Alkanes are organic compounds that consist entirely of single-bonded carbon and hydrogen atoms and lack any other functional groups. Alkanes have the general formula CnH2n+2 and can be subdivided into the following three groups: the linear straight-chain alkanes, branched alkanes, and cycloalkanes.

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