6.1 Kinds of Organic Reactions

Types of Organic Reactions

Organic reactions fall into four main categories: addition, elimination, substitution, and rearrangement. Learning to recognize these reaction types is the first step toward predicting products and understanding how complex molecules are built from simpler ones.

Differentiating the Four Reaction Types

Addition reactions involve atoms or groups being added to a molecule, typically across a double or triple bond. The pi bond breaks, and two new sigma bonds form in its place. No atoms leave the molecule, so the product contains everything the reactants started with.

• Hydrogenation adds $H_2$ across a double bond to form an alkane
• Hydration adds $H_2O$ across a double bond to form an alcohol
• Hydrohalogenation adds $HX$ (where X is a halogen) across a double bond to form an alkyl halide

Elimination reactions are the reverse of addition. A molecule loses atoms or small molecules (like water or a hydrogen halide) to form a new double or triple bond. Think of it as addition running backward.

• Dehydration removes $H_2O$ from an alcohol to form an alkene
• Dehydrohalogenation removes $HX$ from an alkyl halide to form an alkene
• Decarboxylation removes $CO_2$ from a β-keto acid to form a ketone

Substitution reactions swap one atom or group for another. A nucleophile (electron-rich species) replaces a leaving group on the molecule. The carbon skeleton stays intact, but the functional group changes.

• Nucleophilic substitution ($S_N1$, $S_N2$): a nucleophile like $OH^-$ or $CN^-$ replaces a halide leaving group
• Electrophilic aromatic substitution: an electrophile like $NO_2^+$ replaces a hydrogen on a benzene ring

Rearrangement reactions redistribute atoms within the same molecule. No atoms are gained or lost; the connectivity just changes. The molecular formula of the product is identical to the starting material.

• Hydride shifts and alkyl shifts move a hydrogen or alkyl group to an adjacent carbon
• Ring expansions and contractions change the size of a cyclic structure
• Named examples include the Claisen, Beckmann, and pinacol rearrangements

Examples in Processes and Pathways

These four reaction types show up constantly in both lab synthesis and biological chemistry.

• Addition: Hydrogenation of unsaturated fats produces saturated fats (this is how margarine is made). Hydration of ethylene forms ethanol. Addition of $HCN$ to aldehydes or ketones forms cyanohydrins.
• Elimination: Dehydration of ethanol produces ethylene, an important industrial feedstock. Dehydrohalogenation of 2-bromobutane gives 2-butene. Decarboxylation of acetoacetic acid yields acetone and $CO_2$.
• Substitution: Williamson ether synthesis uses nucleophilic substitution to make ethers. Nitration, sulfonation, and halogenation of benzene are all electrophilic aromatic substitutions. Transamination reactions in your body biosynthesize amino acids.
• Rearrangement: The Claisen rearrangement converts allyl vinyl ethers to γ,δ-unsaturated carbonyl compounds. The Beckmann rearrangement converts oximes to amides. The pinacol rearrangement converts 1,2-diols to aldehydes or ketones.

Predicting Organic Reaction Products

For each reaction type, you can predict the product by identifying what's added, removed, swapped, or rearranged.

Addition reactions:

1. Alkene + $H_2$ (with catalyst) → Alkane
2. Alkene + $H_2O$ (with acid catalyst) → Alcohol
3. Alkene + $HX$ (X = halogen) → Alkyl halide

Elimination reactions:

1. Alcohol + heat (with acid catalyst) → Alkene + $H_2O$
2. Alkyl halide + strong base → Alkene + $HX$
3. β-Keto acid + heat → Ketone + $CO_2$

Substitution reactions:

• Alkyl halide + nucleophile ($OH^-$, $CN^-$, $NH_3$) → Substituted product + leaving group
• Benzene + electrophile ($NO_2^+$, $SO_3$, $Br_2$/catalyst) → Substituted benzene + $H^+$
• Amino acid + α-keto acid → New amino acid + new α-keto acid

Rearrangement reactions:

• Allyl phenyl ether + heat → o-Allylphenol
• Oxime + acid catalyst → Amide
• 1,2-Diol + acid catalyst → Aldehyde or ketone

Notice how addition and elimination are essentially mirror images of each other. The same is true of their products: an alkene plus water gives an alcohol (addition), and an alcohol gives an alkene plus water (elimination). Recognizing these pairings makes predicting products much easier.

Understanding Reaction Mechanisms and Kinetics

Reaction mechanisms describe the step-by-step bond-breaking and bond-forming events that turn reactants into products. Each step may involve reactive intermediates like carbocations, carbanions, or radicals that exist only briefly before reacting further. Drawing out the mechanism with curved arrows shows exactly where electrons move at each stage.

Reaction kinetics deals with how fast a reaction proceeds. The rate depends on factors like concentration, temperature, solvent, and the height of the energy barrier (activation energy) the reactants must overcome. For example, $S_N2$ reactions are second-order (rate depends on both the nucleophile and substrate concentrations), while $S_N1$ reactions are first-order (rate depends only on the substrate).

Stereochemistry matters because the three-dimensional arrangement of atoms can change during a reaction. An $S_N2$ reaction inverts the stereocenter (like an umbrella flipping in the wind), while an $S_N1$ reaction typically gives a mixture of both configurations because the planar carbocation intermediate can be attacked from either side.