Post on 18-Dec-2015
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Alkenes Structure, Nomenclature,
and an introduction to Reactivity
• Thermodynamics
and Kinetics
Chapter 5
Paula Yurkanis BruiceUniversity of California,
Santa Barbara
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Molecular Formula and the Degree of Unsaturation
molecular formula = CnH2n+2 – 2 hydrogens for every π bond or ring
degree of unsaturation = the total numbers of π bonds and rings
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Compounds with Molecular Formula = C8H14
CnH2n+2 = C8H18
Therefore, C8H14 has two degrees of unsaturation.
compounds with six hydrogens and two degrees of unsaturation
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Saturated and Unsaturated Hydrocarbons
Saturated hydrocarbons have no double bonds.
Unsaturated hydrocarbons have one or more double bonds.
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Nomenclature of Alkenes
Replace “ane” of alkane with “ene.”
The functional group gets the lowest possible number.
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Stereoisomers are Named Using a cis or trans Prefix
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Nomenclature of Dienes
two double bonds = diene
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Number in the direction so that the functional group
gets the lowest number.
Nomenclature of Alkenes
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Substituents are stated in alphabetical order.
Nomenclature of Alkenes
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Nomenclature of Alkenes
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Nomenclature of Cyclic Alkenes
A number is not needed to denote the position of the functional group;
it is always between C1 and C2.
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Nomenclature of Alkenes
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Nomenclature of Alkenes
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Vinylic and Allylic Carbons
vinylic carbon: the sp2 carbon of an alkene
allylic carbon: a carbon adjacent to a vinylic carbon
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Double Bonds Have Restricted Rotation
Rotation about a double bond breaks the π bond.
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Six Atoms of an Alkene are in the Same Plane
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Alkenes Have Cis–Trans Isomers
Cis: The hydrogens are on the same side of the ring.
Trans: The hydrogens are on opposite sides of the ring.
cis trans
They have different configurations; they can be separated.They have different configurations; they can be separated.
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Which Is Cis and Which Is Trans?
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The E,Z System of Nomenclature
Z = Zusammen (together) E = Entgegen (opposite)
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Relative Priorities
The relative priorities of the two groups depends on the
atomic numbers of the atoms attached to the sp2 carbon.
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The E and Z Isomers
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E and Z
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E and Z
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Isomers for Compounds withTwo Double Bonds
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Reactions of Organic Compounds
Organic compounds can be divided into families.
All members of a family react in the same way.
The family a compound belongs to depends on its functional group.
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Electrophiles
An electrophile has a positive charge,a partial positive charge,or an incomplete octet.
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Nucleophiles
A nucleophile hasa negative chargeor a lone pair.
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A Nucleophile Reacts with an Electrophile
Your first organic reaction
is a two-step reaction.
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Curved Arrows
Curved arrows are used to show the mechanism of a reaction.
The mechanism of a reaction is the step-by-step description of the process by which reactants are converted into products.
first step of the reaction
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second step of the reaction
The curved arrow shows where the electrons start from and where they end up.
Curved Arrows
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How to Draw Curved Arrows
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How to Draw Curved Arrows
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How to Draw Curved Arrows
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How to Draw Curved Arrows
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Thermodynamics and Kinetics
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A Reaction Coordinate Diagram
A reaction coordinate diagram shows the energy changes that take place in each step of a reaction.
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The Equilibrium Constant
The equilibrium constant gives the concentration of reactants and products at equilibrium.
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Exergonic and Endergonic Reactions
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Gibbs Free-Energy Change (∆G°)
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Calculating ∆H°
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Reaction Coordinate Diagrams for Fast and Slow Exergonic and Endergonic Reactions
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Free Energy of Activation (∆G‡)
The free energy of activation is the energy barrier of the reaction.
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Rate of a Reaction
Increasing the concentration increases the rate.
Increasing the temperature increases the rate.
The rate can also be increased by a catalyst.
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The Rate of a Reaction versusThe Rate Constant for a Reaction
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The Rate of a Reaction versusThe Rate Constant for a Reaction
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The Arrhenius Equation
The value of A pertains to the frequency and orientation of collisions.
e–Ea/RT
The term e–Ea/RT provides the reaction of molecules withthe needed activation energy at a given temperature (T).
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An Electrophilic Addition Reaction
Transition states have partially formed bonds.
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Reaction Coordinate Diagram forEach Step of the Addition of HBr to 2-Butene
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Reaction Coordinate Diagram forthe Addition of HBr to 2-Butene
The rate-limiting step of the reaction is the step that has its transition state at the highest point of the reaction coordinate diagram.
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A Catalyst
A catalyst provides a pathway for a reaction with a lower energy barrier.
A catalyst does not change the energy of the starting point (the reactants) or the energy of the end point (the products).
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Enzymes
Most biological reactions require a catalyst.
Most biological catalysts are proteins called enzymes.
The reactant of a biological reaction is called a substrate.
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The Active Site of an Enzyme
An enzyme binds its substrate at its active site.
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Enzyme Side Chains that Bind the Substrate
Some enzyme side chains bind the substrate.
Some enzyme side chains are acids, bases, and nucleophilesthat catalyze the reaction.
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Some enzyme side chains are acids, bases, and nucleophiles that catalyze the reaction.
Enzyme Side Chains that Catalyze the Reaction