Chemistry

Alkane

Session objectives Methods of preparation Kolbe’s electrolysis Physical Properties and Chemical properties Halogenation Refining of petroleum Octane number

General characteristics of alkene Paraffins General formula CnH2n+2 sp3 hybridisation C–C bond length 1.15 4 A0 Chemically unreactive

Methods of preparation Wurtz reaction Follow mainly free radical mechanism Useful in preparing an alkane containing even number of carbon atoms Stepping up reaction

Frankland reaction

Decarboxylation of sodium or potassium salt of fatty acid For example

Kolbe’s electrolysis At anode

Kolbe’s electrolysis For example At anode

From Grignard reagent (RMgX) From unsaturated hydrocarbons Sabatier-Senderens reduction

Methods of preparation From alkyl halides R – I > R – Br > R – Cl > R – F From aluminium carbide Corey House reaction

Physical Properties Boiling point:

Physical Properties Melting point: Zigzag arrangement of carbon atoms in alkanes

Physical Properties (a) Alkane having even carbon atom Symmetrical –higher melting point (a) Alkane having odd carbon atom unsymmetrical – lower melting point

Chemical properties Combustion Oxidation Halogenation CH4 + Cl2 CH3Cl + HCl

Mechanism Initiation Propagation Termination

Features of Halogenation F2 > Cl2 > Br2 > I2. Attack of or on an alkane is selective Order of reactivity is 3° > 2° > 1°

Features of Halogenation

Features of Halogenation Nitration

Features of Halogenation Sulphonation Isomerization

Features of Halogenation Aromatization

Refining of petroleum Sources of hydrocarbons Petroleum Aliphatic hydrocarbons Coal Aromatic hydrocarbons

Approximate composition Refining of petroleum Fraction Boiling range Approximate composition uses Gaseous 113 to 303 K C1 - C5 (2%) For producing carbon black and in preparation of ammonia, methyl alcohol and gasoline. Petroleum ether or Ligroin 303 to 363 C5 - C7 (2%) solvent for oils, fats, rubber and in dry cleaning. Gasoline or petrol 343 to 473 C7 - C12 (32%) Mainly as a motor fuel.

Approximate composition Refining of petroleum Fraction Boiling range Approximate composition uses Kerosene 448 to 548 C12 - C15(18%) Illuminant fuel and for preparing petrol gas. Gas oil, fuel oil and diesel oil 523 to 673 C15 - C18(20%) In furnace oil, fuel for diesel engines and in cracking. Lubricating oils and petroleum jelly 623 and up C16 and up Used mainly as lubricants.

Cracking

Synthetic petrol Fischer-Tropsch process Bergius process

Octane number Percentage by volume of iso-octane in the mixture of iso-octane and n-heptane which has the same anti-knocking qualities as the fuel under examination.

Octane number Straight chain alkanes have low octane numbers. The greater the length of the chain, lower is the octane number. Straight chain alkenes and alkynes, and also cyclic alkanes have higher octane numbers than their corresponding alkanes. Branched chain hydrocarbons have high octane numbers. Aromatic hydrocarbons have very high octane numbers.

Class exercise

Class exercise 1 Ethyl iodide + n-propyl iodide mixture of hydrocarbon. Which of the following hydrocarbons will not be formed? (a) Butene (b) n-hexane (c) n-pentane (d) n-butane

Solution Hence, the answer is (a).

Class exercise 2 Product is Solution: Hence, the answer is (a).

Class exercise 3 Which of the following carbides are used in preparation of methane by action of water? (a) CaC2 (b) Al4C3 (c) Si C (d) All of these Solution: Hence, the answer is (b).

Class exercise 4 Which of the following reactions will give maximum yield? Solution: Reaction proceeds via free radical mechanism and excess of C2H6 is needed as it forms various products. Hence, the answer is (b).

Class exercise 5 Which of the following compounds will have the highest heat of combustion? C3H6 (b) C5H12 (c) C6H14 (d) C10H22 Solution: It has highest number of carbon and hydrogen atoms. Hence, the answer is (d).

Class exercise 6 Which of the following reactions will give unsymmetrical alkanes in good yield? (a) Frankland reaction (b) Wurtz reaction (c) Corey House reaction (d) All of these Solution: Frankland and Wurtz reaction gives alkane having even number of carbon atoms. Hence, the answer is (c).

Class exercise 7 Which of the following is true for above reaction? (a) Compound X and Y are formed in equal quantities (b) Compound Y is formed in excess (c) Compound X is formed in excess (d) None of these

Solution Hence, the answer is (b).

Class exercise 8 The monochlorination of an alkane (molecular formula C8H18) gives only one product. The IUPAC name of alkane is (a) neo octane (b) iso octane (c) 2, 2, 3-trimethylpentane (d) 2, 2, 3, 3-tetramethylbutane Solution: Hence, the answer is (d).

Class exercise 9 Hydrolysis of calcium carbide gives a solution with pH (a) 0 (b) < 7 (c) > 7 (d) 3 Solution: pH is more than 7 due to formation of Ca(OH)2. Hence, the answer is (c).

Class exercise 10 Sodium benzoate + sodia lime X. X is ethane (b) benzene (c) ethene (d) propane Solution: Hence, the answer is (b).

Thank you