Lecture 7: Alkanes

Alkanes

• Hydrocarbons

• Contain only carbon and hydrogen

Alkanes often described as saturated hydrocarbons

• Saturated

• Contain maximum possible number of hydrogens per carbon and have only C-C and C-H single bonds

Alkane

• contains only carbon-carbon single bonds from the sigma-overlap of sp3 hybrid orbitals giving strong sigma bonds

Alkyl

If a hydrogen is removed from an alkane, the resulting partial structure is called an alkyl group

Example: -CH3 Mthyl (Me-)

Classification

• Refer to the number of other carbon atoms attached to the branching carbon atom

• Four possibilities

• Primary (1o)

Secondary( 2o)

Tertiary (3o)

Quaternary (4o)

Conformations of Ethane

Stability of conformations

Staggered

• Lowest energy, most stable conformation

• All six C-H bonds are as far away from one another as possible

Eclipsed

• Highest energy, least stable conformation

• The six C-H bonds are as close to one another as possible

Physico-chemical Properties of alkanes

• Structures do not contain any polar groups capable of forming hydrogen or ion dipole bonds

• Insoluble in water (hydrophobic)

• However they have high solubility in lipids (lipophilic) due to strong van der waals forces of attraction between hydrocarbons

• Alkanes move into the lipid area in an aqueous-lipid interface.

• This movement also occurs in biological systems

• e.g. the general anaesthetic alkanes partition rapidly into the lipid portion of the brain

• Alkanes such as cyclopropane are also absorbed via the lungs

• Alterations in alkyl chain length on drug molecules such as increasing or decreasing chain length, branching and changing ring size

→ Have profound effects on potency and pharmacological activity of the molecule

• Change in chain length alter the lipophilic character of the molecule and therefore its absorption, distribution and excretion properties.

• Branching of a hydrocarbon will result in a less lipophilic compound and can significantly alter biological effect.

• This decrease in lipophilicity as a result of chain branching - the chain becomes more compact and therefore produces less disruption of the H-bonding network of water.

Chemical Properties of alkanes

• Alkanes are exceptionally inert substances.

• Alkanes contain only strong sigma bonds and all bonds (C—C and C—H)

• Unreactive towards most reagents

• Do not react with ionic or polar substances. Also inert to acids or bases

• One of their major reactions is combustion

• Undergo combustion (or oxidation) reaction in oxygen at high temp to give water and carbon dioxide (greenhouse gas).

• Reaction is exothermic, which is why alkanes good fuels.

• Oxidation of saturated hydrocarbons is the basis for their use as energy sources for heat and power e.g. natural gas, LPG, diesel and petrol

• The other common reaction of alkanes is halogenation.

• Such reactions occur in the presence of ultraviolet (UV) light, or at high temps

• These reactions occur through a free radical mechanism and are extremely rapid

• Depending on the amounts of chlorine and methane present, further substitution can happen to give dichloromethane, trichlormethane (chloroform) and tetrachloromethane (carbon tetrachloride).

• Alkanes react readily with radicals e.g. Cl

.

• Radicals are species that have a single(unpaired) electron in its outer shell

• Irradiation of a molecule of chlorine, Cl2, with uv-light causes breaking (photolysis) of the Cl—Cl bond to give two chlorine radicals