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Chapter 25 ORGANIC CHEMISTRY Part II Dr. Al-Saadi 1 25.4 Isomerism Isomers are compounds with identical molecular formula but different structural formulas. For example, propanone (acetone) and propanal both have the same molecular formula: C3H6O but they have different structural formulas. Isomers Dr. Al-Saadi CH3OCH3 CH3CH2COH Propanone Propanal 2 25.4 Classes of Isomers Isomers Constitutional (structural) Isomers Stereoisomers Geometrical Isomers Dr. Al-Saadi Optical Isomers 3 25.4 Constitutional Isomerism Constitutional (structural) isomerism occurs when the same atoms can be connected in two or more different ways. Constitutional isomers have different names and different physical and chemical properties. C5H12 Dr. Al-Saadi 4 25.4 Constitutional Isomerism Exercise: What are the constitutional (structural) isomers of C2H2Cl2 (with C=C bond)? There are two constitutional (structural) isomers: These two drawings show one constitutional isomer because there is no breaking-reconnecting of bonds between atoms. These are rather called cis/trans isomers. Dr. Al-Saadi 5 25.4 Stereoisomerism Stereoisomers are compounds that contain identical bonds but differ in the orientation of those bonds in space. There are two kinds of stereoisomers: Geometrical isomers cis-platin Dr. Al-Saadi Optical isomers trans-platin 6 25.4 Geometrical Isomers Geometrical isomers occur in compounds that have a restricted rotation around a bond (a double or triple bond). They usually have different chemical and physical properties. C2H2Cl2 Same side Opposite side Geometrical isomers Dr. Al-Saadi Structural isomer 7 25.4 Optical Isomers Optical isomers are compounds that are mirror images of each other, but are not superimposable (not identical). They usually have similar chemical and physical properties. Your hands are mirror images of each other mirror But they are NOT superimposable Dr. Al-Saadi 8 25.4 Optical Isomers Such molecules are called chiral ; and a pair of mirror-image molecules are called enantiomers. Enantiomers have most of their chemical and physical properties to be identical. These two molecules are mirror image mirror But you will never be able to superimpose them on top of each other Dr. Al-Saadi 9 25.4 Optical Isomers To represent chirality of molecules when drawing them on a piece of paper, we use wedges and dashes. H COOH COOH C C R R NH2 Dr. Al-Saadi Chiral molecules are optically active H NH2 Chiral molecules exist as mirror images of each other, just as the right hand is a mirror image of the left hand (the word chiral comes from the Greek word for ‘hand’). Chiral molecules have at least one central carbon atom connected to four different groups/atoms. 10 25.4 Optical Activity of Enantiomers A pair of enantiomers rotate the plane of a plane-polarized light in opposite directions. What is plane-polarize light? Plane-polarized light oscillates only in a single plane, while ordinary light (that is not plane-polarized) oscillates in all directions. Dr. Al-Saadi 11 25.4 Optical Activity of Enantiomers A pair of enantiomers rotate the plane of a plane-polarized light in opposite directions. What is plane-polarize light? Plane-polarized light oscillates only in a single plane, while ordinary light (that is not plane-polarized) oscillates in all directions. Non plane-polarized Dr. Al-Saadi plane-polarized 12 25.4 Optical Activity of Enantiomers Measuring the rotation of polarized light by optical isomers. Plane of polarization gets rotated by 90 Plane-polarized light Solution of optically active enantiomers Dr. Al-Saadi Minimal transmission occurs when the plane of polarization of the light is perpendicular to that of the analyzer 13 25.4 Optical Activity of Enantiomers When the two enantiomers of CHFClBr molecule have a net rotation of zero, it means that the mixture of the two enantiomers is equimolar (same number of moles). Such a mixture is called a racemic mixture. Non plane-polarized plane-polarized Net rotation is zero Dr. Al-Saadi 14 25.5 Organic Reactions An organic reaction is the chemical process that involves the addition of an organic molecule or an ion to another organic molecule. Some examples of organic reactions: o Addition reactions. • Electrophilic addition. • Nucleophilic addition. o Substitution reactions. • Electrophilic substitution. • Nucleophilic substitution. – SN1 and SN2 reactions. Dr. Al-Saadi 15 25.5 Electrophiles and Nucleophiles An electrophile “loves electrons” is a species with a region of positive or partial positive charge, such as: + o cations (H ion). o the positive region of polar molecules (HCl). Electrophiles are electron-poor. A nucleophile “loves nucleus” is a species with a region of negative or partial negative charge, such as: o anions (Cl‒ ion) , or o the negative region of polar molecules like HCl. Nucleophiles are electron-rich. The keys to understand organic reactions Dr. Al-Saadi 16 25.5 Electrophilic Addition Reactions H Cl C C H H H H C C H + HCl H H Where are the electron-rich and electron-poor areas? H Nucleophile Electrophilic attack Electrophile Dr. Al-Saadi 17 25.5 Electrophilic Addition Reactions These curved pink arrows show the movement of electron pairs “Mechanism” intermediate Mechanism: 1. The π bond breaks and a new C―H bond is produced. As a result, the H―Cl bond also breaks and a Cl‒ ion is produced. 2. A lone pair on the electron-rich Cl‒ ion forms a new σ bond with the electron-deficient carbocation. 3. The hybridization of the C atoms has changed from sp2 to sp3. Dr. Al-Saadi 18 25.5 Electrophilic Addition Reactions Viewing the atomic orbitals in the previous addition reaction and how the hybridization of C atoms has changed. carbocation A carbocation is showing an sp2 hybridization which implies a planar structure. Dr. Al-Saadi 19 25.5 Electrophilic Addition Reactions Viewing the atomic orbitals in the previous addition reaction and how the hybridization of C atoms has changed. Dr. Al-Saadi 20 25.5 Nucleophilic Addition Reactions O C O Electrophile + H H O O O H O H C Nucleophile Where are the electron-rich and electron-poor areas? Mechanism: o Dr. Al-Saadi Nucleophilic attack. Stabilized by resonance 21 25.5 Nucleophilic Addition Reactions o Breaking an O―H bond and forming H+ ion. o New O―H bond formed leading to the product. Dr. Al-Saadi 22 25.5 Addition Reactions Addition reactions are reactions that involve the addition of a molecule or an ion to another molecule. Dr. Al-Saadi o Electrophilic addition. (species to be added is an electrophile) o Nucleophilic addition. (species to be added is a nucleophile) 23 25.5 Addition Reactions Addition reactions are reactions that involve the addition of a molecule or an ion to another molecule. Dr. Al-Saadi o Electrophilic addition. (species to be added is an electrophile) o Nucleophilic addition. (species to be added is a nucleophile) 24 25.5 Addition Reactions Exercise: Draw the mechanism for the nucleophilic addition of CN ion to CH3CHO. O H3C C H O + C N H3C C H C Nucleophile Dr. Al-Saadi N 25 25.5 Substitution Reactions A substitution reaction is the reaction during which one group is replaced by another group. Electrophilic substitution An electrophile (NO2+) attacks an aromatic molecule and replaces a hydrogen atom. Nucleophilic substitution A nucleophile (OH‒) attacks an electron-deficient C atom and replaces another group on that C atom. Dr. Al-Saadi 26 25.5 Electrophilic Substitution Reactions Mechanism: o Formation of the NO2+ electrophile. Electrophile (E+) o Dr. Al-Saadi Electrophilic attack which breaks a π bond. Carbocation 27 25.5 Electrophilic Substitution Reactions Dr. Al-Saadi o Carbocation is stabilized by resonance. o The π bond is restored and H+ ion is produced. 28 25.5 Electrophilic Substitution Reactions Exercise: Predict the product and give the mechanism for the following electrophilic substitution reaction: H + tert-butyl cation tert-butyl benzene H H H H + C(CH3)3 C(CH3)3 H H First step: + H H H H H H H C(CH3)3 H C(CH3)3 H H Second step: + + H Dr. Al-Saadi H H H H H+ H H 29 25.5 Nucleophilic Substitution Reactions Nucleophilic substitution can be of two types: o SN2 reactions: Direct nucleophilic back-side attack . In SN2 reactions the molecular configuration is inverted. transition state Dr. Al-Saadi 30 10.5 Nucleophilic Substitution Reactions The other type of nucleophilic substitution is: o SN1 reactions: First, a group is leaving the C atom. Then, a nucleophile attacks the carbocation intermediate. Mechanism: o A group is leaving and a carbocation intermediate is formed. carbocation o Dr. Al-Saadi Nucleophilic attack 31 10.5 Nucleophilic Substitution Reactions In SN1 reactions for a molecule with a chiral C atom (bonded to four different atoms/groups), a pair of enantiomers is formed. This is known as racemization. The carbocation intermediate is planar. Thus, nucleophilic attack could also take place from the opposite side of the carbocation. Dr. Al-Saadi The produced two enantiomers which are mirror images of each other “racemization” 32 10.5 Mechanisms of Substitution Reactions Electrophilic substitution Nucleophilic substitution (SN1) Nucleophilic substitution (SN2) Formation of carbocation intermediate Nucleophile attacking from back Dr. Al-Saadi 33 10.5 Other Types of Organic Reactions Elimination reactions are reactions in which a double bond forms and a small molecule is removed. + X‒ o Dr. Al-Saadi + H―Nu Example is the following step in carbohydrate metabolism. 34 10.5 Other Types of Organic Reactions Oxidation-reduction reactions are reactions that involve the loss and gain of electrons. o One example is the conversion (or oxidation) of ethanol to acetaldehyde which takes place in liver. CH3CH2OH CH3CHO + H2 + Gaining O or losing H oxidized Losing O or gaining H reduced Dr. Al-Saadi H2 35 10.5 Other Types of Organic Reactions Isomerization reactions are the reactions in which one isomer is converted to another. o An example is : Aldose Ketose C3O3H6 C3O3H6 Dr. Al-Saadi 36 10.5 Organic reactions Organic Reactions Substitution reactions Electrophilic addition Nucleophilic addition SN1 reactions Dr. Al-Saadi Addition reaction Electrophilic addition Nucleophilic addition Other reactions Elimination Oxidationreduction Isomerization SN2 reactions 37 25.6 Organic Polymers Polymers are molecular compounds that are made up of many repeating units called monomers. Dr. Al-Saadi 38 25.6 Organic Polymers Polymers are molecular compounds that are made up of many repeating units called monomers. Polymers: o can be natural or synthetic. o have physical properties different than the properties of monomers. o have a very high molar mass. o contain monomers attached together by chemical bonds. o Are very important in industrial and biological applications. Dr. Al-Saadi 39 25.6 Types of Organic Polymers Addition polymers form when monomers join end to end. Polyethylene is an example of addition polymer. Condensation polymers form when two different functional groups combine in an elimination reaction. Biological polymers are naturally occurring polymers. Dr. Al-Saadi 40 25.6 Addition Polymers Initiation step Propagation step Termination step Dr. Al-Saadi 41 25.6 Examples of Addition Polymers Dr. Al-Saadi 42 25.6 Condensation Polymers Condensation reactions are the basis of condensation polymers. In a condensation reaction, two or more molecules each with different functional group become connected with the elimination of a small molecule (often water). Carboxylic acid Alcohol Dr. Al-Saadi Alcohol Ester Alcohol ether 43 25.6 Condensation Polymers An example of condensation polymers is Nylon 66. Diamine Dicarboxylic acid New bond formed Monomer New bonds formed Dr. Al-Saadi 44 25.6 Biological Polymers These are naturally occurring polymers, and they include: o Proteins : polymers of amino acids. o Polysaccharides : polymers of sugars. o Nucleic acids : polymers of nucleotides. • DNA (deoxyribonucleic acid). • RNA (ribonucleic acid). Dr. Al-Saadi 45 20.6 Proteins Formation Protein formation. Peptide bonds are also called “amide linkages” since they contain an amide functional group. Very long chains are called proteins, while shorter chains are called polypeptides. o Dr. Al-Saadi 46