Hydrocarbons & Alcohols: Nomenclature and Key Concepts

Core Definitions and Context

• Hydrocarbons
  • Organic compounds containing only carbon (C) and hydrogen (H).
  • Serve as the parent framework for naming most other functionalized molecules.
• Alcohols
  • Hydrocarbons in which ≥ 1 hydroxyl (OH) group replaces a hydrogen.
  • OH substitution increases polarity, hydrogen-bonding capacity, and overall chemical reactivity.
  • Oxidation state of the carbon bearing OH is higher ⇒ gives OH higher naming priority over multiple bonds.
• MCAT Focus
  • Detailed mechanisms of alkene/alkyne reactions are not directly tested, but correct nomenclature and recognition are essential (e.g., unsaturated fatty acids).

Alkanes

• General formula: $C<em>nH</em>{2n+2}$ (where $n$ = number of carbons).
• Names for the first four members (must be memorized):
  • $n=1$ → methane
  • $n=2$ → ethane
  • $n=3$ → propane
  • $n=4$ → butane
• Systematic naming (> 4 carbons)
  • Prefix = Greek/Latin root indicating $n$; suffix = "-ane".
  • $n=5$–$12$ roots: pent-, hex-, hept-, oct-, non-, dec-, undec-, dodec-.
  • Example: $C<em>7H</em>{16}$ → heptane.

Halogen Substituents on Alkanes

• Referred to collectively as alkyl halides.
• Prefixes: fluoro-, chloro-, bromo-, iodo-.
• Example: 2-bromopropane.

Alkenes & Alkynes

• Suffixes convey bond multiplicity:
  • "-ene" → at least one double bond (C=C).
  • "-yne" → at least one triple bond (C≡C).
• Numbering rule: Assign the lowest possible number to the first C of the multiple bond.
  • Can be written before the parent name ("2-butene") or just before the suffix ("but-2-ene"). Both valid.
• Multiple multiple bonds: Separate locants from suffix, e.g. $1,3$-butadiene.
• Common test-day appearance: Unsaturated fatty acids or other biochemical molecules featuring C=C.

Alcohols

• Systematic (IUPAC) naming
  1. Identify the longest chain containing the OH; parent name gets suffix "-ol" (replace terminal "-e").
  2. Number chain so the carbon with OH gets lowest possible locant, overriding double/triple bonds.
  3. If OH is not the highest priority group in the molecule, designate it as the substituent "hydroxy-".
• Examples (IUPAC)
  • Ethanol (2-carbon parent with OH on C-1).
  • 5-methyl-2-heptanol (7-carbon parent, OH on C-2, methyl on C-5).
  • Hept-6-en-1-ol (7-carbon chain, C=C starting at C-6, OH on C-1; OH outranks the alkene when numbering).
• Common (non-IUPAC) names
  • "Ethyl alcohol" (ethanol).
  • "Isopropyl alcohol" (2-propanol). Knowing common names is vital for interpreting passages and lab data on the MCAT.

Diols (Glycols)

• Molecules with exactly two hydroxyl groups.
• Keep the full hydrocarbon name; add suffix "-diol" and give a locant for each OH.
  • Example: Ethane-1,2-diol $\rightarrow$ common name: ethylene glycol.
• Geminal vs. Vicinal Diols
  • Geminal ("gem") diols: Both OH groups on the same carbon.
  • Also called hydrates.
  • Thermodynamically unstable; readily dehydrate to form carbonyl compounds (C=O).
  • Vicinal diols: OH groups on adjacent carbons.

Nomenclature Summary & Priorities (High-Yield)

1. Identify the highest priority functional group (for these notes: OH outranks multiple bonds > alkane backbone).
2. Number to satisfy that priority.
3. Assemble name: prefixes (substituents) + parent + locants + suffix (functional group indication).
4. Maintain correct punctuation: numbers separated by commas; numbers and words separated by hyphens.

Practical / Real-World Significance

• Correctly naming and recognizing these functional groups is crucial for:
  • Interpreting metabolism pathways (e.g., oxidation of ethanol to acetaldehyde $\rightarrow$ acetate).
  • Understanding reagent selectivity (e.g., vicinal diol cleavage by $\text{KMnO}_4$).
  • Communicating in clinical or laboratory settings (labels often use common alcohol names).
• Safety/ethical notes: Mislabeling (e.g., methanol vs. ethanol) can lead to serious toxicological errors.