ORGANIC CHEMISTRY Organic Chemistry • Organic chemistry is the branch of chemistry in which covalent carbon compounds and their reactions are studied. • A wide variety of classes of compounds such as vitamins, drugs, natural and synthetic fibres, as well as carbohydrates, peptides, and fats consist of organic molecules. Organic Chemistry • Organic chemists determine the structures of organic molecules, study their various reactions, and develop procedures for the synthesis of organic substances. • An organic compound is a compound that contains carbon and, in nearly all cases, also contains hydrogen in a covalently bonded structure. • Other elements such as oxygen, nitrogen , sulphur and chlorine are also present, but it is carbon that is the key. Organic Chemistry • Carbon is able to form a large number of compounds than all the other elements put together. • This is because carbon forms four strong covalent bonds with other carbon atoms or with other elements, especially hydrogen. • Carbon’s ability to link to itself to form chains and rings, known as catenation, is one of the main reasons for the vast number of organic compounds that exist. • The simplest organic chemicals, called hydrocarbons, contain only carbon and hydrogen atoms. Classification of Organic Compounds Homologous series: • Organic compounds are classified into families of compounds known as homologous series. • The members of each homologous series possess certain common features, which are described below Types of homologous series 1. 2. 3. 4. 5. 6. Alkanes Alkenes Arenes Alkynes Halogen alkanes Alcohols (including primary, secondary and tertiary) and phenols, aldehydes and ketones 7. Carboxylic acids and esters 8. Amines (primary only), amides and amino acids. Properties of homologous series 1. Successive members of a homologous series differ by a – CH2 – group. This means that the molecular mass increases by a fixed amount as we go up a series. 2. Members of a homologous series can be represented by the same general formula. Other homologous series are charecterised by the presence of a particular functional group, for example the homologous series of alcohols possess the functional group –OH– 3. Members of a homologous series show a gradation in the physical properties. This is because the carbon chains become longer as we go up a series. 4. Members of a series have similar chemical properties. Nomenclature for organic compounds: IUPAC system • For over a hundred years chemists have recognised the need for a specific set of rules for the naming of organic compounds. • IUPAC names are logically based on the chemistry of the compounds, and so give information about the functional groups present and the size of the molecules. Nomenclature for organic compounds: IUPAC system Rule 1: Identify the longest straight chain of carbon atoms. • The longest chain of carbon atoms gives the stem of the name as follows. Number of carbon atoms in longest chain Stem in IUPAC name Example of compound 1 meth- CH4, methane 2 eth- C2H6, ethane 3 prop- C3H8, propane 4 but- C4H10, butane 5 pent- C5H12, pentane 6 hex- C6H14, hexane 7 hept- C7H16, heptane 8 oct- C8H18, octane Nomenclature for organic compounds: IUPAC system Rule 2: Identify the functional group • The functional group usually determines the specific ending or suffix to the name. • The suffixes for some functional groups are shown below. Class Functional group alkane - alkene β βπΆ β = πΆβ Name of functional Suffix in group IUPAC name Example of compound General formula - -ane C2H6, ethane CnH2n+2 alkenyl -ene π»2 πΆ = πΆπ»2 , ethene CnH2n alkyne −πΆ ≡ πΆ − alkynyl -yne π»πΆ ≡ πΆπ» CnH2n-2 alcohol −ππ» hydroxyl -anol πΆπ»3 πΆπ»2 ππ», ethanol CnH2n+1OH aldehyde β«½π −πΆ\π» aldehyde(carbonyl) -anal πΆ2 π»5 πΆπ»π, propanal R – CHO carbonyl -anone πΆπ»3 πΆππΆπ»3 , propanone R – CO – R’ ketone π= /π πΆ\π ′ carboxylic acid β«½π −πΆ\π−π» carboxyl -anoic πΆπ»3 πΆπππ», ethanoic acid CnH2n+1COOH ester β«½π −πΆ\π−π ester -anoate πΆ2 π»5 πΆπππΆπ»3 , methyl propanoate R – COO – R’ O amide C carboxyamide -anamide πΆ2 π»5 πΆπππ»2 , propanamide amine -anamine πΆ2 π»5 ππ»2 , ethanamine phenyl -benzene πΆ6 π»5 πΆπ»3 , methylbenzene H N amine H −ππ»2 arene πΆ6 π»5− Nomenclature for organic compounds: IUPAC system Rule 3: Identify the side chains or substituent groups • Side chains or functional groups in addition to the one used as the suffix, are known as substituents and are given as the first part or prefix of the name. Class Functional group Name of functional group Prefix in IUPAC name Example of compound alkane - - methyl, ethyl, propyl, etc. πΆπ»3 πΆπ» πΆπ»3 πΆ2 π»5 , 2 – methylbutane πΆπ» πΆ2 π»5 3 , 3 – ethylpentane πΆπ» πΆ3 π»7 3 , 4 – propylheptane halogenoalkane - F , - Cl, - Br, -I halogeno flouro, chloro, bromo, iodo πΆ2 π»5 πΆπ, chloroethane πΆπ»3 πΆπ»π΅ππΆπ»3 , 2 - bromopropane amine - NH2 amine amino πΆπ»2 ππ»2 πΆπππ», 2 – aminoethanoic acid Amines • Some examples of amines are given in the table below: Compound methylamine ethylamine propylamine butylamine formula CH3 – NH2 CH3 – CH2 – NH2 CH3 – CH2 – CH2 – NH2 CH3 – CH2 – CH2 – CH2 – NH2 HALOALKANES Production of haloalkanes from alkanes MANUFACTURE OF CHLOROALKANES FROM ALKANES • Chloroalkanes can be made from alkanes in a reaction known as a free radical substitution reaction. • e.g. CH4 + Cπ2 → CH3 Cl + HCl • A substitution reaction is one in which one atom or group of atoms on the organic molecule is directly replaced by another. MANUFACTURE OF CHLOROALKANES FROM ALKANES • In this reaction the H atom on the alkane is replaced by a Cl atom. • Since all the carbon atoms in alkanes are attached to four other atoms, it is not possible to add another species to an alkane unless another species leaves. • Thus addition reactions are not possible and alkanes can undergo substitution reactions only. • A free radical is a species which contains an unpaired electron. • Free radicals are very reactive species and will bond with almost anything they come into contact with. • In this reaction the free radical which starts the reaction is a chlorine atom, formed when a chlorine molecule breaks up in the presence of ultra-violet light: • Free radicals are represented by a dot next to the atom containing an unpaired electron. • They are caused by homolytic fission of covalent bonds. • Homolytic fission is the breaking of a covalent bond in such a way that one electron goes to each atom. Mechanism 1: free radical substitution • Free radical substitution reactions proceed in three stages: initiation, propagation and then termination. 1.Initiation • Initiation is the production of free radicals by homolytic fission of a covalent bond. • Most bonds do not undergo homolytic fission under normal conditions. Generally, UV light is required. • UV 2.Propagation • Propagation is the reaction of a free radical with a molecule to produce another free radical. • The reaction involves two propagation steps: 3.Termination • Termination is the combination of two free radicals to form a single molecule. • Note that the Cl radicals are recycled during the propagation steps, so the reaction only requires a very small number of Cl-Cl bonds to undergo fission for the reaction to proceed. By-products in free radical substitution: • Chloromethane is not generally the only organic product of this reaction. • Free radical reactions tend to result in a variety of different products. 1.Different propagation steps • The propagation steps can continue beyond the formation of methane, and can result in the formation of dichloromethane, trichloromethane or tetrachloromethane: • • These further propagation steps are likely if excess chlorine is used. • If excess methane is used, then chloromethane is likely to be the major product. 2.Different termination steps • It also possible to get other products from alternative termination steps: • • Thus when methane reacts with chlorine, a variety of products are formed including chloromethane, dichloromethane and ethane. • In order to ensure that chloromethane is the major product, it is important to use excess methane.
