These are aliphatic hydrocarbons in which carbons atoms are joined by single covalent bonds. These are saturated organic compounds. C n H 2n+2 The part of an Alkane obtained after the removing the one hydrogen atom CH 4 becomes -CH 3 (Methyl) C 2 H 6 becomes -C 2 H 6 (Ethyl)
Structure H Sp 3 -S H Sp 3 -S C H H o 109A C o 109A H H H H Sigma Bonds H 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.
The Methane Problem The ground state configurations for C can be written as 1s 2 2s 2 2p x1 2p y 1 Using the valence bond picture and the concept of paired electrons in molecular orbitals we might expect C to react with H atoms to form CH 2. The CH 2 molecule does form but is unstable (a transient species). However, carbon happily reacts with H to form the methane, CH 4.
Methane and Hybridization By experiment, as previously discussed, methane has a regular tetrahedral geometry four equal bond distances and all bond angles of 109.5 o. The regular geometry of methane and its ability to form four bonds can be explained using the concept of hybridization.
Methane and Hybridization cont d: In the hybridization picture we imagine methane being formed from C and H atoms in three steps. In the first step we take a ground state C atom and excite one electron (from the 2s orbital) to form the lowest lying ( or first) excited state. Carbon Ground State: 1s 2 2s 2 2p x1 2p y 1 Carbon Excited State: 1s 2 2s 1 2p x1 2p y1 2p z 1
Methane and Hybridization cont d: In the second step we imagine combining the single occupied 2s orbital and the three occupied 3p orbitals in the excited to form four equivalent sp 3 hybrid orbitals (each containing a single unpaired electron). In step three the hybridized C atom reacts with four H atoms to form a CH 4 molecule. The process is represented on the next few slides.
Hybridization of Atomic Orbitals
Bonding and structure of CH4
Hydrogenation of Alkenes or Alkynes Alkenes or Alkynes react with the hydrogen in the presence of nickel catalyst at 200-300 o C to form alkenes. Other catalyst which be used are platinum and palladium. R-CH=CH 2 + H 2 R-CH 2 -CH 3 Alkene Ni Alkane CH 2 =CH 2 + H 2 CH 3 -CH 3 Alkene Ni Ethane CH 3 -CH=CH 2 + H 2 Alkene Ni CH 3 -CH 2 -CH 3 Propane
R C CH + 2H Ni 2 R-CH 2 -CH 3 Alkane Alkyne HC CH + 2H Ni 2 Ethane Acetylene CH 3 -CH 3
Alkyl Halides undergo reduction with nascent hydrogen to form Alkanes R X + 2 H R-H Alkyl Halide Alkane + HX H 3 C I + 2 H CH 4 + HI Methyl Iodide Methane CH 3 -CH 2 -Br+ 2 H Ethyl Bromide CH 3 -CH 3 Ethane + HBr The hydrogen for reduction may be obtained by using any of the following reducing agents Zn+HCl, Zn + CH 3 COOH, Or LiAlH 4
Alkyl magnesium halides (Grignard reagents) are obtained by treating alkyl halide with magnesium in anhydrous ether. These on treatment with water give alkane. R-X +Mg RMgX RMgX + HOH ether Alkylmegnessium Halide R-H Alkane Mg X OH CH 3 MgX Methylmagnessium iodide CH 4 + HOH Mg Alkane I OH
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. ether R-X + 2Na + X-R R-R + 2NaX Alkane ether CH 3 -Br + 2Na + Br-CH 3 CH 3 -CH 3 + 2NaBr Methylbromide Ethane
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. R-COONa + NaOH R-H Na 2 CO 3 Sod salt of Acid Alkane CH 3 -COONa + NaOH CH 4 Na 2 CO 3 Sodium Acetate Methane
When a concentrated solution of sodium salt of carboxylic acid is electrolyzed, an alkane is formed. 2RCOONa + 2H 2 O R-R + 2CO 2 At cathode + 2NaOH at anode + H 2 2CH 3 COONa + 2H 2 O CH 3 -CH 3 At cathode + 2CO 2 + 2NaOH at anode + H 2 This reaction is only suitable for the preparation of symmetrical alkanes i.e. those of the type R-R.
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.
PROPERTIES OF ALKANES CHEMICAL PROPERTIES: Alkanes are called paraffins (low affinity compounds) because they are relatively uncreative. This term describes that alkanes show little chemical affinity for other substances and are chemically and biologically inert. The alkanes are inert due to factors below; a. Strong C-C bond and C-H bonds b. Absence of functional group
The reactivity of alkanes is limited for instance, Combustion needs a flame to get it started. Halogens react in the presence of heat or light. Much of the chemistry of alkanes involve free radical and chain reactions are involved.
1- Reaction with oxygen occurs during combustion in an engine or furnace when the alkane is used as a fuel. They burn in a flame, producing carbon dioxide, water, and heat. CH 4 + 2 O 2 CO 2 + 2 H 2 O + 890 kj/mol (213 kcal/mol) The amount of heat evolved when one mole of a hydrocarbon is burnt to carbon dioxide and water is called heat of combustion.
2- Reaction with Halogens : The reaction of halogen with alkanes is called halogenation. The reaction of an alkane with Cl 2 occurs when a mixture of the two is irradiated with ultraviolet light as (UV) represented as hy, where y is the Greek letter nu). Alkanes also react with fluorine and bromine. However reaction with Iodine is reversible.
Reaction with Halogens UV light CH 4 + Cl 2 CH 3 Cl Methane Methyl Chloride Chloromethane The reaction does not stop at this stage. Depending upon time and Cl 2, the remaining three hydrogen atoms of methyl Chloride can be successively replaced by chlorine atoms.
Reaction with Halogens CH 3 Cl + Cl 2 CH 2 Cl 2 + Methylene Chloride or Dichloromethane HCl CH 2 Cl 2 + Cl 2 CHCl 3 Chlorof orm or trichloromethane + HCl CHCl Cl 3 + 2 CCl 4 Carbon Tetra Chloride or tetrachloromethane + HCl Bromine (Br) reacts with alkanes in a similar manner but less vigorously. Reaction with Iodine is reversible. CH 4 + I 2 CH 3 I + HCl Methane Methyliodide
3- Nitration: Alkanes undergo nitration react at high temperature with the replacement of H- atom by Nitro group (NO2) is called nitration, Possible with conc. HNO3. Example: 400-500 o R H + HO NO 2 R-NO 2 + H 2 O Nitroalkane CH3 -H + HO NO 2 400-500 o CH 3 -NO 2 + Nitromethane H 2 O
4- Sulphonation: It is the replacement of H- atom of Alkanes by SO 3 H group. Sulphonation of alkanes occurs with fuming suphuric acid at moderate temperature. H CH 3 -CH 2 -C H + HO SO 3 H CH 3 -CH 2 -CH 2 -SO 3 H + H 2 O H propane sulphonic acid
5- Pyrolysis or Cracking: The decomposition of a compound by the action of heat alone is called pyrolysis. (pyr = fire and lysis = split up i. e cleavage by heat. The pyrolysis of alkanes is known as cracking. Alkanes decompose into smaller molecule on heating to 500-700 o C. Ethane when heated to 500 o C in the absence of air, gives a mixture of methane, ethylene and hydrogen CH 3 -CH 3 500 o C Absance of air H 2 C CH 2 + CH 4 + H 2 Ethylene 500 o C CH 2 -CH 2 -CH 3 Absance of air H 3 C C CH 2 + H 2 C CH 2 H CH 4 + H 2
6- Dehydrogenation: It is the removal of hydrogen from molecule Benzene and other aromatic hydrocarbons are prepared from petroleum hydrocarbons. Example: n-hexane yields benzene. H 2 C H 2 C CH 2 CH 3 CH 3 Cr 2 O 3 + 4H2 CH 2 500 o C
PHYSICAL PROPERTIES 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. 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.
Stereochemistry Stereochemistry is the branch of chemistry which concerned with the three-dimensional (3D) aspects of molecules. Rotation is possible around carbon carbon (C C) bonds in open-chain molecules. In ethane, for instance, rotation around the C - C bond occurs freely, constantly changing the spatial relationships between the hydrogens on one carbon and those on the other.
Conformational isomers The different arrangements of atoms that result from bond rotation are called conformations, and molecules that have different arrangements are called conformational isomers, or conformers. Different conformers often can not be isolated because they interconvert very rapidly.
Conformational isomers Conformational isomers are represented in two ways; Sawhorse representation: It shows molecules at an angle, showing a molecular model. C-C bonds are at an angle to the edge of the page. Newman projections: Bonds to front carbon are lines going to the center while bonds to back/rear carbon are lines going to the edge of the circle as shown;
Conformational isomers Experiments show that there is a small (12 kj/mol or 2.9 kcal/mol) barrier to rotation and that some conformations are more stable than others. The lowest energy, most stable conformation is the one in which all six C - H bonds are as far away from one another as possible called staggered conformation when viewed end-on in a Newman projection. The highest-energy, least stable conformation is the one in which the six C -H bonds are as close as possible called eclipsed in a Newman projection.
Conformational isomers Conformational situation is more complex for larger alkanes. Not all staggered conformations have same energy, and not all eclipsed conformations have same energy. In butane, the lowest-energy arrangement, called the anti conformation, is the one in which the two methyl groups are as far apart as possible 180 away from each other.
Conformational isomers Butane- Anti conformation Butane-eclipsed conformation (0 kj/mol) (16 kj/mol) Butane gauche conformatio (3.8 kj/mol)
Gasoline Gasoline: The Kingdom of Earth runs on alkanes or runs on oil. The distillation of crude oil is the first step in gasoline production, Straight-chain gasoline are poor fuel in automobiles because of engine knock, an uncontrolled combustion that can occur in a hot engine.
Gasoline The octane number of a fuel is the measure by which its antiknock properties are judged. straight-chain hydrocarbons are more prone to induce engine knock than are highly branched compounds. Heptane, a particularly bad fuel, is assigned a base value of 0 octane number while 2,2,4-trimethylpentane (isooctane) has a rating of 100. CH3CH2 CH2 CH2 CH2 CH2 CH3 Heptane Octane Number = 0