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Alkanes

Alkanes. 1 Introduction 2 Nomenclature of Alkanes 3 Physical Properties of Alkanes 4 Preparation of Alkanes 5 Reactions of Alkanes. Dr Manal F. AbouTaleb. Alkanes. Alkanes are the hydrocarbons of aliphatic row .

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Alkanes

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  1. Alkanes 1Introduction 2Nomenclature of Alkanes 3Physical Properties of Alkanes 4Preparation of Alkanes 5Reactions of Alkanes Dr Manal F. AbouTaleb

  2. Alkanes • Alkanes are the hydrocarbons of aliphatic row. • Alkanes are hydrocarbons in which all the bonds are single covalent bonds (-bonds). • Alkanes are called saturated hydrocarbons. The simplest one, methane (CH4), is also the most abundant. • All carbon atoms are sp3-hybridized • All bond angles are 109.5°

  3. Alkyl groups • Alkyl groups are formed by loss of a hydrogen atom from the corresponding alkane . General formula of alkyl group : CnH2n+1 • ( e.g. CH4 Methane – 1 H = -CH3 Methyl group ) • Alkyl groups are named by dropping the -anesuffix of the alkanes and adding the suffix -yl. Methane becomes a methyl group, ethane an ethyl group, etc.

  4. Unbranched alkyl groups

  5. Three-Carbon Groups Nomenclature Of Branched Alkyl Groups - Methylethylis the systematic name; isopropyl is a common name. - Numbering always begins at the point where the group is attached to the main chain.

  6. Propane Butane

  7. 2. Four-Carbon Groups

  8. 2. Four-Carbon Groups

  9. IUPAC Nomenclature • IUPAC (International Union of Pure and Applied Chemistry) names: • 1- The unbranched alkanes (homologous series) • 2- Branched alkanes • Suffix of Alkanes added ane

  10. Nomenclature of Alkanes 1. Straight-Chain Alkanes

  11. Nomenclature of Alkanes Alkanes Alkanes are molecules(hydrocarbons) that contain only C-C and C-H  bonds. Alkanesare eitheracyclic or cyclic. Acyclicalkanes(not cyclic) have the formula CnH2n+2(where n = an integer)

  12. Nomenclature of Alkanes 2. Branched-Chain Alkanes • 1. Select the longest possible straight chain; this gives the parent name for the alkane • 2. Number the parent chain beginning with the end of the chain nearer the branched chain • 3. Use the number obtained by application of rule 2 to designate the position of the branched chain • When two or more branched chains are present, give each branched chain a number corresponding to its position on the parent chain • When two or more branched chains are identical, indicate this by the use of the prefixes ‘di-’, ‘tri-’, ‘tetra-’, and so on

  13. Nomenclature of Alkanes => 1. Select the longest possible straight chain; this gives the parent name for the alkane 2- methyl hexane 3- methyl heptane 3,4- dimethyl heptane 2. Number the parent chain beginning with the end of the chain nearer the branched chain 7 6 5 4 3 2 1 4-ethyl-3- methyl heptane

  14. Nomenclature of Alkanes 3- Use the numbers obtained by application of rule 2 to designate the position of the branched chain • In writing the full name the root name is placed last; the substituent group, preceded by thenumber indicating its location on the chain, is placed first. 14

  15. 4. When two or more substituents are present, giveeach substituent a numbercorresponding to its location on the longest chain. Nomenclature of Alkanes - The substituent groups are listed alphabetically. • 5. When two substituentsare present on the same carbon, use the number twice.

  16. Nomenclature of Alkanes 6. When two or more branched chains are identical, indicate this by the use of the prefixes ‘di-’, ‘tri-’, ‘tetra-’, and so on. e.g.

  17. Nomenclature of Alkanes 1 2 3 4 5 6 3,3- Dimethyl hexane 10 9 8 7 6 5 4 3 2 1 2,7,8-Trimethyl decane

  18. Nomenclature of Alkanes 7- When two chains of equal length compete for selection as the parent chain, choose the chain with the greater number of substituents.

  19. 8. If two substituents are equidistant, look for the next closest group. 1 3 4 5 2 6 7 Nomenclature of Alkanes

  20. Monocyclic Compounds 1. Cycloalkanes with only one ring: Nomenclature Of Cycloalkanes

  21. 1) Number the ring beginning with the substituent first in the alphabet, and number in the direction that gives the next substituent the lower number possible. 2) When three or more substituents are present, begin at the substituent that leads to the lowest set of locants. Substituted cycloalkanes:

  22. Physical Properties Of Alkanes and Cycloalkanes • 1. A series of compounds, where each member differs from the next member by a constant unit, is called a homologous series. Members of a homologous series are called homologs. • 2. At room temperature ( 25 °C) and 1 atm pressure, the C1-C4 unbranchedalkanes are gases; the C5-C17 unbranched alkanes are liquids; the unbranched alkanes with 18 or more carbon atoms are solids.

  23. Physical Properties of Alkanes Boiling Point • Higher members have higher boiling points • Reason: • Increase in molecular mass • Increase in intermolecular force

  24. Physical Properties of Alkanes Branched-chain alkanes have lower boiling points than straight-chain alkanes ∵ molecule is more compact  surface area  van der Waals’ force   boiling point 

  25. Physical Properties of Alkanes Melting Point • Higher members have higher (less for odd- number of carbons)melting points • Reason: • Increase in molecular mass • Increase in intermolecular force Branched alkanes pack more efficiently into a crystalline structure, so have higher m.p.

  26. Physical Properties of Alkanes Solubility • Alkanes: • non-polar compounds • insoluble in water and highly polar solvents • soluble in non-polar solvents like benzene, 1,1,1-trichloroethane Density All alkanes and cycloalkanes have densities less than 1 g cm–3 at 20°C.  Petroleum floats on water surface Alkanes are less dense than water and swim on top of water

  27. Rings of carbon atoms (CH2 groups) Formula: CnH2n Nonpolar, insoluble in water Compact shape Melting and boiling points similar to branched alkanes with same number of carbons 5- and 6-membered rings most stable Bond angle closest to 109.5 Cycloalkanes

  28. From Alkenes & Alkynes Hydrogenation of unsaturated hydrocarbon:

  29. 2) From alkyl Halides A) Reduction of alkyl halides

  30. B) Hydrolysis of Grignard Reagent Grignard reagent is an alkyl magnesium halide compound, R-Mg-X - The Grignard reagent is formed when a solution of an Alkyl Halide (R-X) is allowed to stand over a metallic magnesium in the presence of dry ether - Then Grignard reagent react with water or alcohol to form alkane

  31. C) Wurtz Reaction تفاعل فورتز This is the reaction of two alkyl halides (R-X) with metallic sodium to give symmetrical alkanes. The wurtz reaction is a poor method for the preparation of unsymmetrical alkanes

  32. Preparation of Alkanes Reforming Reforming is a process in which straight-chain alkanes are heated under pressure in the presence of a platinum catalyst. The chains break up and reform to give branched-chain molecules.

  33. 29.5 Reactions of Alkanes Combustion Complete combustion : Alkanes react with sufficient oxygen to give carbon dioxide and water through a complex series of reaction with the release of a large amount of energy. General formula:

  34. complete combustion incomplete combustion CH3-CH2-CH3 + 4 O2 ——>  CO2   + 2 CO   +  4 H2O   + heat

  35. Reactions of Alkanes Chlorination • Methane reacts with chlorine under diffuse sunlight or heating but not in dark • A mixture of products (CH3Cl, CH2Cl2, CHCl3, CCl4) is formed with the replacement of hydrogen by one or more chlorine atom • If methane is in excess, CH3Cl is major product • If chlorine is in excess, CCl4 is major product

  36. Reactions of Alkanes Reaction Mechanism: Free Radical Substitution Reaction • Mechanism of reaction : • 1.Chain initiation • homolytic fission of chlorine molecules by heat or light into two chlorine radicalsStep 1:

  37. Reactions of Alkanes 2.Chain propagation Step 2: Step 3: • steps 2 and 3 repeat hundreds or thousands of time due to formation of the reactive intermediate in each step chain reaction

  38. Reactions of Alkanes Further substitution occurs:

  39. Reactions of Alkanes

  40. Reactions of Alkanes

  41. Reactions of Alkanes • 3.Chain termination • two free radicals combine to form a neutral molecule • the chain reaction is terminated

  42. Reactions of Alkanes • Solution: • The reaction mechanism is shown as follows: • 1. Chain initiation • Chain propagation Example Write down the reaction mechanism involved in the chlorination of ethane in the presence of diffuse sunlight. Answer

  43. Reactions of Alkanes • Solution: • Chain termination

  44. Mechanism For the Radical Halogenation of Methane

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