alken reaction

Electrophilic Addition of Alkenes the double bond is composed of ao bond and az bond electrophile... Alkyl groups decrease the concentration of positive charge in the carbocation elect

Organic Chemistry

4‘ Edition

Reactions of Alkenes

lrene Lee

Case Western Reserve University

Cleveland, OH

©2004, Prentice Hall

Electrophilic Addition of Alkenes

the double bond is composed

of ao bond and az bond

electrophile

Addition of Hydrogen Halides

What is the product?

Carbocation formation is the rate-limiting step

not formed

a tertiary a secondary a primary methyl cation

carbocation carbocation carbocation

Alkyl groups decrease the concentration of positive charge in the carbocation

electrostatic electrostatic electrostatic electrostatic potential map potential map potential map potential map for the tert-butyl cation _for the isopropyl cation for the ethyl cation for the methyl cation

Molecular Orbital Diagram in a

Va

C—H

Hammond postulate: the transition state will be more

similar to the species that it is closer to energetically

Exergonic reaction: early transition state

Endergonic reaction: late transition state

A

5

°

Progress of the reaction — ` TT CHẾ

I: early transition state lll: later transition state

Il: mid-transition state

Reaction Rate and AG?#

more stable transition state

|

lower AG?

faster reaction rate

In a regioselective reaction, one constitutional isomer is the

major or the only product

tert-Butyl cation is formed faster and it is more stable

Addition of Water to Alkene

addition of addition of proton, regenerating

Acid-Catalyzed Addition of Alcohol

T°

eee ees

Br minor product

addition to the | carbocation addition to the | carbocation

unrearranged [er rearranged [er

Carbocation does not always rearrange

Addition of Halogens to Alkene

Addition of Halogens

in the Presence of Water

mechanism for halohydrin formation

Consider the transition states

7"

H

less stable transition state

Oxymercuration and Mercuration

Demercuration by Reduction

NaBH CH,;CHCH,—He—OAc ———*> CH;CHCH; + Hg + AcO-

propene 1-propanol

2 propene OH

2-propanol

more stable less stable

transition state transition state

Formation of Alkyl Boranes

an alkylborane a dialkylborane

a dialkylborane a trialkylborane

Reaction of Carbene with Alkene

rn,

H,C=CH;

Addition of Radicals to Alkene

Relative Stabilities of Alkyl Radicals

Calculating the Change in H

Cl: + CH,=CH, — CICH;CH; AH° = 63 — 82 = —19 kcal/mol (or —79 kJ/mol) exothermic

CICH;CH; + HCI ——> CỊCH;CH; + Cl: AH° = 103 - 101 = +2 kcal/mol (or +8 kJ/mol)

Bri + CH,=CH, —> BrCH,CH, AH? = 63 — 69 = -6 kcal/mol (or —25 kJ/mol)

exothermic

BrCH,CH, + HBr — BrCH,CH; + Br: AH? = 87 — 101 = —14 kcal/mol (or —59 kJ/mol)

I- + CH,=CH, — ICH;CH; AH° = 63 — 55 = +8 kcal/mol (or +33 kJ/mol)

ICH;CH; + HI -——> ICH;CH; + I: AH° = 71 - 101 = -30 kcal/mol (or -126 kJ/mol)<= exothermic

Addition of Hydrogen to Alkenes

Catalytic Hydrogenation of an Alkene

hydrogen molecules settle

on the surface of the catalyst

and react with the metal atoms

Potential Energies of Pentene Isomers

Relative Stabilities of Alkenes

relative stabilities of alkyl-substituted alkenes

Steric Strain in Alkenes

Relative Stabilities of Dialkyl-

alkyl substituents alkyl substituents alkyl substituents

are trans are CIs are on the same sp? carbon