chem200-unit5(2)
DESCRIPTION
organic chemistry - chem200TRANSCRIPT
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
Characteristic Reactions of Alkanes
1. Oxidation
2. Thermal Cracking
Characteristic Reactions of Alkanes
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
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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+)
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.
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.
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.
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.
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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