9.1.5 Reaction Types in Organic Chemistry

Overview of Reaction Types

In organic chemistry, a comparatively small number of basic reaction types appear again and again. Most named reactions and complex syntheses can be understood as combinations of these few fundamental patterns.

In this chapter, the emphasis is on recognizing what kind of transformation occurs at the level of bonds and electrons, not on detailed mechanisms (which are treated elsewhere). We look at:

• How bonds are broken and formed.
• How the carbon skeleton can change.
• How functional groups can appear, disappear, or trade places.

The most important reaction types in introductory organic chemistry are:

• Substitution
• Addition
• Elimination
• Rearrangement (isomerizations)
• Redox reactions in an organic context

You will often see them combined, e.g. “addition–elimination” sequences.

Throughout, consider “functional group in / functional group out” and “change in unsaturation” (single vs. multiple bonds) as key diagnostic questions.

Substitution Reactions

In a substitution reaction, one atom or group in a molecule is replaced by another, while the overall frame of the molecule (especially the carbon skeleton) usually remains the same.

Generic scheme:
$$
\text{R–X} + \text{Y}^- \rightarrow \text{R–Y} + \text{X}^-
$$

• R = an organic group (e.g. alkyl group).
• X = a leaving group (e.g. halide, tosylate).
• Y = a nucleophile or other incoming group.

Key features:

• The number of $\sigma$-bonds to the central atom (often carbon) stays the same.
• One bond is broken (to the leaving group), another is formed (to the incoming group).
• No net change in the degree of unsaturation (single/double/triple bonds) at that carbon.

You will meet several subtypes later (with details such as rate laws and mechanisms in the kinetics and organic mechanisms chapters):

• Nucleophilic substitutions at saturated carbon (e.g. haloalkanes).
• Electrophilic substitutions in aromatic systems (e.g. substituted benzenes).
• Acyl substitutions at carbonyl derivatives (e.g. in carboxylic acid derivatives).

From the “reaction type” point of view, these subtypes differ in who attacks whom (nucleophile vs electrophile, and which atom), but all share the pattern “group A leaves, group B takes its place.”

Nucleophilic Substitution at Saturated Carbon (Pattern)

Pattern at a carbon atom bearing a leaving group:
$$
\text{R–X} + \text{Nu}^- \rightarrow \text{R–Nu} + \text{X}^-
$$

• Nu^- is a nucleophile (electron-pair donor).
• The reaction center is usually an sp³-hybridized carbon.

Important for:

• Converting one functional group into another (e.g. haloalkane → alcohol, amine, etc.).
• Building carbon–heteroatom bonds.

Mechanistic subtypes (e.g. $S_N1$, $S_N2$) are classified by how the substitution happens in time, but they are the same basic reaction type.

Electrophilic Aromatic Substitution (Pattern)

For aromatic compounds, especially benzene derivatives, a frequent pattern is:
$$
\text{Ar–H} + \text{E}^+ \rightarrow \text{Ar–E} + \text{H}^+
$$

• Ar is an aromatic ring.
• E^+ is an electrophile.

The key feature: the aromatic $\pi$-system is retained (unsaturation pattern is preserved), and a ring hydrogen is replaced by another group.

Addition Reactions

In an addition reaction, two (or more) molecules combine to form a single product, and new bonds are formed without losing atoms as a stable, separate small molecule (like HCl, $H_2O$, etc.).

In organic chemistry, the classic pattern is addition to multiple bonds:

• Addition to C=C (alkenes).
• Addition to C≡C (alkynes).
• Addition to C=O (carbonyl compounds).
• Addition to C≡N (nitriles).

Generic examples:

• Electrophilic addition:
  $$
  \text{C=C} + \text{H–X} \rightarrow \text{H–C–C–X}
  $$
• Nucleophilic addition:
  $$
  \text{C=O} + \text{Nu}^- + \text{H}^+ \rightarrow \text{C(OH)–Nu}
  $$

Key features:

• The degree of unsaturation decreases:
• Double bond $\rightarrow$ two new single bonds.
• Triple bond $\rightarrow$ double bond, or two singles.
• The number of atoms in the main molecule increases.
• Unlike substitution, no group needs to “leave” as a separate molecule.

Electrophilic Addition to C=C (Pattern)

Typical for alkenes:

$$
\text{R}_2\text{C=CR}_2 + \text{H–X} \rightarrow \text{R}_2\text{CH–C(R)}_2\text{–X}
$$

• The $\pi$-bond acts as a nucleophile.
• An electrophile (e.g. $H^+$) attacks first; a nucleophilic anion (e.g. $X^-$) adds second.

Recognize this as an addition because:

• One starting molecule (alkene) plus one molecule (HX) give one product.
• The C=C bond is converted to C–C single.

Nucleophilic Addition to C=O (Pattern)

Frequent in carbonyl chemistry (aldehydes, ketones):

$$
\text{R}_2\text{C=O} + \text{Nu}^- + \text{H}^+ \rightarrow \text{R}_2\text{C(OH)–Nu}
$$

• The carbonyl C is electrophilic.
• The nucleophile adds to this carbon.
• The C=O double bond is formally converted into a single bond C–O in the product (often as an alcohol).

Again, no small molecule departs; the nucleophile is simply added.

Elimination Reactions

In an elimination reaction, a single molecule loses atoms or groups, often forming a multiple bond and a smaller molecule (like $H_2O$, HX, or $H_2$).

Generic example:
$$
\text{R–CH}_2\text{–CH}_2\text{–X} \rightarrow \text{R–CH=CH}_2 + \text{HX}
$$

Key features:

• Degree of unsaturation increases:
• Single bond $\rightarrow$ double or triple bond.
• Two groups (often on adjacent atoms) are removed from the molecule.
• The groups that leave commonly form a neutral small molecule: e.g. $H_2O$, HX.

Elimination and addition are typical reverse processes:

• Addition: multiple bond $\rightarrow$ saturated; two new substituents attached.
• Elimination: saturated centers $\rightarrow$ multiple bond; substituents lost as a small molecule.

Typical Elimination Pattern (β-Elimination)

Common for haloalkanes or alcohol derivatives:

$$
\text{–CH}_2\text{–CH}_2\text{–X} \;\;\xrightarrow{\text{base or heat}}\;\; \text{–CH=CH}_2 + \text{HX}
$$

• One hydrogen and one leaving group (X) are lost from neighboring carbons.
• Called β-elimination (loss from the β-carbon relative to the functional group).

Mechanistic subtypes (e.g. $E1$, $E2$) describe the timing, but the reaction type is the same: elimination leading to a new multiple bond.

Relationship Between Addition and Elimination

Addition and elimination are often mutually reversible under suitable conditions:

• Addition of HX to an alkene (electrophilic addition) can be reversed by elimination of HX (e.g. strong base, heat).
• Dehydration of an alcohol (elimination) produces an alkene; hydration of the alkene (addition) regenerates the alcohol.

When analyzing a reaction:

• Ask whether the number of $\pi$-bonds goes up (elimination) or down (addition).
• Ask whether the molecule gains or loses substituents overall.

Rearrangement Reactions and Isomerizations

In a rearrangement reaction, the overall molecular formula stays the same, but the connectivity of atoms changes. The molecule becomes a structural isomer of itself.

Generic schematic:
$$
\text{R–CH}_2\text{–CH}_2\text{–X} \rightarrow \text{R–CH(CH}_3\text{)–X}
$$

Both sides have the same atoms, but bonded differently.

Key features:

• No net gain or loss of atoms (in pure rearrangements).
• Bonds break and reform within the same molecule.
• Often proceed via intermediates where a group (or hydride) migrates from one atom to another.

You will later see many examples:

• Carbocation rearrangements (hydride shift, alkyl shift).
• Rearrangements of carbon skeletons (ring expansion, ring contraction).
• Tautomerizations (a special case that also involves proton and double-bond shifts).

From a reaction-type viewpoint, rearrangements are distinct because:

• The main change is not simply “group out, group in” or “double bond to single bond,” but repositioning:
• A substituent moves to a different carbon.
• A double bond moves to a different location (isomerization).

Isomerization vs. Rearrangement

• Rearrangement: emphasizes the mechanism and moving parts within a single molecule.
• Isomerization: emphasizes the relationship between initial and final structures (they are isomers).

In practice, many rearrangements can be described as isomerizations, and vice versa.

Redox Reactions in Organic Chemistry (As a Reaction Type)

Redox processes in organic chemistry are characterized by formal changes in the oxidation states of carbon atoms and often by:

• Gain or loss of bonds to more electronegative atoms (O, N, halogens).
• Gain or loss of bonds to less electronegative atoms (H, metals).

You will meet the general theory of oxidation and reduction (including oxidation numbers and electrochemical potentials) in other chapters. Here, focus on how organic transformations are classified as oxidation or reduction.

Oxidation in an Organic Context

Typical features:

• Increase in the number of C–O (or C–N, C–X) bonds.
• Decrease in the number of C–H bonds.
• The carbon center becomes more “electron-poor.”

Examples of patterns:

• Alcohol $\rightarrow$ carbonyl compound (aldehyde/ketone).
• Aldehyde $\rightarrow$ carboxylic acid.

Overall, the carbon atom’s oxidation level increases.

Reduction in an Organic Context

Typical features:

• Increase in C–H bonds.
• Decrease in C–O (or C–N, C–X) bonds.
• The carbon center becomes more “electron-rich.”

Examples of patterns:

• Carbonyl compound $\rightarrow$ alcohol.
• Alkene or alkyne $\rightarrow$ alkane (hydrogenation).

Even if the mechanism is complex, you classify the reaction as a reduction if the organic molecule formally gains electrons (often via gaining H or losing O).

Combined and Multistep Reaction Patterns

Many real reactions combine basic types in a sequence. Being able to decompose them into elementary patterns helps in understanding and predicting outcomes.

Addition–Elimination Sequences

A very common pattern in carbonyl and aromatic chemistry:

1. Addition of a nucleophile or electrophile to a $\pi$-bond system.
2. Elimination of another group to restore a multiple bond or aromaticity.

Overall, the pattern may look like substitution, but mechanistically it passes through an addition intermediate.

Examples of “addition–elimination” as a pattern include:

• Nucleophilic acyl substitution at carboxylic acid derivatives (addition of Nu, elimination of leaving group).
• Certain aromatic substitutions where nucleophiles temporarily add to the ring and another group is expelled.

From the reaction-type viewpoint:

• The net result is substitution.
• The microscopic steps are addition followed by elimination.

Cyclization and Ring-Opening

These can be viewed as special cases of the basic types:

• Cyclization: intramolecular addition (a group in the same molecule adds across a multiple bond, forming a ring).
• Ring-opening: often an elimination-like or addition-like process that converts a cyclic structure into an open-chain structure.

Whether we call them “addition” or not depends on how we follow the electrons, but as a functional type in synthesis, “cyclization” and “ring-opening” are frequently referenced patterns.

How to Recognize Reaction Types in Practice

When you see a transformation:

1. Compare formulas and structures:
• Same formula, different connectivity: rearrangement/isomerization.
• Different formula: consider whether material was added or removed.
2. Check degree of unsaturation:
• Fewer multiple bonds: likely addition (or reduction).
• More multiple bonds: likely elimination (or oxidation).
3. Look for leaving groups and small molecules:
• New small molecule e.g. $H_2O$, HX: elimination, condensation, or substitution.
• Leaving group replaced by new group: substitution.
4. Consider oxidation state changes at carbon:
• More C–O / fewer C–H: oxidation.
• More C–H / fewer C–O: reduction.
5. Ask whether the reaction is intramolecular or intermolecular:
• Intramolecular bond reorganization without formula change: rearrangement.
• New bond between two molecules: addition or substitution.

Developing the habit of classifying reactions by these few types will make it easier to understand unfamiliar transformations, relate them to mechanisms, and plan synthetic routes.