CHEM200-Unit5(2)

Unit V: Reactions of Alkenes and

AlkynesE. D. Gloria

UST – Faculty of pharmacy

Chem200 – Organic Chemistry

Contents

1. Kinds of organic reactions

2. Organic reaction intermediates

3. Electrophilic addition reactions1. Classification tests for hydrocarbons

4. Reaction energy diagrams and transition states

5. Reactions of alkenes, dienes, and alkynes

6. Preparation of alkenes and alkynes

7. Alkyne acidity

8. Structure elucidation

Kinds of Organic Reactions

Characteristic Reactions of Alkanes

1. Oxidation


1. Oxidation

2. Thermal Cracking


1. Oxidation

2. Thermal Cracking

3. Catalytic re-forming

Kinds of Organic Reactions:AlkenesREACTION MECHANISMS

4 General Types of Organic Reactions

1. Additions

2. Eliminations

3. Substitutions

4. Rearrangements

Addition

• Occurs when two reactants add together to form a single product with no atoms “left over”

Characteristic Reactions of Alkenes

• Hydrochlorination (hydrohalogenation)• Hydration• Bromination (halogenation)• Hydroboration • Hydrogenation (reduction)

Halogenation

• Introduction of halogen into the molecule: chlorine, bromine, etc.

Elimination

• Occur when a single reactant splits into two products, often with the formation of a small molecule such as water or HBr.

Stability and Ease of Formation of Alkenes

CH2=CH2 < RCH=CH2 < R2C=CH2 = RCH=CHR < R2C=CHR < R2C=CR2

• Dehydration and dehydrohalogenation. If elimination can result in the formation of more than one alkene, the most stable alkene is formed predominantly. The most stable alkene is the one most highly substituted with alkyl groups.

Dehydrohalogenation

• A reaction in which hydrogen and halogen are eliminated from a molecule

Dehydration

• The reaction in which the elements of water are eliminated from a molecule

Substitution

• occur when two reactants exchange parts to give two new products.

Reaarangement

• occur when a single reactant undergoes a reorganization of bonds and atoms to yield an isomeric product.

Characteristic Reactions of Alkenes

• Addition• Hydrochlorination (hydrohalogenation)

• Hydration

• Bromination (halogenation)

• Hydroboration

• Hydrogenation (reduction)

Reaction Mechanism

Indicating Steps in Mechanisms

• Curved arrows indicate breaking and forming of bonds

• Arrowheads with a “half” head (“fish-hook”) indicate homolytic and homogenic steps (called ‘radical processes’)

• Arrowheads with a complete head indicate heterolytic and heterogenic steps (called ‘polar processes’)

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Reaction Mechanism

• An overall description of how a reaction occurs

• Two ways in which a covalent two-electron bond can break:• Symmetrical – homolytic

• Unsymmetrical – heterolytic

• Two ways in which a covalent two-electron bond can form:• Symmetrical

• Unsymmetrical

Usual sites of reactions:

1. Multiple bonds – more reaction active sites than single bonds. Pi bonds are more prone to attacking species.

2. Polar bonds – covalent bonds with uneven sharing of electron pairs between atoms of different electronegativity

3. Lewis acids and bases

Lewis Acid Lewis Base

• Electrophile•Accepts electrons

•Nucleophile•Donates electrons• Substance with

nonbonding electron pair

Breaking

Bonding

Radical Reactions

• Processes involving symmetrical bon-breaking and bond-making• Radical – also called “free radical”, is a neutral chemical species

that contain an odd number of electrons and thus has a single, unpaired electron in one of its orbital

1

1

2

1

2

3

Polar Reactions

• Processes involving unsymmetrical bon-breaking and bond-making• Involve species that have an even number of electrons and thus

have only electron pairs in their orbitals. More common processes.

Hydrogen atom on HBrattacked by nucleophilic

double bond

Bromide ion donates an electron pair to positively

charges carbon atom

Using curved arrows in polar reaction mechanisms1. Electrons move from a nucleophilic source (Nu: or Nu:-) to an

electrophilic sink (E or E+)



2. The nucleophile can be either negatively charged or neutral.




3. The electrophile can be either positively charges or neutral.




3. The electrophile can be either positively charges or neutral.

4. The octet rule must be followed.

Example

Example

Reaction Intermediates

Reaction Intermediates

• Intermediates are intermediate structures in going from the starting material to the product.

1

2

Kinds of Reaction Intermediates

1. Carbocation

2. Free radical

3. Carboanion

Reasons why an intermediate is unstable:1. The particle is charged (carbocation and carboanion)

2. The particle does not have an octet of electrons in the outer shell (carbocation, free radical)

Energy Diagrams and Transition States

Thermodynamics

Enthalpy

• the amount of heat content used or released in a system at constant pressure.

Energy Diagram

The point on the reaction

coordinate at which the energy is at a maximum.

Heat of Reaction

• The difference in energy between the reactants and products

• Exothermic – if the energy of the products is lower than that of the reactants

• Endothermic – if the energy of the products is higher that that of the reactants

Energy Diagram

Entropy

• Measure of disorder associated with a system.

• The ultimate measure for determining whether or not a reaction can occur.

• The large number of possible states corresponds with a larger entropy

Gibbs free energy

Equilibria

Conclusion

The thermodynamics of a reaction is based on the difference in energy between starting materials and products.

Kinetics

Energy of activation

• The energy barrier between the reactants and the products

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CHEM200-Unit5(2)