Ch 3,4

Alkanes: Structure and Nomenclature

General Properties and Formula

Alkanes are saturated hydrocarbons containing only single bonds between carbon atoms. Their general formula is $C_nH_{2n+2}$, where n is the number of carbon atoms.

• Methane: $CH_4$

• Ethane: $CH_3CH_3$

• Propane: $CH_3CH_2CH_3$

• Butane: $CH_3(CH_2)_2CH_3$

• Pentane: $CH_3(CH_2)_3CH_3$

• Hexane: $CH_3(CH_2)_4CH_3$

• Heptane: $CH_3(CH_2)_5CH_3$

• Octane: $CH_3(CH_2)_6CH_3$

• Nonane: $CH_3(CH_2)_7CH_3$

• Decane: $CH_3(CH_2)_8CH_3$

Condensed Structure	Name	Skeletal Structure
CH4	methane	|
CH3CH3	ethane	–
CH3CH2CH3	propane	∧
CH3(CH2)2CH3	butane	zig-zag
CH3(CH2)3CH3	pentane	zig-zag
CH3(CH2)4CH3	hexane	zig-zag
CH3(CH2)5CH3	heptane	zig-zag
CH3(CH2)6CH3	octane	zig-zag
CH3(CH2)7CH3	nonane	zig-zag
CH3(CH2)8CH3	decane	zig-zag

Structural Isomers

Isomers are compounds with the same molecular formula but different structural arrangements. Alkanes can have several isomers as the number of carbons increases.

• Butane: n-butane and isobutane

• Pentane: n-pentane, isopentane, neopentane

• Hexane: n-hexane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, 2,2-dimethylbutane

IUPAC Nomenclature of Alkanes

The IUPAC system provides rules for naming alkanes to ensure each compound has a unique name.

• Identify the longest continuous carbon chain (this gives the parent name).

• Number the chain from the end nearest a substituent to give the lowest possible numbers to substituents.

• Name substituents as alkyl groups with the suffix -yl.

• List substituents in alphabetical order (ignore prefixes like sec- and tert-; iso- is not ignored).

• Use hyphens to separate numbers from words and commas to separate numbers.

• Use prefixes (di-, tri-, tetra-) for multiple identical substituents.

• If two chains of equal length are possible, choose the one with the most substituents.

Common Alkyl Groups

Structure	Name
CH3–	Methyl
CH3CH2–	Ethyl
CH3CH2CH2–	Propyl
(CH3)2CH–	Isopropyl
CH3CH2CH2CH2–	Butyl
CH3CH2CH(CH3)–	Sec-butyl
(CH3)2CHCH2–	Iso-butyl
(CH3)3C–	Tert-butyl

Examples

• 3-methylpentane: A five-carbon chain with a methyl group on carbon 3.

• 3-ethylheptane: A seven-carbon chain with an ethyl group on carbon 3.

• 2,4-dimethylhexane: A six-carbon chain with methyl groups on carbons 2 and 4.

Cycloalkanes: Structure and Nomenclature

General Properties

Cycloalkanes are saturated hydrocarbons with carbon atoms arranged in a ring. The general formula is $C_nH_{2n}$.

• Cyclopropane: triangle (3 carbons)

• Cyclobutane: square (4 carbons)

• Cyclopentane: pentagon (5 carbons)

• Cyclohexane: hexagon (6 carbons)

IUPAC Nomenclature of Cycloalkanes

• The ring is the parent hydrocarbon unless a substituent has more carbons than the ring.

• If only one substituent, no need to number its position.

• For two substituents, number to give the lowest set of locants; list substituents alphabetically.

• For three or more substituents, number to give the lowest possible numbers to all substituents.

• If a substituent has more carbons than the ring, the ring is named as a substituent (e.g., cyclobutylpentane).

Examples

• 1-ethyl-2-methylcyclopentane: Ethyl and methyl groups on a cyclopentane ring.

• 1,3-dimethylcyclohexane: Methyl groups on carbons 1 and 3 of cyclohexane.

• 1-cyclobutylpentane: Cyclobutyl group attached to pentane.

Alkyl Halides: Structure and Nomenclature

General Properties

Alkyl halides are alkanes in which one or more hydrogen atoms are replaced by halogen atoms (F, Cl, Br, I).

IUPAC Nomenclature of Alkyl Halides

• Name the parent hydrocarbon as for alkanes.

• Number the chain to give the halogen the lowest possible number.

• Halogen substituents are named as prefixes (fluoro-, chloro-, bromo-, iodo-).

• List substituents alphabetically.

• Use prefixes (di-, tri-) for multiple identical halogens.

Examples

• 3-bromopentane: Bromine on carbon 3 of pentane.

• 2-fluoropentane: Fluorine on carbon 2 of pentane.

• 1-chloro-4-methylhexane: Chlorine on carbon 1, methyl on carbon 4 of hexane.

• 1,3-dichlorobutane: Chlorines on carbons 1 and 3 of butane.

• 1,1-dibromocyclobutane: Two bromines on carbon 1 of cyclobutane.

• 4-bromo-2-chloro-1-methylcyclohexane: Bromo, chloro, and methyl groups on cyclohexane.

Conformational Analysis of Alkanes and Cycloalkanes

Carbon-Carbon Single Bond and Conformers

Alkanes have free rotation around their carbon-carbon single bonds due to the symmetrical overlap of sp3 orbitals. This leads to different spatial arrangements called conformational isomers or conformers.

• Newman projections are used to visualize the 3D arrangement of atoms around a single bond.

• Common conformers for butane: anti (methyl groups 180° apart, most stable), gauche (methyl groups 60° apart, less stable).

Example: Butane Conformers

• Anti conformer: Lowest energy, methyl groups farthest apart.

• Gauche conformer: Higher energy, methyl groups closer together.

Cyclohexane Conformations

Cyclohexane adopts a chair conformation to minimize angle and torsional strain. This conformation is almost free of strain and is the most stable.

• Chair conformation has axial (vertical) and equatorial (slanted) positions for substituents.

• Ring flip interconverts axial and equatorial positions.

• Substituents prefer equatorial positions to minimize steric interactions (1,3-diaxial interactions).

Substituted Cyclohexane

• Cis isomer: Substituents on the same side (both up or both down).

• Trans isomer: Substituents on opposite sides (one up, one down).

• More stable conformer is the one with bulky groups in equatorial positions.

Example: 1,3-dimethylcyclohexane

• Cis-1,3-dimethylcyclohexane: Both methyl groups on the same side.

• Trans-1,3-dimethylcyclohexane: Methyl groups on opposite sides.

Summary Table: Conformational Stability

Conformer	Stability	Reason
Anti (butane)	Most stable	Methyl groups far apart
Gauche (butane)	Less stable	Steric hindrance between methyl groups
Chair (cyclohexane)	Most stable	Minimized angle and torsional strain
Boat (cyclohexane)	Less stable	Steric strain (flagpole hydrogens)

Key Terms

• Alkane: Saturated hydrocarbon with only single bonds.

• Cycloalkane: Saturated hydrocarbon with carbon atoms in a ring.

• Alkyl halide: Alkane with one or more halogen substituents.

• Isomer: Compounds with the same molecular formula but different structures.

• Conformer: Different spatial arrangement due to rotation about single bonds.

• Newman projection: Representation of a molecule looking down a bond axis.

• Chair conformation: Most stable form of cyclohexane.

• Axial/equatorial positions: Locations of substituents in cyclohexane chair form.

Example Application: Predicting the most stable conformer of substituted cyclohexane involves placing bulky groups in equatorial positions to minimize steric strain.

Additional info: The notes cover foundational nomenclature and conformational analysis, essential for understanding organic structure and reactivity.