Elements in Cells and Carbon Bonding

Most Abundant Elements in the Human Body

The human body is composed primarily of a few key elements, which play essential roles in biological chemistry.

Carbon is Tetravalent

Carbon is a central element in organic chemistry due to its ability to form four covalent bonds.

• Group IVA: Carbon is in group IVA of the periodic table.

• Valence Electrons: Carbon has 4 valence electrons (2 in 2s, 2 in 2p).

• Bonding: Needs 4 more electrons for a stable octet; forms 4 covalent bonds.

• Does not ionize: Carbon typically does not form ions in biological systems.

Bonding and Polarity

Expected Bonding of Common Elements

Types of Carbon Bonding

Carbon can form single, double, or triple bonds, which determine the geometry of molecules.

• Single bonds: Tetrahedral geometry ()

• Double bonds: Trigonal planar geometry ()

• Triple bonds: Linear geometry ()

Examples:

• Tetrahedral:

• Trigonal planar:

• Linear:

Polarity of Bonds

Bond polarity depends on the difference in electronegativity between atoms.

• Increasing polarity: H—O > H—N > H—Br > H—Cl > H—C

• Polar bonds: Have partial positive and negative charges (e.g., H—O)

Types of Organic Reactions

General Types of Reactions

Organic molecules undergo several types of chemical reactions:

• Addition reaction: Two reactants combine to form a single product ()

• Elimination reaction: A single reactant splits into two or more products ()

• Substitution reaction: Two reactants exchange parts to give new products ()

• Rearrangement reaction: A molecule undergoes bond reorganization to give an isomer ()

Hydrocarbons: Structure and Classification

Saturated Hydrocarbons

Saturated hydrocarbons contain only carbon and hydrogen atoms with single bonds.

• Alkanes: Hydrocarbons with only carbon-carbon single bonds.

Types of Structures

Hydrocarbons can be represented in various ways:

• Condensed:

• Lewis Dot: Shows all atoms and electrons.

• Line Bond (Structural): Shows bonds between atoms.

• Abbreviated Structures: "Line" or "line angle" omits hydrogens for simplicity.

• Wedge and dash: Indicates 3D shape.

Counting Carbons and Hydrogens

• End of chain: 3 hydrogens (-CH3)

• Middle of chain: 2 hydrogens (-CH2-)

• Branch point: 1 hydrogen (-CH-)

Classifying Carbon Atoms

Physical Properties of Hydrocarbons

• Nonpolar: Carbon-hydrogen bonds are considered nonpolar.

• Hydrophobic: Hydrocarbons do not mix well with water.

• Combustion: Alkanes can undergo combustion to form and .

Isomerism and Stereochemistry

Isomers

Isomers have the same molecular formula but different structures.

• Structural (constitutional) isomers: Different connectivity of atoms.

• Stereoisomers: Same connectivity, different spatial arrangement.

Structural Isomers vs. Conformers

• Structural isomers: Different connections between atoms.

• Conformers: Same connections, differ by rotation around single bonds.

How to Distinguish Isomers from Conformers

1. Same numbers of atoms (same molecular formula).

2. Same connections between atoms?

3. Check the length of chains and positions of groups.

4. Are groups and connections in the same place?

Cycloalkanes and Cis-Trans Isomers

• Cycloalkanes: Closed ring structures; limited bond rotation.

• Cis-trans isomers: Same connectivity, different spatial arrangement due to restricted rotation.

• Cis: Groups on the same face.

• Trans: Groups on opposite faces.

Stereoisomers and Chirality

• Chiral carbon: Carbon bonded to four different groups.

• Stereoisomers: Same connections, different locations around a chiral center.

Unsaturated Hydrocarbons and Bond Rotation

Unsaturated Hydrocarbons

• Alkenes: At least one carbon-carbon double bond.

• Alkynes: At least one carbon-carbon triple bond.

• Aromatic: Benzene-like ring (alternating double and single bonds).

Double and Triple Bonds

• No rotation: Double and triple bonds do not rotate due to electron density above and below the plane of atoms.

• Bond shapes: Double bond: planar; triple bond: linear.

Cis-Trans Isomers in Alkenes

• Cis: Groups on the same side of the double bond.

• Trans: Groups on opposite sides.

Hydrogenation of Fatty Acids

Hydrogenation Reactions

• Addition of H2: Converts alkenes to alkanes.

• Reduction: More bonds to H.

Hydrogenation of Fatty Acids

• Saturated: No double bonds; pack tightly; higher melting point.

• Unsaturated: Double bonds; cannot pack tightly; lower melting point.

• Partial hydrogenation: Some double bonds remain; can produce trans fats.

Summary of Key Concepts

• Carbon is most commonly bonded to nitrogen, oxygen, hydrogen, and halogens or sulfur in biological molecules.

• Carbon is tetravalent and can form single, double, or triple bonds, determining geometry and shape.

• Four general reaction types: addition, elimination, substitution, rearrangement.

• Hydrocarbons are nonpolar and hydrophobic; alkanes are least reactive but can combust.

• Isomers have the same molecular formula but different structures; stereoisomers differ in spatial arrangement.

• Double and triple bonds do not rotate, affecting molecular geometry and reactivity.

• Hydrogenation changes the properties of fatty acids, influencing their physical characteristics and health effects.