Chapter 4 ch.pdf

College of Medicine – ‫كلية الطب‬ Saturated Hydrocarbons “ALKANES” College of Medicine – ‫كلية الطب‬ After this lecture, the student will be able to:  Identify alkanes and the several ways to express alkanes  Explain the bonding in alkanes according to VSEPR  Explain how hybridization occurs in c...

College of Medicine – ‫كلية الطب‬ Saturated Hydrocarbons “ALKANES” College of Medicine – ‫كلية الطب‬ After this lecture, the student will be able to:  Identify alkanes and the several ways to express alkanes  Explain the bonding in alkanes according to VSEPR  Explain how hybridization occurs in carbon atom  Illustrate the formation of sigma bond in alkanes  Explain the physical properties of alkanes  Explain the chemical properties of alkanes  Illustrate the reactions of alkanes College of Medicine – ‫كلية الطب‬ Alkane is a hydrocarbon in which there are only single covalent bonds With a general formula Can be acyclic (no rings) or cyclic (cycloalkanes) The simplest alkane is Methane (CH4), it is the major component of natural gas College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ There are several ways to draw the formulas for alkanes Kukle Skeletal College of Medicine – ‫كلية الطب‬  The table below shows the ball-andstick models, and space-filling models for methane and ethane Molecular Orbital Theory College of Medicine – ‫كلية الطب‬   Generate hybrid orbitals by “mixing” atomic orbitals in order to achieve the VSEPR geometries Based on the twin concepts that ◦ atoms are surrounded by regions of electron density ◦ regions of electron density repel each other College of Medicine – ‫كلية الطب‬ The Valence Shell Electron Pair Repulsion Model College of Medicine – ‫كلية الطب‬  Two electron pairs in the valence orbital are arranged linearly  Three electron pairs are organized in a trigonal planar arrangement  Four electron pairs are organized in a tetrahedral arrangement College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ That’s why atomic orbitals undergo hybridization College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ A tetrahedral atom has sp3 hybridization Head to head overlapping College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ Intermolecular forces: College of Medicine – ‫كلية الطب‬     Van der Waals interactions (London dispersion forces)* These are instantaneous induced-dipole interactions Important interactions between nonpolar compounds Increases with the surface of interaction between molecules, and therefore increases with molecular weight Melting and boiling points: College of Medicine – ‫كلية الطب‬  Melting and boiling points increase with increasing molecular weight within a homologous series College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ Melting and boiling points: College of Medicine – ‫كلية الطب‬ Solubility: Solubility – “Like dissolves like” Alkanes are nonpolar, hydrophobic They are soluble in nonpolar solvents and insoluble in water. College of Medicine – ‫كلية الطب‬ Solubility: College of Medicine – ‫كلية الطب‬ Alkanes are rather unreactive due to the presence of only C—C and C—H bonds. Therefore, they make great nonpolar solvents. College of Medicine – ‫كلية الطب‬  In addition to combustion reaction, Alkanes undergo two important reactions:  Halogenation  Dehydrogenation (Substitution) (Elimination) College of Medicine – ‫كلية الطب‬  Halogenation (Substitution Reaction): College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬  Dehydrogenation (Elimination Reaction): College of Medicine – ‫كلية الطب‬ Summary College of Medicine – ‫كلية الطب‬ Colors of Alkanes Summary Methane to butane are colorless gases (propane and butane are easily condensed under pressure and are commonly sold as liquids) Alkanes containing 5 carbons up to about 19 are colorless liquids (petrol and kerosene are mixtures of liquid alkanes, dye is added to the fluids for safety reasons) Alkanes with more than about 20 carbon atoms are colorless, waxy solids (paraffin wax is a mixture of solid alkanes) College of Medicine – ‫كلية الطب‬ Summary Alkanes are less dense than water (alkanes float on top of water) Alkanes are non-polar molecules so they are more soluble in non-polar solvents than they are in polar solvents. Alkanes are insoluble in water The melting and boiling points of the shorter chain alkanes is low, but the melting and boiling of alkanes increase as the number of carbon atoms in the carbon chain increases College of Medicine – ‫كلية الطب‬ Summary VOLATILITY Due to their low boiling points, lower alkanes are highly volatile. Volatility refers to the ability of a liquid to change into vapor state. Among alkanes volatility decreases with increase in chain length. Among isomeric alkanes more the branching, greater is the volatility At room temperature, alkanes from C1 to C4 are gases, from C5 to C16 they are liquids and higher alkanes are solids and wax like to some extent Summary College of Medicine – ‫كلية الطب‬      Alkanes are relatively unreactive Alkanes do not react with strong acids, bases, oxidizing agents (oxidants) or reducing agents (reductants) 1. Alkanes combust (react rapidly with oxygen) releasing energy, which makes alkanes useful as fuels. 2.Alkanes will react with halogens such chlorine gas and bromine in the presence of ultraviolet light 3. Alkanes undergo elimination reactions to produce alkenes.