Overview of Organic Reactions with Alkenes

I. Elimination reactions (Synthesis of Alkenes) Alkanes Alkenes1. Dehydrohalogenation (release of H-X)2. Dehydration (release of H-OH)3. Dehalogenation (X-X)

II. Addition reactions Alkenes Alkanes 1. Addition of Halogens

a. Halogenation b. Hydrohalogenation

2. Addition of Hypohalous Acids (X-OH)3. Hydration (addition)

a. Oxymercurationb. Hydroboration

4. Addition of Carbenes (R2C:)5. Reduction

a. Hydrogenation 6. Oxidation

a. Epoxidationb. Hydroxylationc. Cleavage to Carbonyl compounds

7. Radical Additions

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Synthesis of Alkenes

There are three ways to produce alkenes that involve the elimination of certain groups from an alkane to produce a double bond

1. DehydrohalogenationGeneral Mechanism:

a. Involves the removal/loss of a ‘H-X’ from adjacent carbons b. Needs the use of a strong organic base (alkoxide RO- CH3O-)c. Run by either E1 OR E2

d. Ease of a reaction parallels C+ formation ( 3 > 2 > 1)e. Some problems might occur:

i. Formation of isomeric products

ii. Shifts and rearrangements due to C+ formation

Example:

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2. DehydrationGeneral Mechanism

a. Involves the removal/loss of ‘H-OH’ from adjacent carbonsb. Needs the use of a strong acid (H2SO4)

i. Acid Catalyzed Reactionc. Run by either E1 OR E2

d. Ease of a reaction parallels C+ formation ( 3 > 2 > 1)e. Some problems might occur:

i. Formation of isomeric productsii. Shifts and rearrangements due to C+ formation

Example:

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3. DehalogenationGeneral Mechanism

a. Involves the removal/loss of ‘X-X’ from adjacent carbons of vicinal dihalidesb. Needs the use of Zinc and acetic acid

i. Na, Mg, and Ca also workc. Redox Reactiond. One problem possible:

i. Availability of vicinal dihalides (they are usually made from alkenes)e. Benefits of such a reaction:

i. A vicinal dihalide can be used in a reaction to temporarily protect an alkene while a reaction is carried out

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Reactions with Alkenes

There are a wide amount of ways in which alkenes are reacted, in general to form alkane products.There are two major types of reactions

1. Electrophilic additions—These are two step reactions in which the π electrons of alkenes are reacted with an electrophilic agent

2. Oxidation/Reduction—these

Addition of Halogens

1. Hydrohalogenation (opposite of dehydrohalogenation)

General Mechanism

a. Involves the addition of H-X to form an alkyl halideb. H+ adds first to the least substituted carbon, giving a C+ intermediate that gets

attacked by remaining halogenc. Follows Markovnikov’s Rule (H+ attacks the least substituted carbon, while

the halogen attacks the most substituted carbon

2. HalogenationGeneral Mechanism

a. Involves the addition of X-X to form a vicinal dihalideb. The X+ adds first to the least substituted carbon, giving a halonium intermediate

that gets attacked by the remaining X-

c. Follows Markovnikov’s ruled. Anti-addition to form a trans product (one X adds to the other side of the original

X, due to a backside attack by the second X)

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3. Addition of Hypohalous AcidsGeneral Mechanism

a. Involves the addition of X-OH to form halo-alcohols (halohydrins) i. Takes place indirectly by the reaction of the alkenes with either of the

halogensb. The X+ adds first to the least substituted carbon giving a halonium intermediate

(+) that gets attacked by the remaining OH-

c. Follows Markovnikov’s ruled. Anti-addition (X+ adds to one side, while the OH adds to the other side due to a

backside attack by the OH)e. One of the best sources of bromine comes from N-Bromosuccinimide (NBS)

Example

Hydration to form an alcohol

a. Acid Catalyzed reaction in which the H+ adds via Markovnikov’s Rule that is followed by the addition of the -OH

1. Oxymercuration/Demercuration Cis ProductsGeneral mechanism

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a. Oxymercuration occurs when the electrophilic mercury from mercury acetate (HgOAc) by Markovnikov addition creating a C+ intermediate

b. Water attacks the intermediate via a back side (anti-addition of HgOAc and water)c. Demurcuration occurs when the substance is replaced by a hydrogen atom via the

reaction with sodium borohydride (NaBH4)

Example

2. Hydroboration/Oxidation (using R2BH) Trans product

General Mechanism

a. Hydroboration by the concerted addition of R2BH across the double bond, in which the addition is guided by the addition of the boron to the least sterically hindered carbon (anti-Markovnikov)

b. Oxidation occurs when a hydroxyl group from hydrogen peroxide replaces boronExample

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4. Addition of Carbenes (R2C) to form a cyclopropane

a. To form a carbene, the simplest way is to treat chloroform it with a strong base, like KOH.

a. CHCl3 + KOH [CCl3-] + H-OH :CCl2 + KCl-

b. The carbene then reacts with the alkene in a single, consorted step, in which the double bond breaks and the cyclopropane ring forms

c. Process is sterospecific (if a trans alkene is used, a trans product is formed)

d. Simmons-Smith reaction is one in which a nonhalogenated cyclopropane is created by using carbenoid, a metallic reagent with properties of a carbene.

Oxidation/Reduction reactionsReduction is considered the gain of hydrogen (or the loss of oxygen, nitrogen, or a halogen); increases electron density

1. Hydrogenation

a. Is primarily the syn addition (hydrogen adds on the same side) of H2 across an C=C catalyzed by metals like palladium (Pd/C) and platinum (PtO2)

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Oxidation is considered the gain of oxygen, nitrogen, or a halogen (or a loss of a hydrogen); decreases the electron density around carbon

1. Epoxidation

a. Alkenes that treated with peroxyacid (RCO3H) like meta-chloroperoxybenzoic acid form an Epoxide, which is a cyclic ether with an oxygen atom in a three membered ring

b. Peroxyacids transfer oxygen atom to the alkene with syn stereochemistry—both C-O bonds form on the same side giving a one-step mechanism

c. One-step, consorted reaction

2. Hydroxylation to give diols

a. Basic reagents include Osmium tetroxide (OsO4) or potassium permanganate (KMnO4) under dilute conditions (this removes the intermediate)

b. Epoxidation and hydrolysis occur in a two step reaction to yield a diol, also called a glycol (double alcohol)

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3. Oxidative Cleavage to form Carbonyl compoundsa. Ozonolysis to give Aldehydes and Ketones

b. Potassium Permanganate Oxidation to give Carboxylic acids and/or Ketones

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