Advanced Chemistry. Ch. 12. Physical Properties & Reactions of Alkanes

I. Physical Properties of Alkanes

  A. Common Alkanes

        The three major sources of alkanes are the fossil fuels; natural gas, petroleum & coal.  These account for about

        90% of the total energy consumption in the United States (the rest is mainly nuclear and hydroelectric sources).

Reference: Energy Sources used in the USA Department of Energy:

 reference: Common Alkanes

        1. Light weight.  a.k.a. Petroleum gas used for heating, cooking, making plastics

                small alkanes (1 to 4 carbon atoms);  commonly known by the names methane, ethane, propane, butane

                boiling range = less than 104 degrees Fahrenheit / 40 degrees Celsius

                often liquified under pressure to create LPG (liquified petroleum gas)

                propane (gas grills) and butane (lighter fluid) are easily condensed under pressure & are commonly sold as liquids)

        2. Medium weight. Alkanes that are typically liquids at room temperatures

            a.  Naphtha or Ligroin - intermediate that will be further processed to make gasoline

                mix of 5 to 9 carbon atom alkanes; iso-octane (2,2,4-trimethylpentane)

                boiling range = 140 to 212 degrees Fahrenheit / 60 to 100 degrees Celsius

            b. Gasoline - motor fuel.  mixture of alkanes and cycloalkanes (5 to 12 carbon atoms)

                boiling range = 104 to 401 degrees Fahrenheit / 40 to 205 degrees Celsius

                   Ask a Scientist. How does Ethanol increase octane rating in gasoline? 

            c. Kerosene - fuel for jet engines and tractors; starting material for making other products

                mix of alkanes (10 to 18 carbons) and aromatics. (dye is added to the fluids for safety reasons)

                boiling range = 350 to 617 degrees Fahrenheit / 175 to 325 degrees Celsius

            d. Gas oil or Diesel distillate - used for diesel fuel and heating oil; starting material for making other products

                alkanes containing 12 or more carbon atoms

                boiling range = 482 to 662 degrees Fahrenheit / 250 to 350 degrees Celsius

            e. Lubricating oil - used for motor oil, grease, other lubricants

                long chain (20 to 50 carbon atoms) alkanes, cycloalkanes, aromatics

                boiling range = 572 to 700 degrees Fahrenheit / 300 to 370 degrees Celsius

             f. Heavy gas or Fuel oil - used for industrial fuel; starting material for making other products

                long chain (20 to 70 carbon atoms) alkanes, cycloalkanes, aromatics

                boiling range = 700 to 1112 degrees Fahrenheit / 370 to 600 degrees Celsius

        3. Heavy weight. Alkanes that are typically solids at room temperatures

            a. Residuals - coke, asphalt, tar, waxes; starting material for making other products   

                multiple-ringed compounds with 70 or more carbon atoms

                boiling range = greater than 1112 degrees Fahrenheit / 600 degrees Celsius

    Alkanes are normally refined from crude oil through distillation and other chemical process.

Oils: Refining & their Origins

Resource: Refining Oil. Howstuffworks.  Very cool animated picture of the refining process.

    B. Physical Properties of Alkanes (& other members of a homologous series).  10.1.2 & 10.1.13

 
     1. Alkanes are nonpolar molecules that are insoluble in polar solvents like water and soluble in nonpolar solvents. 

         They dissolve organic substances of very low polarity. (Hydrocarbons containing functional groups change their physical

         properties. The more polar the functional group is, the lower the volatility and higher the solubility. BUT, longer hydrocarbon

         chains can create a greater nonpolarity and reduce the affect of the functional group).

     2. Less dense than water (only C and H), colorless, tasteless, odorless.

         a. Density of straight-chain alkanes increases with increasing molecular mass

         b. Alkanes containing 5 carbons up to about 19 are colorless liquids

         c. alkanes with more than about 20 carbon atoms are colorless, waxy solids (paraffin wax is a mixture of 

             solid alkanes)

     3. Low melting point and boiling points in general as compared to similar weight compounds.

     4. The boiling point and melting points of straight chain alkanes increase with molecular weight.

     5. Straight chain isomers have higher boiling point than their branched-chained isomers. This is due higher London

         dispersion forces with straight-chained molecules which have greater surface areas.

         Discussion of dispersion forces in alkanes

       The properties of alkanes are dependent upon two factors

        a. Magnitude of dispersion forces

         -Dispersion forces (London Forces) are IMAs caused from temporary dipoles

         -The force is proportional to mass (or size). Greater polarization properties exist within larger atoms/molecules.

          ex. IMAs (dispersion forces) between Cl₂ molecules and Br₂ molecules are estimated to be 0.7 kcal/mol and 1.0 kcal/mol.

         Bromine molecules are larger molecules and therefore possess greater polarization potentials (movement of electrons) 

       b. Stacking properties (branching dependence)

        -Dispersion forces are dependent upon the surface area of the molecules.

        -Branching decreases the available surface area for alkanes and thus reduces IMA forces.

C. Chemical nature of alkanes and chemical properties of alkanes

    1. Chemical Properties of Alkanes 10.2.1

        a. Alkanes are composed principally of carbon and hydrogen. Alkanes represent a highly reduced form of carbon,

            due to carbon being more electronegative than hydrogen (but still small enough De.n. to be nonpolar).

        b. Alkanes are usually stable at room temperature toward reactants such as concentrated aqueous acids or 

            bases, and even the most reactive metals. pK_a values of alkanes are above 60 (don't lose hydrogens);

        c. Fluorine, however, attacks virtually all organic compounds, including the alkanes, to give mixtures of products;

        d. Hot nitric acid, chlorine, and bromine can also react with alkanes;

             Alkanes will undergo halogenation by substitution reaction in the presence of ultraviolet light via a 

             radical chain mechanism.

        e. The chlorination of methane can give the following compounds depending on the relative quantities of reactants used;

        f. When heated at high temperatures in the absence of air, alkanes can "crack", meaning that they break up into smaller

             molecules. The cracking of methane gives finely powered carbon and hydrogen gas;
                        

                    CH₄ --> C + 2H₂

        g. The controlled cracking of ethane gives ethene ("ethylene"), which is an important raw material in the organic 

            chemicals industry, used to make polyethylene plastics, ethyl alcohol, and ethylene gycol (in antifreeze).      
                

                    CH₃CH₃ --> CH₂=CH₂ + H₂    (This requires temperatures between 800-900 ^oC.)
                    ethane                ethene

        h. Alkanes will combust in the presence of oxygen: commonly used as fuels since large amounts of energy are 

            released, the longer the chain, the more bonds are broken, the greater the energy that is released.

                * Alkanes with flashpoints below room temperature (the components of petrol for example) should be stored

                in strong metal containers with narrow mouths & tightly sealed lids to prevent the vapour from escaping & to 

                prevent a naked flame or spark from igniting the vapour/air mixture.

    2. Reactions of Alkanes- Redox processes with oxygen and halogens (high e.n. entitites)

        a. Combustion. An oxidation of an alkane with oxygen.  Exothermic reaction that requires and initial source of energy. 10.2.2

            1. Complete combustion. Production of CO₂, H₂O & energy as a result of excess oxygen.

                Ex.      CH_4(g)      +      2 O_2(g)       -->      CO_2(g)      +     2 H₂O_(l)       DH^o = -886 kJ/mol

                           C₃H_8(g)    +      5 O_2(g)       -->     3 CO_2(g)      +    4 H₂O_(l)     DH^o = -2220 kJ/mol

                As the number of carbons increase, the standard enthalpy increases (more bonds = more energy)

            2. Incomplete combustion. Production of CO₂, H₂O, & energy along with CO and C as a result of insufficient

                oxygen.  

                Ex. CH_4(g)     +     O_2(g)     -->     C_(s)     +     2H₂O_(l)  (solid carbon is called soot)

                     2CH_4(g)     +     3O_2(g)  -->     2CO_(g) +     4H₂O_(l)    (CO is carbon monoxide)

                It  is extremely important that any combustion system is as efficient as possible, eg. car engines, gas heaters,

                furnaces, etc. must all have excellent ventilation for complete combustion to harmless water and carbon dioxide. 

                Periodic check of all air filters is essential as plugged filters hinder the accessibility of oxygen to the reaction mixture.

                Carbon monoxide is colorless and odorless and even low concentrations in the air can be fatal.  Carbon monoxide 

                is unfortunately emitted by all car exhausts, though catalytic converters help reduce this by converting nitrogen monoxide 

                (another pollutant) and carbon monoxide into harmless nitrogen and carbon dioxide. 

                    2NO_(g) + 2CO_(g) --> N_2(s) + 2CO_2(l)

Resource:  How does a catalytic converter work?  Howstuffworks        

        b. Halogenation (Cl₂ & Br₂) of Alkanes. 10.2.3 & 10.2.4

            This typically requires light or heat as a source of activation energy.  

            Fluorine is typically not used because fluorinated alkanes are very unstable and the reactions are highly exothermic.

            Iodine  is seldom used because the reaction is endothermic with an equilibrium favoring the iodine/alkane reactants rather

            than the iodoalkane and HI. 

            1. The reaction is characterized as being a substitution reaction. A hydrogen from the alkane will be substituted by 

            a halogen atom.

                Ex. CH₄    +    Cl₂      ^hv  >     CH₃Cl    +    HCl          DH^o = -105 kJ/mol

            2. Mechanism:  Halogenation of alkanes proceeds via a radical chain mechanism

                a. Initiation- light (hn) or heat is needed to break bond in the dihalide (homolysis- splitting a molecule into 2 equal parts)

                b. Propagation- halide radicals singly oxidize alkanes (due to high concentration) thus producing alkyl radicals

                c. Termination- two radicals (alkyl/alkyl radicals, halide/halide radical or alkyl/halide radicals combine)

            - Antioxidants are molecules that form stable radicals, via hyperconjugation & resonance.

                BHT- Butylatedhydroxytoluene (2,6-di-tert-butyl-4-methylphenol)      

                BHA- Butylatedhydroxyanisole (2-tert-butyl-4-methoxyphenol)

            3. Regioselectivity

                a. Product formation of based on probability

                    ex. CH₃CH₂CH₃   +  Br₂  --> CH₃CH₂CH₂Br   +  CH₃CHBrCH₃

                    -There are 6-primary bonded hydrogen atoms and 2 secondary bonded hydrogen atoms, so based on probability

                      1-bromopropane = 6/8 (75%) and 2-bromopropane = 2/8 (25%)

                    -But the actual yield is:  2-bromopropane = 92% and 1-bromopropane = 8%

                b. Regioselectivity:  reactions proceed where a specific carbon is preferred based on its position in the molecule

                    3^o > 2^o > 1^o ; a more substituted carbon is preferred because the radical is more stable.

-Carbon radicals are stabilized by electron-releasing substituents (low e.n.) and destabilized by electron-withdrawing substituents (high e.n.).

D. Properties of stereoisomers 20.6.4 & 20.6.7

     - Physical properties (m.p., b.p., solubilities, densities, etc.) are dependent upon the degree of intermolecular attractions. Chemical properties are

        very much dependent upon bond arrangement and energies.

    1. Enantiomers-

        a. physical properties- The physical properties are commonly identical between two enantiomers, due to similarities in IMA potentials.

        b. chemical properties- The two enantiomeric forms of a compound will be chemically identical, unless they are reacting with another specific

             stereoisomer or catalyst (such as an enzyme).

Resource: Chiral Drugs

    2. Diastereomers (geometric isomers)

        a. physical & chemical properties of diastereomers are different due to the differences in polarities within the molecules. 

Resources of Information

    1. How Oil Refining Works. (http://www.howstuffworks.com/oil-refining2.htm)