Chapter 25 Organic Chemistry Part II Dr. Al-Saadi
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 CH3OCH3 CH3CH2COH Propanone Propanal Dr. Al-Saadi
Constitutional (structural) Isomers 25.4 Classes of Isomers Isomers Stereoisomers Geometrical Isomers Optical Isomers Constitutional (structural) Isomers Dr. Al-Saadi
Constitutional Isomerism 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
Constitutional Isomerism 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
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 Optical isomers cis-platin trans-platin Dr. Al-Saadi
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 Opposite side Same side Geometrical isomers Structural isomer Dr. Al-Saadi
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
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
Chiral molecules are optically active 25.4 Optical Isomers To represent chirality of molecules when drawing them on a piece of paper, we use wedges and dashes. 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. Chiral molecules are optically active Dr. Al-Saadi
Optical Activity of Enantiomers 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
Optical Activity of Enantiomers 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 plane-polarized Dr. Al-Saadi
Optical Activity of Enantiomers 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 Minimal transmission occurs when the plane of polarization of the light is perpendicular to that of the analyzer Solution of optically active enantiomers Dr. Al-Saadi
Optical Activity of Enantiomers 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
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: Addition reactions. Electrophilic addition. Nucleophilic addition. Substitution reactions. Electrophilic substitution. Nucleophilic substitution. SN1 and SN2 reactions. Dr. Al-Saadi
Electrophiles and Nucleophiles 25.5 Electrophiles and Nucleophiles An electrophile “loves electrons” is a species with a region of positive or partial positive charge, such as: cations (H+ ion). 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: anions (Cl‒ ion) , or the negative region of polar molecules like HCl. Nucleophiles are electron-rich. The keys to understand organic reactions Dr. Al-Saadi
Electrophilic Addition Reactions 25.5 Electrophilic Addition Reactions + HCl Where are the electron-rich and electron-poor areas? Nucleophile Electrophilic attack Electrophile Dr. Al-Saadi
Electrophilic Addition Reactions 25.5 Electrophilic Addition Reactions These curved pink arrows show the movement of electron pairs “Mechanism” intermediate Mechanism: 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. A lone pair on the electron-rich Cl‒ ion forms a new σ bond with the electron-deficient carbocation. The hybridization of the C atoms has changed from sp2 to sp3. Dr. Al-Saadi
Electrophilic Addition Reactions 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. intermediate Dr. Al-Saadi
Electrophilic Addition Reactions 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
Nucleophilic Addition Reactions 25.5 Nucleophilic Addition Reactions O H H H O C O + O O C O H Electrophile Nucleophile Where are the electron-rich and electron-poor areas? Mechanism: Nucleophilic attack. Stabilized by resonance Dr. Al-Saadi
Nucleophilic Addition Reactions 25.5 Nucleophilic Addition Reactions Breaking an O―H bond and forming H+ ion. New O―H bond formed leading to the product. Dr. Al-Saadi
25.5 Addition Reactions Addition reactions are reactions that involve the addition of a molecule or an ion to another molecule. Electrophilic addition. (species to be added is an electrophile) Nucleophilic addition. (species to be added is a nucleophile) Dr. Al-Saadi
25.5 Addition Reactions Addition reactions are reactions that involve the addition of a molecule or an ion to another molecule. Electrophilic addition. (species to be added is an electrophile) Nucleophilic addition. (species to be added is a nucleophile) Dr. Al-Saadi
Addition Reactions Exercise: 25.5 Addition Reactions Exercise: Draw the mechanism for the nucleophilic addition of CN ion to CH3CHO. Nucleophile Dr. Al-Saadi
Substitution Reactions 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
Electrophilic Substitution Reactions 25.5 Electrophilic Substitution Reactions Mechanism: Formation of the NO2+ electrophile. Electrophilic attack which breaks a π bond. Electrophile (E+) Dr. Al-Saadi Carbocation
Electrophilic Substitution Reactions 25.5 Electrophilic Substitution Reactions Carbocation is stabilized by resonance. The π bond is restored and H+ ion is produced. Dr. Al-Saadi
Electrophilic Substitution Reactions 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 Dr. Al-Saadi
Nucleophilic Substitution Reactions 25.5 Nucleophilic Substitution Reactions Nucleophilic substitution can be of two types: SN2 reactions: Direct nucleophilic back-side attack . In SN2 reactions the molecular configuration is inverted. transition state Dr. Al-Saadi
Nucleophilic Substitution Reactions 10.5 Nucleophilic Substitution Reactions The other type of nucleophilic substitution is: SN1 reactions: First, a group is leaving the C atom. Then, a nucleophile attacks the carbocation intermediate. Mechanism: A group is leaving and a carbocation intermediate is formed. Nucleophilic attack carbocation Dr. Al-Saadi
Nucleophilic Substitution Reactions 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. The produced two enantiomers which are mirror images of each other “racemization” Dr. Al-Saadi
Mechanisms of Substitution Reactions 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
Other Types of Organic Reactions 10.5 Other Types of Organic Reactions Elimination reactions are reactions in which a double bond forms and a small molecule is removed. Example is the following step in carbohydrate metabolism. + X‒ + H―Nu Dr. Al-Saadi
Other Types of Organic Reactions 10.5 Other Types of Organic Reactions Oxidation-reduction reactions are reactions that involve the loss and gain of electrons. One example is the conversion (or oxidation) of ethanol to acetaldehyde which takes place in liver. CH3CH2OH CH3CHO + H2 + H2 Gaining O or losing H oxidized Losing O or gaining H reduced Dr. Al-Saadi
Other Types of Organic Reactions 10.5 Other Types of Organic Reactions Isomerization reactions are the reactions in which one isomer is converted to another. An example is : Aldose Ketose C3O3H6 C3O3H6 Dr. Al-Saadi
Organic reactions Organic Reactions Substitution reactions 10.5 Organic reactions Organic Reactions Substitution reactions Electrophilic addition Nucleophilic addition SN1 reactions SN2 reactions Addition reaction Other reactions Elimination Oxidation-reduction Isomerization Dr. Al-Saadi
25.6 Organic Polymers Polymers are molecular compounds that are made up of many repeating units called monomers. Dr. Al-Saadi
25.6 Organic Polymers Polymers are molecular compounds that are made up of many repeating units called monomers. Polymers: can be natural or synthetic. have physical properties different than the properties of monomers. have a very high molar mass. contain monomers attached together by chemical bonds. Are very important in industrial and biological applications. Dr. Al-Saadi
Types of Organic Polymers 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
Addition Polymers Initiation step Propagation step Termination step 25.6 Addition Polymers Initiation step Propagation step Termination step Dr. Al-Saadi
Examples of Addition Polymers 25.6 Examples of Addition Polymers Dr. Al-Saadi
Condensation Polymers 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 Ester Alcohol Alcohol ether Dr. Al-Saadi
Condensation Polymers 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
25.6 Biological Polymers These are naturally occurring polymers, and they include: Proteins : polymers of amino acids. Polysaccharides : polymers of sugars. Nucleic acids : polymers of nucleotides. DNA (deoxyribonucleic acid). RNA (ribonucleic acid). Dr. Al-Saadi
Proteins Formation Protein formation. 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. Dr. Al-Saadi