Organic Chemistry Chapter 4An Introduction to Organic
Reactions
Slide 2
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.
Slide 3
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).
Reaction Mechanisms The reaction equation tells you WHAT
happens in a chemical reaction. The reaction mechanism tells you
HOW it happens; its a step-by-step description of how the chemical
changes occurs. Addition of HBr to propene: 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 its needed electrons) in the
second step by a negative bromide ion.
Slide 6
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): If the reaction is endothermic
(absorbs heat): Energy of reactants is higher than products Energy
of products is higher than reactants
Slide 7
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: 1) Carbocations 2) Free radicals 3) Carboanions 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). Carbocation (+)Carboanion (-)Free
radical
Slide 8
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 carbocation
Slide 9
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.
Slide 10
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:
Slide 11
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.
Slide 12
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 (Cl 2 ) or bromine (Br 2 ) in the presence
of heat ( ) or light (hv). General Reaction Equation for
Halogenation of Alkanes: C H+ X 2 C X + HX alkane alkyl halide
Where X 2 = Cl 2 or Br 2 ; HX = HCl or HBr heat light
Slide 13
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 chain-
propagation steps. Once all of the hydrogens have been replaced,
the chain reaction stops.
Slide 14
Mechanism of Halogenation
Slide 15
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: C C C = C + A B B A Alkynes: C CC = C + 2 A B B
A
Slide 16
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:
Slide 17
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 H 2 SO
4 is used as the dehydrating agent. This reaction does not work
well to form alkynes.
Slide 18
Dehydration of Alcohols
Slide 19
Mechanism of Dehydration Reaction
Slide 20
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 bonds in addition to a bond; the double bond has one bond, and
the triple bond has two bonds. bonds are formed by p orbital
overlapthey 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.
Slide 21
Addition Reaction Double bonds can undergo addition once while
alkynes can undergo twice. Addition to alkenes: Addition to
alkynes: C = C + E AC E A C + 2 E AC E A
Slide 22
Addition Reactions of Alkenes General Reaction for Addition to
Alkenes: 1) Addition of hydrogen halides: (E = H, A = X; HX = HCl,
HBr, HI) C = C + E AC E A C = C + H ClC H Cl 2) Halogenation: (E =
X, A = X; X 2 = Cl 2, Br 2, F 2 is too reactive; I 2 is not
reactive) C = C + Cl ClC Cl
Slide 23
Addition Reactions of Alkenes 3) Hydration: (E = H, A = OH; H 2
SO 4 is the catalyst) 4) Hydrogenation: (E = H, A = H; metal
catatlyst such as Ni, Pt, or Pd with reaction lead under pressure)
C = C + H OHC H OH H 2 SO 4 C = C + H HC H Ni, Pt, Pd pressure
Slide 24
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.