1. Saturated hydrocarbons
Alkanes
Saturated hydrocarbons

2. Structure of alkanes Contain only C & H
Single bonds only – all carbons are sp3 and tetrahedral
All bond angles close to 109.5º
General formula – CnH2n+2
Constitutional (structural) isomerism
Compounds with the same empirical formula but different structural formulas

3. Structure of alkanes Straight chain (n-) CH3CH2CH2CH3 n-butane
Branched
CH3 |
CH3CHCH3
i-butane (isobutane)

4. Nomenclature of alkanes
IUPAC system
Prefixes
1=meth =pent =oct
2=eth =hex =non
3=prop 7=hept 10=dec
4=but
CH3CH2CH2CH2CH3
pentane

5. Nomenclature of alkanes
Common names
isobutane
isopentane

6. Nomenclature of alkanes
Endings
-ane – all single bonds
-ene – at least one double bond
-yne – at least one triple bond
H2C=CHCH3
propene
HC≡CCH3
propyne

7. Nomenclature of alkanes
Branches are named after the number of carbons in the branch, + -yl
-CH3 = methyl
-CH2CH3 = ethyl
-CH2CH2CH3 = propyl
Common names for substituents |
CH3CHCH3
Isopropyl (methylethyl)

8. Nomenclature of alkanes|
CH3CHCH2CH3
sec-butyl (1-methylpropyl)
-CH2CHCH3
CH3
Isobutyl (2-methylpropyl)

9. Nomenclature of alkanes|
CH3CCH3
CH3
tert-butyl
(1,1-dimethylethyl)

10. Nomenclature of alkanes
Numbering – the longest chain is numbered so that the first branch has the lowest possible number.
2-methylpentane

11. Nomenclature of alkanes
3-methylpentane
Numbering is applied to the longest possible chain
incorrect

12. Nomenclature of alkanes
3-methylhexane
Multiple identical substituents use prefixes
2 = di 3 = tri 4 = tetra
5 = penta = hexa, etc.

13. Nomenclature of alkanes
Each substituent is designated by number
2, 4-dimethylpentane

14. Nomenclature of alkanes
Different substituents are named in alphabetical order
4-ethyl-2-methylhexane
Cycloalkanes
Add “cyclo-“ in front of the chain name

15. Nomenclature of alkanes
cyclopropane
Can also be represented by a line-angle drawing

16. Nomenclature of alkanes
cyclopropane cyclobutane
Substituents are named as in other alkanes
Draw a structure for methylcyclohexane

17. Nomenclature of alkanes
methylcyclohexane
1,4-dimethylcyclohexane
1-ethyl-4-methylcyclohexane

18. Nomenclature of alkanes
When the ring has fewer carbons than the longest straight chain, the ring is named as a substituent.
2-cyclopropylbutane

19. Physical properties Most alkanes are liquids or gases (C4 or below).
Alkanes are nonpolar due to nonpolar nature or C-H bond.
Only intermolecular forces are weak London dispersion forces (a type of van der Waals interaction) – dispersion forces increase with increasing molecular surface area
Boiling point increases with molar mass
Boiling point decreases with increased branching

20. Physical properties
Solubility – insoluble in water or smaller alcohols, soluble in each other or any nonpolar solvent (carbon tetrachloride, benzene, ether)
Density – less than water
Strong odor
Insulator

21. Conformations
Straight chain alkanes
Ethane
Newman projections

22. Conformations Energy barrier to rotation is 2.8-3.0 kcal/mol.
At RT there is enough thermal energy to allow rapid rotation about the C-C bond.
1 kcal/mol for each H-H interaction

23. Conformations

24. Conformations
Butane conformations along the central C-H bond

25. Conformations methyl-methyl interactions are 3kcal/mol
methyl-H interactions are 1.25 kcal/mol

26. Conformations of butane

27. Conformations of cycloalkanes
Cyclopentane – envelope conformation – all bond angles  109.5º
Cyclohexanes
Chair conformation – most stable

28. Conformations of cycloalkanes
Boat and twist conformations – less stable
twist
boat

29. Conformations of cycloalkanes
Ring positions – equatorial and axial

30. Conformations of cycloalkanes
Equatorial positions are less sterically hindered.
If large groups are attached to the ring they will most always be in the equatorial position.
Large groups in the axial position interfere with groups in the other axial positions.

31. Conformations of cycloalkanes

32. Cis-trans isomerism in cycloalkanes
Cyclobutanes and cyclopentanes
trans-1,2-dimethylcyclobutane
cis-1,2-dimethylcyclobutane

33. Cis-trans isomerism in cycloalkanes
Cyclohexanes
cis-1,2-dimethylcyclohexane

34. Cis-trans isomerism in cycloalkanes
Adjacent (1,2) cis substituents are equatorial-axial
cis-1,3 substituents are equatorial/equatorial
cis-1,4 substituents are equatorial-axial

35. Chemical reactions Mostly unreactive Combustion
2C6H14+19O2  12CO2+14H2O
Halogenation
2C3H8 + Cl2  2CH3CHClCH3
F2>Cl2>Br2>I2
3º>2º>1º

36. Sources of Alkanes Petroleum – products of refining process
Natural gas