The document provides an overview of alkanes, including their molecular structure, properties, nomenclature, and reactions. Alkanes are a family of hydrocarbons with a general formula of CnH2n+2, characterized by single bonds and varying physical states based on carbon chain length. The document also discusses the preparation and chemical reactions of alkanes, highlighting their low reactivity and sources such as natural gas and petroleum.

1. ALKANES
Md. Saiful Islam
BPharm, MSc
North South University
Fb Group: Pharmacy Universe

2. Carbon
• Carbon has four valence electrons; hydrogen has one.
•
• C • H •
•
• To obtain an octet, carbon forms four bonds.
H H
• •
H  C  H H C H CH4 , methane
• •
H HH
Review

3. Hydrocarbons
Large family of organic compounds
Composed of only carbon and hydrogen
Saturated hydrocarbons
Alkanes
Unsaturated hydrocarbons
Alkenes, Alkynes & Aromatics
C - C C = C C C
C
C
C
C
C
C
H
H
H
H
H
H
A Kekulé structure
showing all atoms
A Kekulé structure
as a line-angle formula

4. Methane
Tetrahedral
Expanded structural formula (Lewis):
showing each bond line.
CH4Molecular formula

5. -The most simple of hydrocarbons species
-Entirely composed of single bond and
saturated in Hydrogen
-Suffixe –ane
-General formula:
CnH2n+2 (non-cyclic)
ALKANES Characteristics:

6. -Same for all alkane structure (following CH4)
-Covalent, weakly polar bonds: C, H same
electronegativity
-Boiling point increase by 20 or 30 degrees for each
carbon added (except for very small alkanes, same
tendency for M.P but less regular).
-C1 to C4: gases, C5-C17: liquid, +C18: solid
-Insoluble in water, dilute acids and bases and
concentrated H2SO4
Physical properties:

7. Ethane
C2H6 Molecular formula
Expanded structural formula
CH3 – CH3
Condensed structural formula: with each carbon atom and
its attached hydrogen atoms.

8. Alkanes
CnH2n+2 n: number of carbon atoms

9. Naming of Alkanes
CnH2n+2Prefix + ane

10. Line-angle Formula
CH3-CH2-CH2-CH2-CH3
Propane
Butane
Pentane
CH3-CH2-CH2-CH3
CH3-CH2-CH3

11. In the IUPAC system:
• Removing a H from an
alkane is called alkyl
group.
-ane
• Halogen atoms are
named as halo.
-ine
Naming Substituents
-OH Hydroxyl
-NO2 Nitro
-yl
-O

12. STEP 2
Number the carbon atoms starting
from the end nearest a substituent.
STEP 3
Give the location and name of each
substituent (alphabetical order) as a
prefix to the name of the main chain.

13. Give the name of: CH3

CH3─CH─CH2─CH3
STEP 1 Longest chain is butane.
STEP 2 Number chain. CH3

CH3─CH─CH2─CH3
1 2 3 4
STEP 3 Locate substituents and name.
2-Methylbutane

14. Give the name of: CH3 CH3
 
CH3─CH─CH─CH3
STEP 1 Longest chain is butane.
STEP 2 Number chain. CH3 CH3
 
CH3─CH─CH─CH3
1 2 3 4
STEP 3 Locate substituents and name.
2,3-dimethylbutane

15. STEP 1 Longest chain is pentane.
STEP 2 Number chain from end nearest substituent.
Cl CH3
 
CH3─CH2─CH─CH─CH3
5 4 3 2 1
STEP 3 Locate substituents and name alphabetically.
3-Chloro-2-methylpentane
Cl CH3
 
CH3─CH2─CH─CH─CH3

16. CH3 CH3
| |
CH3─CH─CH2 ─CH─CH3 2,4-dimethylpentane
1 2 3 4 5
Cl CH3
| |
CH3─CH2─CH─CH2─C─CH2─CH3
|
Cl
7 6 5 4 3 2 1
3,5-dichloro-3-methylheptane

17. CH2 CH3

CH3─CH─CH2─CH3
STEP 1 Longest chain has 5 carbon atoms (Pentane).
STEP 2 Number chain from end nearest substituent.
CH2 CH3

CH3─CH─CH2─CH3
STEP 3 Locate substituent and name.
3-Methylpentane
12
3 4 5

18. Constitutional Isomers
• Have the same molecular formula.
• Have different atom arrangements (different structural formula).
CH3CH2CH2CH3
Butane
CH3CHCH3
2-Methylpropane
CH3
C4H10
C4H10

19. =
=
=
Cyclic Hydrocarbon - Cycloalkane
Cyclobutane
Cyclopentane
Cyclohexane
Carbon atoms are joined to form a ring.

20. Naming of Cycloalkanes
Prefix “cyclo-” + the name of the open-chain alkane
Substituents:
- One substituent: no location number
- Two or more substituents: number the ring beginning
with the substituent of lower alphabetical order.
Chlorocyclohexane 1-Bromo-2-methylcyclohexane
1-Chloro-3-hydroxyl-2-nitrocyclopentane
Cl CH3
Br Cl NO2
OH
1
1
2
2
3

21. Conformation - 3D shapes
Any three-dimensional arrangement of atoms in a molecule that results by rotation
about a single bond.
– The following are three conformations for a butane molecule.
Most crowded
conformation
rotate
by 120°
rotate
by 60°
Least crowded
conformation
Intermediate
crowding
2 3
4
1

22. Conformation - 3D shapes
• The most stable conformation of a cyclopentane is the envelope
conformation.
• The most stable conformation of a cyclohexane is the chair conformation.
– All bond angles are approximately 109.5°.

23. • In a chair conformation,
– six C-H bonds are equatorial bonds.
– six C-H bonds are axial bonds.
Conformation - 3D shapes
H
H
H
HH
H
(a) Ball-and-stick model
showing all 12 hydrogens
axis through the
centerof the ring
H H
H
H
H
H
(b) The six equatorial
C-H bonds
(c) The six axial
C-H bondsEquatorial Axial

24. – The more stable conformation of a substituted cyclohexane ring has the
substituent group(s) as equatorial rather than axial.
Conformation - 3D shapes
CH3
Axial methylcyclohexane
CH3
Equatorial methylcyclohexane
H H H
H
More Stable

25. The same molecular formula and the same connectivity of their atoms,
but a different arrangement of their atoms in space.
mp & bp of cis < mp & bp of trans
Cis & Trans Stereoisomers
trans-1,2-Dimethyl-
cyclopentane
cis-1,2-Dimethyl-
cyclopentane
CH3
H
CH3
H
H
H
H
H
H
H
H
H
CH3
H3 C
H
H
H
H
H
H

26. Cis & Trans Stereoisomers
trans-1,4-Dimethylcyclohexane cis-1,4-Dimethylcyclohexane
H
H3 C
CH3
H
H
H3 C
H
CH3
or or
CH3
CH3
CH3
CH3
Stereoisomers: Isomers that have the same molecular formulas and the same
connectivity of their atoms but a different orientation of their atoms in space.

27. Boiling & melting points of Alkanes
Number of carbon atoms ↑ bp & mp ↑
Number of branches ↑ bp & mp ↓
CH3CH2CH2CH3
CH3CHCH3
CH3

28. Preparation of alkanes
1- Hydrogenation of unsaturated hydrocarbon:
2- Hydrolysis of Grignard reagent
CH2 CH2
Ni or Pd or Pt / H 2
200, 300
CH3 CH3
CH3CH2Br + Mg
2+ Dry ether CH3CH2MgBr
Grignard reagent
CH3CH2MgBr
H3O
+
CH3CH3 + Mg(OH)Br

29. 3- Reduction of alkyl halides
a) by metal and acid or by metal hydrides
b) By sodium metal (Coupling reaction)
c) By lithium dialkyl cuprate
CH3 Br2 + 2 Na CH3 CH3 + 2 NaBr
(CH3CH2)2CuLi + CH3Br CH3CH2CH3
CH3CH2CH2Br + Zn H CH3CH2CH3 + ZnBr2
CH3CH2CH2CH2Br
1) LiAlH4 / ether
2) H3O
CH3CH2CH2CH3

30. Chemical reactions of Alkanes
Low reactivity
1- Combustion:
• Alkanes react with oxygen.
• CO2, H2O, and energy are produced.
• Alkane + O2 CO2 + H2O + heat
CH4 + 2O2 CO2 + 2H2O + energy

31. 2- Halogenation:
Alkanes react with Halogens.
CH4 + Cl2 CH3Cl + HCl
Heat or light
Chemical reactions of Alkanes
Low reactivity
CH3Cl+ Cl2 CH2Cl2 + HCl
CH2Cl2 + Cl2 CHCl3 + HCl
CHCl3 + Cl2 CCl4 + HCl
Heat or light
Heat or light
Heat or light
Chloromethane
Dichloromethane
Trichloromethane
Tetrachloromethane

32. • Natural gas
– 90 to 95 percent methane.
– 5 to 10 percent ethane, and
– a mixture of other low-boiling alkanes, chiefly propane, butane, and 2-
methylpropane.
• Petroleum
– A thick liquid mixture of thousands of compounds, most of them
hydrocarbons, formed from the decomposition of marine plants and
animals.
Sources of Alkanes