Hydrocarbons (Alkanes)

Alkanes

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Alkanes are saturated hydrocarbons, meaning they contain only carbon-carbon single bonds and carbon-hydrogen bonds. They form the simplest class of organic compounds and serve as the building blocks for more complex organic molecules.

Nomenclature And Isomerism

Nomenclature:

• Parent Name: Based on the longest continuous carbon chain (meth-, eth-, prop-, but-, pent-, hex-, etc.).
• Suffix: '-ane' for alkanes.
• General Formula: $C_nH_{2n+2}$.

Isomerism in Alkanes:

• Chain Isomerism: Alkanes exhibit chain isomerism starting from butane ($C_4H_{10}$). Butane has two chain isomers: $n$-butane and isobutane (2-methylpropane). Pentane ($C_5H_{12}$) has three chain isomers. The number of isomers increases rapidly with chain length.

Preparation

Alkanes can be prepared through various methods:

1. From Unsaturated Hydrocarbons (Reduction):
• Catalytic Hydrogenation: Alkenes and alkynes react with hydrogen gas in the presence of metal catalysts (like Ni, Pt, Pd) to form alkanes.

$CH_2=CH_2(g) + H_2(g) \xrightarrow{Ni} CH_3CH_3(g)$

$CH \equiv CH(g) + 2H_2(g) \xrightarrow{Ni} CH_3CH_3(g)$

2. From Alkyl Halides:
• Reduction with $Zn$/Acid or $Zn$/Base: Alkyl halides can be reduced to alkanes using reducing agents.

$CH_3CH_2I + 2[H] \rightarrow CH_3CH_3 + HI$

• Wurtz Reaction: Reaction of an alkyl halide with sodium metal in the presence of dry ether. This is useful for preparing symmetrical alkanes with an even number of carbon atoms.

$2CH_3Cl + 2Na \xrightarrow{dry \ ether} CH_3CH_3 + 2NaCl$

Note: Wurtz reaction cannot be used to prepare alkanes with an odd number of carbon atoms directly, as it leads to a mixture of products (alkane, alkane with double the carbons, and $H_2$ from side reaction).

3. Decarboxylation of Sodium Salts of Fatty Acids: Heating sodium salts of carboxylic acids with soda lime (a mixture of $NaOH$ and $CaO$).

$CH_3COONa(s) + NaOH(s) \xrightarrow{CaO, heat} CH_4(g) + Na_2CO_3(s)$

4. From Aluminium Carbide ($Al_4C_3$) and Beryllium Carbide ($Be_2C$): Reaction with water or dilute acids produces methane ($CH_4$).

$Al_4C_3(s) + 12H_2O(l) \rightarrow 4Al(OH)_3(s) + 3CH_4(g)$

$Be_2C(s) + 2H_2O(l) \rightarrow 2BeO(s) + CH_4(g)$

Properties

Physical Properties:

• State: Gaseous at room temperature ($C_1-C_4$), liquid ($C_5-C_{17}$), solid ($C_{18}$ onwards).
• Odor: Generally odorless, but impure samples may have a slight odor.
• Solubility: Insoluble in water (due to nonpolar nature) but soluble in nonpolar organic solvents (like benzene, gasoline, carbon tetrachloride).
• Density: Less dense than water.
• Boiling Points: Increase with increasing molecular weight due to strengthening van der Waals forces. Branching in isomers lowers the boiling point.
• Melting Points: Generally increase with molecular weight, but the trend is less regular than boiling points due to differences in crystal packing efficiency.

Chemical Properties:

1. Inertness: Alkanes are generally unreactive due to the presence of strong, nonpolar $C-C$ and $C-H$ single bonds with high bond dissociation enthalpies. They do not react with acids, bases, oxidizing agents, or reducing agents under ordinary conditions.

2. Combustion: They burn in the presence of air or oxygen to produce $CO_2$ and $H_2O$, releasing a large amount of energy. This makes them excellent fuels.

$C_nH_{2n+2} + \frac{3n+1}{2}O_2 \rightarrow nCO_2 + \frac{n+1}{2}H_2O$

3. Controlled Oxidation: Under specific conditions, controlled oxidation can occur.

4. Halogenation (Substitution Reaction):

• Alkanes react with halogens (except iodine) in the presence of UV light or heat via a free radical mechanism.
• This is a substitution reaction where hydrogen atoms are replaced by halogen atoms.
• The reaction proceeds stepwise, potentially replacing multiple hydrogen atoms.

$CH_4 + Cl_2 \xrightarrow{UV \ light} CH_3Cl + HCl$

$CH_3Cl + Cl_2 \xrightarrow{UV \ light} CH_2Cl_2 + HCl$

5. Cracking: At high temperatures (around 773 K) and in the absence of air, long-chain alkanes decompose into smaller alkanes and alkenes.

$C_{16}H_{34} \xrightarrow{high \ T, \ catalyst} C_8H_{18} + C_8H_{16}$ (Example of cracking)

6. Isomerization: In the presence of catalysts like anhydrous $AlCl_3$ or $HF$, straight-chain alkanes can rearrange to form branched-chain alkanes.

$n-butane \xrightarrow{AlCl_3, HCl} iso-butane$

7. Reforming: Heating alkanes in the presence of catalysts (like platinum) at high temperatures converts them into aromatic hydrocarbons.

$n-hexane \xrightarrow{Pt, 500^\circ C} Benzene + 4H_2$

Conformations

Conformational Isomerism: Conformations are different spatial arrangements of atoms in a molecule that can be interconverted by rotation around single bonds. They are not true isomers because they can interconvert freely at room temperature.

Ethane Conformations:

• Staggered Conformation: The hydrogen atoms on the front carbon are positioned exactly between the hydrogen atoms on the back carbon.
• Anti/Totally Staggered: Dihedral angle between identical groups is 180°. Most stable.
• Gauche: Dihedral angle between identical groups is 60°. Less stable than anti due to torsional strain.
• Eclipsed Conformation: The hydrogen atoms on the front carbon are directly in front of the hydrogen atoms on the back carbon.
• Totally Eclipsed: Identical groups are directly superimposed. Least stable due to steric repulsion.
• Eclipsed: Identical groups are offset by 120° but still aligned. Less stable than staggered.
• Stability Order: Anti-staggered > Gauche-staggered > Eclipsed > Totally eclipsed.

Newmann Projection: Used to represent conformations by viewing the molecule along the bond axis.

Cyclohexane Conformations: Cyclohexane exists predominantly in the chair conformation, which is the most stable due to minimizing torsional strain and steric repulsion (all bonds are either axial or equatorial).