Transcript Orbitals - drjosephryan.com

John E. McMurry
http://www.cengage.com/chemistry/mcmurry
Chapter 6
An Overview of Organic
Reactions
Richard Morrison • University of Georgia, Athens
Organic Chemical Reactions
Organic chemical reactions broadly organized in two
ways:
1.
What kinds of reactions occur
2.
How those reactions occur
6.1 Kinds of Organic Reactions
Addition reactions
•
Occur when two reactants add together to form a single product
with no atoms “left over”
•
Reaction of fumarate with water to yield malate (a step in the
citric acid cycle of food metabolism)
Kinds of Organic Reactions
Elimination reactions
• Occur when a single reactant splits into two products
(usually with the formation of a small molecule such as
water)
• Reaction of hydroxybutyryl ACP to yield trans-crotonyl
ACP and water (a step in the biosynthesis of fat
molecules)
Kinds of Organic Reactions
Substitution reactions
• Occur when two reactants exchange parts to give two new
products
• Reaction of an ester such as methyl acetate with water to
yield a carboxylic acid and an alcohol
• In biological pathways this type of reaction occurs in the
metabolism of dietary fats
Kinds of Organic Reactions
Rearrangement reactions
• Occur when a single reactant undergoes a reorganization
of bonds and atoms to yield an isomeric product
• Rearrangement of dihydroxyacetone phosphate into its
constitutional isomer glyceraldehyde 3-phosphate (a step
in the metabolism of carbohydrates)
6.2 How Organic Reactions Occur:
Mechanisms
Reaction Mechanism
• An overall description of how a reaction occurs at each
stage of a chemical transformation
• Which bonds are broken and in what order
• Which bonds are formed and in what order
• What is the relative rate of each step
• A complete mechanism accounts for all reactants consumed
and all products formed
How Organic Reactions Occur: Mechanisms
All chemical reactions involve bond breaking and bond making
Two ways a covalent two-electron bond can break:
1.
2.
Symmetrical - HOMOLYTIC
•
One electron remains
with each product
fragment
Unsymmetrical - HETEROLYTIC
•
Both bonding electrons
remain with one
product fragment,
leaving the other with
a vacant orbital
Half-headed arrow, “fishhook”,
indicates movement of one
electron
Full-headed arrow indicates
movement of two electrons
How Organic Reactions Occur: Mechanisms
Two ways a covalent two-electron bond can form:
1.
Symmetrical
•
One electron is donated
to the new bond by each reactant (radical)
2.
Unsymmetrical
•
Both bonding electrons
are donated by one reactant (polar)
How Organic Reactions Occur: Mechanisms
Radical reaction
• Process that involves symmetrical bond breaking and
bond making
• Radical (free radical)
• A neutral chemical species that contains an odd
number of electrons and has a single unpaired
electron
Polar reactions
• Process that involves unsymmetrical bond breaking and
bond making
• Involve species that have an even number of electrons
• Common in both organic and biological chemistry
Radical Reactions
Radical
•
Highly reactive because it contains an atom with an odd
number of electrons
•
Can achieve stability through:
•
Radical substitution reaction
•
Radical abstracts an atom and one bonding electron
from another reactant
Radical Reactions
•
Radical addition reaction
• A reactant radical adds to a double bond, taking one
electron from double bond and leaving one behind to
form a new radical
Radical Reactions
Industrial radical reaction
• The chlorination of methane to yield chloromethane
• A substitution reaction
• First step in the preparation of the solvents dichloromethane
(CH2Cl2) and chloroform (CHCl3)
Radical Reactions
Radical chlorination of methane requires three kinds of
steps: initiation, propagation, and termination
Initiation
1.
•
Ultraviolet light breaks Cl-Cl bond to generate chlorine
radicals
Radical Reactions
Propagation
2.
•
Reaction with CH4 to generate new radicals and propagate
the chain reaction
Radical Reactions
Termination
3.
•
•
Two radicals combine to end the chain reaction
No new radical species is formed
Radical Reactions
Carbon radicals are categorized as primary (1°), secondary (2°)
and tertiary (3°) based on the number of attached R groups.
RCH2
1°
R2CH
R3CH
2°
3°
A carbon radical is sp2 hybridized with a trigonal planar geometry
with the unpaired electron in the unhybridized p orbital.
Radical Reactions
Bond dissociation energy is used as a measure of radical stability.
Radical Reactions
Radical Reactions
What type of radical are each of the following?
•
H3C
•
H3C
CH3
H3C
CH3
CH
CH2
H3C
H2C
CH3
CH
•CH
H3C
Of these three radicals, which is the most stable?
CH2
Which C-H bond in each compound is most reactive?
Radical Reactions
Predict the products from the monobromination of the following compound?
H3C
CH3
CH C
H
H3C
CH3
Br2
hv/ heat
Radical Reactions
Chlorination is faster and nonselective. This is due to it’s rate determining
step being exothermic.
Bromination is slower and chooses the most stable radical. This due to it’s
rate determining step being endothermic.
Bond Dissociation Energies
Bond
Bond Energy (kj/mol)
H—F
565
H—Cl
427
H—Br
363
H—I
295
C—H
413
C—F
485
C—Cl
339
C—Br
276
C—I
240.
Polar Reactions
Bond polarity
• Certain bonds within a molecule are polar
•
•
Consequence of an unsymmetrical electron distribution in a
bond
One atoms preference over the electrons between the
bonded atoms.
Polar reactions
• electrical attraction between positive and negative
centers on functional groups in molecules
Two electron type reactions
• Double headed arrows used to show two electron flow
in mechanisms
Polar Reactions
Certain bonds within molecules with functional groups are
polar
• Carbon is positively polarized (d+) when bonded to more
electronegative elements
• Carbon is negatively polarized (d ) when bonded to metals
Polar Reactions
Polar Reactions
Polar Reactions
Polar of bonds can be enhanced!
•
Interactions of functional groups with acids or bases
• Methanol
•
•
In neutral methanol the carbon atom is somewhat electron-poor
Protonation of the methanol oxygen by an acid makes carbon much
more electron-poor
Polar Reactions
Polarizability of the atom
•
•
•
The measure of change in electron distribution around the atom to an
external electrical influence
Larger atoms (more, loosely held electrons) – more polarizable
Smaller atoms (fewer, tightly held electrons) – less polarizable
Effects of polarizability on bonds
•
Although carbon-sulfur and carbon-iodine bonds are nonpolar according
to electronegativity values, they usually react as if
they are polar because sulfur and iodine are highly polarizable
Polar Reactions
Nucleophile – A Lewis Base
• Substance that is “nucleus-loving”
• Has a negatively polarized electron-rich atom
• Can form a bond by donating a pair of electrons to a
positively polarized, electron-poor atom
• May be either neutral or negatively charged
Electrophile – A Lewis Acid
• Substance that is “electron-loving”
• Has a positively polarized, electron-poor atom
• Can form a bond by accepting a pair of electrons from a
nucleophile
• May be either neutral or positively charged
Worked Example 6.1
Identifying Electrophiles and Nucleophiles
Which of the following species is likely to behave as a
nucleophile and which as an electrophile?
(a) (CH3)3S+
(b)
-CN
(c) CH3NH2
An Example of a Polar Reaction:
Addition of H2O to Ethylene
Addition of water to ethylene
• Typical polar process
• Acid catalyzed addition reaction (Electophilic addition reaction)
An Example of a Polar Reaction:
Addition of H2O to Ethylene
Addition of water to ethylene
•
1)
•
2)
•
3)
An Example of a Polar Reaction:
Addition of HBr to Ethylene
Addition of HBr to ethylene
•
1)
•
2)
•
3)
Example 6.2
Using Curved Arrows in Reaction Mechanisms
Add curved arrows to the following polar reactions to
show the flow of electrons
Describing a Reaction:
Equilibrium, Rates, and Energy Changes
Every chemical reaction can proceed in either the forward or
reverse direction

 cC + dD
aA + bB 

•
The position of the resulting chemical equilibrium is expressed by the
equilibrium constant equation Keq
Keq
•
•
•
K>1
K=1
K<1
•
∆G = Gproducts – Greactant
C  D

=
 A  B 
c
d
a
b
Describing a Reaction: Equilibria, Rates, and
Energy Changes
The free-energy change ∆G made up of two terms:
Enthalpy ∆H
2. Entropy T∆S (temperature depended)
1.
∆Gº = ∆Hº - T∆Sº (standard conditions)
Reaction of ethylene with H2O at 298 K
Describing a Reaction: Equilibria, Rates, and
Energy Changes
Keq
• Tells position of equilibrium
• Tells how much product is theoretically possible
• Does not tell the rate of reaction
• Does not tell how fast equilibrium is established
Rate → Is the reaction fast or slow?
Equilibrium → In what direction does the reaction
proceed?
Describing a Reaction:
Bond Dissociation Energies
Bond strength is a measure of the heat change that
occurs on breaking a bond, formally defined as bond
dissociation energy
• Each bond has its own characteristic strength
Bond Dissociation Energy (D)
• The amount of energy required to break a given bond to
produce two radical fragments when the molecule is in the
gas phase at 25ºC
Describing a Reaction: Bond Dissociation
Energies
Describing a Reaction: Bond Dissociation
Energies
Describing a Reaction:
Energy Diagrams and Transition States
For a reaction to take place
• Reactant molecules must collide
• Reorganization of atoms and bonds must occur
Describing a Reaction: Energy Diagrams and
Transition States
Chemists use energy diagrams to graphically depict the
energy changes that occur during a chemical
reaction
•
Vertical axis
• the total energy
of all reactants
• Horizontal axis
• “reaction coordinate”
the progress of the
reaction from
beginning to end
Addition of water to ethylene
Describing a Reaction: Energy Diagrams and
Transition States
Activation Energy (∆G‡)
• The energy difference between reactants and
transition state
• Determines how rapidly the reaction occurs at a given
temperature
•
•
Large activation energy results in a slow reaction
Small activation energy results in a rapid reaction
• Many organic reactions have activation energies in the
range of 40 – 150 kJ/mol
• If ∆G‡ less than 80 kJ/mol the reaction takes place at or
below room temperature
•
If ∆G‡ more than 80 kJ/mol the reaction requires heating
above room temperature
Describing a Reaction: Energy Diagrams and
Transition States
Activation energy leads to transition state
The Transition State
• Represents the highest-energy structure involved
in the reaction
• Unstable and cannot be isolated
A hypothetical transition–state
structure for the first step of
the reaction of ethylene with
H3O+
•
•
the C=C bond about to break
the C-H bond is beginning to form
Describing a Reaction: Energy Diagrams and
Transition States
Once transition-state is reached the reaction either:
• Continues on to give carbocation product
• New C-H bond forms fully
• Amount of energy corresponding to difference between
transition-state (∆G‡) and carbocation product is released
• Since carbocation is higher in energy than the starting alkene,
the step is endergonic (+∆Gº, absorbs energy)
• Reverts back to reactants
• Transition-state structure comes apart
• Amount of free-energy (-∆G‡) is released
Describing a Reaction:
Energy Diagrams and Transition States
Each reaction has its
own profile
(a) a fast exergonic
reaction (small G‡,
negative G°);
(b) a slow exergonic
reaction (large G‡,
negative G°);
(c) a fast endergonic
reaction (small G‡,
small positive G°);
(d) a slow endergonic
reaction (large G‡,
positive G°).
Describing a Reaction: Intermediates
Reaction Intermediate
• A species that is formed during the course of a multi-step
reaction but is not final product
• More stable than transition states
• May or may not be stable enough to isolate
• The hydration of ethylene proceeds through two reaction
intermediates, a carbocation intermediate and a
protonated alcohol intermediate
Describing a Reaction: Intermediates
Each step in a multi-step process can be considered separately
(each step has ∆G‡ and ∆Gº)
Overall ∆Gº of
reaction is the
energy difference
between initial
reactants and
final products
Overall energy diagram for the
reaction of ethylene with water
Describing a Reaction: Intermediates
Biological reactions occur at physiological conditions
•
Must have low activation energy
• Must release energy in relatively small amounts
Enzyme catalyst
changes the
mechanism of reaction
to an alternative
pathway which proceeds
through a series of
smaller steps rather
than one or two large
steps
Worked Example 6.3
Drawing Energy Diagram for Reactions
Sketch an energy diagram for a one-step reaction that
is fast and highly exergonic