Organic Chemistry Functional Groups

Why This Matters

Functional groups are the heart of organic chemistry—they're the reactive sites that determine how molecules behave, what reactions they undergo, and what properties they exhibit. When you're being tested on organic chemistry, you're rarely asked to simply identify a functional group in isolation. Instead, you're expected to predict reactivity, explain solubility patterns, compare acidity or basicity, and recognize how polarity, hydrogen bonding, and electron distribution shape molecular behavior.

Think of functional groups as the "personality" of organic molecules. The carbon backbone is just the skeleton, but functional groups dictate whether a compound dissolves in water, reacts with acids or bases, or participates in addition versus substitution reactions. Don't just memorize structures—know why alcohols are more water-soluble than ethers, why carboxylic acids are acidic while amines are basic, and how carbonyl chemistry connects aldehydes, ketones, esters, and amides. That conceptual understanding is what earns you points on FRQs.

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Hydrocarbon Frameworks: The Foundation

These groups form the carbon backbone of organic molecules. While they're generally unreactive compared to other functional groups, their degree of saturation determines reactivity and geometry.

Alkyl (R-)

• Saturated hydrocarbon substituent—contains only single bonds between carbons, making it relatively unreactive
• General formula: $C_nH_{2n+1}$—the "$2n+1$" reflects maximum hydrogen saturation
• Electron-donating group—stabilizes carbocations and influences reactivity when attached to functional groups

Alkene ($R-CH=CH-R$)

• Contains at least one carbon-carbon double bond—the pi bond creates a region of high electron density
• General formula: $C_nH_{2n}$—fewer hydrogens than alkanes due to the double bond
• Exhibits geometric isomerism—restricted rotation around the double bond creates cis/trans (or E/Z) isomers

Alkyne ($R-C \equiv C-R$)

• Contains at least one carbon-carbon triple bond—two pi bonds make this highly electron-rich
• General formula: $C_nH_{2n-2}$—even fewer hydrogens reflect greater unsaturation
• More reactive than alkenes—the concentrated electron density attracts electrophiles readily

Phenyl ($C_6H_5-$)

• Aromatic ring derived from benzene—one hydrogen removed to attach as a substituent
• Exhibits resonance stabilization—delocalized pi electrons create exceptional stability
• Resists addition reactions—prefers substitution to preserve aromaticity

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Oxygen-Containing Groups: Polarity and Hydrogen Bonding

Oxygen's electronegativity creates polar bonds and enables hydrogen bonding. The arrangement of oxygen determines whether a group can donate H-bonds, accept them, or both.

Alcohol ($-OH$)

• Hydroxyl group attached to carbon—the $O-H$ bond is highly polar and can both donate and accept hydrogen bonds
• Amphiprotic behavior—can act as a weak acid (donating $H^+$) or weak base (accepting $H^+$)
• High water solubility for small alcohols—hydrogen bonding with water decreases as carbon chain lengthens

Ether ($R-O-R$)

• Oxygen bonded to two carbon groups—no $O-H$ bond means it can only accept hydrogen bonds
• Lower boiling point than comparable alcohols—cannot donate H-bonds, so weaker intermolecular forces
• Excellent solvent properties—polar enough to dissolve many compounds but relatively unreactive

Aldehyde ($-CHO$)

• Carbonyl group at the terminal carbon—the $C=O$ is polar and electrophilic at the carbon
• General structure: $R-CHO$—the hydrogen on the carbonyl carbon makes it easily oxidized
• Undergoes nucleophilic addition—the electrophilic carbonyl carbon attracts nucleophiles

Ketone ($R-CO-R$)

• Carbonyl group within the carbon chain—flanked by two carbon groups, not hydrogen
• More stable than aldehydes—electron donation from alkyl groups reduces carbonyl reactivity
• Cannot be easily oxidized—no hydrogen on carbonyl carbon to remove (unlike aldehydes)

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Carbonyl Derivatives: Acids, Esters, and Amides

These groups all contain the carbonyl ($C=O$) but differ in what's attached to it. The atom bonded to the carbonyl carbon determines acidity, reactivity, and biological function.

Carboxylic Acid ($-COOH$)

• Carboxyl group combines carbonyl and hydroxyl—resonance stabilization of the conjugate base makes it acidic
• Releases $H^+$ ions in solution—the conjugate base (carboxylate) is stabilized by delocalization across both oxygens
• General formula: $R-COOH$—found in fatty acids, amino acids, and many metabolic intermediates

Ester ($R-COO-R$)

• Formed from carboxylic acid + alcohol—a condensation reaction releases water
• Carbonyl adjacent to ether-like oxygen—no acidic hydrogen, so neutral compounds
• Responsible for fragrances and flavors—also forms the backbone of fats and oils (triglycerides)

Amide ($-CONH_2$)

• Carbonyl bonded directly to nitrogen—resonance delocalizes the nitrogen lone pair into the carbonyl
• Very stable bond—the resonance makes amides resistant to hydrolysis under mild conditions
• Peptide bonds are amides—this linkage connects amino acids in proteins

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Nitrogen-Containing Groups: Basicity and Biological Relevance

Nitrogen's lone pair makes these groups basic and capable of forming hydrogen bonds. The availability of that lone pair determines base strength.

Amine ($-NH_2$)

• Nitrogen bonded to carbon and hydrogen(s)—the lone pair on nitrogen is available to accept protons
• Acts as a base—primary ($RNH_2$), secondary ($R_2NH$), and tertiary ($R_3N$) amines vary in basicity
• Forms hydrogen bonds—contributes to water solubility and higher boiling points than comparable hydrocarbons

Amide ($-CONH_2$)

• Nitrogen's lone pair is delocalized—resonance with the carbonyl makes amides much weaker bases than amines
• Critical in biochemistry—peptide bonds linking amino acids are amide linkages
• Planar geometry around nitrogen—resonance creates partial double-bond character

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Sulfur and Halogen Groups: Reactivity and Synthesis

These groups introduce heteroatoms that dramatically alter reactivity. Halogens are good leaving groups; sulfur mimics oxygen but with key differences.

Halide ($-X$, where X = F, Cl, Br, or I)

• Halogen bonded to carbon—the $C-X$ bond is polar, making the carbon electrophilic
• Excellent leaving groups—weak $C-X$ bonds (especially $C-I$) break easily in substitution reactions
• Key substrates for $S_N1$, $S_N2$, $E1$, and $E2$ reactions—the type of halide influences mechanism

Thiol ($-SH$)

• Sulfur analog of alcohol—the $S-H$ bond is weaker and more acidic than $O-H$
• Forms disulfide bonds—two thiols can oxidize to form $R-S-S-R$, critical for protein structure
• Distinctive strong odors—responsible for the smell of garlic, onions, and skunk spray

Nitro ($-NO_2$)

• Nitrogen bonded to two oxygens—a strongly electron-withdrawing group
• Deactivates aromatic rings—pulls electron density away, making electrophilic aromatic substitution slower
• Found in explosives and pharmaceuticals—the high oxygen content contributes to energetic decomposition

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Quick Reference Table

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Self-Check Questions

1. Which two functional groups both contain a carbonyl but differ in their acidity? Explain why one is acidic and the other is not.

2. Arrange the following in order of increasing boiling point: an ether, an alcohol, and an alkane of similar molecular weight. What intermolecular force explains your ranking?

3. Why are amides much weaker bases than amines, even though both contain nitrogen? Use resonance in your explanation.

4. Compare and contrast aldehydes and ketones: What structural difference makes aldehydes easier to oxidize?

5. If an FRQ asks you to identify a compound that can form disulfide bonds in proteins, which functional group should you discuss, and what reaction occurs?