Organic chemistry-Some basic principles and techniques CHAPTER ORGANIC CHEMISTRY-SOME BASIC PRINCIPLES AND TECHNIQUES 12 LEARNING OBJECTIVES (i) Understand reasons for tetravalence of carbon and shapes of organic molecules. In organic compounds can be described in terms of orbitals hybridisation concept, according to which carbon can have sp3, sp2 and sp hybridised orbitals. The sp3, sp2 and sp hybridised carbons are found in compounds like methane, ethene and ethyne respectively. The tetrahedral shape of methane, planar shape of ethene and linear shape of ethyne can be understood on the basis of this concept. (ii) Write structures of organic molecules in various ways. Organic compounds can be represented by various structural formulas. The three dimensional representation of organic compounds on paper can be drawn by wedge and dash formula. (iii) Classify the organic compounds. Organic compounds can be classified on the basis of their structure or the functional groups they contain. A functional group is an atom or group of atoms bonded together in a unique fashion and which determines the physical and chemical properties of the compounds. (iv) Name the compounds according to IUPAC system of nomenclature and also derive their structures from the given names. The naming of the organic compounds is carried out by following a set of rules laid down by the International Union of Pure and Applied Chemistry (IUPAC). In IUPAC nomenclature, the names are correlated with the structure in such a way that the reader can deduce the structure from the name. (v) Understand the concept of organic reaction mechanism. Organic reactions involve breaking and making of covalent bonds. A covalent bond may be cleaved in heterolytic or homolytic fashion. A heterolytic cleavage yields carbocations or carbanions, while a homolytic cleavage gives free radicals as reactive intermediate. Reactions proceeding through heterolytic cleavage involve the complimentary pairs of reactive species. These are electron pair donor known as nucleophile and an electron pair acceptor known as electrophile. (vi) Explain the influence of electronic displacements on structure and reactivity of organic compounds. The inductive, resonance, electromeric and hyperconjugation effects may help in the polarisation of a bond making certain carbon atom or other atom positions as places of low or high electron densities. (vii) Recognise the types of organic reactions. Organic reactions can be broadly classified into following types; substitution, addition, elimination and rearrangement reactions (viii) Learn the techniques of purification of organic compounds. Understand the principles involved in quantitative analysis of organic compounds. Purification, qualitative and quantitative analysis of organic compounds are carried out for determining their structures. The methods of purification namely : sublimation, distillation and differential extraction are based on the difference in one or more physical properties. Chromatography is a useful technique of separation, identification and purification of compounds. It is classified into two categories : adsorption and partition chromatography. Adsorption chromatography is based on differential adsorption of various components of a mixture on an adsorbent. Partition chromatography involves continuous partitioning of the components of a mixture between stationary and mobile phases. After getting the compound in a pure form, its qualitative analysis is carried out for detection of elements present in it. Nitrogen, sulphur, halogens and phosphorus are detected by Lassaigne’s test. Carbon and hydrogen are estimated by determining the amounts of carbon dioxide and water produced. Nitrogen is estimated by Dumas or Kjeldahl’s method and halogens by Carius method. Sulphur and phosphorus are estimated by oxidising them to sulphuric and phosphoric acids respectively. The percentage of oxygen is usually determined by difference between the total percentage (100) and the sum of percentages of all other elements present. INTRODUCTION Organic chemistry is the study of the preparation, properties, identification, and reactions of those compounds of carbon not classified as inorganic. The latter include the oxide of carbon, the bicarbonates and carbonates of metal ions, the metal cyanides, and a handful of other compounds. There are several million known carbon compounds, and all but a very few are organic. Virtually all plastics, synthetic and natural fibers, dyes and drugs, insecticides and herbicides, ingredients in perfumes and flavouring agents and all petroleum products are organic compounds. All the foods you eat consist chiefly of organic compounds in the families of the carbohydrates, fats and oils, proteins, and vitamins. The substances that make up furs and feathers, hides and skins, and all cell membranes are also organic. Originally, the distinction between inorganic and organic substances was based on whether or not they were produced by living systems. For example, until the early nineteenth century, it was believed that organic compounds had some sort of “life force” and could only be synthesized by living organisms. The view was dispelled in 1828 when German chemist Friedrich Wohler prepared urea from the inorganic salt ammonium cyanate by simple heating : Gyaan Sankalp 1 Organic chemistry-Some basic principles and techniques Heat NH4 OCN H 2 N C NH 2 Ammonium cyanate || O Urea Since urea is a component of urine, it is clearly an organic material, yet here was clear evidence that it could be produced in the laboratory as well as by living things. STRUCTURE AND SHAPES OF ORGANIC MOLECULES : 1. Tetracovalency of carbon : The atomic number of carbon is 6 and it has four electrons in its valence shell. In order to acquire stable inert gas configuration, it can share its electrons with the electrons of other to form four covalent bonds. Thus, carbon has a covalency of four or is tetracovalent. In 1874, Vant Hoff and Le Bel predicted that the four bonds of carbon in methane and other saturated compounds do not lie in a plane but are directed towards the four corners of a regular tetrahedral. 109º28' C C A Regular Space Model of The Normal Direction Tetrahedron Carbon Atom of Valencies with 4similar Faces Figure : Tetrahedral representation of carbon valencies 2. Formation of organic molecules : We know that organic compounds are carbon compounds and carbon atom does not take part as such in bond formation but its 2s and 2p orbital (s) mixed together to form new orbitals known as hybrid orbitals and the process in turn is known as hybridization. The types of hybridizations encountered in organic compounds are sp3 (involved in saturated organic compounds containing only single covalent bonds, eg. methane), sp2 (involved in organic compounds having carbon linked by double bonds, e.g. ethylene) and sp (involved in organic compounds having carbon linked by a triple bond, e.g acetylenes). The bond angles and geometry associated with the three types of hybridization are summarized below : Hybridisation sp3 sp 2 sp Angle 109º28' 120º 180º Geometry Tetrahedral Trigonal Linear Example Alkane, Cycloalkane Alkenes Alkynes and all other and in saturated part and other compounds compounds containing of all organic molecules containing C=C, C=O, C C and C N triple bonds, C=N and C=S double bonds Bond Four- Three- Two- One- Two- s% 25 33.3 50 p% 75 66.7 50 Electronegativity 2.48 2.75 3.25 2 Gyaan Sankalp Organic chemistry-Some basic principles and techniques 3. Types of bonds : Organic compounds normally contain only two types of carbon–carbon bonds, i.e, –(sigma) and –(pi) (i) Sigma bond : (a) A carbon–carbon, –bond can be formed by overlap of two sp3, sp2 or sp–hybridized orbitals of carbon atoms. Alkanes and cycloalkanes contain only sp3–sp3 C—C, –bonds, alkenes contains sp2–sp2, C—C –bonds while alkynes contain sp—sp, C—C, –bonds. (b) In a similar way, carbon can form sigma bonds with hydrogen atoms, while alkanes contain only sp3–s, C – H, –bonds, alkenes contain sp2–s, C–H, –bonds and alkynes contain sp–s, C–H, –bonds. (ii) –bond : (a) A pi–bond is formed by sideways or lateral overlap of two p–orbitals, Thus, alkenes contain one sp2 — sp2, C – C, –bond and one –bond. (b) In alkynes, the carbon atoms are sp–hybridized. Therefore, alkynes contain one sp–sp, C – C, –bond and two –bonds which are mutually perpendicular to each other. (c) The two carbon atoms and two hydrogen atoms of acetylene molecule lie along a line with C – C bond angle of 180º. Thus, acetylene is a linear molecule. 4. Effect of hybridization on bond length and bond strengths : The bond length and bond strength of any bond depends upon the size of the hybrid orbitals involved. (i) Bond lengths : Since a p–orbital is much bigger in size than a s–orbital of the same shell, therefore, as we go from sp3 sp2 sp, the percentage of p–character decreases from 75 66.7 50%. Accordingly, the size of the orbital decreases in the same order : sp3 > sp2 > sp. Since a bigger orbital forms a longer bond, therefore, C–C single bond lengths decrease in the order: C(sp3) – C(sp3) > C (sp2) – C(sp2) > C(sp) – C(sp) 1.54 Å 1.34Å 1.20 Å (ii) Bond strengths : Shorter the bond, greater is its strength. Thus, the -bond formed by sp–hybridized carbon is the strongest (i.e. maximum bond energy) while that formed by sp³–hybridized carbon is the weakest (i.e. minimum bond dissociation energy). For example, C(sp) – H > C(sp2)–H > C(sp3)–H 121 kcal mol–1 106 kcal mol–1 98.6 kcal mol–1 C(sp) – C(sp) > C(sp2) – C(sp2) > C(sp3) – C(sp3) 200 kcal mol–1 142 kcal mol–1 80-85 kcal mol–1 Since the extent of overlap in sideways overlap is low, a carbon–carbon –bond is always weaker than a carbon–carbon –bond. A carbon–carbon double bond is, however, stronger than a carbon–carbon single bond since it consists –bond and a weak – bond.