1. 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

2. "triyne" 3 triple bonds etc.
The Alkynes (CnH2n-2)
These are named in identical fashion to alkenes except that "ene" in the alkene name is replaced by: "yne" 1 triple bond, "diyne" 2 triple bonds
"triyne" 3 triple bonds etc. 2

3. 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
CH becomes -CH3 (Methyl)
C2H becomes -C2H6 (Ethyl)
Alkanes

4. 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.

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

7. From Alkynes

8. 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

9. 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.

10. 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.

11. 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

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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

18. 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.

19. They burn in a flame, producing carbon dioxide, water, and heat.
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 O2 → CO H2O 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.

20. 2- Reaction with Halogens : The reaction of halogen with alkanes is called halogenation.
The reaction of an alkane with Cl2 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.

21. Reaction with Halogens
The reaction does not stop at this stage. Depending upon time and Cl2, the remaining three hydrogen atoms of methyl Chloride can be successively replaced by chlorine atoms.

22. Reaction with Halogens
Bromine (Br) reacts with alkanes in a similar manner but less vigorously.
Reaction with Iodine is reversible.

23. 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:

24. 4- Sulphonation:
It is the replacement of H- atom of Alkanes by –SO3H group. Sulphonation of alkanes occurs with fuming suphuric acid at moderate temperature.

25. 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 oC .
Ethane when heated to 500 oC in the absence of air, gives a mixture of methane, ethylene and hydrogen

26. Example: n-Hexane yields benzene.
6- Dehydrogenation:
It is the removal of hydrogen form molecule Benzene and other aromatic hydrocarbons are prepared from petroleum hydrocarbons.
Example: n-Hexane yields benzene.

27. 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.

28. 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.

29. 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;

30. 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.

31. 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.

32. Conformational isomers
Butane- Anti conformation Butane-eclipsed conformation
(0 kJ/mol) (16 kJ/mol)
Butane—gauche conformation (3.8 kJ/mol)

35. 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.

36. CH3-CH2- CH2- CH2- CH2- CH2- CH3
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 (iso-octane) has a rating of 100.
CH3-CH2- CH2- CH2- CH2- CH2- CH3
Heptane
Octane Number = 0

37. ALKENES
Alkenes are also called Olefins (CnH2n) “unsaturated” hydrocarbons. Alkenes occur abundantly in nature. Ethylene (H2C=CH2) is a plant hormone that induces ripening in fruit.
Functional group = carbon-carbon double bond
sp2 hybridization => flat, 120o bond angles
σ bond & π bond => H2C=CH2
No rotation about double bond!

38. ALKENES
Examples:
C3H6 propylene
C4H8 butylenes

39. Cis/Trans Isomerism:
When disubstituted (two substituents other than hydrogen) alkenes contain two substituents on the same side of the double bond they are called cis alkenes and when substituents are attached to opposite side are called trans alkenes.
Two methyl groups on the same side of double bond = Cis

40. (CH3)2C=CHCH3 CH3CH=CCH2CH3
Cis–trans isomerism is not possible If one of the double-bond carbons (vinyl carbon) is attached to two identical groups.
CH3CH=CHCH CH3CH2CH=CH2
Yes No
CH3
(CH3)2C=CHCH3 CH3CH=CCH2CH3
No Yes

41. E and Z Alkenes
For trisubstituted (three substituents) and tetrasubstituted (four substituents) double bonds, a more general method is needed for describing double-bond geometry.
If the Higher mass ranked groups are on the same side, then alkene has Z geometry (German zusammen, meaning “together.”) If the higher-ranked groups are on opposite sides, while the alkene has E geometry (German entgegen, meaning “opposite).

42. E and Z Alkenes

43. Examples: (3-methyl-cis-2-pentene Expanded into becomes
(Z)-3-methyl-2-pentene or
(3-methyl-cis-2-pentene
(E)-1-bromo-1-chloropropene
Expanded into
becomes

44. Stability of Alkenes
The trans isomer is more stable than the cis isomer by 2.8 kJ/mol (0.66 kcal/mol) at room temperature, corresponding to a 76;24 ratio. Cis alkenes are less stable due to steric strains between two larger substituents on the same side of the double bond.