Topic I

What’s a Transition State?

A transistion state is a very short-lived configuration of atoms at a local energy maximum in a reaction energy diagram

a transition has partial bonds, an extremely short lifetime, and cannot be isolated

do not confuse this with an intermediate. An intermediate is a species at an energy minimum in a reaction coordinate

transition state = the point of maximum pain

think of a werewolf transtioning from man to wolf

at the point where the mystical creature is both man and wolf

The Transition State in the SN2 Reaction

When you take a primary alkyl halide and add a good nucleophile, you end up forming a new carbon-nucleophile bond and breaking the carbon-halide bond(aka a substitution reaction)

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This happens at the same time, but since a 5-bonded carbon is impossible, there has to be some point where you have partial bonds or the transition state

This point will be be the highest energy( the most unstable) due to the multiple partial bonds. Since this is pretty painful for both molecules, the transition state is extremely short.

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this point of maximum pain is called the “activation energy”

so given a diagram for the progress of a reaction, you can figure out the activation energy by subtracting the energy of the transition state from the energy of the products

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Introductions to Nucleophilic Substitution Reactions

the key to these reactions is just to understand what bonds are formed and what bonds are broken

here are some examples of the SN2 reaction:

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pattern:

we break a bond at the carbon, and corm a new bond at the same carbon

Observations:

observation 1: in some substitution reactions the rate is proportional to the concentration of two species but in other reactions the rate is proportional to the concentration of one

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observation 2: in certain cases the reaction rate depends on the type of alkyl halide. primary halides are slow and tertiary halides are fast

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observation 3: sometimes the tertiary alkyl halide is fast and the primary alkyl halide is slow( this is known to be true in elimination reactions)

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