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ORGANIC - KLEIN 3E

CH.6 - CHEMICAL REACTIVITY AND MECHANISMS

CONCEPT: FREE ENERGY DIAGRAMS

□ Atoms save energy by forming bonds. Free energy diagrams show overall changes in potential energy during reactions.

□ Free energy diagrams give us information on the spontaneity and rate of reactions

● Thermodynamics describes favorability. Determined by Gibbs Free Energy

● Kinetics describes rate. Determined by the activation energy of the rate determining step.

EXAMPLE: Describe the favorability and rate of the following free energy diagrams.

ORGANIC - KLEIN 3E

CH.6 - CHEMICAL REACTIVITY AND MECHANISMS

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CONCEPT: GIBBS FREE ENERGY

□ Predicts ____________________________ of reactions. Composed of three terms.

● Enthalpy is the sum of bond dissociation energies for the reaction.

_____ = ___________ bonds = Exothermic _____ = ___________ bonds = Endothermic

● Entropy is a measure of disorder in the system.

_____ = More ordered _____ = More disordered

● Temperature amplifies the effect of entropy on the overall favorability.

□ Some reactions require more than one step to go to completion. The ΔG⁰ is the sum of all the steps.

● Transition states ____________ be isolated. They involve bonds being broken and made at the same time.

● Intermediates ____________ be isolated. They rest at a higher energy state than normal.

ORGANIC - KLEIN 3E

CH.6 - CHEMICAL REACTIVITY AND MECHANISMS

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CONCEPT: ENTHALPY

□ Bond dissociation energies describe the strength of chemical bonds. They can be determined experimentally.

● Enthalpy is the sum of bond dissociation energies for the reaction.

_____ = ___________ bonds = Exothermic _____ = ___________ bonds = Endothermic

EXAMPLE: Predict the sign and magnitude of ΔH⁰ for the following reaction. Give your answer in units of kilojoules per

mole, and identify whether the reaction is expected to be endothermic or exothermic.

PRACTICE: Predict the sign and magnitude of ΔH⁰ for the following reaction. Give your answer in units of kilojoules per

mole, and identify whether the reaction is expected to be endothermic or exothermic.

ORGANIC - KLEIN 3E

CH.6 - CHEMICAL REACTIVITY AND MECHANISMS

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CONCEPT: ENTROPY

□ Even if a reaction is highly exothermic, the level of order it requires may make it statistically improbable.

● Entropy is the tendency of a system to take its most probable form.

_____ = More ordered _____ = More disordered

□ There are three phenomena that make reactions more probable. □ All are favored by ___ _____

1. Increasing the Number of Molecules

Reactions that create extra molecules are more probable since there are more ways to arrange them.

2. Phase Transition

Transformation of solid to liquid or liquid to gas is more probable since the molecules will have a greater vibrational freedom.

3. Increasing Molecular Freedom of Motion

Converting cyclic molecules to acyclic molecules are more probable since it increases freedom of rotation.

ORGANIC - KLEIN 3E

CH.6 - CHEMICAL REACTIVITY AND MECHANISMS

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CONCEPT: THE HAMMOND-POSTULATE

□ The Hammond-Postulate more accurately describes what transition states look like. Paraphrased version:

● Transition state that resembles reagents = ____________ ● Transition state that resembles products = ___________

EXAMPLE: Determine which species has higher energy in the following reactions. Identify the transition state as early or

late, and correctly draw it.

ORGANIC - KLEIN 3E

CH.6 - CHEMICAL REACTIVITY AND MECHANISMS

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CONCEPT: THE MECHANISM- REACTIVITY

The currency of organic chemistry is ____________________________ ● Stability and reactivity generally have an ___________________ relationship. ● The 4 common indicators of reactivity: 1. ____________________________ 2. ____________________________

3. ____________________________ 4. ____________________________ EXAMPLE: Identify which of the following molecules would be expected to be reactive. We can categorize almost all reactive molecules into two massive subtypes. These will display similar behaviors.

● Negatively charged species are known as ___________________

● Positively charged species are known as ____________________

The side of the dipole with the ______ bonding preference can be used to predict nucleophilicity or electrophilicity

EXAMPLE: Identify which of the above molecules are nucleophilic or electrophilic

ORGANIC - KLEIN 3E

CH.6 - CHEMICAL REACTIVITY AND MECHANISMS

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CONCEPT: THE MECHANISM- ELECTRON MOVEMENT Reactive molecules share electrons to become more stable. _________ are used to show which direction they are going.

● Arrows always move from regions of _________ electron density to _________ electron density ● By that logic, _________________________ must always attack ____________________________ ● Each attacking arrow represents ____ electrons being shared. Replace that arrow with a new ____ - __________ EXAMPLE: Determine the initial direction of electron movement by drawing the first arrow of each mechanism: a. b. c.

□ Bond breaking is sometimes also required in mechanisms, but only when it is required to preserve octets.

There are two ways to break chemical bonds: EXAMPLE: Identify which of the above reactions require bond breaking and/or more arrows to satisfy all octets.

ORGANIC - KLEIN 3E

CH.6 - CHEMICAL REACTIVITY AND MECHANISMS

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CONCEPT: CARBOCATION INTERMEDIATES — STABILITY

□ Carbocations are stabilized by a phenomenon called _________________________________

● Hyperconjugation is the delocalization of charge by the interaction of an empty p-orbital with an adjacent, eclipsed σ-bond

● Since this is only possible with -R groups, the more substituted the carbocation, the more ________________

EXAMPLE: Which of the following alkyl halides would generate the most stable carbocation?

ORGANIC - KLEIN 3E

CH.6 - CHEMICAL REACTIVITY AND MECHANISMS

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CONCEPT: CARBOCATION INTERMEDIATES — REARRANGEMENTS

□ Carbocations will rearrange to an adjacent, more stable position if possible. This is called a ___________

● 1,2-Hydride Shift occurs when there is a ____________ located on an adjacent, more stable carbon.

● 1,2-Alkyl Shift occurs when only small __________ groups are located on an adjacent, more stable carbon.

● Ring Expansion occurs when a carbocation is adjacent to a 3, 4 or 5- membered ring.

ORGANIC - KLEIN 3E

CH.6 - CHEMICAL REACTIVITY AND MECHANISMS

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PRACTICE: Which of the following carbocations would be likely to rearrange? Draw each rearranged structure below.

ORGANIC - KLEIN 3E

CH.6 - CHEMICAL REACTIVITY AND MECHANISMS

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