ch. 10 - electrophilic additions to...

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ORGANIC - JONES 5E

CH. 10 - ELECTROPHILIC ADDITIONS TO ALKENES

CONCEPT: GENERAL MECHANISM

□ Addition reactions are ones in which 1 ______ bond is broken and 2 new ______ bonds are formed.

● They are the inverse of ___________________ reactions

EXAMPLE: Provide the mechanism for the following addition reaction.

ORGANIC - JONES 5E


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 - JONES 5E


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.

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

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CONCEPT: MARKOVNIKOV’S RULE

□ When we try to perform addition on asymmetric double bonds, we now get two possible products.

● Markovnikov’s Rule predicts that the carbocation will ALWAYS form on the _________ _______________ carbon


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CONCEPT: HYDROHALOGENATION

General Reaction:


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CONCEPT: ACID-CATALYZED HYDRATION

● Same reagents as acid-catalyzed dehydration, except starting with a _____________ __________ instead of an alcohol.

● Same as the general mechanism hydrohalogenation, except with _____________ as the nucleophile

● Remember that EVERY acid-cat. mechanism begins with ________________ and ends with ________________

General Reaction:


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PRACTICE: Provide the mechanism and predict the product of the following reaction.

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CONCEPT: HYDROBORATION-OXIDATION

General Reaction:

Mechanism:

1. Electrophilic Addition

2. Oxidation

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PRACTICE: Provide the mechanism and predict the product of the following reactions.

a.

b.

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CONCEPT: INTRODUCTION TO CONJUGATION

Conjugation exists when three or more atoms with the ability to resonate are adjacent to each other (overlapping).

● Conjugation provides an electron “highway” to ______________________ from one side of the molecule to the other.

● Conjugated molecules display unique chemical reactivity. The higher the conjugation, the ________ the UV wavelength.

EXAMPLE: Which of the following molecules exists in a conjugated state?

● Allylic carbocations, carbanions, and radicals are unusually stable due to ________________________

ORGANIC - JONES 5E


CONCEPT: STABILITY OF CONJUGATED INTERMEDIATES

Regardless of the type of reactive intermediate, conjugation increases the stability.

Carbocations

Radicals

Due to the stability of the allylic position, radical and carbocation intermediated allylic reactions are common.

ORGANIC - JONES 5E


CONCEPT: CONJUGATED HYDROHALOGENATION

Recall the addition of a strong halohydric acid on a double bond. This reaction is called hydrohalogenation.

● Carbocation rearrangements are possible

Conjugated hydrohalogenation, also known as hydrohalogenation of dienes, or 1,2 vs. 1,4 addition to dienes, is the same

reaction, except with a possibility of multiple products due to the presence of a conjugated intermediate.

This reaction undergoes kinetic vs. thermodynamic control.

● Temperatures above 40° C favor the __________________, also called the thermodynamic product.

● Temperatures below 0° C favor the ___________________, also called the kinetic product.

EXAMPLE: Products of conjugated hydrohalogenation at different temperatures.

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CONCEPT: CONJUGATED HYDROHALOGENATION – KINETIC VS THERMODYNAMIC CONTROL

Conjugated hydrohalogenation is one of the reactions that undergoes kinetic vs. thermodynamic control.

● Hot reaction conditions favor the thermodynamic product __________________

● Cold reaction conditions favor the kinetic product ___________________

EXAMPLE: Simplified Conjugated Hydrohalogenation Energy Diagram

Summarizing Temperature Control:

The kinetic pathway has a more stable intermediate _________________ but less stable product ___________________

The thermo pathway has a less stable intermediate _________________ but more stable product ___________________

ORGANIC - JONES 5E


ch. 10 - electrophilic additions to...

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