Hydrocarbons revision notes

The Wurtz reaction is an efficient method for synthesizing any desired alkane, regardless of the carbon chain length.

The Wurtz reaction is highly preferred only for preparing symmetrical alkanes with an even number of carbon atoms. Attempting to couple two different alkyl halides to create an odd-carbon alkane generates a complex mixture of three distinct alkanes that are nearly impossible to separate due to similar boiling points.

Friedel-Crafts alkylation attaches the alkyl group directly to the benzene ring via the specific carbon atom that originally held the halogen.

The reaction proceeds through a free carbocation intermediate, which will undergo hydride or methyl shifts to maximize its thermodynamic stability before attacking the ring. For example, reacting benzene with 1-chloropropane yields isopropylbenzene (cumene) as the major product, not $n$-propylbenzene.

The anti-Markovnikov peroxide effect (Kharasch effect) alters the addition mechanism for all hydrogen halides ($\text{HX}$).

The free-radical mechanism applies strictly to $\text{HBr}$. It fails with $\text{HCl}$ because the $\text{H}-\text{Cl}$ bond is thermodynamically too strong to break homolytically. It fails with $\text{HI}$ because, while the $\text{H}-\text{I}$ bond is weak, the resulting iodine radicals preferentially combine with each other to form $\text{I}_2$ rather than attacking the alkene.

Electrophilic aromatic substitution directing effects always run parallel to activation properties; all deactivating groups must direct meta.

Halogens are a unique anomaly. Due to their intense electronegativity ($-\text{I}$ effect), they withdraw electron density and are strongly deactivating. However, because they possess lone pairs capable of delocalization ($+\text{R}$ effect), they uniquely act as ortho/para-directing groups.

All hydrogen atoms in an alkyne molecule possess enhanced acidity due to the nearby triple bond.

Acidity is strictly restricted to hydrogen atoms attached directly to the triply bonded, $sp$-hybridized carbon atoms. Therefore, only terminal alkynes exhibit weak acidity and react with strong bases like $\text{NaNH}_2$ or Tollenis reagent. Internal alkynes completely lack acidic hydrogens.

Reducing an alkyne to an alkene yields a random equilibrium mixture of geometric cis and trans isomers.

The stereochemical outcome depends entirely on the chosen reducing agent. Hydrogenation using Lindlar's catalyst ($\text{H}_2/\text{Pd}/\text{BaSO}_4$ poisoned with quinoline) performs syn-addition to yield a cis-alkene. Conversely, reduction using Birch conditions ($\text{Na}$ or $\text{Li}$ in liquid $\text{NH}_3$) undergoes a radical-anion mechanism to yield a trans-alkene.

Isomeric alkanes with more branching have higher boiling points due to a more compact, dense molecular structure.

Highly branched alkanes have lower boiling points than their straight-chain isomers. Increased branching forces the molecule to adopt a spherical geometry, which drastically minimizes its outer surface area. This reduction in surface area weakens the intermolecular Van der Waals forces, lowering the energy required to vaporize the liquid.

Alkanes are completely inert and can never be oxidized by chemical oxidizing agents like aqueous $\text{KMnO}_4$.

While ordinary linear and secondary alkanes resist oxidation, any alkane possessing a tertiary ($3^\circ$) hydrogen atom is an exception. The weak $3^\circ$ $\text{C}-\text{H}$ bond is selectively targeted and oxidized by $\text{KMnO}_4$ to synthesize a tertiary alcohol.

Methane ($\text{CH}_4$), being the simplest alkane, can easily be prepared using primary hydrocarbon coupling reactions.

Methane cannot be synthesized via the Wurtz reaction or Kolbe's electrolytic method. Both synthetic pathways rely fundamentally on the homolytic coupling of two distinct alkyl radicals, meaning the absolute smallest possible hydrocarbon product they can generate is ethane ($\text{CH}_3-\text{CH}_3$).

Staggered and eclipsed conformational isomers of ethane can be physically isolated at exceptionally low room temperatures.

Conformational isomers cannot be isolated at room temperature. The energy barrier separating the staggered and eclipsed forms is only about $12.5\text{ kJ/mol}$, which is easily and continuously overcome by the ambient thermal kinetic energy of the molecules, resulting in rapid, continuous rotation.

Determining reductive ozonolysis products requires drafting the complex, unstable cyclic ozonide ring intermediate every time.

Use the direct structural shortcut: conceptually slice the reactive $\text{C}=\text{C}$ double bond completely in half. Affix a carbonyl oxygen atom ($\text{=O}$) directly to each of the two newly exposed carbon ends to instantly reveal the resulting aldehydes or ketones.

Benzene reacts via addition reactions just as readily as open-chain conjugated dienes or simple alkenes do.

Benzene strictly favors electrophilic aromatic substitution over addition. Substitution allows the ring to preserve its highly stable $(4n+2)\pi$ Huckel aromatic resonance. Forcing benzene into an addition reaction requires destroying this aromaticity, which demands aggressive, high-energy conditions like intense UV radiation or extreme heat and pressure.