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Organic Chemical Reactions
Organic chemical reactions
• Organic reactions are chemical reactions
involving organic compounds.
• The number of possible organic reactions is
basically infinite.
• Organic reactions can be categorized based on
the type of functional group involved in the
reaction as a reactant and the functional group
that is formed as a result of this reaction.
Organic chemical reactions
• Almost every component of living organisms
involves organic chemistry – carbohydrates,
enzymes, fats, lipids, proteins, nucleic acids
etc.
• The balance of attractive forces between atoms
when they share the electrons.
• It is known as covalent bonding.
• Depending upon the order of the bond millions
of compound connections, all with potentially
different properties, can be prepared.
Organic Reactions
Writing Equations for Organic Reactions:
• Equations for organic reactions are usually drawn
with reagents to the left of a reaction arrow ()
and products to the right.
• A reagent, the chemical substance with which an
organic compound reacts, may be placed to the left
of the arrow or on the arrow itself.
Organic Reactions
Writing Equations for Organic Reactions:
• The solvent and temperature are often
omitted but at times is also placed
above or below the arrow.
• The symbols “h” and “” are placed
above or below the arrow for reactions
that require light and heat energy,
respectively.
Understanding Organic Reactions
Writing Equations for Organic Reactions:
Different ways of writing organic reactions
Here, all are needed together.
Organic Chemical Reactions
• Organic compounds are those that have carbon
atoms.
• An organic molecule is a collection of atoms
held together by covalent bonds.
Organic Chemical Reactions
• Four lines connecting a carbon atom to other
atoms, each line representing a pair of shared
electrons (one electron from carbon and one
from another atom).
• The pair of electrons that form a bond between
two atoms are called bonding electrons.
Organic Chemical Reactions
• All chemical reactions are
bond-breaking or bond-making
reactions.
Organic Chemical Reactions
• Bond formation or breakage can be
symmetrical or unsymetrical:
• Symmetrical- homolytic
(radicalic)
• Unsymmetrical- heterolytic
(ionic)
Bond-breaking
• Symmetrical bond-braking is also named
radicalic barking.
• One bonding electron stays with each product.
radical
radical
Bond-breaking
• Unsymmetrical bond –braking is also named
ionic braking.
• Two bonding electrons stay with one product.
+ ion
- ion
Homolysis
Heterolysis
Ionic braking products
• Different molecules occurs by ionic bond braking:
A) Anions – possess an electron pair that can be
introduced into an electron-deficient substrate.
Those are nucleophilic reactans.
• anions (H–, OH–)
B) Cations– electron-deficient  bind to substrate
centres with a higher electron density.
Those are ectrophilic reactans.
• cations (Br+)
Radicals,Carbocations and Carbanions
Three reactive intermediates resulting
from homolysis and heterolysis of a C – Z bond
Understanding Organic Reactions
Radicals, Carbocations and Carbanions
• Radicals and carbocations are electrophiles
because they contain an electron deficient
carbon (sp2 carbon atoms).
• Carbanions are nucleophiles because they
contain a carbon with a lone pair (sp3)
carbon atoms.
Lewis acids and bases
• Lewis base: acts as an electron-pair donor;
••
e.g. ammonia: NH3
• Lewis acid: can accept a pair of electrons;
e.g.: AlCl3, FeCl3, ZnCl2. These
compounds – important catalysts: generate
ions that can initiate a reaction:
CH3–Cl + AlCl3  CH3+ + AlCl4-
Bond making
• Bond making also two types
• Symmetrical- homolytic (radicalic)
One bonding electron is donated by each reactant
• Unsymmetrical- heterolytic (ionic) (polar)
Two bonding electrons are donated by one reactant.
Radical Reactions
• Radicals react to complete electron octet of
valence shell
– A radical can break a bond in another
molecule and abstract a partner with an
electron, giving substitution in the original
molecule.
Radical Reactions
– A radical can add to an alkene to give a
new radical, causing an addition reaction.
Categories of organic reactions
•
•
•
•
•
Addition Reactions
Substitution Reactions
Elimination Reactions
Oxidation – Reduction Reaction
Rearrangement reaction
Addition Reactions
• Addition is a reaction in which elements are
added to the starting material. (Opposite of
elimination.)
Addition Reactions
• In an addition reaction, new groups X and Y are
added to the starting material. A  bond is
broken and two  bonds are formed.
Addition Reaction
In alkenes and alkynes
• In addition reactions, reactants are added to the
carbon atoms in the double or triple bond.
• The double or triple bond is easily broken, since
it is highly reactive.
• In an addition reaction, new groups H and Br are
added to the starting material.
• A  bond is broken and two  bonds are formed.
25
Radicalic addition
• 1. Initiation (creation of radicals),
• 2. Propagation (radicals attack neutral molecules,
producing more and more radicals),
• 3. Termination (radicals react with each other, forming
a stable product; the chain reaction is terminated)
Electrophilic addition
• An electrophile forms a covalent bond by
attacking an electron-rich unsaturated C=C bond
• Typical of alkenes and alkynes
• Markovnikov´s rule: the more positive part of the
agent (hydrogen in the example below) becomes
attached to the carbon atom (of the double bond)
with the greatest number of hydrogens:
Nucleophilic addition
• In compounds with polar unsaturated bonds,
such as C=O:
– carbon atom carries +
• Nucleophiles – water, alcohols, carbanions – form a
covalent bond with the carbon atom of the carbonyl
group:
used for
synthesis
of alcohols
Hemiacetals
• Addition of alcohol to the carbonyl group yields
hemiacetal:
hemiacetal
• As to biochemistry, hemiacetals are formed by
monosaccharides:
hemiacetals
glucose
Hydrogenation
(addition of Hydrogen)
In hydrogenation,
• Hydrogen atoms add to the carbon atoms of a double bond
or triple bond.
• Converts unsaturated molecule to saturated
• alkene or alkyne + H2 → alkane
• Unsaturated vegetable oils reduced to produce saturated fats
used in margarine and cooking products
H H
H2C CH2 + H2
Pt
H2C CH2
H H
HC CH + 2H2
Ni
HC CH
H H
Hydration
(addition of water H2O)
•
•
•
•
An acid H+ catalyst is required.
Water (HOH) adds to a double bond.
An H atom bonds to one C in the double bond.
An OH bonds to the other C.
H+
CH3─CH=CH─CH3 + H─OH
H OH
│ │
CH3─CH─CH─CH3
Esterification
• Alcohol + Organic Acid = Water + Ester
• Used to make perfumes, scents and flavors
• Combination reaction which involves
dehydration.
• The alcohol becomes the alkyl group and the
acid becomes -oate
Polymers
The joining together of many smaller repeating
Units to form a very high MW molecule
- Polymers range from 10,000 amu to more
than 1,000,000 amu (Atomic Mass Unit)
The small repeating units used to build the
polymer are known as monomers
Monomers
Sometimes just one monomer is used to make the Polymer
(example: ethylene (a) to form polyethylene)
a
a
a
a
a
a
a
And sometimes several monomers are used (example:
adipic acid (a) and 1,6-diaminohexane (b) to form nylon)
Polymerization
There are two methods we’ll quickly look at for
Forming Polymers.
- Addition polymerization
- Condensation polymerization
Addition Polymerization
-All the atoms present in the monomer are
retained in the polymer
-This type of reaction involves monomers with
double or triple bonds
-An initiator is required to produce a free radical
-A very reactive substance having a free e-Peroxides are typically used to produce this free radical
Peroxide  Rad
Free radical induced addition polymerization of
Ethylene to form polyethylene
Rad
Rad
Free radical induced addition polymerization of
Propylene to form polypropylene
Free radical induced addition polymerization of
Styrene to form polystyrene
Condensation Polymerization
- Monomers that join together by the loss of water
- each monomer has two functional groups that are
the same
- monomer 1 and monomer 2 functional groups
are different
- reaction occurs between the two pairs of
dissimilar functional groups
Condensation Polymerization - Dacron
Condensation Polymerization - Nylon
Substitution Reaction
In a substitution reaction, one atom, ion or
group is replaced (substituted) by another.
Steps in Radical Substitution
• Three types of steps
– Initiation – homolytic formation of two
reactive species with unpaired electrons
• Example – formation of Cl atoms form Cl2 and light
Steps in Radical Substitution
• Three types of steps
– Propagation – reaction with molecule to
generate radical
• Example - reaction of chlorine atom with methane to give
HCl and CH3.
Steps in Radical Substitution
• Three types of steps
– Termination – combination of two radicals to
form a stable product.
Substitution Reactions
are characteristic reaction of saturated
compounds such as alkanes.
CH4 + Cl2
CH3Cl + Cl2
CH2Cl2 + Cl2
CHCl3+ Cl2
CH3Cl + HCl
CH2Cl2 + HCl
CHCl3 + HCl
CCl4 + HCl
Radical Substitution
49
Electrophilic substitution
• An electron-deficient agent reacts with an
electron-rich substrate.
• The substrate retains the bonding electron
pair, a cation (proton) is removed:
R–X + E+  R–E + X+
.
Nucleophilic substitution
• Electron-rich nucleophile introduces an
electron pair into the substrate.
• The leaving atom/group retains the originally
bonding electron pair:
|Nu– + R–Y  Nu–R + |Y–
• This reaction is typical of haloalkanes:
alcohol is produced
• Nucleophiles: HS–, HO–, Cl–
Elimination Reactions
• Elimination: removal of atoms or group of atoms
from adjacent carbon to form a carbon-carbon
double bond.
• These are the opposite of additions
• This used to prepare alkenes
Addition and Elimination Reactions
• Addition and elimination reactions are exactly
opposite.
• A  bond is formed in elimination reactions, whereas
a  bond is broken in addition reactions.
Elimination reactions
• Any reaction in which atoms are eliminated
from another molecule
• This can be done by
– Elimination of H2
– Elimination of HX
– Elimination of H20
– Elimination of RH
Elimination Reactions
• In most cases, the two atoms/groups are removed
from the neighbouring carbon atoms and a double
bond is formed (-elimination)
• Elimination of water = dehydration – used to
prepare alkenes:
– H2O
• In biochemistry
e.g. in glycolysis:
2-phosphoglycerate
phosphoenolpyruvate
Elimination Reaction
Loss of H2O
a) The acid protonates the –OH group, water leaves
Positive carbon remains behind
b) An adjacent proton (H+) leaves next leaving the
electron pair to form the double bond
Elimination Reactions
Loss of H2
-This process is often referred to as
Dehydrogenation
H H
H-C-C-H

H2C=CH2 + H2
H H Heat, catalyst
Elimination Reactions
Loss of RH
H H
H-C-C-H
H R

H2C=CH2 + HR
Elimination Reactions
Loss of HX
-Alkyl halides can also undergo elimination.
-This is as known as dehydrohalogenation
HH
H-C-C-H
HX

H2C=CH2 + HX
Base (ex KOH)
The base extract a proton (H+) and X- leaves
Oxidation – Reduction Reaction
• Oxidation: the loss of electrons
– alternatively, the loss of H, the gain of O, or OH
• Reduction: the gain of electrons
– alternatively, the gain of H, the loss of O, or OH
Oxidation-Reduction
Reactions involve electron transfers
• Oxidation of biomolecules often occurs as
dehydrogenation, electron acceptor are needed for
such reactions to occur.
• Generally electron acceptors are coenzymes.
• Oxidation of Alcohols
– Oxidation of Primary Alcohols to Aldehydes
• A primary alcohol can be oxidized to an
aldehyde or a carboxylic acid
– The oxidation is difficult to stop at the aldehyde stage
and usually proceeds to the carboxylic acid
Oxidation and Reduction
hydrocarbons oxidation- reduction levels
Oxidation - Reduction
Oxidation - Reduction
Oxidation - Reduction
Rearrangement Reaction
67
Fermentation
• Fermentation is the process by which glucose is
broken down by an enzyme (a catalyst) in the
absence of oxygen into an alcohol and carbon
dioxide.
• One enzyme used is Zymase (Found in baker yeast)
– If Zymase is used the alcohol produced is ethanol
• The oldest chemical reaction practiced by man
– In place of glucose, starches from grains can be used.
Hence the name grain alcohol
C6H12O6
Glucose

Zymase
2C2H5OH + 2CO2
Ethanol
Carbon dioxide
Saponification
• The hydrolysis of the ester bonds (back to acid +
alcohol) in triglycerides using an aqueous sol’n of a
strong base to form carboxylate salts and glycerol
• Hydrolysis of fats by a strong base (KOH or NaOH)
– Products are soap and glycerol (a triol)
O
CH2-0-C-(CH2)14CH3
|
O
CH2-O-C-(CH2)14CH3 + 3KOH 
|
O
CH2-0-C-(CH2)14CH3
A TRIGYCERIDE
CH2-0H
|
CH2-OH
|
CH2-0H
+
O
K+ -O-C-(CH2)14CH3
O
K+ -O-C-(CH2)14CH3
O
K+ -O-C-(CH2)14CH3
GLYCEROL 3 SOAP MOLECULES
Combustion
A reaction in which a compound (often
carbon) reacts with oxygen
C + O2
CO2
CH4 + 2O2
CO2 + 2H2O
C3H8 + 5O2
3CO2 + 4H2O
C6H12O6 + 6O2
6CO2 + 6H2O