Alkanes Lecture Notes

Return

Nomenclature

Prefixes

• Methyl

• Ethyl

• Propyl

• Butyl

• Pentyl

• Hexyl

• Heptyl

• Octyl

• Nonyl

• Decyl

• Undecyl

• Dodecyl

• Tridecyl

• Tetradecyl

• Pentadecyl

• Hexadecyl

• Heptadecyl

• Octadecyl

• Nonadecyl

• Eicosyl

Common Names of Alkyl Groups

• 3-carbon

– n-propyl

– isopropyl

• 4-carbon

– n-butyl

– sec-butyl

– t-butyl

Common Names of Alkyl Groups

• 5-carbon

– n-pentyl or n-amyl

– isopentyl or isoamyl

– t-pentyl or t-amyl

– neopentyl or neoamyl

IUPAC

International Union of Pure & Applied Chemistry

• Select parent structure: longest chain

• When necessary; indicate by a number the carbon attached to the alkyl group

• When numbering the parent chain, use the lowest set of numbers

• If the same alkyl group is used more than once as a side chain, indicate by the appropriate prefix (di-, tri-, tetra-, etc.) and indicate by number the positions of each group

• With several different alkyl groups, name in order of increasing size or in alphabetical order

Classification of Atoms

• Classification of each carbon atom with respect to the number of other carbon atoms which are attached is extremely useful.

• Primary (1^o)

• Secondary (2^o)

• Tertiary (3^o)

• Each hydrogen is given the same designation as the carbon atom to which it is attached.

Classification of Atoms

• Classification of each carbon atom with respect to the number of other carbon atoms which are attached is extremely useful.

• Primary (1^o)

• Secondary (2^o)

• Tertiary (3^o)

• Each hydrogen is given the same designation as the carbon atom to which it is attached.

Physical Properties

• Nonpolar or slightly polar

• Molecular forces

– Van der Waals

• weak

• short range

– The larger the molecule -- the stronger the intermolecular forces

• melting &boiling points vary with molecular weight

• 20-30^oC for each C atom

• increases in melting point are more variable

Physical Properties

– C₁ through C₄ are gases at 20^oC

– n-alkanes C₅ - C₁₇ are liquids at 20^oC

– n-alkanes greater than C₁₈ are solids at 20^oC

– Branched isomers have a lower boiling point than straight-chain isomers

• Alkanes dissolve in nonpolar solvents and do not dissolve in polar solvents such as H₂O

• Alkanes are less dense than H₂O

Conformational Considerations

Ethane

• Carbon atoms are connected by s bonds

• Free rotation about the C-C bond

– eclipsed

– staggered

– skewed

– Newman projections

– Minimal potential energy in the staggered conformation

– Potential energy increased with rotation to a maximum of 3 kcal/mol at the eclipsed conformation

– The 3 kcal/mol barrier is minimal; therefore, at room temperature all conformations are present and can enter into a reaction

– The energy required for rotation is called torsional energy

Conformational Considerations

Propane

• The rotational barrier is 3.3 kcal/mol

• Newman projections

Conformational Considerations

Butane

• Two possible structures

– straight chain

– branched

• Anti conformation

• Gauche conformation

• See figure 3.8 on page 85

PREPARATION OF ALKANES

Page 98

The Grignard Reagent

• Victor Grignard

• Nobel Prize in 1912

• General name of alkylmagnesium halide

REACTIONS OF ALKANES

Page 103

Halogenation

Chlorination - light @ 25^oC

Halogenation

Bromination - light @ 127^oC

Mechanism

Ease of abstraction of hydrogen atoms

3^o > 2^o > 1^o > CH₄

Energy of Activation, kcal/mol

R X=Cl X=Br

CH₄ 4 18

1^o 1 13

2^o 0.5 10

3^o 0.1 7.5

Stability of Free Radicals