hydrocarbon aliphatic alkanes alkenes aromatic alkynes n n

hydrocarbon aliphatic alkanes alkenes aromatic alkynes

n n n Hydrocarbon: Compound composed of only carbon and hydrogen Saturated Hydrocarbons: Compound with only single bonds Unsaturated Hydrocarbons: Compounds with AT LEAST one double or triple

Alkanes represent the most basic functional group n within organic chemistry. They contain only carbon and hydrogen n all carbons are sp 3 n all bond angle are 109. 5 o n

Methane n n n Methane (CH 4) the simplest alkane Again, methane is tetrahedral with dihedral angles of 109. 5 o.

Physical Properties of Methane n n n n nonpolar, insoluble in water, but very soluble in benzene, CCl 4, ether and gasoline. intermolecular force is Van der Waals the boiling point = -161. 5 o. C the melting point = - 183 o. C it is gas at room temperature Colorless combustible nontoxic when inhaled, but it can produce suffocation by reducing the concentration of oxygen inhaled

Sources of Methane n n Decayed plants produces methane {manufactured by the distillation of bituminous coal. Coal is a combustible rock formed from the remains of decayed vegetation}. Sources can be anthropogenic or natural Can be produced in the laboratory by heating sodium acetate with sodium hydroxide Produced by the reaction of aluminum carbide (Al 4 C 3) with water.

Common name of methane n n n firedamp march gas biogas

Common Uses of Methane 1. Important source of heat n Complete combustion: n CH 4 + 2 O 2 CO 2 + 2 H 2 O +heat( 213 kcal) 2. Use in the manufacture of CH 3 OH and other alcohol n Incomplete combustion: n CH 4 + O 2 850 o, Ni CO + H 2 CH 3 OH + other alc

3. Use in the manufacture of ammonia 3 H 2 + N 2 2 NH 3 4. A mixture of CH 4, H 2 O, NH 3, and H 2 are allowed to pass thru electric discharge converted to large molecules Example: amino acid , the building block of protein

Chemical Properties of Methane: 1. Combustion/ Oxidation to produced heat n Complete combustion: CH 4 + 2 O 2 CO 2 + 2 H 2 O + heat( 213 kcal) n Incomplete combustion: CH 4 + 2 O 2 CO( soot) 2. Halogenation CH 4 X 2 CH 3 X Light/ heat n Where X: F 2 > Cl 2 > Br 2 > I 2

Mechanism of the reaction CH 4(g) + Cl 2(g) CH 3 Cl(g) + HCl(g) This reaction has the following characteristic properties. n It doesn't take place in the dark or at low temperatures. n It occurs in the presence of ultraviolet light or at temperatures above 250 o. C. n Once the reaction gets started, it continues after the light is turned off. n The products of the reaction include CH 2 Cl 2 (dichloromethane), CHCl 3 (chloroform), and CCl 4 (carbon tetrachloride), as well as CH 3 Cl (chloromethane). n The reaction also produces some C 2 H 6.

These facts mechanism initiation, are consistent with a chain-reaction that involves three processes: chain propagation, and chain termination. Chain Reaction Mechanism 1. Chain Initiation A Cl 2 molecule can dissociate into a pair of chlorine atoms by absorbing energy in the form of either ultraviolet light or heat. Cl 2 2. Cl ∆Ho = 243. 4 k. J/mol n The chlorine atom produced in this reaction is an example of a free radical an atom or molecule that contains one or more unpaired electrons.

The reaction doesn't occur in the dark or at low temperatures because energy must be absorbed to generate the free radicals that carry the reaction. The reaction occurs in the presence of ultraviolet light because a UV photon has enough energy to dissociate a Cl 2 molecule to a pair of Cl atoms. The reaction occurs at high temperatures because Cl 2 molecules can dissociate to form Cl atoms by absorbing thermal energy.

2. Chain Propagation Free radicals, such as the Cl atom, are extremely reactive. When a chlorine atom collides with a methane molecule, it can abstract a hydrogen atom to form HCl and a CH 3 radical. CH 4 +. Cl . CH 3 + HCl ∆Ho = -16 k. J/mole

If the CH 3 radical then collides with a Cl 2 molecule, it can remove a chlorine atom to form CH 3 Cl and a new Cl radical. n. CH 3 + Cl 2 CH 3 Cl +. Cl ∆Ho = -87 k. J/mole Because a Cl atom is generated in the second reaction for every Cl atom consumed in the first, this reaction continues in a chain-like fashion until the radicals involved in these chain-propagation steps are destroyed.

3. Chain Termination If a pair of the radicals that keep the chain reaction going collide, they combine in a chain-terminating step. Chain termination can occur in three ways. n 2. Cl Cl 2 ∆Ho = -243. 4 k. J/mole n. CH 3 +. Cl CH 3 Cl ∆Ho = -330 k. J/mole n 2. CH 3 CH 3 ∆Ho = -350 k. J/mole n Because the concentration of the radicals is relatively small, these chain-termination reactions are relatively infrequent.

Uses of Halogenated Compounds n n The chlorinated derivatives of methane have been known for so long that they are frequently referred to by the common names shown in the figure below. Methyl chloride, Methylene chloride, Chloroform, Carbon tetrachloride, BP = - 24. 2 OC, BP = 40 OC, BP = 61. 7 OC BP = 76. 5 OC , respectively gas liquid

n n These chlorinated hydrocarbons make excellent solvents for the kind of nonpolar solutes that would dissolve in hydrocarbons. They have several advantages over hydrocarbons; they are less volatile and significantly less flammable. Chloroform (CHCl 3) and carbon tetrachloride (CCl 4) react with hydrogen fluoride to form a mixture of chlorofluorocarbons, such as CHCl 2 F, CHCl. F 2, CCl 3 F, CCl 2 F 2, and CCl. F 3, which are sold under trade names such as Freon and Genetron. The freons are inert gases with high densities, low boiling points, low toxicities, and no odor. As a result, they once found extensive use as propellants in antiperspirants and hair sprays. Controversy over the role of chlorofluorocarbons in the depletion of the Earth's ozone layer led the Environmental Protection Agency to ban the use of CCl 2 F 2 and CCl 3 F in aerosols in 1978. CCl 2 F 2, CCl 3 F and CHFCl 2 are still used as refrigerants in the airconditioning industry, however.

Thank You By: Maridit C. Pedrosa