Carbon and Functional Groups

Review and Learning Objectives

This section begins with a review of fundamental concepts from previous lectures, including the definitions of acids and bases, their differences, characteristics of the pH scale, how to determine if a solution with a given pH (e.g., pH 5.5) is acidic or basic, the mechanism of buffers, and their biological importance. It also covers the definition of a salt.

Key questions from the textbook's "Test your understanding" section on page 55 include:

Identifying hydrophobic materials (e.g., paper, table salt, wax, sugar).
Understanding that a mole of any substance, such as table sugar and vitamin C, contains the same number of molecules.
Calculating the hydrogen ion concentration ([H^{+}]) of a lake with a pH of 4.0. The relationship is [H^{+}] = 10^{-pH}, so (10^{-4}M).
Calculating the hydroxide ion concentration ([OH^{-}]) of the same lake. Given pH = 4.0, pOH = 14 - pH = 14 - 4 = 10. Thus, [OH^{-}] = 10^{-pOH} = 10^{-10}M.
An explanation of how farmers use water to protect crops from freezing, involving the properties of water and the role of hydrogen bonds in releasing heat upon freezing.

Upon completion of this section (pages 56-66), students should be able to:

Describe the unique qualities of Carbon that make it the fundamental atom for life.
Identify and understand different forms of carbon skeletons.
Be familiar with the concept of Isomers.
Know the 7 major functional groups and provide examples of each.

Carbon: The Basis for All Biological Organisms

Carbon is the foundation of all biological organisms due to its extraordinary bonding capabilities:

Versatility in Compound Formation: Carbon can form large, complex, and diverse compounds.
Number of Bonds: A carbon atom can form 4 covalent bonds.
Bonding with Other Carbon Atoms: Carbon atoms can readily bond with other carbon atoms, creating extensive chains and rings.
Organic Compounds: Compounds containing both Carbon (C) and Hydrogen (H) are defined as organic. These organic compounds typically feature C and H in rings or long chains.

Carbon's Electron Configuration and Bonding

Electrons: Carbon has 6 electrons in total.
Valence Electrons: It possesses 4 valence electrons, which dictate its bonding capacity.
Covalent Bonds: Carbon can form 4 covalent bonds. These can be:
- 4 single covalent bonds, or
- 2 double covalent bonds, or a combination (e.g., one double and two singles, one triple and one single).
Covalent Compatibility: The electron configuration of carbon provides it with covalent compatibility with a variety of other elements. The number of covalent bonds an element forms corresponds to its number of valence electrons.
Significance of Shapes: The ability to form various bond types (single, double) leads to different molecular shapes, like those seen in simple organic molecules (e.g., methane is tetrahedral, ethene is planar, ethyne is linear).

Examples of Carbon Bonding

Carbon Dioxide (CO_2): An inorganic molecule. It consists of a single carbon atom joined to two oxygen atoms via double covalent bonds (O=C=O).
Urea (CO(NH2)2): An organic molecule found in urine. It features a single carbon atom bonded to two amino functional groups (NH2) and one oxygen atom. This involves two single bonds (to NH2 groups) and one double bond (to oxygen).

These examples illustrate that even molecules with only one carbon atom can demonstrate diverse bonding arrangements.

Carbon Skeletons

Carbon's ability to form covalent bonds with other carbon atoms allows for the creation of diverse carbon skeletons. These skeletons can vary in several fundamental ways, leading to immense molecular complexity and diversity:

Length: Carbon chains can be short or very long.
Branching: Skeletons can be unbranched (linear) or branched.
Double Bond Position: The location of double bonds within a carbon chain can vary.
Presence of Rings: Carbon atoms can form cyclic structures (rings).

These variations are crucial for generating the vast array of organic molecules essential for life.

Hydrocarbons

Hydrocarbons are organic molecules composed exclusively of carbon and hydrogen atoms.

Examples: Methane (CH4), propane (C3H_8), and various components of gasoline and jet fuel are hydrocarbons.
Occurrence: While hydrocarbons are the main components of petroleum, they are not common as isolated molecules in living organisms.
Biological Relevance: However, long hydrocarbon chains are found within other biological molecules, such as the fatty acid tails of fat molecules.
Polarity and Hydrophobicity: The carbon-hydrogen linkages in hydrocarbons are largely nonpolar. This characteristic makes hydrocarbon compounds hydrophobic, meaning they do not readily dissolve in water.

Isomers

Isomers are molecules that share the same molecular formula but possess different structural arrangements of atoms.

There are three main types of isomers:

Structural Isomers: These isomers differ in the covalent arrangement of their atoms. For example, two molecules might have the same number of carbons and hydrogens but have different branching patterns or functional group positions.
Cis-Trans Isomers (Geometric Isomers): These isomers arise from the restricted rotation around a double bond. Atoms or groups of atoms attached to the carbons of the double bond can be arranged on the same side (cis configuration) or on opposite sides (trans configuration).
Enantiomers: These are a type of stereoisomer that are mirror images of each other but are non-superimposable. They often involve an asymmetric carbon (a carbon atom bonded to four different groups). Enantiomers are significant in biology because often only one enantiomeric form is biologically active.

Functional Groups

The properties of an organic molecule are not solely determined by its carbon skeleton but are also profoundly influenced by functional groups.

What are Functional Groups?

They are small groups of atoms consistently found attached to the carbon skeleton of organic molecules.
They are generally involved in chemical reactions, serving as the chemically active parts of organic molecules.

Impact of Functional Groups on Molecular Properties:

Reactivity: Functional groups largely dictate how a molecule will react with other molecules.
Structural Stability: They can influence the overall stability and conformation of a molecule.
Hydrogen Bonding: Many functional groups contain electronegative atoms (like oxygen or nitrogen) that can form hydrogen bonds with water or other molecules, affecting solubility and intermolecular interactions.
Solubility: The presence of polar functional groups often increases a molecule's solubility in water.
Hydrophilic and Hydrophobic Groups: Functional groups can make a molecule more hydrophilic (water-loving) or contribute to its hydrophobic character.
Polarity: They introduce regions of polarity within molecules.
Acid/Base Capacity: Certain functional groups can donate (H^{+}) ions (acting as acids) or accept (H^{+}) ions (acting as bases), thereby influencing the pH of solutions.

The Seven Major Functional Groups in Living Organisms

1. Hydroxyl Group

Structure: (–OH) or (HO–)
Example: Ethanol (H-C-C-OH). Isopropanol and glycerol also contain hydroxyl groups.
Compound Name: Alcohol
Properties: Polar, due to the electronegative oxygen. Forms hydrogen bonds with water molecules, increasing solubility.

2. Carbonyl Group

Structure: (C=O) (a carbon atom double-bonded to an oxygen atom)
Location: Can be at the end of a carbon chain or within a chain.
Compound Name: Ketone (if the carbonyl group is within a carbon skeleton) or aldehyde (if the carbonyl group is at the end of the carbon skeleton).
Examples: Acetone (a ketone), Propanal (an aldehyde).
Biological Relevance: Sugars with ketone groups are called ketoses, and those with aldehyde groups are called aldoses.

3. Carboxyl Group

Structure: (–COOH) or (–C(=O)OH)
Example: Acetic acid (CH_3COOH), which gives vinegar its sour taste.
Compound Name: Carboxylic acid (or organic acid)
Properties: Acts as an acid because the hydrogen atom of the hydroxyl group is easily donated (due to the polarity created by the two oxygen atoms). In cells, it typically exists in its ionized form, the carboxylate ion ((–COO^{-})), having lost an (H^{+}) ion.

4. Amino Group

Structure: (–NH_2)
Example: Glycine (H2N-CH2-COOH), which is an amino acid (note that it also contains a carboxyl group).
Compound Name: Amine
Properties: Acts as a base by accepting (H^{+})) ions from the surrounding solution. In cells, it is often found in its ionized form ((–NH_3^{+})), having accepted an (H^{+})) ion.

5. Sulfhydryl Group

Structure: (–SH) or (HS–)
Example: Cysteine, a sulfur-containing amino acid.
Compound Name: Thiol
Properties: Two sulfhydryl groups can react with each other to form a covalent bond called a "disulfide bridge" ((–S-S–) linkage). These cross-links are crucial for stabilizing the tertiary structure of proteins, influencing their three-dimensional shape and function.

6. Phosphate Group

Structure: (–OPO_3^{2-}) (a phosphorus atom bonded to four oxygen atoms, with one oxygen bonded to the carbon skeleton and two negative charges).
Example: Glycerol phosphate, a molecule involved in many important cellular chemical reactions.
Compound Name: Organic phosphate
Properties: Contributes a negative charge to the molecule. When attached, it confers on a molecule the ability to react with water, releasing energy. Phosphate groups are vital components of energy molecules like ATP (adenosine triphosphate) and ADP (adenosine diphosphate), as well as structural components of DNA and RNA.

7. Methyl Group

Structure: (–CH_3)
Example: 5-Methylcytosine, a modified component of DNA.
Compound Name: Methylated compound
Properties: While often non-reactive itself, the addition of a methyl group can significantly affect the expression of genes by altering DNA (epigenetics). It also influences the shape and function of sex hormones.

The Stanley Miller Experiment (1953)

This landmark experiment, conducted by Stanley Miller in 1953, addressed the question of whether organic molecules could form under conditions simulating those on early Earth. Miller designed a closed system to mimic these hypothesized conditions, including an ocean, atmosphere, and lightning.

Results:

The experiment successfully demonstrated the abiotic (non-biological) formation of various organic molecules.
These included formaldehyde, hydrogen cyanide, amino acids, and hydrocarbons.
This provided crucial evidence supporting the hypothesis that organic molecules, the building blocks of life, could synthesize spontaneously on the early Earth from inorganic precursors.