Alkanes: Nomenclature, Physical Properties, and Conformations Study Notes

LECTURE OVERVIEW AND CONTACT INFORMATION

Lecturer: ChM. Ts. Dr. Azalina Mohamed Nasir.
Slide Credit: Adopted from Sharifah Zati Hanani Syed Zuber.

LECTURE CONTENT

IUPAC Alkanes Nomenclature.
Physical properties of alkanes.
Conformations of Alkanes: Rotation about Carbon–Carbon Bonds.
Cycloalkanes.
Conformations of monosubstituted cyclohexanes.
Reactions of alkanes: Chlorination and bromination.

CLASSIFICATION OF HYDROCARBONS

Hydrocarbons are divided into two primary classes based on their bonding nature:

Saturated Hydrocarbons: These contain only single bonds.
- Alkanes: Contain only carbon-carbon single bonds. Example: Ethane ( $H_3C-CH_3$ ).
Unsaturated Hydrocarbons: These contain one or more multiple bonds or rings.
- Alkenes: Contain one or more carbon-carbon double bonds. Example: Ethene ( $H_2C=CH_2$ ).
- Alkynes: Contain one or more carbon-carbon triple bonds. Example: Acetylene ( $HC \equiv CH$ ).
- Arenes: Contain one or more benzene-like rings. Example: Benzene ( $C_6H_6$ ).

INTRODUCTION TO ALKANES

Definition: Hydrocarbons containing only single bonds.
Straight-chain Alkanes: Carbons are arranged in a continuous chain without branches.
General Formula: $C_nH_{2n+2}$ .
Structural Representations:
- Ball-and-stick model.
- Line-angle formula.
- Structural formula (e.g., $CH_3CH_2CH_3$ for propane).
- Condensed structural formula.

Nomenclature and Molecular Formulas for First 20 Unbranched Alkanes

1 Carbon: Methane ( $CH_4$ ); Prefix: meth-.
2 Carbons: Ethane ( $C_2H_6$ ), $CH_3CH_3$ ; Prefix: eth-.
3 Carbons: Propane ( $C_3H_8$ ), $CH_3CH_2CH_3$ ; Prefix: prop-.
4 Carbons: Butane ( $C_4H_{10}$ ), $CH_3(CH_2)_2CH_3$ ; Prefix: but-.
5 Carbons: Pentane ( $C_5H_{12}$ ), $CH_3(CH_2)_3CH_3$ ; Prefix: pent-.
6 Carbons: Hexane ( $C_6H_{14}$ ), $CH_3(CH_2)_4CH_3$ ; Prefix: hex-.
7 Carbons: Heptane ( $C_7H_{16}$ ), $CH_3(CH_2)_5CH_3$ ; Prefix: hept-.
8 Carbons: Octane ( $C_8H_{18}$ ), $CH_3(CH_2)_6CH_3$ ; Prefix: oct-.
9 Carbons: Nonane ( $C_9H_{20}$ ), $CH_3(CH_2)_7CH_3$ ; Prefix: non-.
10 Carbons: Decane ( $C_{10}H_{22}$ ), $CH_3(CH_2)_8CH_3$ ; Prefix: dec-.
11 Carbons: Undecane ( $C_{11}H_{24}$ ), $CH_3(CH_2)_9CH_3$ ; Prefix: undec-.
12 Carbons: Dodecane ( $C_{12}H_{26}$ ), $CH_3(CH_2)_{10}CH_3$ ; Prefix: dodec-.
13 Carbons: Tridecane ( $C_{13}H_{28}$ ), $CH_3(CH_2)_{11}CH_3$ ; Prefix: tridec-.
14 Carbons: Tetradecane ( $C_{14}H_{30}$ ), $CH_3(CH_2)_{12}CH_3$ ; Prefix: tetradec-.
15 Carbons: Pentadecane ( $C_{15}H_{32}$ ), $CH_3(CH_2)_{13}CH_3$ ; Prefix: pentadec-.
16 Carbons: Hexadecane ( $C_{16}H_{34}$ ), $CH_3(CH_2)_{14}CH_3$ ; Prefix: hexadec-.
17 Carbons: Heptadecane ( $C_{17}H_{36}$ ), $CH_3(CH_2)_{15}CH_3$ ; Prefix: heptadec-.
18 Carbons: Octadecane ( $C_{18}H_{38}$ ), $CH_3(CH_2)_{16}CH_3$ ; Prefix: octadec-.
19 Carbons: Nonadecane ( $C_{19}H_{40}$ ), $CH_3(CH_2)_{17}CH_3$ ; Prefix: nonadec-.
20 Carbons: Eicosane ( $C_{20}H_{42}$ ), $CH_3(CH_2)_{18}CH_3$ ; Prefix: eicos-.

IUPAC NOMENCLATURE OF ALKANES

System: Systematic nomenclature designed by the International Union of Pure and Applied Chemistry (IUPAC) in Geneva, 1892.

Alkyl Substituents

Removing one hydrogen atom from an alkane results in an alkyl substituent ( $R-$ ):

Methyl: $-CH_3$ (Me)
Ethyl: $-CH_2CH_3$ (Et)
Propyl: $-CH_2CH_2CH_3$ (Pr)
Isopropyl (1-Methylethyl): $-CH(CH_3)_2$ (iPr)
Butyl: $-CH_2CH_2CH_2CH_3$ (Bu)
Isobutyl (2-Methylpropyl): $-CH_2CH(CH_3)_2$ (iBu)
sec-Butyl (1-Methylpropyl): $-CH(CH_3)CH_2CH_3$ (s-Bu)
tert-Butyl (1,1-Dimethylethyl): $-C(CH_3)_3$ (t-Bu)
Pentyl: $-CH_2CH_2CH_2CH_2CH_3$
Isopentyl (3-Methylbutyl): $-CH_2CH_2CH(CH_3)_2$
Neopentyl (2,2-Dimethylpropyl): $-CH_2C(CH_3)_3$

Rules for Systematic Naming

Determine the Longest Continuous Chain: This is the parent hydrocarbon. Its carbon count yields the alkane's "last name."
Number the Chain: Assign numbers to the carbon atoms starting from the end that gives the substituent the lowest possible number. Use a hyphen to connect the number to the substituent name.
Assign Lowest Numbers for Multiple Substituents: If multiple substituents exist, number the chain in the direction that produces the lowest set of numbers. List substituents in alphabetical order.
Handle Identical Substituents: Use prefixes such as di- (2), tri- (3), or tetra- (4). The positions of all groups must be indicated by numbers.
- Note: Prefixes (di, tri, tetra, sec, tert) are ignored in alphabetization. However, "iso" and "cyclo" are NOT ignored.
Alphabetization Tie-breaker: If the same substituent numbers result from both ends, give the lower number to the group that appears first alphabetically (e.g., 3-ethyl-5-methylheptane instead of 5-ethyl-3-methylheptane).
Multiple Chains of Equal Length: If two different chains have the same length, choose the one with the greater number of substituents.
Complex Substituents: Systematic substituent names are obtained by numbering the alkyl group starting from the carbon attached to the parent chain (always Carbon 1).

PHYSICAL PROPERTIES OF ALKANES

Boiling Point (bp)

Definition: The temperature at which liquid becomes vapor.
Trend: Boiling points increase as the number of carbon atoms and molecular weight increase.
Intermolecular Forces: Alkanes are held together by Van der Waals forces (induced-dipole-induced-dipole interactions).
- These are the weakest intermolecular forces.
- Strength depends on the contact area between molecules. Greater area equals stronger forces and higher boiling points.
Branching Effect: Branched alkanes have lower boiling points than unbranched alkanes of the same molecular weight.
- Branched alkanes are more compact and spherical, resulting in a lower contact area.
- Metaphor: Pentane (unbranched) acts like a cigar with large surface contact, whereas neopentane (branched) acts like a tennis ball with minimal surface contact.

Melting Point (mp)

Definition: The temperature at which a solid converts to liquid.
Influence of Packing: Controlled by how well molecules fit into a crystal lattice. Tighter packing requires more energy to break.
Even vs. Odd Rule: Alkanes with an even number of carbons pack more tightly and have higher melting points than alkanes with an odd number of carbons (which pack less effectively).

Solubility

Rule: "Like dissolves like."
Polarity: Alkanes are non-polar compounds with no net charge.
Solvent Interactions:
- Alkanes are insoluble in water (polar).
- Alkanes are soluble in organic solvents such as benzene ( $C_6H_6$ ), acetonitrile ( $CH_3CN$ ), and chloroform ( $CHCl_3$ ).
Dissolution Process: Dissolving requires breaking Van der Waals forces in the alkane and hydrogen bonds in water; water-water attractions are too strong to allow non-polar alkanes to penetrate the structure.

CONFORMATIONS OF ALKANES

Theoretical Basics

Conformations: Different spatial arrangements of atoms resulting from rotation about a single ( $C-C$ ) bond.
Conformers: Molecules with different conformations (also called conformational isomers).

Ethane Conformations

Staggered Conformer: Hydrogens are as far apart as possible. This is the energy minimum and the most stable state.
Eclipsed Conformer: Hydrogens on adjacent carbons are aligned with each other. This is the energy maximum.
Stability: The eclipsed state is less stable due to bond repulsion between hydrogens. The energy of the eclipsed state is higher by $2.9\,kcal/mol$ compared to the staggered state.
Representations: Perspective drawings and Newman projections (viewing the bond end-on).

Butane Conformations

Rotation occurs about the $C-1-C-2$ , $C-2-C-3$ , and $C-3-C-4$ bonds. Rotation about the central $C-2-C-3$ bond produces distinct conformers:

Anti-conformer: The two large methyl groups are $180^{\circ}$ apart. This is the most stable staggered conformer.
Gauche-conformer: Methyl groups are $60^{\circ}$ apart. It is a staggered conformer but less stable than anti due to steric strain.
Steric Strain: Repulsion experienced when atoms/groups are close enough for their electron clouds to overlap. Increases with the size of the substituent.

Energy Profile of Butane Rotation

Totally Eclipsed (Methyl-Methyl alignment): $4.5\,kcal/mol$ ( $19\,kJ/mol$ ).
Partially Eclipsed (Methyl-Hydrogen alignment): $3.8\,kcal/mol$ ( $16\,kJ/mol$ ).
Gauche: $0.87\,kcal/mol$ ( $3.6\,kJ/mol$ ).
Anti: $0\,kcal/mol$ .

EXERCISES AND APPLICATIONS

Exercise 1 (Naming/Drawing): Draw 3-ethyl-5-isobutyl-3-methylheptane and 4-(1,1-dimethylethyl)octane.
Exercise 2 (Acceptable Names): Provide systematic names for specific skeletal structures.
Exercise 3 (Complex Substituents): Draw 5-(1,2-dimethylpropyl)-2-methyldecane.
Exercise 4 (Boiling Point Trends): Arrange butane, decane, and hexane in decreasing order of boiling point. (Answer: Decane > hexane > butane due to chain length).
Comparison Example: Explain why isobutane has a lower boiling point than $n$ -butane ( $CH_3CH_2CH_2CH_3$ ). (Answer: $n$ -butane has a greater contact area/Van der Waals forces).
Conformation Task: Draw all staggered and eclipsed conformers for the rotation of the $C-2$ and $C-3$ bond of pentane and create a potential energy diagram for a $360^{\circ}$ rotation.