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Chapter 5. Reactions of Alkenes and Alkynes

Learning objectives:

1. Identify the followings from a reaction coordinate diagram when applicable:

endothermic or exothermic reactions, activation energy, heat of reaction, locations of

transition states, locations of intermediates, and rate-limiting step.

2. Identify electrophiles and nucleophiles

3. Differentiate primary, secondary, and tertiary carbocations, and recognize the order of

stability for these carbocations.

4. Write the electron-pushing (arrow-pushing) mechanisms for hydrohalogenation,

hydration and halogenation.

5. Draw the reaction coordinate diagram and provide all the applicable information in

hydrohalogenation, hydration and halogenation.

6. Explain the regio- and stereoselectivity issues involving in hydrohalogenation,

hydration and halogenation of alkenes.

7. Know the products from hydrogenation of alkenes and alkynes.

Sections to be covered (in the order of delivery):

5.1 Electron-pushing Mechanism and Reaction Coordinate Diagram

5.2 Electrophilic Addition: Hydrohalogenation

5.3 Hydration

5.4 Halogenation and Hydrogenation

5.5 An Introduction to Alkynes*

*: section to be skipped

5.1 Electron-pushing Mechanism and Reaction Coordinate Diagram

Electron-pushing mechanism is the key to understand organic reactions!

Understand the movement of electron may help you understand organic

reaction better!

A. Arrows in Organic Reactions

Use to represent two electrons movement.

Use to represent one electron movement.

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B. Before Deriving an Electron-pushing Mechanism

(i) Identify electrophiles and nucleophile.

Use Lew structures when needed.

(ii) Recognize electron-rich and electron-deficient atoms.

The sites where electron movement will occur. Octet rule! Inductive effect!

Electronegativity!

(iii) Compare starting compounds and products and recognize the chemical bonds

that need to be broken or formed.

Identify what needs to be changed (broken or formed).

C. Derive an Electron-pushing Mechanism

Example: electrophilic addition of alkenes

HCl

Cl

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D. Reaction Coordinate Diagram

Reaction coordinate diagram describes the energy changes that takes place during a

reaction

Important Terminologies: starting materials, products, reaction mechanism, energy

diagram, reaction coordinate, heat of reaction (H), exothermic reaction, endothermic

reaction, activation energy (Ea) or free energy of activation (G‡), reaction intermediate,

rate-determining step (rate-limiting step) and transition state (TS‡).

I

II

III

IV

V

potential energy

HCl

A B

C

reaction coordinate

Cl

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E. Self-assessment Question

Can you identify what are electrophile and nucleophile?

Can you use arrow to indicate the movement of electron-pair for an electrophilic

addition?

Can you identify the followings from a reaction coordinate diagram when

applicable: endothermic or exothermic reactions, activation energy, heat of

reaction, locations of transition states, locations of intermediates, and rate-

limiting step.

Can you derive the structures of transition states and intermediates from an

electron-pushing mechanism?

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5.2 Electrophilic Addition: Hydrohalogenation

Know what is electrophilic addition reaction.

A. General Reaction:

RCH CH2 + H X RCH CH3

X

X = F, Cl, Br, I

R: alkyl group (example: CH3-, CH3CH2CH2-)

B. Electron-pushing Mechanism:

C. Two possible products can be formed (Regioselectivity of Hydrohalogenation)

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D. Stability of Carbocation Intermediate

Carbocation stability depends on the number of alkyl substituents attached to the

positively charged carbon.

Relative stability of carbocations:

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E. Regioselectivity of Hydrohalogenation

Know what is regioselectivity.

(i). Markovnikov’s rule:

For the electrophilic addition, hydrogen (atom prefers to attach to the double-

bonded carbon with more hydrogen atoms.

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(ii) Regioselectivity of governed by the stability of carbocation

Examples

F. Self-assessment Question

Can you differentiate primary, secondary, and tertiary carbocations, and

recognize the order of stability for these carbocations?

Can you write the electron-pushing (arrow-pushing) mechanisms for

hydrohalogenation?

Can you explain the regioselectivity of hydrohalogenation?

Can you describe what is Markovnikov’s rule?

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5.3. Hydration (The addition of water and alcohols to an alkene)

A. The addition of water to an alkene is an acid-catalyzed hydration

General Reaction:

RCH CH2 + H2O RCH CH3

HO

H+

Commonly used acid catalysis:

H+ (HX), H3O+ (acid in water), H2SO4, H3PO4

B. Electron-pushing Mechanism:

C. Reaction Energy Diagram

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D. Self-assessment Questions

Can you write the electron-pushing (arrow-pushing) mechanisms for acid-

catalyzed hydration?

Can you explain the regioselectivity of acid-catalyzed hydration?

5.4 Halogenation and Hydrogenation

A. Addition of Bromine and Chlorine (Halogenation)

(i) General Reaction:

RCH CH2 + X X RCH CH2

X XX = Cl, Br, I

(ii) Electron-pushing Mechanism of halogenation:

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(iii) Anti-selectivity (Anti addition): Issue in the Stereoselectivity of Halogenation

Selection between stereoisomers.

Halide attacks from the opposite of the X+ ion.

B. Hydrogenation

Addition of molecular hydrogen to double-bond or triple-bond.

Hydrogenation is a type of reduction.

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(i) Stereoselectivity of Hydrogenation

C. Self-assessment Questions

Can you write the electron-pushing (arrow-pushing) mechanisms for

halogenation?

Can you explain the stereoselectivity (anti-addition) of halogenation?

Can you explain the stereoselectivity (syn-addition) of hydrogenation?

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5.5 An introduction to alkynes

OHN

H2N

O

Parsalmide (analgesic)

O

HN

O

O

O

HO

H

S

S

S

O

O

OO

O

O

NHHO

O

HN

S

O

I

O

O

O

OH

OHO

HO

Calicheamicin γ1 (anticancer)

A. The nomenclature of alkynes

Br

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B. The structure of alkynes

C CH H

C CH H C CH H C CH H C CH H

C. The physical properties of unsaturated hydrocarbons

D. The addition of a hydrogen halide to an alkyne

R H + H X

R H

X H

+ H X

R H

X H

R CX2 CH3

X = F, Cl, Br, I

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Homework for Chapter 5 (optional)

Name: A number: