Transcript Chapter 4—An Introduction to Organic Reactions

Chapter 4—An Introduction to Organic
Reactions
Organic Chemistry
General Principles of Organic
Reactions
 A reaction equation is an equation that shows what happens in
a chemical reaction by showing reactants and products.
The amount of chemical reactions in organic chemistry is large, but
most fall into one of three basic categories.
Types of Organic Reactions
Most organic
reactions can
fall into one of
the following
categories:
1) Substitution
2) Elimination
3) Addition
 In a substitution reaction, an atom or
group of atoms is replaced by another
species.
 In an elimination reaction, an atom or
group of atoms is eliminated from adjacent
carbon atoms, usually resulting in the
formation of a multiple bond.
 In an addition reaction, atoms or groups
of atoms add to the multiple bond (sort
of the reverse of an elimination
reaction).
Types of Organic Reactions
Substitution Reaction Examples:
Elimination Reaction Example:
Addition Reaction Example:
Reaction Mechanisms
The reaction
equation tells
you WHAT
happens in a
chemical reaction.
The reaction
mechanism tells
you HOW it
happens; it’s a
step-by-step
description of how
the chemical
changes occurs.
Addition of HBr to ethene:
This reaction proceeds by a two step mechanism:
1) The hydrogen adds first as a positive ion to form a
short-lived intermediate called a carbocation.
2) The carbocation is neutralized (or gets it’s
needed electrons) in the second step by a negative
bromide ion.
Reaction Mechanisms and Energy
Diagrams
Potential energy
diagrams are used
to show energy
changes during
chemical reactions.
Energy is required
to break bonds and
this raises the
potential energy
as these bonds break
in the initial stages
of the reaction. As
new bonds form,
energy is then
released.
If the reaction is exothermic (gives off heat):
Energy of
reactants is
higher
than
products
If the reaction is endothermic (absorbs heat):
Energy of
products
is higher
than
reactants
Reaction Intermediates
When a organic
reaction occurs,
bonds must break
and new bonds
form. As bonds
break, unstable,
short-lived species
called reaction
intermediates
form. There are
three types:
These species is unstable for one or both of the following:
1) The particle is charged (carbocation, carboanion);
or
2) The particle does not have an octet of electrons in the
outer shell (carbocation, free radical).
1) Carbocations
2) Free radicals
3) Carboanions
Carbocation (+)
Free radical
Carboanion (-)
Reaction Intermediates
Intermediates can
be formed in a variety
of ways:
1) Homolytic
cleavage—
cleavage in which
shared electrons
are evenly
divided between
the parting atoms
(“shared
custody”).
2) Heterolytic
cleavage—
cleavage in which
shared electrons
are unevenly
divided between
the parting atoms
(“sole custody”)
Heterolytic cleavage:
Forms a
carbocation
Homolytic cleavage:
Forms a
free
radical
Heterolytic cleavage:
Forms a
carboanion
Sites of Organic Reactions
Why do organic
molecules react?
The reactivity of
an organic
compound is
determined by its
structure;
specifically at
places in the
molecule where
there is an
availability of
finding electrons
or where there is a
lack of electrons.
Electrophiles (means “electron-loving”) is an
electron deficient species that accepts electrons
from electron rich species in a chemical
reaction. Electrophiles are Lewis acids.
Nucleophiles (means “nucleus-loving”) is a
electron rich species that donates electrons to
electron deficient species in a chemical
reaction. Nucleophiles are Lewis bases.
Lewis Acids and Lewis Bases
A Lewis base is a
species that has a
nonbonding pair of
valence electrons that
is can share in a
chemical reaction. A
Lewis base is known
as an “electron pair
donator”.
A Lewis acid is a
species that can
accept a pair of
electrons for sharing
in a chemical
reaction. A Lewis
acid is known as an
“electron pair
acceptor”.
Lewis Acid-Base Reactions:
How to Predict Reactions & Products
Organize your study of reactions as follows:
1) General reaction equation. Learn it and identify the
reaction as substitution, elimination, or addition.
2) Predominant product. Learn to determine which product
predominates when more than one is present.
3) Reaction mechanism. Learn the step-by-step mechanism as
best you can so you can apply it to other examples.
4) Specific examples and practice problems. Be sure to
include as many examples of each type of reaction in your notes
and write out entire reactions in your practice problems.
Reactions of Alkanes: Halogenation
Halogenation is a type of substitution reaction in which a
hydrogen atom is replaced or substituted by a halogen.
The reaction occurs when an alkane is combined with chlorine
(Cl2) or bromine (Br2) in the presence of heat (Δ) or light
(hv).
General Reaction Equation for Halogenation of Alkanes:
heat
C
H
alkane
+
X2
light
C
X + HX
alkyl halide
Where X2 = Cl2 or Br2; HX = HCl or HBr
Chlorination of Methane
The chlorination of methane (and other alkanes) occurs by a
free radical chain reaction. A chain reaction is a
reaction that sustains itself through repeating chainpropagation steps.
Once all of the
hydrogens have
been replaced,
the chain
reaction stops.
Mechanism of Halogenation
Preparation of Alkenes and Alkynes:
Elimination Reactions
Elimination reactions are used to introduce carbon-carbon
double or triple bonds into a molecule. To do this, two atoms
or groups of atoms from two adjacent carbon atoms must be
eliminated.
General Equations of Elimination Reactions for Preparing Alkenes & Alkynes
Alkenes:
Alkynes:
C
C
B
B
A
A
C
C
B
A
C = C
+
A B
C = C
+ 2A B
Preparation of Alkenes and Alkynes:
Elimination Reactions
One type of elimination reaction is called
DEHYDROHALOGENATION. In it, a hydrogen halide
(HX) is removed to form the double or triple bond:
General Equations for Dehydrohalogenation:
Alkenes:
Alkynes:
Preparation of Alkenes and Alkynes:
Elimination Reactions
Another type of elimination reaction is called
DEHYDRATION. In this reaction, the eliminated product
is water, H-OH.
General Reaction Equation for Preparation of Alkenes by
Dehydration:
*Usually H2SO4 is used as the dehydrating agent. This reaction
does not work well to form alkynes.
Dehydration of Alcohols
Mechanism of Dehydration Reaction
Reactions of Alkenes and Alkynes:
Addition Reactions
Why are alkenes and alkynes reactive and why is addition the
characteristic reaction?
1) Carbon-carbon double & triple bonds are composed of π
bonds in addition to a σ bond; the double bond has one
π bond, and the triple bond has two π bonds. Π bonds are
formed by p orbital overlap—they are loosely held and
susceptible to attack by electrophiles.
2) Alkenes and alkynes are unsaturated. This means the two
carbons do not have the maximum possible number of
atoms or groups bonded to them.
Addition Reaction
Double bonds can undergo addition once while alkynes can
undergo twice.
Addition to alkenes:
C=C
+
E
A
Addition to alkynes:
C C
+
2E
A
C
C
E
A
E
A
C
C
E
A
Addition Reactions of Alkenes
General Reaction for Addition to Alkenes:
C=C
+
E
A
C
C
E
A
1) Addition of hydrogen halides: (E = H, A = X; HX = HCl, HBr, HI)
C=C
+
H
Cl
C
C
H
Cl
2) Halogenation: (E = X, A = X; X2 = Cl2, Br2, F2 is too reactive; I2 is
not reactive)
C=C
+
Cl
Cl
C
C
Cl Cl
Addition Reactions of Alkenes
3) Hydration: (E = H, A = OH; H2SO4 is the catalyst)
C=C
+
H
OH
H2SO4
C
C
H OH
4) Hydrogenation: (E = H, A = H; metal catatlyst such as Ni, Pt, or Pd
with reaction lead under pressure)
C=C
+
H
H
Ni, Pt, Pd
pressure
C
C
H
H
Mechanism of Electrophilic Addition
Mechanism for Addition of Hydrogen Halogen (HX):
Electrophilic Addition is an addition reaction
initiated by an electron deficient species or an
electrophile. In this case, the electrophile is H
and the nucleophile is X.
Unsymmetrical Reagents & Addition
How do you know which product of an addition reaction will be the
major product?
By using MARKOVNIKOV’S RULE!
It states that when an unsymmetrical reagent (like H-Cl,
H-I, H-Br, H-OH) adds to an unsymmetrical double or
triple bond, the positive (E+) portion of the reagent
adds to the carbon with the most hydrogens (or fewest
alkyl groups).
Reactions such as these in which one product predominates is called
regioselective. If one product is formed exclusively, the reaction
is termed regiospecific.
Unsymmetrical Reagents & Addition
Unsymmetrical reagent
H is E+
Cl is Nu-
Unsymmetrical
or “uneven”
double bond
Notice carbon
on right has
most hydrogens
Carbon
with most
hydrogens
gets E+ of
reagent