Synthesis, Properties and Reactions of Alkanes ORGANIC CHEMISTRY- 1 Synthesis, Properties and Reactions of Alkanes By Dr. Sulaiman Al Sulaimi Assistant Professor UNIVERSITY OF NIZWA

These are aliphatic hydrocarbons in which carbons atoms are joined by single covalent bonds. These are saturated organic compounds, General Formula CnH2n+2 Alkyl The part of an Alkane obtained after the removing the one hydrogen atom CH4 becomes -CH3 (Methyl) C2H6 becomes -C2H6 (Ethyl) Alkanes

Structure In Methane, carbon forms single bonds with four hydrogen atoms. Since the carbon atom is attached to four other atoms. It uses sp3 hybrid orbitals to form these bonds. Each C-H bond is the result of the overlap of an sp3 orbital from carbon and an s orbital from Hydrogen. All C-H bonds are sigma bonds.

Methods of Preparation Hydrogenation of Alkenes or Alkynes Alkenes or Alkynes react with the hydrogen in the presence of nickel catalyst at 200-300 oC to form alkenes. Other catalyst which be used are platinum and palladium.

From Alkynes

From Alkyl Halides Alkyl Halides undergo reduction with nascent hydrogen to form Alkanes The hydrogen for reduction may be obtained by using any of the following reducing agents Zn+HCl, Zn + CH3COOH, Or LiAlH4

Hydrolysis of Grignard Reagents Alkyl magnesium halides (Grignard reagents) are obtained by treating alkyl halide with magnesium in anhydrous ether. These on treatment with water give alkane.

Wurtz Synthesis Higher alkanes are produced by heating an alkyl halide (RX) with sodium metal in dry ether solution. Two molecules of the alkyl halide lose their halogen atom as NaX. The net results is the joining of two alkyl group to yield a symmetrical alkane (R-R type) having an even number of carbon atoms.

By Decarboxylation of Carboxylic Acid When the sodium salt of carboxylic acid is heated strongly with sodalime (NaOH + CaO), a molecule of carbon dioxide is split of as carbonate and an alkane is formed. Notice that alkane produced contains one carbon less then the original carboxylic acid. By Decarboxylation of Carboxylic Acid

Kolbe’s Synthesis When a concentrated solution of sodium salt of carboxylic acid is electrolyzed, an alkane is formed. This reaction is only suitable for the preparation of symmetrical alkanes i.e. those of the type R-R.

Properties of Alkanes Physical Properties of alkanes depends on the number and on the way in which the C atoms are linked together. Under standard condition, the normal hydrocarbons are gases (from C1 to C 4). They are liquid from C5 to C17. They are wax like solids from C18 and above.

PHYSICAL PROPERTIES Boiling points and melting points increase as size of alkane increases. The rise in boiling point is due to the increased attraction (temporary dipoles, dispersion) between molecules which increase as molecule size increases, accounting for the higher melting and boiling points of larger alkanes.

Melting and Boiling Points of Alkanes Branching of the chain always results in the lowering of the boiling point Which can be explained as follow “As branching increases the molecule becomes more compact, decreasing its surface area.

Solubility: Alkanes are nonpolar compounds Solubility: Alkanes are nonpolar compounds. Their solubility may be predicted by “Like dissolve like rule” which means nonpolar compounds are soluble in nonpolar solvent and polar compounds are generally soluble in polar solvent.