Exhaustive Guide to Alkanes and Alkenes: Nomenclature, Properties, and Synthesis

IUPAC Rules for the Nomenclature of Alkanes

The systematic naming of alkanes follows specific IUPAC (International Union of Pure and Applied Chemistry) rules to ensure each unique structure has a unique name.

• Rule 2: Numbering the Chain: Number the longest continuous carbon chain beginning with the end that is nearer to a substituent. This ensures that the substituent is assigned the lowest possible number.

• Rule 3: Designating Location and Name: Use the numbers obtained from Rule 2 to identify the location of substituent groups.

  • The parent name (based on the longest chain length) is placed last.

  • The substituent group name, preceded by its location number, is placed first.

  • Numbers are separated from words by a hyphen.

  • Example: A six-carbon chain with a methyl group on the second carbon is named 2-methylhexane. A seven-carbon chain with a methyl group on the third carbon is 3-methylheptane.

• Rule 4: Multiple Substituents: When two or more substituents are present, each group receives a number corresponding to its carbon position on the longest chain.

  • Selection path check: To determine the correct numbering direction, add the locants (position numbers). The path with the lower sum is preferred. For example, in 4-ethyl-2-methylhexane, numbering from left to right gives locants $2 + 4 = 6$, while numbering from right to left gives $3 + 5 = 8$. Thus, left-to-right is correct.

  • Alphabetical Order: Substituent groups must be listed alphabetically (e.g., ethyl before methyl).

  • Prefixes: Disregard multiplying prefixes such as 'di-', 'tri-', and 'tetra-' when determining alphabetical order. For instance, in 4-ethyl-2,2-dimethylheptane, ethyl comes before methyl (ignoring 'di').

  • Italics: Structure-defining prefixes written in italics and separated by a hyphen (like tert-) are ignored in alphabetizing (so tert-butyl precedes ethyl). However, prefixes that are not separated by hyphens and italics, like iso, are not ignored (ethyl precedes isobutyl).

• Rule 5: Multiple Substituents on One Carbon: If two substituents are attached to the same carbon atom, that position number must be used twice in the name. Example: 3-ethyl-3-methylhexane.

• Rule 6: Identical Substituents: Use prefixes (di-, tri-, tetra-, etc.) to indicate multiple identical groups. A position number must be provided for every single substituent, and numbers are separated by commas. Examples include 2,3-dimethylbutane, 2,3,4-trimethylpentane, and 2,2,4,4-tetramethylpentane.

• Rule 7: Equal Chain Lengths: If two different chains of the same length compete to be the parent chain, choose the one with the greater number of substituents. Example: 2,3,5-trimethyl-4-propylheptane is preferred over 4-(1,2-dimethylpropyl)-5-methylheptane.

• Rule 8: Equal Distance Branching: If branching occurs at an equal distance from both ends of the chain, select the numbering that gives the lower number at the first point of difference. Example: 2,3,5-trimethylhexane is correct, while 2,4,5-trimethylhexane is incorrect.

• Rule 9: Complex Substituents: Sub-branched substituents are named as if they were a separate compound. Numbering for these alkyl groups begins at the carbon directly attached to the parent chain.

  • Example: 2,3-Dimethyl-6-(2-methylpropyl)decane (also known as 2,3-Dimethyl-6-isobutyldecane).

  • Common complex alkyl names include: 3-methylbutyl, 2-methylbutyl, 1,2-dimethylpropyl, and 1,1-dimethylpropyl.

Classification of Carbon and Hydrogen Atoms

Carbon and hydrogen atoms within alkanes can be categorized based on their connectivity:

• Primary ($1^{\circ}$): A carbon attached to only one other carbon; hydrogens attached to it are primary hydrogens.

• Secondary ($2^{\circ}$): A carbon attached to two other carbons; hydrogens attached to it are secondary hydrogens.

• Tertiary ($3^{\circ}$): A carbon attached to three other carbons; hydrogens attached to it are tertiary hydrogens.

• Quaternary ($4^{\circ}$): A carbon attached to four other carbons.

• Example: 2-methylbutane contains primary, secondary, and tertiary hydrogens.

• Note: 2,2-dimethylpropane (neopentane) possesses only primary hydrogen atoms.

Isomerism in Alkanes

Isomerism describes compounds that share the same molecular formula but differ in the arrangement of their atoms.

• Structural (Constitutional) Isomers: Compounds with the same molecular formula but different connectivity of atoms. They have distinct physical properties, such as different melting points, boiling points, and densities.

• Alkane Isomer Counts:

  • Methane ($CH_4$), Ethane ($C_2H_6$), and Propane ($C_3H_8$) have only one possible structure.

  • Butane ($C_4H_{10}$) has two isomers: n-butane (linear) and isobutane (2-methylpropane/branched).

  • Pentane ($C_5H_{12}$) has three isomers.

• Formula for Isomers: For alkanes with 4 to 7 carbon atoms, the number of isomers can be determined using: $2^{n-4} + 1$

• Etymology: The term "isomer" comes from the Greek isos + meros, meaning "made of the same parts."

Preparation of Alkanes

Alkanes are primary constituents of petroleum and natural gas, typically isolated via fractional distillation. They can also be synthesized in the laboratory through several methods:

1. Hydrogenation of Alkenes: Reacting an alkene with hydrogen gas in the presence of a metal catalyst ($Pt$) and heat.

   • Reaction: $\text{Alkene} + H_2 \xrightarrow[\text{heat}]{Pt} \text{Alkane}$

   • Example: $C_2H_4 + H_2 \rightarrow C_2H_6$

2. Würtz Synthesis: The reaction of a halogenated alkane (alkyl halide) with sodium metal. This reaction is exothermic and named after Charles-Adolphe Würtz (1817–1884).

   • General Reaction: $2R-X + 2Na \rightarrow R-R + 2NaX$

   • Example: $2CH_3Cl + 2Na \rightarrow CH_3-CH_3 + 2NaCl$

3. Decarboxylation: Heating the sodium salt of an organic acid with soda lime (a mixture of $NaOH$ and $CaO$). This removes the carboxylate group.

   • General Reaction: $RCOONa + NaOH \xrightarrow[\text{heat}]{CaO} R-H + Na_2CO_3$

   • Example (Preparation of Methane): $CH_3COONa + NaOH \rightarrow CH_4 + Na_2CO_3$

Chemical Properties of Alkanes

Alkanes are often called paraffins, derived from the Latin parum affinis, meaning "little affinity." They are chemically inert toward most acids, bases, and redox agents.

1. Combustion Reaction: The most significant reaction for alkanes, where they react with oxygen to release CO2, H2O, and high amounts of heat energy.

   • General Equation: $C_nH_{2n+2} + [\frac{3n+1}{2}] O_2 \rightarrow nCO_2 + (n+1)H_2O$

   • Example: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + 890\,kJ/mol \, (213\,kcal/mol)$.

2. Substitution Reaction (Halogenation): Replacement of a hydrogen atom by a halogen (specifically chlorine or bromine). This is a photochemical reaction requiring light or heat through a reaction mechanism:

   • Chain Initiation: Generation of free radicals. $Cl-Cl \xrightarrow{\text{light}} 2Cl\cdot$

   • Chain Propagation: Reaction of a radical with the alkane to generate a new radical. $Cl\cdot + CH_4 \rightarrow HCl + CH_3\cdot$

   • Chain Termination: Consumption of radicals to form stable products. $CH_3\cdot + Cl\cdot \rightarrow CH_3Cl$

   • A mixture of products (e.g., $CH_3Cl$, $CH_2Cl_2$, $CHCl_3$, $CCl_4$) is often produced.

3. Elimination Reaction: Removal of small molecules (like $H_2$) to form unsaturated compounds containing multiple bonds.

   • Example: $CH_3-CH_3 \xrightarrow{600-800\,^{\circ}C} CH_2=CH_2 + H_2$

Cycloalkanes and Uses of Alkanes

• Cycloalkanes (Alicyclic Hydrocarbons): Saturated hydrocarbons where carbons are arranged in a ring.

  • General formula: $C_nH_{2n}$ ($n \geq 3$).

  • Simplest member: Cyclopropane.

  • Examples: Cyclobutane, cyclopentane, cyclohexane.

• Uses: Alkanes serve primarily as fuels (gasoline, diesel, natural gas). They are also utilized as solvents and raw materials for synthesizing alkenes, alcohols, soaps, detergents, and plastics.

Unsaturated Hydrocarbons: Alkenes (Olefins)

Alkenes contain at least one carbon-carbon double bond ($C=C$) and are classified as unsaturated hydrocarbons.

• General Formula: $C_nH_{2n}$ ($n = 2, 3, 4, ...$).

• Ethene ($C_2H_4$): The simplest alkene. It acts as a plant hormone for fruit ripening and is a feedstock for ethanol and polyethylene.

• Propene ($C_3H_6$): Used to produce polypropylene, acetone, and cumene.

• Isomerism in Alkenes:

  • Chain Isomerism: Differences in the carbon skeleton (e.g., 1-pentene vs. 2-methyl-1-butene).

  • Position Isomorphism: Displacement of the double bond (e.g., 1-pentene vs. 2-pentene).

  • Geometrical Isomerism (cis-trans): Results from restricted rotation around the double bond.

    • cis: Similar groups on the same side.

    • trans: Similar groups on opposite sides.

    • Condition: Cannot occur if two identical groups are attached to the same carbon of the double bond.

Questions & Discussion

• Activity 6.7: How to identify two similar dogs? In chemistry, compounds with the same formula and weight but different structures are called isomers.

• Activity 6.8: Two gases, A and B ($C_4H_{10}$), have boiling points of $-1.0\,^{\circ}C$ and $-11.7\,^{\circ}C$ respectively. Relationship: They are structural isomers. Conclusion: Isomers have different physical properties.

• Activity 6.9: Do saturated alkanes have affinity for others? No, they are "paraffins." What is their most important reaction? Combustion.

• Activity 6.11: Are alkenes polar? No, they are nonpolar. Are they soluble in water? No, but they are soluble in nonpolar solvents like carbon tetrachloride ($CCl_4$).