Chapter 4 Part B (Key Chemical Groups and Functional Groups in Organic Molecules)

Key Chemical Groups and Their Role in Molecular Function

Introduction to Chemical Groups

Chemical groups, also known as functional groups, are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The unique properties of organic molecules depend on both the carbon skeleton and the chemical groups attached to it. These groups can replace hydrogens on the carbon skeleton and may participate directly in chemical reactions or influence molecular shape and function indirectly, such as in the case of sex hormones like estradiol and testosterone.

• Distinctive properties of organic molecules arise from the arrangement and type of chemical groups attached.

• Functional groups are the most commonly involved in chemical reactions.

• Some groups, like methyl, are not chemically reactive but still affect molecular function.

• Example: The difference between estradiol and testosterone is due to the presence of different functional groups, which leads to differences in biological activity.

Functional Groups Important in the Chemistry of Life

Overview of Functional Groups

There are seven chemical groups that are most important in the chemistry of life. Six of these are considered functional groups because they are chemically reactive. The methyl group is not reactive but still influences molecular function.

• Chemically reactive (functional groups):

  • Hydroxyl (–OH)

  • Carbonyl (>C=O)

  • Carboxyl (–COOH)

  • Amino (–NH2)

  • Sulfhydryl (–SH)

  • Phosphate (–OPO32–)

• Not chemically reactive: Methyl (–CH3)

Descriptions and Properties of Major Functional Groups

Hydroxyl Group (–OH)

The hydroxyl group consists of an oxygen atom bonded to a hydrogen atom. It is polar due to the electronegativity of oxygen and can form hydrogen bonds with water, increasing the solubility of organic molecules.

• Compound name: Alcohol

• Example: Ethanol (C2H5OH), the alcohol present in alcoholic beverages

• Properties: Polar group, forms hydrogen bonds

Carbonyl Group (>C=O)

The carbonyl group consists of a carbon atom double-bonded to an oxygen atom. It is found in two major types of compounds: ketones and aldehydes.

• Compound names: Ketone (if the carbonyl is within the carbon skeleton), Aldehyde (if the carbonyl is at the end of the skeleton)

• Examples: Acetone (a ketone), Propanal (an aldehyde)

• Properties: Polar group, increases reactivity

Carboxyl Group (–COOH)

The carboxyl group consists of a carbon atom double-bonded to an oxygen atom and also bonded to a hydroxyl group. It acts as an acid, donating a proton (H+) to solution, and is found in organic acids.

• Compound name: Carboxylic acid, or organic acid

• Example: Acetic acid (gives vinegar its sour taste)

• Properties: Charged group (negatively charged when ionized), acidic

• Note: A carboxyl group is not simply a combination of a carbonyl and a hydroxyl group; it is a distinct functional group.

Amino Group (–NH2)

The amino group consists of a nitrogen atom bonded to two hydrogen atoms. It acts as a base, picking up a proton from the surrounding solution.

• Compound name: Amine

• Example: Glycine (an amino acid)

• Properties: Charged group (positively charged when ionized), basic

Sulfhydryl Group (–SH)

The sulfhydryl group consists of a sulfur atom bonded to a hydrogen atom. It is important in stabilizing protein structure by forming disulfide bonds (cross-links) between cysteine residues.

• Compound name: Thiol

• Example: Cysteine (an amino acid containing a sulfhydryl group)

• Properties: Polar group, can form covalent bonds (disulfide bridges)

Phosphate Group (–OPO32–)

The phosphate group consists of a phosphorus atom bonded to four oxygen atoms, with two carrying negative charges. It is involved in energy transfer and contributes negative charge to molecules.

• Compound name: Organic phosphate

• Example: Glycerol phosphate (important in cellular reactions), ATP (adenosine triphosphate)

• Properties: Charged group (negatively charged), acidic, can react with water to release energy

Methyl Group (–CH3)

The methyl group consists of a carbon atom bonded to three hydrogen atoms. It is not chemically reactive but affects the expression of genes and the shape and function of molecules such as hormones.

• Compound name: Methylated compound

• Example: 5-Methyl cytosine (a component of DNA)

• Properties: Non-polar, hydrophobic, not chemically reactive

Summary Table: Properties of Major Chemical Groups

Group	Structure	Compound Name	Properties	Example
Hydroxyl	–OH	Alcohol	Polar, forms hydrogen bonds	Ethanol
Carbonyl	>C=O	Ketone, Aldehyde	Polar, increases reactivity	Acetone, Propanal
Carboxyl	–COOH	Carboxylic acid	Acidic, negatively charged when ionized	Acetic acid
Amino	–NH2	Amine	Basic, positively charged when ionized	Glycine
Sulfhydryl	–SH	Thiol	Polar, forms disulfide bonds	Cysteine
Phosphate	–OPO32–	Organic phosphate	Acidic, negatively charged, energy transfer	ATP, Glycerol phosphate
Methyl	–CH3	Methylated compound	Non-polar, affects gene expression	5-Methyl cytosine

Additional Academic Context

• Isomers: Organic molecules can exist as isomers, which are compounds with the same molecular formula but different structures and properties. Types include structural isomers, cis-trans isomers, and enantiomers.

• ATP (Adenosine Triphosphate): The primary energy currency of the cell. Hydrolysis of ATP releases energy used by cells to perform work: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Role of Carbon: The versatility of carbon allows for the diversity of organic molecules, which is the foundation of biological diversity.