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

• 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.

Writing Equations for Organic Reactions:

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.

Writing Equations for Organic Reactions:

Different ways of writing organic reactions

Understanding Organic Reactions

Writing Equations for 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+)

Three reactive intermediates resulting from homolysis and heterolysis of a C – Z bond

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.

Understanding Organic ReactionsRadicals, Carbocations and Carbanions

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 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 Reactions

Addition Reaction

25

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.

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:

• Nucleophiles – water, alcohols, carbanions – form a covalent bond with the carbon atom of the carbonyl group:

used for synthesisof alcohols

– carbon atom carries +

Hemiacetals• Addition of alcohol to the carbonyl group yields hemiacetal:

• As to biochemistry, hemiacetals are formed by monosaccharides:

glucose

hemiacetals

hemiacetal

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

Hydrogenation(addition of Hydrogen)

HC CH + 2H2Ni

HC CH

H H

H H

H2C CH2

H HPt

H2H2C CH2 +

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 OH

H+ │ │CH3─CH=CH─CH3 + 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

PolymersThe 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)

And sometimes several monomers are used (example:adipic acid (a) and 1,6-diaminohexane (b) to form nylon)

a a a a a a a

Polymerization

There are two methods we’ll quickly look at forForming 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 ReactionIn 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 + HCl

CH3Cl + Cl2 CH2Cl2 + HCl

CH2Cl2 + Cl2 CHCl3 + HCl

CHCl3+ Cl2 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:

• Nucleophiles: HS–, HO–, Cl–alcohol is produced

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 are exactly opposite.

• A bond is formed in elimination reactions, whereas a bond is broken in addition reactions.

Addition and Elimination 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:

• In biochemistry e.g. in glycolysis:

– H2O

2-phospho-glycerate

phosphoenol-pyruvate

Elimination Reaction

a) The acid protonates the –OH group, water leaves Positive carbon remains behindb) An adjacent proton (H+) leaves next leaving the electron pair to form the double bond

Loss of H2O

Elimination ReactionsLoss 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 H2C=CH2 + HR H R

Elimination Reactions

Loss of HX-Alkyl halides can also undergo elimination. -This is as known as dehydrohalogenation

H H H-C-C-H H2C=CH2 + HX H X 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-ReductionReactions 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 Reductionhydrocarbons 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 2C2H5OH + 2CO2

Glucose Ethanol Carbon dioxide

Zymase

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)

OCH2-0-C-(CH2)14CH3

| OCH2-O-C-(CH2)14CH3

| OCH2-0-C-(CH2)14CH3

+ 3KOH

OCH2-0H K+ -O-C-(CH2)14CH3

| OCH2-OH K+ -O-C-(CH2)14CH3

| OCH2-0H K+ -O-C-(CH2)14CH3

A TRIGYCERIDE 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