Alkane 11


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Alkane 11

  1. 1. Alkanes • Introduction • Alkanes are aliphatic saturated hydrocarbons. • They are represented by the general formula CnH2n+2 • Where ‘n’ is the no. of carbon atoms. • They contain C-C single bonds. • They are also called as paraffin’s(means less affinity) • Because they do not undergo chemical reactions easily.
  2. 2. • Structural formula of a compound gives the exact arrangement of atoms various elements in a molecule. • It can be represented in the different ways. • E.g. Methane CH4 Alkanes.swf CH4 Methane.swf
  3. 3. Structural formula • Condensed formula • In this – (dash) representing covalent bonds are omitted and identical groups attach to ‘C’ atoms indicated by subscript. • E.g. Hexane C6H14
  4. 4. Structural formula • Bond line formula • A molecule can be represented by lines representing C-C bond in zig - zag manner. In this terminal denotes – CH3 group and line junction indicates – CH2 group. • E.g. n-hexane C6H14
  5. 5. Structural formula
  6. 6. Structural formula
  7. 7. Structural formula
  8. 8. Classification of Alkanes Straight and Branched Chain Alkanes.swf
  9. 9. Isomerism • The compounds having same molecular formula but different structural formulae are called isomers of each other. • This phenomenon is called as isomerism. • First three alkanes do not show isomerism. • From butane and onwards alkanes show isomerism. • Isomers have different physical and chemical properties.
  10. 10. Isomers of Pentane • Pentane C5H12.
  11. 11. Types of ‘C’ atoms • There are four types of ‘C’ atoms Primary (10 ) C atom: which is attached to one ‘C’ atom. Secondary (20 ) C atom: which is attached to 2 more ‘C’ atoms. Tertiary (30 ) C atom: which is attached to 3 more ‘C’ atoms. Quaternary (40 ) C atom: which is attached to 4 more ‘C’ atoms.
  12. 12. Confirmations of Ethane • Conformation : The phenomenon of different spatial arrangements of atoms that can be converted into one – another by free rotation of atoms about C – C single bonds is called conformation. • And the compounds are called conformers or rotamers. • These conformational isomers interconvert rapidly and cannot be isolated easily. • Ethane (S).swf
  13. 13. Conformations of ethane • Ethane (C2H6) has two carbon atoms joined by single covalent bond. Each carbon atom has three hydrogen atoms. • Considering free rotation of one of the carbon atom around the C – C bond axis, infinite number of spatial arrangements of hydrogen atoms (attached to one carbon atom with respect to the hydrogen atoms attached to another carbon atom) are possible. • These are called conformational isomers. • As they all have nearly the same energy, they can change from one form to another freely.
  14. 14. Conformations of ethane • Thus two extreme arrangements are considered: • viz. Staggered and • Eclipsed conformations. • a. Staggered conformation: In this arrangement hydrogen atoms attached to two carbon atoms are as far apart as possible. • b. Eclipsed conformation: In this arrangement hydrogen atoms attached to two carbon atoms are as close as possible. • The intermediate conformations during rotation are called skew conformation.
  15. 15. Representation of conformation • Sawhorse projection of ethane: In this representation, C – C bond is viewed from an oblique angle which indicates spatial arrangements by showing all C – H bonds. Ethane (staggered) - sawhorse projection.swf Ethane (eclipsed) - sawhorse projection.swf
  16. 16. Representation of conformation • Newmann projection of ethane: In this representation, C – C bond is viewed directly end – on and represents two carbon atoms by a circle. • Ethane (staggered) - newman projection.swf • Ethane (eclipsed) - newman projection.swf
  17. 17. Alkyl Group • An alkyl group is a free radical obtained by homolytic fission of C – H bond in alkane.
  18. 18. Alkyl groups are classified 1. Primary alkyl group : In this alkyl group, valency on the primary carbon atom is available. Example: methyl radical 2.Secondary alkyl group : In this alkyl group valency of a secondary carbon atom is available. Example: isopropyl radical, 3.Tertiary alkyl group : In this alkyl group, valency of a tertiary carbon atom is available. Example: tert – butyl radical,
  19. 19. IUPAC nomenclature of alkanes • The rules of IUPAC nomenclature which you have studied in chapter 12 are applied. • Common names of first 4 alkanes are used in IUPAC nomenclature. • Methane, Ethane, Propane and Butane • All normal chain alkanes are considered as parent alkanes. • Common nomenclature system.swf
  20. 20. IUPAC nomenclature of alkanes
  21. 21. Nomenclature of Alkanes
  22. 22. IUPAC nomenclature of alkanes 1. Locate the longest continuous chain of carbon atoms; this chain determines the parent name for the alkane. We designate the following compound, for example, as a hexane because the longest continuous chain contains six carbon atoms: The longest continuous chain may not always be obvious from the way the formula is written. Notice, for example, that the following alkane is designated as a heptane because the longest chain contains seven carbon atoms:
  23. 23. IUPAC nomenclature of alkanes • 2. Number the longest chain beginning with the end of the chain nearer the substituent. Applying this rule, we number the two alkanes that we illustrated previously in the following way:
  24. 24. IUPAC nomenclature of alkanes 3. Use the numbers obtained by application of rule 2 to designate the location of the substituent group. The parent name is placed last, and the substituent group, preceded by the number designating its location on the chain, is placed first. Numbers are separated from words by a hyphen. Our two examples are 2-methylhexane and 3-methylheptane, respectively:
  25. 25. IUPAC nomenclature of alkanes 4. When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain. For example, 4-ethyl-2-methylhexane: The substituent groups should be listed alphabetically (i.e., ethyl before methyl).* In deciding on alphabetical order, disregard multiplying prefixes such as “di” and “tri.” 5. When two substituents are present on the same carbon atom, use that number twice:
  26. 26. IUPAC nomenclature of alkanes 6. When two or more substituents are identical, indicate this by the use of the prefixes di-, tri-, tetra-, and so on. Then make certain that each and every substituent has a number. Commas are used to separate numbers from each other: 7. When two chains of equal length compete for selection as the parent chain, choose the chain with the greater number of substituents:
  27. 27. IUPAC nomenclature of alkanes 8. When branching first occurs at an equal distance from either end of the longest chain, choose the name that gives the lower number at the first point of difference:
  28. 28. Methods of preparation of alkanes • From unsaturated hydrocarbons by hydrogenation • Decarboxylation of Na – salt of fatty acids • From alkyl halides by reduction • Wurtz synthesis
  29. 29. Methods of Preparation of Alkane Alkanes are prepared by the following methods: 1. Reduction Reactions: Alkanes can be prepared by the reduction of various organic compounds as follows:
  30. 30. From unsaturated hydrocarbons by hydrogenation • Alkenes and alkynes are unsaturated hydrocarbons. When mixture of alkenes/alkynes and H2(g) passed over Raney Ni catalyst at 473K to 573K form corresponding alkane by addition reaction. Raney Ni 2 2 2 3 3 Ethene Ethane CH CH H CH CHD = + -¾¾¾¾® Raney Ni 2 3 3 Ethyne Ethane CH CH 2H CH CHD + -º ¾ ¾ ¾ ¾®
  31. 31. Decarboxylation of Na – salt of fatty acids • When anhydrous Na- salt of fatty acid is heated with soda lime (mixture of NaOH + CaO) forms alkane by decarboxylation. Alkane containing one atom less than acid is obtained. 2 5 2 6 2 3 CaO sodium propionate ethane C H COONa NaOH C H Na COD+ +¾¾¾®
  32. 32. From alkyl halides by reduction • Alkyl halides when treated with reducing agent like Zn – Cu couple and alcohol form corresponding alkane. 3 4 Zn Cu alcohol methaneMethyl bromide CH Br 2H CH HBr- D + +¾¾¾¾® 2 5 2 6 Zn Cu alcohol Ethyl bromide Ethane C H Br 2H C H HBr- D + +¾¾¾¾®
  33. 33. Wurtz synthesis • When alkyl halide is treated with sodium metal in the presence of dry ether as solvent gives higher alkanes. Dry 2 5 2 5 4 10ether Ethyl bromide Butane C H Br 2Na Br C H C H 2NaBr+ + +¾¾¾®
  34. 34. Drawbacks' of Wurtz’s Reaction
  35. 35. Physical Properties of Alkane 1. In normal alkanes, as the no. of C atoms increases, melting point and boiling point increase due to increase in intermolecular forces. 2. Branched alkanes have lower boiling points than straight chain. More the no. of branches lower is the boiling point because increased branching increases surface area and decreases intermolecular forces. 3. Alkanes are insoluble in H2O, but soluble in non polar solvents like C6H6, CHCI3 etc. 4. First 4 are gases, C5 to C17 are liquids, remaining are solids.
  36. 36. Chemical properties of alkane • 1. Halogenation Alkanes react with halogens like chlorine or bromine to form alkyl halide in presence of diffused sunlight or U.V. light or by heating. Chlorination of methane: • In this reaction one by one H atoms replaced by Chlorine atoms to form mixture of products. • Alkyl halide is obtained by limiting supply of chlorine.
  37. 37. Chlorination of methane 4 2 3 U.V. light methane Methyl chloride CH Cl CH Cl HClD+ +¾¾¾¾® 3 2 2 2 Methylene dichloride U.V. light CH Cl Cl CH Cl HClD+ +¾¾¾¾® 2 2 2 3 U.V. light Chloroform CH Cl Cl CHCl HClD+ +¾¾¾¾® 3 2 4 U.V. light Carbon tetrachloride CHCl Cl CCl HClD+ +¾¾¾¾® Mechanism of halogenation.swf
  38. 38. Chlorination of methane • Mechanism of halogenations – Chlorination of CH4. It is a chain reaction, involves a series of steps. Each step generates a reactive species that brings about the next step. i. Chain initiation step – ii. Chain Propagation step – iii. Chain termination step –
  39. 39. Chlorination of methanei. Chain initiation step – • CI2 molecule absorbs energy; bond breaks homolytically to give chlorine free radicals.
  40. 40. Chlorination of methane ii. Chain Propagation step – Chlorine free radical is highly reactive. It abstracts a H atom of CH4 and forms Methyl free radical and HCI. • Methyl free radical attacks chlorine to form CH3CI and Chlorine free radical. • These steps are repeated many times. Overall reaction is
  41. 41. Chlorination of methane iii. Chain termination step – After some time, reaction stops due to combination of free radicals. may further get chlorinated.
  42. 42. Bromination, Nitration of alkanes Bromination is carried out in presence of AlBr3 3 2 6 2 2 5 AlBr Ethyl bromide C H Br C H Br HBrD+ +¾¾¾® Nitration of Alkanes: In this reaction one H atom of alkane is replaced by nitro ( - NO2 ) group. 423 to 698 k 2 6 3 2 5 2 2 Ethane Conc.nitricacid nitroethane C H HNO C H NO H OD + +¾¾¾¾®
  43. 43. Pyrolysis of alkanes • The thermal decomposition of alkanes in absence of air to give lower alkanes, alkenes and H2 is called as pyrolysis. It takes place as follows. Dehydrogenation : It involves breaking of C-H bond in alkanes to form alkenes by dehydrogenation. 3 3 2 2 2 etheneethane CH CH CH CH HD - = +¾¾® Cracking : It involves breaking of C - C bonds and C - H bonds to form lower alkanes and alkenes. Cracking.swf 3 2 3 2 2 4 ethene MethanePropane CH CH CH CH CH CHD - - = +¾¾®
  44. 44. Pyrolysis of alkanes Cracking : It involves breaking of C - C bonds and C - H bonds to form lower alkanes and alkenes. 3 2 3 2 2 4 ethene MethanePropane CH CH CH CH CH CHD - - = +¾¾® Aromatization : Alkanes containing more than 5 ‘C’ atoms get cyclised to benzene and it’s homologues on heating under 10 to 20 atm. at 773K in presence of Cr2O3.
  45. 45. Combustion • Alkane when heated in air, combine with oxygen to give CO2(g) and water vapour. • It is a exothermic reaction because large amount of heat is evolved. ( ) ( ) ( ) ( )24 g 2 g 2 g g Methane CH 2 O 2 H O heatCOD + + +¾¾®
  46. 46. Uses of alkanes 1) As fuels e.g. LPG, CNG, Petrol, Diesel etc. 2) Liquid alkanes used as solvents. 3) C17 to C20 as lubricants. 4) C21 to C30 as a lubricant base for preparation of cosmetics and candles. 5) As a source of Hydrogen. 6) Incomplete combustion gives carbon black for manufacture of printing ink, polishes, black pigments etc. Uses of alkanes.swf