Organic Chemistry Practice Quiz For Students

Alkanes are called saturated hydrocarbons because each carbon atom forms only single covalent bonds, allowing it to hold the maximum number of hydrogen atoms permitted by valency rules. No additional atoms can be added without breaking existing bonds. This “saturation” makes alkanes generally less reactive than unsaturated hydrocarbons, which contain double or triple bonds. For this reason, alkanes are described as fully saturated with hydrogen under normal structural conditions.

In organic substitution reactions, halogen atoms replace hydrogen atoms because both occupy monovalent positions and form single covalent bonds. Halogenation typically occurs in alkanes under radical conditions or in aromatic systems via electrophilic substitution. Oxygen, nitrogen, or carbon cannot be directly substituted in the same manner without altering functional groups or molecular frameworks. Thus, hydrogen is the atom most commonly replaced by halogens in standard organic substitution reactions.

Butane is the only gas among the listed hydrocarbons at room temperature due to its relatively low molecular mass and weaker intermolecular London dispersion forces. Hexane and pentane have higher molar masses, creating stronger dispersion forces that hold them in the liquid state at room temperature. The boiling point of butane is approximately −1°C, well below room temperature, ensuring it exists as a gas under typical laboratory conditions.

Alcohols contain an –OH (hydroxyl) group bonded to a carbon atom, distinguishing them from other functional groups. This hydroxyl group contributes to hydrogen bonding, affecting boiling points and solubility. Aldehydes contain a carbonyl group (–CHO), and amines feature nitrogen bonded to carbon or hydrogen, neither of which includes an O–H bond characteristic of alcohols. Thus, only alcohols match the functional group description involving an oxygen–hydrogen bond.

Alkynes are hydrocarbons containing at least one carbon–carbon triple bond, giving them the general formula CₙH₂ₙ₋₂. The triple bond contributes significant electron density and unique reactivity patterns, distinguishing alkynes from alkanes, which have only single bonds, and alkenes, which contain double bonds. Arenes contain aromatic rings rather than multiple bonds in linear chains. Therefore, any compound with a C≡C bond is classified specifically as an alkyne.

A constitutional isomer has the same molecular formula but a different connectivity of atoms. Butane (C₄H₁₀) has two constitutional isomers: n-butane and isobutane (2-methylpropane). The structure CH₃CH(CH₃)₂ represents isobutane, which rearranges the carbon skeleton into a branched form. Other listed options either repeat the straight-chain structure or have incorrect formulas. Hence, the branched structure correctly represents a constitutional isomer of butane.

Nonane has the highest boiling point because boiling point increases with molecular size and surface area. Larger molecules experience stronger London dispersion forces due to increased electron cloud surface interactions. Nonane, with nine carbon atoms, has the largest molar mass among the listed alkanes, causing stronger intermolecular attractions. These increased forces require more energy to overcome, resulting in a higher boiling point than hexane, heptane, or octane.

Alkanes are classified as saturated hydrocarbons because their carbon atoms are connected only by single covalent bonds, permitting the maximum number of hydrogen atoms. They follow the general formula CₙH₂ₙ₊₂ and exhibit low reactivity due to the strength and stability of their C–C and C–H bonds. Aromatic hydrocarbons contain benzene rings, while unsaturated hydrocarbons include alkenes and alkynes with double or triple bonds. Therefore, alkanes belong to the saturated class.

Combustion of propane follows the balanced chemical equation: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Balancing requires three carbon dioxide molecules and four water molecules, which together demand ten oxygen atoms, corresponding to five O₂ molecules. Thus, burning one mole of propane consumes exactly five moles of molecular oxygen. This reaction releases energy and represents complete oxidation, ensuring formation of only carbon dioxide and water as combustion products.

The double bond in alkenes makes them susceptible to addition reactions, where atoms or groups add across the π bond. This reaction breaks the double bond and transforms the alkene into a more saturated molecule. Common additions include hydrogenation, halogenation, hydrohalogenation, and hydration. Although alkenes can undergo oxidation or substitution under certain conditions, addition reactions are the most fundamental and characteristic process associated with the presence of a carbon–carbon double bond.