# Lecture 20: Introduction to Organic Functional Groups
## General Chemistry II
---
## Learning Objectives
By the end of this lecture, students will be able to:
1. Explain why carbon is uniquely suited to form the backbone of organic molecules
2. Identify and name the major organic functional groups
3. Distinguish between alkanes, alkenes, alkynes, and aromatic hydrocarbons
4. Recognize oxygen-containing and nitrogen-containing functional groups
5. Predict basic physical properties based on functional group identity
6. Identify functional groups relevant to biochemistry (amino acids, carbohydrates, lipids, nucleotides)
---
## Lecture Content
### I. Why Carbon?
Carbon occupies a unique position in chemistry because of its ability to form the structural backbone of an enormous diversity of molecules. With four valence electrons, carbon forms four covalent bonds, allowing it to link to other carbon atoms in chains, branched structures, and rings of virtually unlimited size. Carbon forms strong single bonds (C-C), double bonds (C=C), and triple bonds, as well as strong bonds with hydrogen, oxygen, nitrogen, sulfur, and the halogens. This unparalleled bonding versatility gives rise to millions of known organic compounds. Organic chemistry is formally defined as the chemistry of carbon-containing compounds, excluding simple inorganic species such as CO2, CO, carbonates, and cyanides.
### II. Hydrocarbons
Hydrocarbons are the simplest organic compounds, containing only carbon and hydrogen. They are classified by the types of carbon-carbon bonds they contain.
Alkanes are saturated hydrocarbons in which all bonds are single bonds (C-C and C-H). Their general formula for acyclic (non-ring) compounds is C_nH_(2n+2). Each carbon is sp^3 hybridized with tetrahedral geometry and 109.5-degree bond angles. The naming sequence begins with methane, ethane, propane, butane, pentane, hexane, and so on. Alkanes are relatively unreactive, undergoing primarily combustion and halogenation reactions.
Alkenes contain at least one carbon-carbon double bond. For acyclic alkenes with one double bond, the general formula is C_nH_(2n). The carbons of the double bond are sp^2 hybridized with trigonal planar geometry and 120-degree bond angles. The restricted rotation around the double bond gives rise to geometric (cis/trans) isomerism. Alkenes are more reactive than alkanes and characteristically undergo addition reactions.
Alkynes contain at least one carbon-carbon triple bond. For one triple bond in an acyclic molecule, the general formula is C_nH_(2n-2). The carbons of the triple bond are sp hybridized with linear geometry and 180-degree bond angles.
Aromatic hydrocarbons contain a benzene ring (C6H6) or a related ring system with delocalized pi electrons that confer exceptional stability, a property known as aromaticity. Unlike alkenes, aromatic compounds preferentially undergo substitution reactions rather than addition reactions.
A set of four molecular structures representing the four hydrocarbon classes. Panel A: Ethane (C2H6), showing the C-C single bond with all sp3 carbons and a tetrahedral ball-and-stick model. Labeled "Alkane -- saturated, single bonds only." Panel B: Ethene (C2H4), showing the C=C double bond with planar geometry. Labeled "Alkene -- one or more double bonds." Panel C: Ethyne (C2H2), showing the C-C triple bond with linear geometry. Labeled "Alkyne -- one or more triple bonds." Panel D: Benzene (C6H6), showing the hexagonal ring with alternating double bonds (or a circle inside the ring to represent delocalization). Labeled "Aromatic -- delocalized pi system." Each structure includes bond angles and hybridization labels.
### III. Oxygen-Containing Functional Groups
Alcohols (R-OH) contain a hydroxyl group bonded to a saturated carbon and are classified as primary, secondary, or tertiary depending on the number of carbon groups attached to the carbon bearing the OH. The ability of the hydroxyl group to participate in hydrogen bonding gives alcohols higher boiling points than comparable alkanes. Methanol (CH3OH) and ethanol (C2H5OH) are common examples.
Ethers (R-O-R') contain an oxygen atom bonded to two carbon groups. Because they lack an O-H bond for hydrogen bonding, ethers have lower boiling points than alcohols of similar molecular weight. Diethyl ether (C2H5OC2H5) is a representative example.
Aldehydes (R-CHO) contain a carbonyl group (C=O) at the end of a carbon chain and are named with the "-al" suffix. Formaldehyde (HCHO) and acetaldehyde (CH3CHO) are familiar examples. Ketones (R-CO-R') contain a carbonyl group bonded to two carbon groups and are named with the "-one" suffix, as in acetone (CH3COCH3).
Carboxylic acids (R-COOH) contain the carboxyl group, which combines a carbonyl and a hydroxyl on the same carbon. These compounds are weak acids (Ka approximately 10^-5) that can donate the proton from the O-H bond. They are named with the "-oic acid" suffix. Acetic acid (CH3COOH) and citric acid are common examples.
Esters (R-COO-R') are formed by the condensation reaction of a carboxylic acid with an alcohol. Named with the "-oate" suffix, esters often have pleasant, fruity odors. Ethyl acetate (CH3COOC2H5) is a widely used example.
### IV. Nitrogen-Containing Functional Groups
Amines (R-NH2, R2NH, R3N) are organic derivatives of ammonia. The lone pair on nitrogen makes amines basic: they readily accept protons. Amines are classified as primary, secondary, or tertiary depending on the number of carbon groups attached to nitrogen. Primary and secondary amines can form hydrogen bonds. Examples include methylamine (CH3NH2) and trimethylamine ((CH3)3N).
Amides (R-CO-NH2) contain a carbonyl group bonded to a nitrogen atom. The peptide bond that links amino acids in proteins is an amide bond, making this functional group centrally important in biochemistry. Amides are much less basic than amines because the nitrogen lone pair is delocalized into the carbonyl group. Acetamide (CH3CONH2) is a simple example.
### V. Other Important Functional Groups
Thiols (R-SH) are the sulfur analogs of alcohols. They often have strong, unpleasant odors. In biochemistry, the thiol group of the amino acid cysteine can form disulfide bonds (R-S-S-R), which are critical for stabilizing protein structure.
Halides (R-X, where X = F, Cl, Br, or I) contain a carbon-halogen bond. The polarity of this bond makes alkyl halides reactive toward nucleophilic substitution and elimination reactions.
Phosphate esters (R-O-PO3^2-) are of enormous biochemical importance. ATP, DNA, and phospholipids all contain phosphate ester linkages. The so-called high-energy phosphoanhydride bonds in ATP store the chemical energy that drives cellular processes.
A reference chart of major organic functional groups organized in a grid. Each cell contains: the functional group name, general structure, an example molecule with its name, and a key property. Row 1: Alcohol (R-OH, ethanol, H-bonding), Ether (R-O-R, diethyl ether, relatively inert), Aldehyde (R-CHO, acetaldehyde, easily oxidized), Ketone (R-CO-R, acetone, polar solvent). Row 2: Carboxylic acid (R-COOH, acetic acid, weakly acidic), Ester (R-COO-R, ethyl acetate, fruity odor), Amine (R-NH2, methylamine, basic), Amide (R-CONH-R, peptide bond, stable linkage). Row 3: Thiol (R-SH, cysteine, disulfide bonds), Alkyl halide (R-X, chloromethane, polar C-X bond), Phosphate ester (R-OPO3, ATP, energy storage). Functional groups are color-coded by heteroatom: red for oxygen, blue for nitrogen, yellow for sulfur, green for halogens.
### VI. Isomerism in Organic Chemistry
Organic compounds exhibit rich isomerism. Constitutional (structural) isomers share the same molecular formula but differ in the connectivity of their atoms. For example, C4H10 can be butane (a straight chain) or 2-methylpropane (isobutane, a branched chain). Similarly, C2H6O can be either ethanol (CH3CH2OH) or dimethyl ether (CH3OCH3), two compounds with very different properties.
Stereoisomers have the same connectivity but differ in spatial arrangement. Geometric (cis-trans) isomers arise from restricted rotation around a double bond or within a ring, with groups on the same side (cis) or opposite sides (trans). Enantiomers are non-superimposable mirror images that require a chiral center, a carbon bonded to four different groups. Chirality is fundamentally important in biochemistry: all amino acids except glycine are chiral, and biological systems are highly stereospecific in their interactions with chiral molecules.
### VII. Functional Groups in Biochemistry
The functional groups introduced in this lecture are the building blocks of the four major classes of biomolecules. Amino acids contain both an amine (-NH2) and a carboxylic acid (-COOH) group. The 20 common amino acids are linked by peptide (amide) bonds to form proteins.
Carbohydrates are polyhydroxy aldehydes or ketones, containing multiple hydroxyl groups alongside an aldehyde or ketone functionality. Glucose is an aldohexose and fructose is a ketohexose.
Lipids include triglycerides, which are esters of fatty acids with glycerol, and phospholipids, which contain a phosphate ester group and serve as the main structural components of cell membranes.
Nucleotides consist of a nitrogen-containing base, a sugar (ribose or deoxyribose), and a phosphate group. They are the building blocks of DNA and RNA. ATP (adenosine triphosphate), a nucleotide with three phosphate groups, serves as the primary energy currency of cells.
A diagram showing the four major classes of biomolecules and their key functional groups. Panel A: An amino acid (generic structure with NH2, alpha-carbon with R group, and COOH). Two amino acids are shown joining via a peptide (amide) bond with loss of water. Panel B: Glucose in its open-chain form, highlighting the aldehyde group and multiple hydroxyl groups. Panel C: A triglyceride (fat) showing three fatty acid chains linked to glycerol via ester bonds. Panel D: A nucleotide (adenosine monophosphate) showing the adenine base, ribose sugar with hydroxyl groups, and phosphate ester linkage. Each functional group is circled and labeled.
---