Lecture notes · Organic Chemistry II · Lecture 1

Mechanism: How Energies and Kinetic Order Influence Reaction Rates

Yale CHEM 125b Freshman Organic Chemistry II, lecture 1, explained in plain English

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Part 1 · 0:00–10:00

This lecture introduces how chemists use energy to predict whether a reaction can happen and how fast it will happen, starting with the simplest reactions, which break bonds.

Key ideas

The course moves from simple reactions to harder ones, and it explains how chemists figure out reaction mechanisms, which are the step-by-step paths reactions take. Structure and energy decide what chemistry happens. 0:44
Energy also controls equilibrium, meaning where a reaction settles. The lesson gives a rule of thumb that ties the energy difference to the equilibrium constant at room temperature. 3:51
Whether a reaction will happen is a separate question from whether it is favorable. Diamond is less stable than graphite, but it does not turn into graphite in any useful time, because speed (kinetics) is its own question. 4:27
The energy of the transition state, the hard-to-reach point between starting materials and products, sets how fast a reaction goes. Chemists use the starting materials and products to learn about it. 6:08
Breaking a bond takes energy, and the size of that energy depends on the atoms involved. Bigger halogen atoms make weaker bonds to hydrogen because they overlap less with it. 9:22

Words to know

free radical: A molecule piece with an unpaired electron. It has no overall charge, so it is not strongly affected by the liquid it sits in.
transition state: The highest-energy point a reaction must pass through on the way from starting materials to products. It is very short-lived and hard to observe directly.
bond dissociation energy: The energy needed to break a bond in the gas phase so that it splits into two pieces, such as an R-H bond becoming a radical and a hydrogen atom.
kinetics: The study of how fast a reaction happens, as opposed to how far it goes before it settles.

Example

Think of a ball sitting in a valley next to a hill. The valley is lower, so the ball would like to roll there, but it still needs a push to get over the hill. A tall hill means the ball rolls slowly or not at all, even if the valley is much lower. The hill is like the transition state.

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Part 2 · 10:00–20:00

This part explains why some carbon-hydrogen bonds are unusually easy or hard to break, and uses bond strengths to check whether halogenation of methane could make money.

Key ideas

Bond enthalpy is the energy cost of breaking a bond. Most C-H bonds on methyl, ethyl, isopropyl, and t-butyl groups cost about 100 kilocalories per mole, so they are all close to the same. 10:46
When a bond is unusual, the cause can be the starting bond or the leftover radical. Vinyl and phenyl C-H bonds are unusually strong because the carbon is sp2 and the bond overlaps well. 13:31
In allyl and benzyl radicals, the single electron mixes with the pi and pi* orbitals next to it. This extra stability makes the bond easy to break, at about 89 to 90 kilocalories per mole. 16:15
To test a reaction, add up the cost of breaking bonds and the return from making new bonds. A net gain means the reaction is favorable. 17:21
Fluorine is favorable by 109 kilocalories per mole, but chlorine and bromine are only slightly favorable, and iodine is 12 kilocalories per mole uphill, so it would not be a profitable process. 19:26

Words to know

bond enthalpy: The amount of energy needed to break a chemical bond. A bigger number means a stronger bond that is harder to break.
SOMO: Short for singly occupied molecular orbital. It is an orbital that holds just one electron, so it can interact with other orbitals.
orthogonal: At right angles to each other. Orthogonal orbitals do not overlap, so they cannot mix or stabilize each other.
allylic: Describes a carbon next to a double bond. A radical on such a carbon gets extra stability from the nearby pi system.

Example

Think of a bond as a rope tying two things together. Pulling the rope apart takes energy, and a stronger rope takes more energy. If the rope is tied to something that can spread out the pull, like a loose knot that stretches, it becomes easier to undo.

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Part 3 · 20:00–30:00

This part explains why a reaction that breaks two bonds in one step is too slow, why a chain of smaller make-as-you-break steps works better, and how the rate of a reaction depends on how much material is present.

Key ideas

A one-step reaction that breaks two bonds first and then makes two new ones has a very high activation energy, the energy you must climb before the reaction can happen. At room temperature the rate it gives is almost zero. 20:40
Raising the temperature speeds the reaction up a lot, but very high heat brings other problems, so this route is still not a good answer. 21:45
A better path is to make a new bond at the same moment an old bond breaks. Each of the two steps then has a much smaller energy barrier that can be crossed. 22:58
Free radicals (atoms or molecules with an unpaired electron) let this work as a repeating cycle, so no new bond has to break on its own except to start it. 24:41
The rate of a reaction depends on how concentrated the materials are, and doubling the amount of material or the size of the container doubles the rate. 28:14

Words to know

activation energy: The amount of energy you have to climb up before a reaction can go forward and come down to the products.
transition state: The highest-energy point on the way from starting materials to products, where bonds are partly broken and partly made.
free radical: An atom or group of atoms with an unpaired electron, which makes it very eager to react.
mechanism: The step-by-step path a reaction takes, showing which bonds break and which form along the way.

Example

Think of a faucet filling a bucket. Two faucets fill it about twice as fast as one, and a faucet twice as wide does too. In the same way, using twice as much material in a reaction, or a container with twice the volume, doubles how fast the reaction runs.

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Part 4 · 30:00–40:00

This part explains how the number of molecules involved (the concentration) changes how fast a reaction goes, and how half-lives and equilibrium work for simple reactions.

Key ideas

A rate law shows how the reaction rate depends on the amount of each starting material. The power that a concentration is raised to is called the kinetic order, and it can only be found by experiment. 30:19
In a zero order reaction, adding more starting material does not speed things up, because a catalyst is already working as fast as it can. The rate then depends on how much catalyst there is. 31:52
In a first order reaction, the rate is proportional to the amount of starting material. The amount left falls off exponentially, and the half-life is 0.69 divided by the rate constant k. 34:14
In a reversible first order reaction, the mixture still changes exponentially but levels off at an equilibrium mix. The time to approach equilibrium depends on the sum of the forward and reverse rate constants. 37:23
A second order reaction can look first order if one reagent is in large excess or is a solvent. This is called pseudo first order, because that reagent's concentration barely changes. 39:20

Words to know

kinetic order: The power (exponent) that a concentration is raised to in the rate law. It tells you how much the rate changes when that concentration changes.
catalyst: A substance that helps a reaction happen and is not used up by the end of it.
half-life: The time it takes for half of a starting material to disappear.
pseudo first order: A reaction that acts like a first order reaction because one reagent is in such large amounts that its concentration hardly changes, even though it is really involved.

Example

Think of sheep going through a gate. If the gate is already packed and each sheep can only pass as fast as the gate opens, adding more sheep will not get them through any faster. This is like a zero order reaction, where the catalyst sets the speed.

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Part 5 · 40:00–48:45

This part explains how to tell a second-order reaction apart from a first-order one, and how one slow step (the rate-limiting step) controls how fast a multi-step reaction runs.

Key ideas

A second-order reaction does not decay exponentially, so it has no constant half-life. It slows down more and more as it goes, because its rate depends on the square of the concentration. 40:13
In a two-step reaction, the reactive intermediate (a short-lived molecule between starting material and product) hardly builds up if the first step is slow and the second step is fast. The slow first step then sets the overall rate. 41:18
If the first step is fast and reaches equilibrium, the second step is the slow one. You can then work out the rate from the starting material alone, without worrying about the intermediate. 42:57
Once the intermediate reaches a steady state, most of it falls back to starting material, and only about one in ten molecules goes on to product. The overall rate is then the rate of forming the intermediate times about 1/10. 47:21

Words to know

second-order reaction: A reaction whose speed depends on the square of how much of a substance is present, usually because two molecules must meet.
reactive intermediate: A short-lived molecule that forms partway through a reaction and quickly turns into the next product, so only a little of it exists at any time.
rate-limiting step: The slowest step in a multi-step reaction. It controls how fast the whole reaction goes.
steady state: A point where the amount of the intermediate stays about the same, because it forms and disappears at the same pace.

Example

Think of a one-lane bridge that cars must cross after a slow toll booth. The toll booth controls how many cars get through per minute, and making the bridge faster does not help. Likewise, the slow step in a reaction sets the overall speed.

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Notes written by AI (Claude) from the lecture's official transcript, so check anything important against the lecture. Open Yale Courses · CC BY-NC-SA 3.0 · not affiliated with or endorsed by Yale. These notes are shared under the same license.

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