert-butyl cation, (CH3)3C+, the species formed partway through some substitution reactions. What does that tell you about where it sits on those reactions' energy profiles?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
The energy profile of a mechanism shows $1$ valley(s) between its reactants and its products. How many transition states does it have?
The mechanism has a transition states.
You can tell a hilltop from a valley on an energy profile. Tell someone why a transition state cannot be bottled. Next: reading activation energies off the profile.
8. Your turn: the one-step reaction CH3Br + OH−, step 3