Organic Compounds: Structure and Properties

• Organic chemistry studies the relationship between molecular structure and properties
• Study of organic chemistry should start with the common structural features of organic compounds
• Need to review chemical ideas, especially related to atomic structure and bonding for a better understanding of molecular structures
• Organic compound classes are examined, aiming to enhance understanding in further studies

Module Objectives

• Recognize the vast number and structural diversity of organic compounds
• Accurately draw Lewis structures
• Identify and classify functional groups in organic compounds

Pre-Competency Assessment

• Organic Chemistry studies carbon-containing compounds
• Organic compounds primarily have covalent bonds
• Priestly disproved the vital force theory
• Carbon (atomic number 6) has 4 valence electrons
• Functional groups are the most reactive parts of organic compounds

The Nature of Organic Compounds

• Kekulé and Couper determined carbon is tetravalent, forming four bonds with other atoms (H, O, N, and C)
• A key feature of carbon atoms is their ability to form strong covalent bonds with each other
• Carbon atoms can link to form linear, branched, or cyclic backbones in organic molecules

Isomerism

• Isomerism is exhibited by Organic molecules
• The term "isomerism" is derived from the Latin "iso" (equal) and "mer" (part)
• Structural isomerism: same molecular formula, different structural formulas
• Stereoisomerism: same molecular and structural formulas, different three-dimensional structures (stereos - solid)

Behavior of Organic Compounds

• Organic compounds, largely covalent, typically have low melting points and limited water solubility
• Some organic compounds are gases, others are liquids at room temperature, and solids usually melt below 400 °C
• Ionic inorganic salts have melting points above 1000 °C and dissolve readily in water

Physical Properties of Organic Compounds

• Primarily combustible (flammable) and/or sensitive to heat (decompose when heated)
• Sucrose isn't combustible but sensitive to heat, decomposes to carbon
• Organic chemistry is an empirical science based on laboratory observations
• August Kekulé, Archibald Scott Couper, and Alexander M. Butlerov developed the structural theory (1858-1861)
  • Atoms in organic molecules form a fixed number of bonds (valence)
  • Carbon atoms can participate in single, double, and triple bonds

Electronic Structure and Bonding

• Electrons in the outermost shell of an element are involved in chemical bonds and reactions
• Valence electrons exist in the valence shell, the energy level of outermost shell electrons
• Lewis structures use atomic symbols surrounded by dots to show the number of valence electrons

Formation of Chemical Bonds

• Gilbert N. Lewis noted noble gases' chemical inertness indicates stable electron configurations
  • Atoms bond to achieve a completed valence shell electronic configuration
  • Atoms acquire completely filled valence shells by gaining/losing electrons (ionic bond) or sharing electrons (covalent bond)
• Ionic bonds form when atoms transfer electrons
  • Atoms with low ionization energy (IE) lose electrons easily
  • Ionization energy (IE) is needed to remove the most loosely held electron from an isolated atom in its ground state
• Atoms with high electron affinity (EA) gain electrons easily
  • Electron affinity (EA) is the energy released when adding an electron to an isolated atom in its ground state
  • Electron affinity generally increases across the periodic table

Covalent Bonds

• Formed through the sharing of electrons by two atoms
• Atoms obtain a noble gas electron configuration
• Atoms in the middle of the periodic table share electrons due to the energy needed to gain/lose electrons

Lewis Structures

• Show covalent bonds as a pair of dots between bonded atoms
• Shorthand notation represents shared electrons as a dash or line -Unshared electron pairs (lone pairs) are shown as a pair of dots
• An atom forms as many covalent bonds with other atoms as needed to fill its valence shell
• Carbon forms four covalent bonds, nitrogen forms three, oxygen forms two, and halogens form one

Writing Lewis Structures

• Used for representing electronic structures of atoms, ions, and molecules
• Provide an accurate accounting of electrons
• A correct Lewis structure needs:
  • Valence shell electrons assigned correctly
  • Formal charges assigned correctly Sometimes covalently bonded atoms have a charge called formal charge

Classification of Organic Compounds

• Organic compounds are divided into hydrocarbons and heteroatom-containing compounds
• Hydrocarbons contain only carbon and hydrogen
• Elements other than carbon and hydrogen in organic compounds are heteroatoms
• Hydrocarbons are aliphatic or aromatic, with aliphatics including alkanes, alkenes, alkynes, and cyclic analogs
• Heteroatoms present include organic halides (halogens), alcohols/phenols/ethers/aldehydes/ketones/carboxylic acids/anhydrides/esters (oxygens), amines (nitrogen), acid halides (oxygen & halogen), amides/amino acids (oxygen & nitrogen), and thiols/sulfides (sulfur)

Functional Groups

• Classifies organic compounds by structure
• Functional group: the atom or group of atoms that defines the structure and properties of a class
• The behavior of a functional group is nearly the same whether in a small, big, or complex molecule
• Organic chemistry is largely functional group chemistry
• Functional groups have similar physical and chemical properties
• Studying a few compounds as representatives can characterize thousands more
• Familiarizing functional groups is a good way to start learning organic chemistry