ORGANIC CHEMISTRY CHM 207

1. ORGANIC CHEMISTRY CHM 207 CHAPTER 2: ALKANES NOR AKMALAZURA JANI

2. TOPICS • Nomenclature • Structure • Reactions: - free radicals substitution - combustion • Industrial source and uses of aliphatic hydrocarbons - petroleum and natural gas - petroleum fractions: cracking and reforming and their uses.

3. General formula: CnH2n+2, where n = 1, 2, …. • Only single covalent bonds are present • Known as saturated hydrocarbons because contain the maximum number of hydrogen atoms that can bond with the number of carbon atoms present. • Can be assumed to be sp3-hydridized

4. Structures of the first four alkanes

5. Homologous Series  • Definition: • A series of compounds in which each member differs from the next by a specific number and kind of atoms. • Alkanes: Differ only at number of (CH2) • Series of compounds that has the same functional group.

6. INITIAL NAMES OF THE HOMOLOGOUS SERIES

7. NAMING ALKANES • Alkyl groups are used to name organic compounds. • The general formula of an alkyl group is CnH2n+1. • The letter “R” is often used in formulas to represent any of the possible alkyl groups. R= CnH2n+1 (any alkyl group) R = CH3 — methyl group R = CH3CH2 — ethyl group

9. IUPAC RULES International Union of Pure and Applied Chemistry Consider all alkyl groups attached to it as branch chains or substituents that have replaced hydrogen atoms of the parent hydrocarbon. If two chains of equal length are found, use the chain that has the larger number of substituents attached to it. The alkane’s name consists of the parent compound’s name prefixed by the names of the alkyl groups attached to it. RULE 1. Select the longest continuous chain of carbon atoms as the parent compound.

10. This chain has 6 carbon atoms. 1 2 3 4 6 5 This chain has 4 carbon atoms. 1 2 3 4 This structure has 2 chains.

11. This is the longest continuous chain. 1 2 3 4 5 6 Select this chain as the parent compound.

12. 1 2 3 4 5 6 This is a methyl group. It is a branch chain and can be considered to have replaced a hydrogen on the parent compound.

13. 3 The name of the compound is 3-methylhexane. 3 1 2 3 4 5 6

14. If the first subsitutent from each end is on the same-numbered carbon, go to the next substituent to determine which end of the the chain to start numbering. RULE 2. Number the carbon atoms in the parent carbon chain starting from the end closest to the first carbon atom that has an alkyl group substituted for a hydrogen atom.

15. 1 2 3 4 5 6 7 8 If the chain is numbered left to right, the isopropyl group is on carbon 5. isopropyl group

16. 8 7 6 5 4 3 2 1 If the chain is numbered right to left, the isopropyl group is on carbon 4. Use right to left numbering so that the isopropyl group is on the lowest numbered carbon. 4-isopropyloctane isopropyl group

17. RULE 3. Name each alkyl group and designate its position on the parent carbon chain by a number (e.g., 2-methyl means group attached to C-2). 5 4 3 2 1 2-isopropyl pentane

18. RULE 4. When the same alkyl-group branch chain appears more than once, indicate this repetition by a prefix (di-, tri-, tetra- and so forth) written in front of the alkyl group name (e.g. dimethyl indicates two methyl groups). • –The numbers indicating the alkyl-group positions are separated by a command and followed by a hyphen and are placed in front of thename (e.g., 2,3-dimethyl). 5 4 3 2 1 The methyl group appears twice 2,3-dimethylpentane

19. RULE 5. When several different alkyl groups are attached to the parent compound, list them in alphabetical order (e.g. ethyl before methyl in 3-ethyl-4-methyloctane). Prefixes are not included in alphabetical ordering (ethyl comes before dimethyl).

20. methyl ethyl 3-ethyl-4-methyloctane 1 2 3 3 4 4 5 6 7 8

21. Alkanes can have many different types of substituents. • For example:

22. CYCLIC HYDROCARBONS • A hydrocarbon that contains carbon atoms joined to form a ring. • Cycloalkanes – all carbons of the ring are saturated

23. NOMENCLATURE OF CYCLOALKANES Similar to that alkanes. For examples:

24. CYCLIC HYDROCARBONS • When the acyclic portion of the molecule contains more carbon atoms than the cyclic portion (or when it contains an important fuctional group), the cyclic portion is named as a cycloalkyl substituent. • Example:

25. ISOMERISATION Structural isomers: • Molecules that have the same molecular formula, but different structure Three isomers of pentane (C5H12)

26. STRUCTURE ISOMERS FOR ALKANES

27. PHYSICAL PROPERTIES OF ALKANES Solubilities and densities Boiling points Melting points

28. SOLUBILITIES AND DENSITIES OF ALKANES 1) Solubilities: • The C-H bond having only a very weak dipole moment. • Alkanes are weak polar molecules and considered as non-polar molecules. • Soluble in non-polar solvents such as benzene and weak non-polar organic solvents such as dimethyl ether (CH3-O-CH3). • Insoluble in water: - alkanes are non-polar and do not form hydrogen bonds with water molecules. - described as ‘hydrophobic’ (water hating).

29. 2) Densities: • alkanes have densities around 0.7 g/mL, compared to density of water (1.0 g/mL). • alkanes is less dense than water and insoluble in water. • - water combined with alkanes will form two phase with the alkanes on the top.

30. oil water

31. BOILING POINTS AND MELTING POINTS OF ALKANES

32. BOILING POINTS OF ALKANES Effect of relative molecular mass on boiling point. - the first four chain alkanes are gases. - alkanes from C5H12 to C18H38 are liquids at room temperature because their melting point are lower than 28oC (301K). - alkanes above C18H38 are solids at room temperature. - the boiling points of straight chain alkanes increase steadily with relative molecular mass (due to increasing forces of attraction between molecules). * A larger molecule, with greater surface area and greater van der Waals attractions, boils at higher temperature *

33. Boiling points of the straight chain of alkanes

34. Effect of branching on boiling point. - Branched chain alkanes boils at a lower temperature (more volatile) than the straight chain alkane with the same number of carbon atoms. - Examples: hexane boils at 68.7oC, 3-methylpentane (one branch) boils at 63.3oC and 2,3-dimethylbutane (two branches) boils at 58oC. - Reason: the branched chain alkanes are more compact (nearly spherical), have smaller surface area, smaller van der Waals forces of attraction and boils at lower temperature.

35. MELTING POINTS OF ALKANES • The melting points increase with increasing of molecular weight. • Alkanes with even numbers of carbon atoms pack better into a solid structure, so higher temperatures are needed to melt them (high melting point). • Alkanes with odd numbers of carbon atoms do not pack as well, and melt at lower temperatures (low melting points). • Branched chain alkanes melts at a higher temperature than n-alkanes (straight alkanes) with same numbers of carbon atoms. -Reason: 3D-structure of branched alkanes are more compact, pack more easily into solid structure and melt at higher temperatures.

36. Formula: C6H14 Melting points increase, boiling points decrease Shape of the molecule become more highly branched and compact

37. UNREACTIVITY OF ALKANES • Alkanes is chemically inert to most reagents. • For example, acids, alkalis, and oxidising agents such as potassium manganate (VII) or potassium dichromate (VI) do not react with alkanes. • Alkanes reacts with oxygen and halogens in suitable conditions. • Why alkanes has low reactivity? - lack of electron-deficient or electron-rich sites on the alkanes molecules. - polar molecules, positive and negative ions (such as H+ and OH-), do not react with alkanes because the C-H bond is weak polar and C-C bond is non-polar.

39. REACTIONS OF ALKANES • COMBUSTION - Alkanes burn in a plentiful supply of air or oxygen to produce water and CO2 only. - for example: C3H8 + 5 O2→ 3CO + 4H2O

40. C2H6 + 3 O2→ 2C + 3H2O 2 C2H6 + 5 O2→ 2CO + 3H2O 2 - In a limited supply of air, combustion of alkanes produces carbon monoxide and water - In a very limited supply of air, alkanes burn to form carbon as one of the product.

41. REACTIONS OF ALKANES • HALOGENATION OF ALKANES - At RT, alkanes do not react with chlorine or bromine in the dark. - if the mixture of alkanes and chlorine or bromine is heated at high temparature (300-400oC), or irradiated by ultraviolet light, the hydrogen atoms in the alkanes are successively replaced by chlorine or bromine atoms to produce a mixture of products (halogenated alkanes).

42. Equations for the reactions of methane with chlorine: • Equation 1: reaction with limited supply of chlorine and excess of methane. Tha major product is chloromethane. • Equation 2: reaction with the excess of chlorine. The major product is tetrachloromethane. * Bromine reacts with alkanes in the same way of chlorine, but iodine do not react well with alkanes *

43. This reaction is called a substitution reaction (an atom or a group atom in an organic compound is replaced by another atom or a group of atoms). • Involves a halogen - called halogenation • If the halogen is chlorine – called chlorination. • If the halogen is bromine – called bromination. * Condition of reaction: light or heat (high temperature) or ultraviolet radiation (provides energy that is absorbed by reactant molecules to produce free radicals).

44. MECHANISM OF FREE RADICAL SUBSTITUTION REACTIONS

45. hv = radiation energy = movement of single electron (radical) MECHANISM OF FREE RADICAL SUBSTITUTION REACTIONS • INITIATION STEP - homolytic fission ΔH = +242 kJMol-1

46. 2) PROPAGATION STEPS • Free radical species produce another free radical species. • Free radicals is highly reactive.

47. - Methyl radical propagates a chain reaction as the methyl free radical then reacts with another chlorine molecule to form chloromethana and a chlorine free radical. • Chlorine free radical produced then react with another methane molecule and the cycle is repeated.

48. 3) TERMINATION STEPS - The reaction stops when two free radicals collide and combine. - highly exothermic.

49. - If a large excess of methane is used, CH3Cl is obtained as the main organic product. - In excess of chlorine, the propagation steps may proceed with the reaction between a chlorine free radical with chloromethane to produce dichloromethane. - The reaction may continue to produce trichloromethane and finally tetrachloromethane.