SEMESTER III Course Code: CHE3C03 Complementary Course III: ORGANIC CHEMISTRY Module I: Organic Chemistry – Some Basic Concepts (9 hrs) Introduction: Origin of organic chemistry – Uniqueness of carbon – Homologous series – Nomenclature of alkyl halides, alcohols, aldehydes, ketones, carboxylic acids and amines. Structural isomerism: Chain isomerism, position isomerism, functional isomerism and metamerism. Hybridisation in organic molecules (a brief study) - Curved arrow formalism - Homolysis and heterolysis of bonds – Electrophiles and nucleophiles. Electron Displacement Effects: Inductive effect: Definition - Characteristics - +I and -I groups. Applications: Explanation of substituent effect on the acidity of aliphatic carboxylic acids. Mesomeric effect: Definition – Characteristics - +M and -M groups. Applications: Comparison of electron density in benzene, nitrobenzene and aniline. Hyperconjugation: Definition – Characteristics. Example: Propene. Applications: Comparison of stability of 1-butene & 2-butene. Electromeric effect: Definition - Characteristics - +E effect (addition of H+ to ethene) and -E effect (addition of CN- to acetaldehyde). Steric effect (causes and simple examples). Reaction Intermediates: Carbocations, carbanions and free radicals (types, hybridization and stability). Carbon is the only element that can form so many different compounds because each carbon atom can form four chemical bonds to other atoms, and because the carbon atom is just the right, small size to fit in comfortably as parts of very large molecules. Having the atomic number 6, every carbon atom has a total of six electrons. Two are in a completed inner orbit, while the other four are valence electrons — outer electrons that are available for forming bonds with other atoms. The carbon atom's four valence electrons can be shared by other atoms that have electrons to share, thus forming covalent (shared-electron) bonds. They can even be shared by other carbon atoms, which in turn can share electrons with other carbon atoms and so on, forming long strings of carbon atoms, bonded to each other. Carbon's ability to form long carbon-to-carbon chains is the first reason that there can be so many different carbon compounds; a molecule that differs by even one atom is, of course, a molecule of a different compound. The second reason for carbon's astounding compound-forming ability is that carbon atoms can bind to each other not only in straight chains, but in complex branchings. They can even join "head-to-tail" to make rings of carbon atoms. There is practically no limit to the number or complexity of the branches or the number of rings that can be attached to them, and hence no limit to the number of different molecules that can be formed. The third reason is that carbon atoms can share not only a single electron with another atom to form a single bond, but it can also share two or three electrons, forming a double or triple bond. This makes for a huge number of possible bond combinations at different places, making a huge number of different possible molecules The fourth reason is that the same collection of atoms and bonds, but in a different geometrical arrangement within the molecule, makes a molecule with a different shape and hence different properties. These different molecules are called isomers. The fifth reason is that all of the electrons that are not being used to bond carbon atoms together into chains and rings can be used to form bonds with atoms of several other elements. The most common other element is hydrogen, which makes the family of compounds known as hydrocarbons. But nitrogen, oxygen, phosphorus, sulphur, halogens, and several other kinds of atoms can also be attached as part of an organic molecule. There is a huge number of ways in which they can be attached to the carbon-atom branches, and each variation makes a molecule of a different compound. These atoms when attached to the rest of the carbon frame work are known as a functional groups and it defines the chemical and physical properties of that particular molecule. The sixth reason is that carbon has the ideal atomic size to form chemical bonds with even larger sized atoms to be part of large molecules yet avoid considerable steric effect in it. This enables carbon to make large but stable complex structured molecules. Catenation: Catenation is the binding of an element to itself through covalent bonds to form chain or ring molecules. Carbon is the most common element that exhibits catenation. It can form long hydrocarbon chains through the formation large number of C-C bonds and even cyclic structures possible if a head to tail bond formation has taken place inside the molecule. carbon is not the only one with catenation property. Silicon shows it to quite a good extent, sulphur and boron has also been shown to catenate. Carbon has highest degree of catenation because: high C-C bond energy tetravalency (large number of bonds) small atomic size hence less diffused orbital Broad Classification of organic compounds There are a large number of organic compounds and therefore a proper systematic classification was required. Organic compounds can be broadly classified as acyclic (open chain) or cyclic (closed chain). Moving on to their classification in detail: Acyclic or open chain compounds: Organic compounds in which all the carbon atoms are linked to one another to form open chains (straight or branched) are called acyclic or open chain compounds. These may be either saturated or unsaturated. These compounds are also called as aliphatic compounds. Alicyclic or closed chain or ring compounds: These are cyclic compounds which contain carbon atoms connected to each other in a ring (homocyclic). When atoms other than carbon are also present then it is called as heterocyclic. They exhibit some properties similar to aliphatic compounds. Examples of this type are as follows: Aromatic compounds: These compounds consist of at least one benzene ring, i.e., a six-membered carbocyclic ring having alternate single and double bonds. Generally, these compounds have some fragrant odour and hence, named as aromatic (Greek word aroma meaning sweet smell).Similar to alicyclic, they can also have hetero atoms in the ring. Such compounds are called as heterocyclic aromatic compounds. Some of the examples are as follows: Benzenoid aromatic compounds Non-benzenoid aromatic compounds: There are aromatic compounds, which have structural units different from benzenoid type and are known as Non-benzenoid aromatics e.g. Tropolone, Tropolone Heterocyclic aromatic compounds: When atoms of more than one kind make up the ring in the compounds, they are known as heterocyclic compounds or heterocycles. In these compounds generally one or more atoms of elements such as nitrogen 'N', oxygen 'O', or sulphur 'S' are present. The atom other than that of carbon viz., N, O or S, present in the ring is called hetero atom. Heterocyclic compounds with five and six atoms in the ring are termed as five-membered, and six- membered heterocycles respectively. Hydrocarbons can be further classified into four types on the basis of their structures. These are: Alkanes: Hydrocarbons that contain only C-C single bonds in their molecules are called alkanes. These include open chain as well as closed chain (cyclic) hydrocarbons. For example, ethane, propane cyclopentane.Alkanes are further divided into: Open chain or acyclic (simple alkanes not having any closed chains). They have the general formula CnH2n+2. Examples are methane(CH4), propane(C3H8) and butane(C4H10). Cycloalkanes or cyclic alkanes (having a closed chain or rings in their molecules). They have the general formula CnH2n. Examples are cyclopropane(C3H6) and cyclobutane(C4H8). Alkenes: These are hydrocarbons that contain at least one carbon-carbon double bond. For example, ethene, but-2-ene, but-1-ene. Alkynes: These hydrocarbons contain at least one carbon-carbon triple bond. For example, ethyne, propyne. Arenes: These are hydrocarbons that contain at least one special type of hexagonal ring of carbon atoms with three double bonds in their alternate positions. The ring is called aromatic or benzene ring. For example, benzene, toluene, o-xylene. They also contain more than one benzene rings. For example, naphthalene (2 rings) and anthracene (3 rings). Hydrocarbons can also be classified into: Saturated hydrocarbons: Those that contain carbon-carbon single bonds e.g. alkanes Unsaturated hydrocarbons; Those that contain carbon-carbon double or triple bonds e.g. alkenes(C=C), alkynes(C=C). Classification of organic compounds based on functional groups Functional group: A specific grouping of elements that is characteristic of a class of compounds, that give a compound certain physical and chemical properties. A functional group is a specific group of atoms or bonds within a compound that is responsible for the characteristic chemical reactions of that compound. The same functional group will behave in a similar fashion, by undergoing similar reactions, regardless of the compound of which it is a part. Functional groups also play an important part in organic compound nomenclature; combining the names of the functional groups with the names of the parent alkanes provides a way to distinguish compounds. The atoms of a functional group are linked together and to the rest of the compound by covalent bonds. The first carbon atom that attach to the functional group is referred to as the alpha carbon; the second, the beta carbon; the third, the gamma carbon, etc. Similarly, a functional group can be referred to as primary, secondary, or tertiary, depending on if it is attached to one, two, or three carbon atoms. Homologous Series A homologous series is a group of organic compounds (compounds that contain C atoms) that differ from each other by one methylene (CH2) group. For example, methane, ethane, and propane are part of a homologous series. The only difference among these molecules is that they have different numbers of CH2 groups.Each member of a homologous series is called a homologue.