Carbon and functional groups Knowledge-Based Study

Study notes

1. The Carbon Atom and Molecular Diversity

   Carbon forms the structural backbone of virtually all biological molecules. A carbon atom possesses six electrons: two in an inner core and four in its valence shell. To complete its octet, carbon forms four covalent bonds with other atoms, a property known as tetravalence.

   When a carbon atom forms four single covalent bonds, its electron orbitals undergo hybridization to produce a tetrahedral geometry with bond angles of approximately 109.5 degrees. When two carbon atoms form a double covalent bond, the hybridized orbitals adopt a planar arrangement with bond angles of roughly 120 degrees. This spatial flexibility enables carbon chains to vary extensively in length, degree of branching, presence of double bonds, and formation of closed rings.

   Structural Variations and Isomerism

   Molecules consisting solely of carbon and hydrogen are designated as hydrocarbons. The nonpolar covalent bonds between carbon and hydrogen atoms make hydrocarbons hydrophobic and rich in potential energy.

   Differences in the spatial architecture of carbon skeletons produce isomers—compounds possessing identical molecular formulas but distinct structural arrangements and properties:

   • Structural isomers: Molecules that differ in the covalent arrangement of their carbon framework or double-bond positions.
   • Cis-trans (geometric) isomers: Molecules that maintain identical covalent partners but differ in spatial orientation around an inflexible carbon-carbon double bond. In a cis configuration, identical functional groups lie on the same side of the double bond; in a trans configuration, they lie on opposite sides.
   • Enantiomers: Molecules that are non-superimposable mirror images of one another due to an asymmetric carbon bonded to four distinct chemical species.

   Polar and Ionic Functional Groups

   While hydrocarbons provide a stable structural framework, specific chemical motifs called functional groups substitute for hydrogen atoms and directly participate in biochemical reactions. Oxygen- and nitrogen-containing groups introduce polarity and charge:

   • Hydroxyl group ($-OH$): Consists of a hydrogen atom covalently bonded to an oxygen atom. Hydroxyl groups impart polarity because electronegative oxygen draws electrons away from hydrogen, enabling hydrogen bonding with aqueous solvents. Molecules bearing this group are classified as alcohols.
   • Carbonyl group ($>C=O$): Consists of a carbon atom joined to an oxygen atom by a double bond. When located within a carbon skeleton, it defines a ketone; when positioned at the terminus of a skeleton, it defines an aldehyde. Carbonyls are essential in sugar chemistry (ketoses and aldoses).
   • Carboxyl group ($-COOH$): Consists of an oxygen atom double-bonded to a carbon atom that is also bonded to a hydroxyl group. Carboxyl groups behave as organic acids because the covalent bond between oxygen and hydrogen can dissociate, releasing a hydrogen ion ($H^+$) and yielding a negatively charged carboxylate ion ($-COO^-$).
   • Amino group ($-NH_2$): Consists of a nitrogen atom bonded to two hydrogen atoms. Amino groups act as bases by accepting a proton ($H^+$) from the surrounding medium, resulting in an ionized form ($-NH_3^+$).

   Sulfhydryl, Phosphate, and Methyl Groups

   Other major functional groups contribute critical chemical properties to proteins, nucleic acids, and regulatory mechanisms:

   • Sulfhydryl group ($-SH$): Consists of a sulfur atom bonded to a hydrogen atom. Two sulfhydryl groups can react covalently to form a disulfide bridge ($-S-S-$), a linkage crucial for stabilizing the tertiary and quaternary structures of many proteins. Molecules containing sulfhydryl groups are called thiols.
   • Phosphate group ($-OPO_3^{2-}$): Consists of a phosphorus atom covalently bonded to four oxygen atoms (one double bond and three single bonds), with two oxygen atoms carrying negative charges at physiological pH. Phosphate groups transfer energy between biochemical pathways (as seen in ATP) and confer a negative charge to nucleic acid polymers.
   • Methyl group ($-CH_3$): Consists of a carbon atom bonded to three hydrogen atoms. Unlike the other functional groups, the methyl group is completely nonpolar and hydrophobic. The addition of methyl groups to DNA bases and chromatin-associated proteins acts as an epigenetic marker regulating gene expression.