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 (sp 2 carbon atoms). Carbanions are nucleophiles because they contain a carbon with a lone pair (sp 3 ) carbon atoms. Understanding Organic Reactions Radicals, Carbocations and Carbanions

Lewis acids and bases Lewis base: acts as an electron-pair donor; e.g. ammonia: NH 3 Lewis acid: can accept a pair of electrons; e.g.: AlCl 3, FeCl 3, ZnCl 2. These compounds – important catalysts: generate ions that can initiate a reaction: CH 3 –Cl + AlCl 3  CH AlCl 4 -

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 synthesis of 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 + H 2 → alkane Unsaturated vegetable oils reduced to produce saturated fats used in margarine and cooking products Hydrogenation (addition of Hydrogen)

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 + │ │ CH 3 ─CH=CH─CH 3 + H─OH CH 3 ─CH─CH─CH 3

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) 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 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

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 Cl 2 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 CH 3.

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. CH 4 + Cl 2 CH 3 Cl + HCl CH 3 Cl + Cl 2 CH 2 Cl 2 + HCl CH 2 Cl 2 + Cl 2 CHCl 3 + HCl CHCl 3 + Cl 2 CCl 4 + 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 H 2 –Elimination of HX –Elimination of H 2 0 –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: – H 2 O 2-phospho- glycerate phosphoenol- pyruvate

Elimination Reaction 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 Loss of H 2 O

Elimination Reactions Loss of H 2 -This process is often referred to as Dehydrogenation H H H-C-C-H  H 2 C=CH 2 + H 2 H H Heat, catalyst

Elimination Reactions Loss of RH H H H-C-C-H  H 2 C=CH 2 + 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  H 2 C=CH 2 + 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-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

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 C 6 H 12 O 6  2C 2 H 5 OH + 2CO 2 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) O CH 2 -0-C-(CH 2 ) 14 CH 3 | O CH 2 -O-C-(CH 2 ) 14 CH 3 | O CH 2 -0-C-(CH 2 ) 14 CH 3 + 3KOH  O CH 2 -0H K + - O-C-(CH 2 ) 14 CH 3 | O CH 2 -OH K + - O-C-(CH 2 ) 14 CH 3 | O CH 2 -0H K + - O-C-(CH 2 ) 14 CH 3 A TRIGYCERIDE GLYCEROL 3 SOAP MOLECULES +

Combustion A reaction in which a compound (often carbon) reacts with oxygen C + O 2 CO 2 CH 4 + 2O 2 CO 2 + 2H 2 O C 3 H 8 + 5O 2 3CO 2 + 4H 2 O C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O