Practice questions Chapter 4

Functional Groups and the Molecular Diversity of Life

Introduction

This set of practice questions focuses on the identification and properties of functional groups in organic molecules, a key topic in understanding the molecular diversity of life. Mastery of these concepts is essential for interpreting the structure and function of biological macromolecules.

Identifying Functional Groups

Common Functional Groups in Biological Molecules

• Hydroxyl group (-OH): Found in alcohols; makes molecules polar and can form hydrogen bonds with water.

• Carbonyl group (C=O): Found in aldehydes (at the end of a carbon skeleton) and ketones (within the skeleton).

• Carboxyl group (-COOH): Acts as an acid; can donate H+ because the covalent bond between oxygen and hydrogen is so polar.

• Amino group (-NH2): Acts as a base; can pick up H+ from the surrounding solution.

• Sulfhydryl group (-SH): Two sulfhydryl groups can form a disulfide bond, stabilizing protein structure.

• Phosphate group (-OPO32-): Contributes negative charge; found in ATP and nucleic acids.

• Methyl group (-CH3): Affects gene expression when attached to DNA or proteins.

Isomerism in Organic Molecules

Types of Isomers

• Structural isomers: Differ in the covalent arrangements of their atoms.

• Cis-trans isomers (geometric isomers): Differ in spatial arrangement due to inflexible double bonds.

• Enantiomers: Mirror images of each other; important in pharmaceuticals due to different biological activities.

Example Table: Types of Isomers

Isomer Type	Description	Example
Structural	Different covalent arrangements	Butane vs. isobutane
Cis-trans	Different spatial arrangement around double bond	Cis-2-butene vs. trans-2-butene
Enantiomers	Mirror images, non-superimposable	L- and D-glucose

Functional Groups: Properties and Biological Importance

Key Properties

• Acidity: Carboxyl groups can donate protons, making molecules acidic.

• Basicity: Amino groups can accept protons, making molecules basic.

• Polarity: Hydroxyl, carbonyl, and phosphate groups increase polarity and solubility in water.

• Reactivity: Sulfhydryl groups can form covalent bonds (disulfide bridges) important in protein structure.

Examples and Applications

• ATP (adenosine triphosphate): Contains phosphate groups; hydrolysis releases energy for cellular work.

• Proteins: Disulfide bridges (from sulfhydryl groups) stabilize tertiary structure.

• Carbohydrates: Aldehyde and ketone groups distinguish between glucose (an aldose) and fructose (a ketose).

Practice Questions Overview

Sample Question Types

• Identify and label functional groups in given molecular structures.

• Classify pairs of molecules as structural isomers, cis-trans isomers, or enantiomers.

• Determine which functional group is responsible for specific chemical properties (e.g., acidity, basicity, ability to form hydrogen bonds).

• Recognize the presence of functional groups in biologically important molecules (e.g., ATP, amino acids).

Example Question

Which functional group can act as an acid and donate a proton (H+)?

• Answer: Carboxyl group (-COOH)

Example Question

Which type of isomerism is shown by two molecules that are mirror images of each other?

• Answer: Enantiomers

Summary Table: Functional Groups and Their Properties

Functional Group	Structure	Properties	Example
Hydroxyl	-OH	Polar, forms hydrogen bonds	Ethanol
Carbonyl	C=O	Polar, found in sugars	Acetone, formaldehyde
Carboxyl	-COOH	Acidic, donates H+	Acetic acid
Amino	-NH2	Basic, accepts H+	Glycine
Sulfhydryl	-SH	Forms disulfide bonds	Cysteine
Phosphate	-OPO32-	Negative charge, energy transfer	ATP
Methyl	-CH3	Nonpolar, gene expression	5-methyl cytosine

Key Equations and Concepts

• Acid dissociation (carboxyl group):

$\ce{R-COOH <=> R-COO^- + H^+}$

• Base association (amino group):

$\ce{R-NH2 + H^+ <=> R-NH3^+}$

Conclusion

Understanding functional groups and isomerism is fundamental to predicting the behavior of biological molecules. These concepts are foundational for further study in biochemistry, molecular biology, and related fields.