Properties &preparation of alkanes

Properties &preparation of alkanes Lec.3

Physical properties of alkanes: • Alkanes are insoluble in water. This is because water molecules are polar, whereas alkanes are nonpolar(C-C and C-H bonds )SO it is soluble in non-polar solvents such as benzene, ether and chloroform . • Note: • All the C-C and C-H bonds are nearly purely covalent bonds. • The O-H bond in a water molecule is strongly polarized by the high electronegativity of oxygen. • Alkanes have lower boiling points for a given molecular weight than most other organic compounds. This is because they are nonpolar molecules.

Note: The boiling points of alkanes rise as the chain length increases and fall as the chains become branched • Alkanes ordinarily do not react with most common acids, bases

Cycloalkane Nomenclature • Cycloalkanes are saturated hydrocarbons that have at least one ring of carbon atoms. A common example is cyclohexane

EXAMPLES:

Cis–Trans Isomerism in Cycloalkanes • Cis–trans isomerism (sometimes called geometric isomerism). • The methyl groups are said to be cis(on the same side) or trans (across to each other.) • Cis–trans isomers differ from one another only in the way that the atoms or groups are positioned in space. • Yet this difference is sufficient to give them different physical and chemical properties.

Reactions of Alkanes 1.Oxidation and Combustion; Alkanes as Fuels: • Alkanes burn to form carbon dioxide and water. 2. Halogenation of Alkanes When a mixture of an alkane and chlorine gas is stored at low temperatures in the dark, no reaction occurs. In sunlight or at high temperatures, One or more hydrogen atoms of the alkane are replaced by chlorine atoms. The reaction is called chlorination. This process is a substitution reaction, as a chlorine is substituted for a hydrogen. CH4+CL2 Heat or U.V NOTE: An analogous reaction, called bromination, occurs when the halogen source is bromine. CH3CL+CH2CL2+CHCL3+CCL4+HCL

The Free-Radical Chain Mechanism of Halogenation A free-radical chain reaction includes a chain-initiating step, chain-propagating steps, and chain-terminating steps 1-The chain-initiating step is the breaking of the halogen molecule into two halogen atoms The Cl-Clbond is weaker than either the C-H bond or the C-C bond ,when the molecular chlorine (Cl2) absorbs visible light but alkanes do not. Thus, the Cl-Clbond would break first.

2-The chain-propagating steps are: • Chlorine atoms are very reactive, because they have an incomplete valence shell (seven electrons instead of the required eight) They may either recombine to form chlorine molecules or, if they collide with an alkane molecule, abstract a hydrogen atom to form hydrogen chloride and an alkyl radical R •. • Radical is a fragment with an odd number of unshared electrons ,so the alkyl radical was to collide with a chlorine molecule (Cl2), • it could form an alkyl chloride molecule and a chlorine atom ,The chlorine • atom formed in this step can then react to repeat the sequence.

3- chain-terminating steps No new radicals are formed in these reactions, so the chain is broken or, as we say, terminated

Preparation of alkanes: (1)-Hydrolysis of grinard reagent

(2)-Coupling of alkyl halides with grinard reagent (3)-Reduction of alkyl halides using metal and acid:

(4)-Hydrogenation of alkenes

Properties &preparation of alkanes