Organic Compounds: Structure, Nomenclature, Isomerism, and Hydrocarbons

Posted on Jan 23, 2025 in Chemistry

Organic Compounds: Key Concepts

Most organic compounds contain six main elements: C, H, O, N, S, and P.

Characteristics of the Carbon Atom

  • Valence of 4 (forms 2 single bonds in CH2 and CO).
  • Capable of forming long chains: C-C-C-C…
  • Forms single (C-C), double (C=C), and triple (C≡C) bonds.
  • Single bonds: tetrahedral geometry (109.5°).
  • Double bonds: planar geometry (120°).
  • Triple bonds: linear geometry (180°).
  • Bond Energies: Etriple > Edouble > Esingle.
  • Reactivity: C≡C > C=C > C-C

Carbon Chain Types

  • Linear, branched, and closed chains.
  • Primary (1°) carbon: bonded to one other carbon.
  • Secondary (2°) carbon: bonded to two other carbons.
  • Tertiary (3°) carbon: bonded to three other carbons.
  • Quaternary (4°) carbon: bonded to four other carbons.

Functional Groups

Functional groups are always united in the same way, giving compounds their characteristic properties. They are identified by suffixes and prefixes:

  • -ane (alkane)
  • -ene (alkene)
  • -yne (alkyne)
  • -ol (alcohol)
  • -al (aldehyde)
  • -one (ketone)
  • -oic acid (carboxylic acid)
  • -amine (amine)
  • -amide (amide)

Homologous Series

A homologous series is a set of compounds with the same functional group, ordered by the number of carbon atoms. They are distinguished by a root:

  • 1: met-
  • 2: et-
  • 3: pro-
  • 4: but-
  • 5: pent-
  • 6: hex-
  • 7: hept-
  • 8: oct-
  • 9: non-
  • 10: dec-

Nomenclature of Organic Compounds

Rules for naming organic compounds:

  1. Identify the main chain:
  • Contains the main functional group.
  • Has the most double and triple bonds.
  • Has the most carbon atoms.
  • Has the most double bonds.
Number the main chain:
  • Give the main group the lowest possible number.
  • Give the remaining functional groups the lowest possible numbers.
  • Give double and triple bonds the lowest possible locators.
  • If double and triple bonds are in equivalent positions, give the lower number to the double bond.
Name the compound:
  • The main group is named by a suffix, others by prefixes.
  • Side chains are named as radicals.
  • Secondary groups are named alphabetically with a number indicating their position.

Structural Isomerism

  • Chain isomerism: Different carbon chain arrangements.
  • Positional isomerism: Same functional group in different positions on the carbon chain.
  • Functional isomerism: Different functional groups with the same number and type of atoms.

Stereoisomerism

  • Cis-trans isomerism (or Z/E): Determined by a double bond or a cycle that prevents rotation.
  • Optical isomerism (or R/S): Occurs in carbons with single bonds to four different atoms or groups.
  • Enantiomers: Optical isomers that are mirror images of each other.
  • Diastereomers: Optical isomers that are not mirror images.

Hydrocarbons

Hydrocarbons are compounds formed only by C and H. They are fuels, have low polarity, and their state depends on the number of carbons (gases < 4C, liquids < 16C, solids > 16C). They are less dense than water.

Alkanes

  • Formula: CnH2n+2.
  • No double or triple bonds.
  • Typical substitution reactions of H by halogens, NO2, or SO3H groups.

Alkenes

  • Formula: CnH2n.
  • Contain double bonds.
  • Typical addition reactions to the double bond:
    • Hydrogenation: CH2=CH2 + H2 → CH3-CH3
    • Addition of H+X-: CH2=CH2 + HCl → CH3-CH2Cl
    • Addition of H+OH: CH2=CH2 + H2O → CH3-CH2OH

Alkynes

  • Formula: CnH2n-2.
  • Contain triple bonds.
  • Addition reactions to the triple bond, similar to alkenes.

Alicyclic Hydrocarbons

  • Closed chain hydrocarbons.
  • Stable from 5 carbon atoms.

Aromatic Hydrocarbons

  • Closed chain with alternating double bonds and additional stability.
  • Benzene is a key example.