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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
CH. 10 - ELECTROPHILIC ADDITIONS TO ALKENES
Page 2
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
CH. 10 - ELECTROPHILIC ADDITIONS TO ALKENES
Page 3
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 - JONES 5E
CH. 10 - ELECTROPHILIC ADDITIONS TO ALKENES
Page 4
PRACTICE: Which of the following carbocations would be likely to rearrange? Draw each rearranged structure below.
ORGANIC - JONES 5E
CH. 10 - ELECTROPHILIC ADDITIONS TO ALKENES
Page 5
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
EXAMPLE: Provide the mechanism for the following addition reaction.
ORGANIC - JONES 5E
CH. 10 - ELECTROPHILIC ADDITIONS TO ALKENES
Page 6
CONCEPT: HYDROHALOGENATION
General Reaction:
EXAMPLE: Provide the mechanism for the following addition reaction.
ORGANIC - JONES 5E
CH. 10 - ELECTROPHILIC ADDITIONS TO ALKENES
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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:
EXAMPLE: Provide the mechanism for the following addition reaction.
ORGANIC - JONES 5E
CH. 10 - ELECTROPHILIC ADDITIONS TO ALKENES
Page 8
PRACTICE: Provide the mechanism and predict the product of the following reaction.
ORGANIC - JONES 5E
CH. 10 - ELECTROPHILIC ADDITIONS TO ALKENES
Page 9
CONCEPT: HYDROBORATION-OXIDATION
General Reaction:
Mechanism:
1. Electrophilic Addition
2. Oxidation
ORGANIC - JONES 5E
CH. 10 - ELECTROPHILIC ADDITIONS TO ALKENES
Page 10
PRACTICE: Provide the mechanism and predict the product of the following reactions.
a.
b.
ORGANIC - JONES 5E
CH. 10 - ELECTROPHILIC ADDITIONS TO ALKENES
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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
CH. 10 - ELECTROPHILIC ADDITIONS TO ALKENES
Page 12
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
CH. 10 - ELECTROPHILIC ADDITIONS TO ALKENES
Page 13
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.
ORGANIC - JONES 5E
CH. 10 - ELECTROPHILIC ADDITIONS TO ALKENES
Page 14
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 ALKENES
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