Physical Properties of Alkanes

Transcription

1 Physical Properties of Alkanes The common physical properties that we will focus on are: Melting point Boiling point Solubility However, any inferences drawn on these may also extend to other properties such as density. It is noteworthy that among the homologous series of alkanes (C n H 2n+2 ), there is a smooth change in physical properties (melting and boiling point) attributed to the van der Waals forces of the molecules. As the number of carbon and hydrogen atoms increase, the additive effects of these weak intermolecular forces become significant, as evidenced by the increase in boiling and melting points from methane to hexane to icosane. 1

2 Boiling Points of Alkanes The boiling points of hexane and heptane are 69 o C and 98 o C, respectively, a difference of 29 o C for one CH 2 unit. We would expect the boiling point of octane to be 98 o C + 29 o C = 127 o C, which is close to the actual boiling point of 126 o C. The boiling points ( o C) below are illustrative of this homology. 2

3 Graph of Boiling Points of Alkanes Boiling points of alkanes. For boiling point, as n increases, each additional CH 2 group contributes a fairly constant increase in boiling point. 3

4 Boiling Points of Alkanes (Exceptions) For branched alkanes, there is too much variation in structure for regularities to be apparent. Refer to the boiling points of the isomeric pentanes below. Note that volatility appears to increase with increased branching. 4

5 Boiling Points of Alkanes (Exceptions) Observe the same variability among isomeric hexanes below. Straight chain alkanes have a higher boiling point than branched alkanes due to the greater surface area in contact and therefore greater van der Waals interactions. 5

6 Melting Points of Alkanes Among the homologous series of alkanes (C n H 2n+2 ), the melting points also increase but not as smoothly as boiling points. For melting point, as n increases, there appears to be a progressive increase among odd numbered and among the even numbered alkanes. 6

7 Solubility of Alkanes Since alkanes are hydrocarbons and there is no significant differences in electronegativity between carbon and hydrogen, they are nonpolar. Being nonpolar, alkanes dissolve in nonpolar solvents because the van der Waals interactions between nonpolar alkane molecules and nonpolar solvent molecules are about the same as the alkane-alkane and nonpolar solvent-nonpolar solvent molecule interactions. Alkanes are thus soluble in nonpolar solvents (i.e. other alkanes) and insoluble in polar solvents (water). The liquid alkanes are therefore good solvents for nonpolar covalent compounds. 7

8 Occurrence of Alkanes Highlights The most important source of alkanes is natural gas and fossil oil. Natural gas contains primarily methane and ethane alongside some propane and butane. Methane is also present in biogas produced by methanogenic bacteria acting on decaying animal and vegetable matter. Fossil oil is a mixture of liquid alkanes and other hydrocarbons. 8

9 Occurrence of Alkanes Highlights Some pine trees produce exceptionally high amounts of n- heptane and burn readily during bush fires. The tsetse fly uses 2-methylheptadecane as a pheromone. It can detect it over long distances in locating a mate. The plant cuticle contains solid long-chain alkanes that protect the plant against water loss and leaching of important minerals by rain. 9

10 Fossil Oil What is Fossil Oil Fossil oil/petroleum is a mixture of alkanes/hydrocarbons of varying length commonly used as fuels. Note that fossil fuels include coal, oil and natural gas. Whereas coal is a solid hydrocarbon, oil is a mixture of liquid hydrocarbons while natural gas is a gaseous hydrocarbon. Fossil fuels are produced by anaerobic decomposition of buried dead plant and animal matter. From plant matter, the process begins with photosynthesis where plants using solar energy convert carbon dioxide and water into oxygen and carbohydrates. When the plants die and the sediments containing them become buried under rocks, heat and pressure transform the carbohydrates into various hydrocarbons. 11

11 Fossil Oil Occurrence Oil and gas occur trapped deep in rocks at high pressure. Extraction of the oil and gas requires drilling through the rocks to provide wells that would be used for recovery of the crude oil. 12

12 Fossil Oil Extraction Oil and gas occur trapped deep in rocks at high pressure. With oil and gas trapped in rocks, extraction of the oil and gas requires drilling of the rocks to provide wells that would be used for recovery. 13

13 Oil Refining The Rationale Crude oil, being a mixture of liquid alkanes, must undergo several separation processes into components that can be used as fuels. This separation process is called oil refining. The separation into its component alkanes takes advantage of their differences in boiling points. Recall that the boiling points of alkanes change gradually with molecular weight. 14

14 Oil Refining Fractional Distillation Distillation is based on the fact that the vapour of a boiling mixture will be richer in the components that have lower boiling points. Thus, when this vapour is cooled and condensed, the condensate will contain the more volatile components. At the same time, the original mixture will contain more of the less volatile components. Oil refining by fractional distillation involves heating the crude oil to vapor then letting the vapor to condense at different levels of the distillation tower. When the fractions at collected at different levels of the tower, they attract different applications. See overleaf, the schematics of a typical distillation tower in a oil refinery. 15

15 Oil Refining Fractionating Tower 16

16 Uses of Alkanes Highlights Hydrocarbons (alkanes) are traditionally called paraffin's in recognition of their use as fuels and oils. The applications of alkanes depend on the carbon chain, Alkanes upto 4 carbons (methane butane) are mainly for heating and cooking purposes. Methane and ethane are the main components of natural gas. Propane and butane can be readily liquified at low pressure and are the chief components of liquified petroleum gas (LPG). 17

17 Uses of Alkanes Highlights Pentane to octane reasonably volatile liquids that are used as fuels in internal combustion engines. They are also good solvents for nonpolar substances. Nonane to hexadecane are used as kerosene and jet fuel. Alkanes from hexadecane are liquids of high viscosity and are commonly used as lubricating oil. Petroleum jelly, a semi-solid mixture of hydrocarbons (mainly higher than 25 carbons), is used as a skin protectant in cosmetic skin care. 18

18 Uses of Alkanes Highlights Many solid alkanes find use as parafin wax in candles. Some synthetic polymers such as polyethene are chains containing hundreds of thousands of carbon atoms. 19

19 Synthesis of Alkanes Laboratory Preparation Catalytic hydrogenation of alkenes or alkynes is a common strategy to alkanes. Catalytic hydrogenation is the common method for converting unsaturated oils to saturated fats in a process commonly refered as hardening of oils. 20

20 Synthesis of Alkanes Laboratory Preparation Organocuprates (Gilman reagents) couple with alkyl halides to provides alkanes. The reaction of organocuprates with alkyl halides is a substitution reaction made possible by the polarization of the C-I as a result of the differences in electronegativities of carbon and iodine. Note that the C-Cu bond is less polarized and therefore more covalent compared with the C-Li or C-Mg bonds. 21

21 Synthesis of Alkanes Laboratory Preparation Reactions of ionic organometallic reagents with water. The most common organometallic reagents used are organolithium and organomagnesium reagents. Hydrolysis of metal carbides provide methane gas 22

22 Synthesis of Alkanes Laboratory Preparation Reduction (deoxygenation) of aldehydes and ketones also provides access to hydrocarbons. One of the most common method for deoxygenation of aldehydes and ketones is the Wolff Kishner reduction. It involves heating an aldehyde or ketone with hydrazine in the presence of a base (KOH). Toluene (methylbenzene) is used as a common industrial solvent. 23

23 Synthesis of Alkanes Mechanism of the Wolff-Kishner Reduction Toluene, common industrial solvent, can be prepared from benzaldehyde based on the Wolff-Kishner reduction. 24

24 Synthesis of Alkanes Mechanism 25

25 Practice Questions Synthesis of Alkanes i. Propane gas is used as a fuel and acetone as a common industrial solvent. Propose a reasonable and stepwise reaction mechanism for the conversion of acetone to propane based on the Wolff Kishner reduction shown below: ii. Explain why hydrogenation turns long chain alkenes into solid alkanes yet it increases the molecular mass by only two mass units. 26