Che 176 Organic Chemistry 1 PDF

CHE 176-ORGANIC CHEMISTRY 1 (BASIC CONCEPTS OF ORGANIC CHEMISTRY) DR. GANIYAT K. OLOYEDE OFFICE: B18, DEPARTMENT OF CHEMISTRY ,  COURSE LECTURERS (2020/2021) PROF. O. O. SONIBARE DR. GANIYAT K. OLOYEDE DR. DORCAS O. MORONKOLA DR. SHERIFAT A. ABOABA...

CHE 176-ORGANIC CHEMISTRY 1 (BASIC CONCEPTS OF ORGANIC CHEMISTRY) DR. GANIYAT K. OLOYEDE OFFICE: B18, DEPARTMENT OF CHEMISTRY ,  COURSE LECTURERS (2020/2021) PROF. O. O. SONIBARE DR. GANIYAT K. OLOYEDE DR. DORCAS O. MORONKOLA DR. SHERIFAT A. ABOABA  COURSE OUTLINE CHECK THE FACULTY PROSPECTUS Lecture outline - Dr.G. K. Oloyede SYNTHESIS, REACTIONS AND USES OF ALKANES INCLUDING PETROLEUM, ALKENES, ALKYNES, BENZENE AND AROMATIC COMPOUNDS;  CHE CHEMISTRY OF ALKANES Organic compounds which contain only C and H are called Hydrocarbons. On the basis of structure, hydrocarbons are divided into two main classes – aliphatic and aromatic. Aliphatic hydrocarbons are further divided into alkanes, alkenes, alkynes and their cyclic analogous, viz:  Hydrocarbons Aliphatic Aromatic Alkanes Alkenes Alkynes Cyclic Aliphatic(Alicyclic)  ALKANES Alkanes are compounds of C and H which do not possess double bonds, triple bonds or rings. The general formula is CnH2n+2. NOMENCLATURE OF ALKANES The most definitive set of organic nomenclature rules currently in use were evolved through several international conferences and are known as International Union of Pure and Applied Chemistry Rules (IUPAC rules). The old trivial names derived from possible sources of the organic compounds e.g. acetic acid from vinegar or formic acid from ants, (formica in latin) are now being gradually discharged because of  IUPAC NOMENCLATURE (a) These names are based on the “straight” or “continuous chain” in which one or more carbon atoms are linked as in normal alkanes: CnH2n+2 The table below shows the names and formulae of the first twenty alkanes and representative alkanes up to C100: n Name Formulae n Name Formula 1 Methan CH4 11 Undecane CH3(CH2)9CH3 e 2 Ethane CH3CH3 12 Dodecane CH3(CH2)10CH3 3 Propane CH3CH2CH3 13 Tridecane CH3(CH2)11CH3 4 Butane CH3CH2CH2CH3 14 Tetradecan CH3(CH2)12CH3 e 5 Pentane CH3CH2CH2CH2CH3 15 Pentadecan CH3(CH2)13CH3 e 6 Hexane CH3CH2CH2 CH2CH2CH3 20 Eicosane CH3(CH2)18CH3 7 Heptan CH3CH2CH2 CH2 30 Triacontane CH3(CH2)28CH3 e CH2CH2CH3 8 Octane CH3(CH2)6CH3 40 Tetracontan CH3(CH2)38CH3 e 9 Nonane CH (CH ) CH 50 Pentaconta CH (CH ) CH (b) In the case of ‘branched chains’, the longest continuous chain of carbon atoms is taken as the parent hydrocarbon e.g. CH3 3 CH3-CH2-CH-CH3 1 2 4 2,3-dimethyl pentane CH2 5 CH3 A  The parent hydrocarbon is then numbered starting from the end of the chain, and the substituent groups are assigned numbers corresponding to their positions on the chain. The direction of numbering is chosen to give the lowest numbers to the side-chain substituents. Thus, A above is 2,3-dimethyl pentane and not 3,4-dimethyl pentane  CH3 3 CH3-CH2-CH-CH3 5 4 2 3,4-dimethylpentane CH2 1 CH3 The prefixes used to designate the number of substituents follow up to ten: 1-mono, 2-di, 3-tri, 4-tetra, 5- penta, e.t.c. (d) Where there are two identical substituents at one position, numbers are supplied for each and the prefix di-, tri- e.t.c is included to signify the number of groups of the same kind e.g. CH3 CH3 CH3-C CH-CH3 2,2,3-trimethyl butane CH3 (e) If there are several different alkyl groups attached to the parent chain, they are named in order of increasing size or alphabetical order e.g. CH3-CH2 CH3 CH3-CH2-CH2-CH2-CH2-CH2-CH3 7 6 5 4 3 2 1 4-ethyl-3-methyl heptane Note: ethyl is cited before methyl Note: Parentheses are used to separate the numbering of the substituents and of the main hydrocarbon chain.  PHYSICAL PROPERTIES OF ALKANES (CONCEPT OF HOMOLOGY) The series of straight chain alkanes in which n is the number of carbons in the chain shows a remarkably smooth graduation of physical properties. As ‘n’ increases, each additional CH2 group contributes a fairly constant increment to the boiling point and density and to a lesser extent to the melting point. This makes it possible to estimate the properties of unknown members of the series from those of its neighbours.  For example, the boiling points of hexane and heptane are 69 oC and 98 o C respectively. Thus, the difference in the structure of one CH2­ group is 29 oC in boiling point. We could predict the boiling point of the next higher member octane to be (98 + 29) oC = 127 oC which is close to the actual boiling point of 126 oC.  Members of a group of compounds such as the alkanes that have similar chemical structure and graded physical properties which differ from one another by the number of atoms in the structural backbone are said to constitute a “homologous series”. When used to forecast the properties of unknown members of the series, the concept of homology works most satisfactorily for the higher molecular weight members because the introduction of additional CH2 groups makes a smaller relative change in the overall composition of such molecules.  PETROLEUM AND SOURCES OF ALKANES As a class, alkanes generally are unreactive. The names saturated hydrocarbons or “paraffin” which literarily means “enough affinity” [Latin: par(um) enough, and affins, affinity] arise because their chemical “affinity” for most common reagents may be regarded as ‘saturated’ or satisfied. Thus, none of the C-H or C-C bonds in a typical saturated hydrocarbon, e.g. ethane is attacked at ordinary temperatures by a strong acid such as H2SO4 or by an oxidizing agent such as bromine in the dark, oxygen or KMnO4. Under ordinary conditions, ethane is similarly stable to reducing agents such as hydrogen, even in the presence of catalysts such as Pt, Pd or Ni.  However, all saturated hydrocarbons are attacked by oxygen at elevated temperatures and if oxygen is in excess, complete combustion to carbon dioxide and water occurs. Thus, vast quantities of hydrocarbons from petroleum are utilized as fuels for the production of heat and power by combustion. Petroleum on distillation yields the following fractions: 1. Gas fraction: (Boiling point up to 40o), contains normal and branched alkanes from C1 to C5. Natural gas is mainly methane and ethane. Bottled gas (liquefied petroleum gas) is mainly propane and butane. This fraction represents 11% of daily use of petroleum. 2. Gasoline: (Boiling point from 40o to 180oC), contains mostly hydrocarbons from C6 to C10. Over 100 compounds have been identified in gasoline and these include continuous chain and branched alkanes, cycloalkanes and alkylbenzenes (arenes). The branched alkanes make better gasoline than their continuous isomers because they give less ‘knock’ in high compression gasoline engines. Gasoline constitutes 39% of daily consumption of petroleum. 3. Kerosene: (Boiling point 180o – 230oC), contains hydrocarbons from C11 to C12. Much of this fraction is utilized as jet fuels or is cracked to simpler alkanes and alkenes. It represents only 6 % of daily use. 4. Light gas oil: (Boiling point 230o to 305oC, C13 to C17 hydrocarbons) They are utilized as diesel and furnace fuels. Of these uses, heating oil represents 14 %, fuels for industrial power plants 6 %, for trains, ships and diesel trucks 6 % and electric generating plants 6 % giving a total daily use of 26 %. 5. Heavy gas oil and light lubricating distillate: (Boiling point 305o – 405oC). A total daily use of 8 %. 6. Lubricants: (Boiling point 405 – 515oC, C26 to C35 hydrocarbons). Familiarly encountered as paraffin wax and petroleum jelly (Vaseline). Daily consumption amounts to 8 %. 7. The distillation residues are known as asphalt and they are used in roofing and road building. In addition to being used directly as just described, certain petroleum fractions are converted into other kinds of chemical compounds. Catalytic isomerisation changes straight chain alkanes into branched-chain ones. The cracking process converts higher alkanes into smaller alkanes and alkenes and thus increases the gasoline yield, it can even be used for the production of natural gas. In addition, the alkenes formed are perhaps the most important raw  PETROL AND SULPHUR Sulphur is usually an undesirable component in crude petroleum. It affects the smooth combustion of petrol in the internal combustion engines because it deactivates tetraethyl lead. It is usually removed during refining. The Nigerian crude petroleum has low sulphur content.  KNOCKING, OCTANE NUMBER AND TETRAETHYL LEAD [(C2H5)4Pb] Knocking arises when a fuel burns in an internal combustion engine leaving residual proportion which burns with difficulty. The engine cuts off making a metallic sound. Knocking is diminished by adding tetraethyl lead. The ‘octane’ rating for petrol involves a scale on which 2,2,4-trimethyl pentane (isooctane) is the standard for good anti knock behavior (100) and heptane is the standard for poor fuel (0). Hence, octane number in petrol refers to the % of isooctane and tetraethyl lead often increases the octane number but causes pollution of the environment. CH3 H CH3 C CH2 C CH3 CH3 CH3 2,2,4-trimethylpentane (isooctane)  ORIGIN OF PETROLEUM Petroleum is considered to arise from dead marine organisms which become buried under rock sediments some hundred million years ago. The effect of temperature and pressure on the organic compounds of these organisms led to their decomposition and maturation to form hydrocarbons.  PREPARATION OF ALKANES Each of the smaller alkanes from methane through n-pentane and isopentane can be obtained in pure form by fractional distillation of petroleum and natural gas. Above the pentanes, the number of isomers of each homolog becomes so large and the boiling point differences become so small that it is no longer feasible to isolate individual compounds. These alkanes must now be synthesized by one of the methods outlined below:  (1). Hydrogenation of alkenes General ly: H2 + Pt CnH2n CnH2n + 2 Pd or Ni Alkene Catalysts Alkane This is the most important method for the preparation of alkanes with the alkenes reacting with H2 under a slight pressure and in the presence of a catalyst. The alkene is converted smoothly and quantitatively into alkane of the same carbon skeleton.  Since the volume of hydrogen consumed can be easily measured, hydrogenation is frequently used as an analytical tool for example to determine the number of double bonds in a compound. Although hydrogenation is an exothermic reaction, it proceeds at a negligible rate in the absence of a catalyst even at elevated temperatures. The function of the catalyst is to lower the energy of activation so that the reaction can proceed rapidly at room  (2). Reduction of alkyl halides (a) Hydrolysis of Grignard Reagent Grignard Reagent is an organo-metallic compound formed from the reaction of magnesium with an alkyl halide in dry ether. e.g. CH3I + Mg ether CH3MgI methyl iodide methylmagnesium iodide OR generally RX + Mg ether RMgX (X = halogen) Grignard Reagents react with water to form an alkane. RMgX + HOH R-H + Mg(OH)X i.e the halogen atom is simply replaced by H in the overall reaction, e.g. Mg H2 O CH3 CH2 CH CH3 CH3 CH2 CH CH3 CH3 CH2 CH CH3 Br MgBr H  (b) Reduction by metal and acid RX + Zn + H RH + Zn + X Zn E.g, CH3 CH2 CH CH3 CH3 CH2 CH CH3 H Br H 2-Bromobutane Butane  2. Halogenations of Alkanes Under the influence of ultraviolet light at 250 – 400oC, chlorine or bromine converts alkanes into chloroalkanes (alkyl chlorides) or bromoalkanes (alkylbromide); an equivalent amount of hydrogen chloride or hydrogen bromide is formed at the same time.  CHEMICAL REACTIONS OF ALKANES 1. Oxidation Reaction or Combustion All hydrocarbons are attacked by oxygen at elevated temperatures and if oxygen is in excess, complete combustion occurs to carbon dioxide and water. CH4 + 2O2 CO2 + 2H2O + Energy The heat evolved in the process, i.e. – the heat of combustion reaction (ΔH) - is a measure of the amount of energy stored in the C-C and C-H bonds. The heat is measured by the use of a calorimeter or calculated from bond energies. Depending on which hydrogen atom is replaced, any of a number of isomeric products can be formed from a single alkane: Light CH4 + Cl2 o CH3 + HCl 25 C Light CH3 CH3 + Cl2 CH3 CH2 Cl + HCl 25o C Light CH3 CH2 CH3 + Cl2 25o C CH3 CH2 CH2 Cl + CH3 CH CH3 Cl Propane 1- Chloropropane 2 - Chloropropane 45 % 55 % Cl2 CH3 CH2 CH2 CH3 CH3 CH2 CH2 CH2 Cl + CH3 CH2 CH CH3 Cl Butane 1- Chlorobutane 2- Chlorobutane 28 % 72 %  In the case of chlorination of methane, methyl chloride can itself undergo further substitution to form more hydrogen chloride and CH2Cl2 – dichloromethane. Cl2 Cl2 Cl2 Cl2 CH4 CH3 Cl CH2 Cl2 CHCl3 CCl4 + HCl Heat Heat Heat Heat Dichloro Chloro or or + HCl or methane or Light Light Light form Light + HCl + HCl Chloroform is an anesthetic and carbon tetrachloride is used as a non-flammable cleaning agent and as the fluid in certain fire extinguishers.  MECHANISM OF HALOGENATION Halogenation of alkanes proceed by the chain reaction (Radical) mechanism which involves a series of reactions each generating a reactive substance (radical) that brings about the next step. It takes place in 3 steps as follows: STEP 1 Chain initiation step: 250 - 400o C X2 or 2X UV light The reactive substance (radical) is generated by cleavage i.e. homolytic bond fission. STEP 2: Chain propagation steps: (a) X + RH HX + R (b) R + X2 RX + X Then a, b, a, b, e.t.c. until finally: STEP 3: Chain terminating steps: (c) X + X X2 (d) R + R R R (e) R + X R X  NITRATION OF ALKANES Nitration of alkanes in vapour phase gives a mixture of nitroalkanes HNO3 CH3 CH2 CH3 400oC CH3 CH2 CH2 NO2 + CH3 CH2 NO2 or 1- nitropropanee (25 %) Nitroethane (10 %) N2 O 4 + CH3NO2 Nitromethane (25%) + CH3CHCH3 (2-nitropropane (40%) NO2  SULPHONATION OF ALKANES Alkanes react with fuming H2SO4 to give alkane sulphonic acids. H replacement: Tertiary (3o) >> Secondary (2o) >> primary (1o). e.g. CH3 CH3 CH3 CH + H2SO4 -SO3 CH3 C SO3H CH3 CH3 2- methylpropane  Octane number in relation to Complete alkanes? combusti Give on oftwo methods alkanes of give will 3 laborator what What y are compoun hydrocar preparati ds? on of bons? List the alkanes? In the distillatio Equation case of n only. s ‘branched products chains’, Give one of 2 the equation petroleu longest each of m? chlorinati continuou Catalytic sonchain and isomeris of carbon sulphona atoms ationofis tion changes taken as alkanes? straight the chain parent alkanes hydrocar 1 bon. into Yes branched or No -chain Distinguis ones? hYes or between No. SHORT QUIZ ON CHEMISTRY OF ALKANES 4-ethyl-3- methyl heptane