1. Structure and Stereochemistry of Alkanes
Organic Chemistry, 8th Edition L. G. Wade, Jr.
Chapter 3 Lecture
Structure and Stereochemistry of Alkanes

2. Lecture 7: Overview Hydrocarbons Alkanes (General Characteristics)
Common Names, Formulas, Properties
IUPAC Nomenclature
Physical Properties
Alkane Sources & Major Uses

3. Hydrocarbons are molecules that are made of carbon
and hydrogen ONLY.
Table: 03-T01
Chapter 3

4. Alkanes General formula: CnH2n+2 Saturated with hydrogens.
Found in everything from natural gas to petroleum.
The smaller alkanes have very low boiling points (b. p.); therefore, they are gases.
CH C2H C3H8
b. p oC oC oC
Chapter 3

5. Alkane Examples The –CH2– group is called a methylene group.
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The –CH2– group is called a methylene group.
A series of compounds that differ only in the number of methylene groups is known as a homologous series.
Chapter 3

6. Common Names Isobutane, “isomer of butane”
Isopentane, isohexane, etc. methyl branch on next to last carbon in chain
Neopentane, most highly branched
Five possible isomers of hexane, 18 possible isomers of octane and 75 for decane!

7. Butane: C4H10 C H C H C H C H C H n-butane isobutane 3 2 3 3 3
Constitutional isomers are compounds with the same
molecular formula but the carbons are connected differently.
Chapter 3

8. Pentanes: C5H12 C H 3 2
n-pentane
isopentane
neopentane
Chapter 3

9. Formulas and Physical Properties of Alkanes
Table: 03-T02.jpg
Chapter 3

10. IUPAC or Systematic Names
International Union of Pure and Applied Chemistry.
Standard method used internationally to name compounds.
Uses the longest chain of carbons as the main chain.
Common names kept: methane, ethane, propane, and butane.
Chapter 3

11. IUPAC Rules
Rule 1: Find the longest continuous chain of carbon atoms, and use the name of this chain as the base name of the compound.
Rule 2: Number the longest chain, beginning with the end of the chain nearest a substituent.
Rule 3: Name the groups attached to the longest chain as alkyl groups. Give the location of each alkyl group by the number of the main-chain carbon atom to which it is attached.
Write the alkyl groups in alphabetical order regardless of their position on the chain.
Chapter 3

12. Rule 1: The Main Chain Find the longest chain of consecutive carbons.
The longest chain is six carbons: hexane
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13. Main Chain
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Figure: 03_01-04UN.jpg
Title:
Nomenclature: Same Length Chains
Caption:
When looking for the longest continuous chain, look to find all the different chains of that length. Often, the longest chain with the most substituents is not obvious.
Notes:
When there are two carbon chains of the same length, the one that has the most substituents must be chosen to name the compound.
When there are two longest chains of equal length, use the chain with the greatest number of substituents.
Chapter 3

14. Hint When looking for the longest continuous chain (to give the
base name), look to find all the
different chains of that length.
Often, the longest chain with
the most substituents is not
obvious.
Chapter 3

15. Rule 2: Numbering the Main Chain
Number the longest chain beginning at the end of the chain nearest a substituent.
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16. Rule 3: Naming Alkyl Groups
Alkane name
Alkyl name
CH4
methane
–CH3
methyl group
CH3CH3
ethane
–CH2CH3
ethyl group
CH3CH2CH3
propane
–CH2CH2CH3
propyl group
CH3CH2CH2CH3
butane
–CH2CH2CH2CH3
butyl group
CH3CH2CH2CH2CH3
–CH2CH2CH2CH2CH3
pentyl group
Chapter 3

17. Common Alkyl Groups Chapter 3 File Name: AAAKPKM0 Figure: 03_02.jpg
Title:
Nomenclature of Alkyl Groups
Caption:
Some common alkyl groups and their names.
Notes:
Substituents on a carbon chain are called alkyl groups. They are named by replacing the -ane ending of the alkane with -yl. The “n” and “iso” groupings are used to describe an alkyl chain attached by a primary carbon. The name of an alkyl chain attached by a secondary carbon is “sec,” and that of chains attached by a tertiary carbons “tert.”
Chapter 3

18. Why do we name them that way?

19. Applying the Naming Rules
4-ethyl
2-methyl
Name the groups attached to the longest chain as alkyl groups.
Give the location of each alkyl group by the number of the main-chain carbon atom to which it is attached.
Write the alkyl groups in alphabetical order regardless of their position on the chain.
4-ethyl-2-methylhexane
Chapter 3

20. Multiple Groups
When two or more of the same substituents are present, use the prefixes di-, tri-, tetra-, etc. to avoid having to name the alkyl group twice.
Three methyl groups at positions 2, 5, and 7.
2,5,7-trimethyldecane
Chapter 3

21. Solved Problem 3-1
Give the structures of 4-isopropyloctane and 5-t-butyldecane.
Solution: 4-Isopropyloctane has a chain of eight carbons, with an isopropyl group on the fourth carbon. 5-t-Butyldecane has a chain of ten carbons, with a t-butyl group on the fifth.
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22. Complex Substituents
Complex alkyl groups are named by using the longest carbon chain.
Carbon number 1 of the alkyl group is the carbon attached to the main chain.
a (1-ethyl-2-methylpropyl) group
a (1,1,3-trimethylbutyl) group
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23. Solved Problem 3-2
Give a systematic (IUPAC) name for the following compound.
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Figure: sp3.2
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24. Solved Problem 3-2: Solution
The longest carbon chain contains eight carbon atoms, so this compound is named as an octane. Numbering from left to right gives the first branch on C2; numbering from right to left gives the first branch on C3, so we number from left to right.
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Figure: sp3.2
4-isopropyl-2,2,3,6-tetramethyloctane
Chapter 3

25. Hint
When substituents are alphabetized, iso- is used as part of the alkyl group name, but the hyphenated prefixes are not.
Thus isobutyl is alphabetized with i, but n-butyl, tert-butyl, and sec-butyl are alphabetized with b.
Chapter 3

26. Physical Properties Solubility: Hydrophobic
Density: Less than 1 g/mL (about 0.7 g/mL)
Boiling points increase with increasing carbons (little less for branched chains)
Melting points increase with increasing carbons (less for odd-number of carbons)

27. Boiling Points of Alkanes
As the number of carbons in an alkane increases, the boiling point increases due to the larger surface area and the increased van der Waals attractions.
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Title:
Boiling Points of Branched and Unbranched Alkanes
Caption:
Alkane boiling points. Comparison of the boiling points of the unbranched alkanes (blue) with those of some branched alkanes (red). Because of their smaller surface areas, branched alkanes have lower boiling points than unbranched alkanes.
Notes:
The only intermolecular force of nonpolar molecules are London dispersion forces which result from induced dipole attractions. Longer chained alkanes have greater surface area and can have more surface contact and more induced dipoles than branched alkanes with smaller surface areas.
Chapter 3

28. Melting Points of Alkanes
Branched alkanes pack more efficiently into a crystalline structure, so have higher m.p.

29. Alkane Sources
Alkanes are obtained from petroleum and petroleum by-products.
Fractional distillation will separate the crude oil into mixtures of alkanes with a range of boiling points.
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30. Major Uses of Alkanes C1 – C2: gases (natural gas)
C3 – C4: liquified petroleum (LPG)
C5 – C8: gasoline
C9 – C16: diesel, kerosene, jet fuel
C17 – up: lubricating oils, heating oil
Origin: petroleum refining

31. Chapter 3, Table 3.3

32. Alkane Information
From petroleum (crude oil) – refined using fractional distillation to separate alkanes of varying sizes.
Natural gas (~70% methane, ~10% ethane, ~15% propane); once a waste, now used for heating, electricity, and starting materials for fertilizer.
Alkanes have very low reactivities, called paraffins
Comes from Latin words Parum meaning “too little” and Affinis which means “affinity”

33. Catalytic Cracking and Hydrocracking
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