Comprehensive Organic Chemistry Functional Groups and Lewis Acid-Base Reactivity

Hydrocarbons and Unsaturation: Alkanes, Alkenes, and Alkynes

• Alkanes:

  • Alkanes represent saturated hydrocarbon structures lacking reactive functional groups relative to other classes of organic compounds.
  • General aliphatic alkane molecular formula:     $C_n H_{2n+2}$
  • Example: Octane contains an unbranched eight-carbon chain:     $CH_3-CH_2-CH_2-CH_2-CH_2-CH_2-CH_2-CH_3$

• Alkenes:

  • Functional group: Carbon-carbon double bond ($C=C$).
  • Lewis Base Character:
  • Alkenes behave as Lewis bases because they can donate a pair of $\pi$ electrons to electron-deficient species.
  • A Lewis base is defined as any chemical species capable of donating a pair of bonded, $\pi$, or non-bonding electrons.
  • Electronic Structure and Geometry:
  • Both carbon atoms involved in a double bond are $sp^2$ hybridized.
  • Counting electron domains around each carbon atom gives three groups of electrons, creating a planar geometry around both $sp^2$ hybridized carbon centers with bond angles of approximately $120^\text{o}$.

• Alkynes:

  • Functional group: Carbon-carbon triple bond ($C\equiv C$).
  • Lewis Base Character:
  • Alkynes function as Lewis bases because they possess two $\pi$ bonds capable of donating electron pairs.
  • Structural Subdivisions:
  • Terminal Alkynes: Possess a triple bond at the end of the carbon chain ($R-C\equiv C-H$).
  • Non-Terminal Alkynes: Possess an internal triple bond flanked by carbon atoms on both sides ($R-C\equiv C-R'$).

Aromatic Systems and Oxygen-Containing Functional Groups

• Phenyl Rings and Aromatic Stability:

  • Structural feature: Phenyl ring ($-C_6H_5$).
  • Phenyl rings display enhanced aromatic stability due to cyclic electron delocalization.

• Single-Oxygen Functional Group Classes:

  • Four major classes of organic compounds contain a single oxygen atom:
  • Alcohols: Contain a hydroxyl group ($-OH$) attached to an aliphatic $sp^3$ carbon ($R-OH$).
  • Phenols: Contain a hydroxyl group ($-OH$) attached directly to an $sp^2$ carbon of an aromatic phenyl ring ($Ar-OH$). Phenols form a distinct reactive class separate from aliphatic alcohols.
  • Ethers: Contain an oxygen atom single-bonded between two carbon atoms ($R-O-R'$).
  • Carbonyl Compounds:
    • Aldehydes: Contain a carbonyl group ($C=O$) bonded to at least one hydrogen ($R-C(=O)H$).
    • Ketones: Contain a carbonyl group ($C=O$) bonded to two carbon groups ($R-C(=O)-R'$).
  • Universal Lewis Base Behavior:
  • All oxygen-containing classes act as Lewis bases because oxygen contains non-bonding electron pairs (lone pairs) capable of coordinate covalent bonding.

Classification of Carbon Centers

• Carbon Substitution Levels:

  • Primary ($1^\circ$) Carbon: A carbon atom bonded directly to exactly one other carbon atom.
  • Secondary ($2^\circ$) Carbon: A carbon atom bonded directly to two other carbon atoms.
  • Tertiary ($3^\circ$) Carbon: A carbon atom bonded directly to three other carbon atoms.
  • Quaternary ($4^\circ$) Carbon: A carbon atom bonded directly to four other carbon atoms.

• Structural Classes of Alcohols:

  • Primary ($1^\circ$) Alcohol: The hydroxyl group ($-OH$) is attached to a primary carbon ($R-CH_2-OH$).
  • Secondary ($2^\circ$) Alcohol: The hydroxyl group ($-OH$) is attached to a secondary carbon ($R_2CH-OH$).
  • Tertiary ($3^\circ$) Alcohol: The hydroxyl group ($-OH$) is attached to a tertiary carbon ($R_3C-OH$).

• Allylic and Benzylic Positions:

  • Allylic Carbon: The carbon directly attached to an $sp^2$ carbon of a carbon-carbon double bond ($C=C-C_{\text{allylic}}$).
  • An allylic alcohol places its hydroxyl group on the allylic carbon ($C=C-C_{\text{allylic}}-OH$).
  • Benzylic Carbon: The carbon directly attached to an $sp^2$ carbon of an aromatic phenyl ring ($Ar-C_{\text{benzylic}}$).
  • A benzylic alcohol places its hydroxyl group on the benzylic carbon ($Ar-C_{\text{benzylic}}-OH$).

Structural Isomers and Cyclic Ethers

• Isomeric Ethers of Formula $C_4H_{10}O$:

  • Structure requirement: Oxygen single-bonded to two carbon groups ($R-O-R'$).
  • Isomeric configurations:
  • $1$-Methoxypropane: $CH_3-O-CH_2-CH_2-CH_3$
  • Ethoxyethane: $CH_3-CH_2-O-CH_2-CH_3$
  • $2$-Methoxypropane: $(CH_3)_2CH-O-CH_3$
  • Systematic Structural Drawing Method:
  1. Construct the carbon and heteroatom framework.
  2. Attach required functional groups at specific positions.
  3. Add hydrogen atoms to satisfy carbon tetravalency ($4$ covalent bonds per carbon).

• Unsaturated and Cyclic Ethers ($C_4H_8O$):

  • For molecular formula $C_4H_8O$ with no double bonds ($0$ $\pi$ bonds):
  • The degree of unsaturation requires a cyclic structure.
  • Examples include tetrahydrofuran and methyl-substituted cyclic ethers.

Lewis Acid-Base Theory and Formal Charge Calculations

• Identification of Lewis Acids and Lewis Bases:

  • Lewis Base: Electron pair donor containing lone pairs or $\pi$ electrons. Heteroatoms like oxygen retain non-bonding electron pairs even when omitted in skeletal formulas.
  • Lewis Acid: Electron pair acceptor featuring an incomplete octet, positive charge, or partial positive charge.
  • Group $13$ (Group III) Elements (e.g., Boron $B$, Aluminum $Al$):
  • Neutral group $13$ compounds are trivalent, forming $3$ covalent bonds with $6$ valence electrons around the central atom.
  • Possessing an incomplete octet ($6$ electrons instead of $8$), group $13$ central atoms are highly electron-deficient Lewis acids.

• Coordinate Bond Formation and Formal Charge Equations:

  • Reaction mechanism representation: Curved arrows originate at non-bonding electron pairs on the Lewis base and point to the electron-deficient atom of the Lewis acid to show new covalent bond formation.
  • Formal Charge Formula:     $\text{Formal Charge} = \text{Group Number} - \text{Non-Bonding Electrons} - \frac{1}{2}(\text{Bonding Electrons})$
  • Formal Charge on Oxygen in a Coordinate Adduct ($R_2O \rightarrow LA$):
  • Valence electrons for oxygen = $6$
  • Non-bonding electrons = $2$ ($1$ lone pair)
  • Shared bonding electrons = $6$ ($3$ covalent bonds)     $\text{Formal Charge} = 6 - 2 - \frac{1}{2}(6) = +1$
  • Formal Charge on Boron in a Coordinate Adduct ($LB \rightarrow BR_3$):
  • Valence electrons for boron = $3$
  • Non-bonding electrons = $0$
  • Shared bonding electrons = $8$ ($4$ covalent bonds)     $\text{Formal Charge} = 3 - 0 - \frac{1}{2}(8) = -1$

Aldehydes, Carboxylic Acids, and Amines

• Aldehydes:

  • Functional group: Carbonyl group bonded to hydrogen ($-CHO$ or $-CH=O$).
  • Structure and Lewis Base Behavior:
  • Structural representation $R-C(=O)H$ accurately depicts carbonyl bonding.
  • Non-bonding electrons on oxygen are more readily donated than carbonyl $\pi$ electrons in standard Lewis acid-base interactions.
  • Cucumber Volatiles and Pest Control:
  • Naturally occurring aldehyde molecules produce the characteristic odor of fresh cucumber slices.
  • These cucumber aldehyde compounds act as natural cockroach repellents; placing cucumber slices around surfaces deters cockroaches.

• Carboxylic Acids:

  • Functional group: Carboxy group ($-COOH$ or $-C(=O)OH$), consisting of a hydroxyl group attached to a carbonyl carbon.

• Amines:

  • Nitrogen-containing organic bases derived from ammonia ($NH_3$).
  • Classification by Substitution Degree:
  • Primary ($1^\circ$) Amine: Nitrogen bonded to one carbon group ($R-NH_2$).
  • Secondary ($2^\circ$) Amine: Nitrogen bonded to two carbon groups ($R_2NH$).
  • Tertiary ($3^\circ$) Amine: Nitrogen bonded to three carbon groups ($R_3N$).
  • Classification by Electronic Structure:
  • Aromatic Amine: Nitrogen atom is bonded directly to a carbon atom of an aromatic phenyl ring.
  • Aliphatic Amine: Nitrogen atom is bonded exclusively to non-aromatic carbon atoms.