Table of Contents
Roles in Organic Reactions: Reagents, Substrates, and Products
In organic chemistry we usually describe reactions not just with formulas, but in terms of the roles played by the reacting species. Three central terms are:
- Substrate – the organic molecule that is being transformed (the “main actor”).
- Reagent – the substance that causes the change in the substrate (often added from outside).
- Product(s) – the new substances formed from the substrate (and reagent) during the reaction.
These are roles in a reaction, not permanent properties of a molecule. The same compound can be substrate in one reaction and reagent in another.
Substrate: The Molecule Being Modified
The substrate is usually:
- the organic compound whose skeleton you are interested in changing,
- present in relatively larger amount (often the “starting material”),
- the species whose structure is usually drawn in full and whose transformation is followed step by step.
Example: Hydrochlorination of ethene:
$$
\ce{CH2=CH2 + HCl -> CH3-CH2Cl}
$$
- Substrate: $\ce{CH2=CH2}$ (ethene) – it undergoes a structural change.
- Product: $\ce{CH3-CH2Cl}$ (chloroethane).
Hydrogen chloride, HCl, here is the reagent that adds across the double bond.
In many organic mechanisms, the substrate is the molecule on which bonds are broken and formed at a specific site (the reaction center or functional group introduced in another chapter).
Reagents: Substances That Induce Change
A reagent is any substance used to bring about a specific chemical transformation of the substrate. Reagents can:
- donate or accept electrons,
- donate or accept protons,
- deliver atoms or groups to the substrate,
- remove atoms or groups from the substrate.
Types of Reagents by Their Role
Without going into detailed electronic effects (covered elsewhere), three frequently used classes of reagents in organic chemistry are:
- Electrophilic reagents: seek electron-rich sites.
- Nucleophilic reagents: seek electron-poor sites.
- Radical reagents: participate via unpaired electrons.
These categories describe how the reagent interacts with the substrate.
Electrophilic Reagents
- “Electron-loving”: attracted to regions of high electron density (e.g., $\pi$ bonds, lone pairs).
- Often positively charged or strongly polarized.
Typical examples (details of their electronic structure belong elsewhere):
- $\ce{HBr, HCl, H2SO4}$ in addition to double bonds,
- $\ce{Br2, Cl2}$ in electrophilic aromatic substitution (as activated reagents),
- $\ce{NO2^+}$ in nitration of benzene (formed from mixed acid).
Electrophilic reagents most often appear in:
- Electrophilic additions to C=C,
- Electrophilic substitutions on aromatic rings.
Nucleophilic Reagents
- “Nucleus-loving”: attracted to positive or electron-poor centers.
- Often negatively charged or neutral with lone pairs that can be donated.
Typical examples:
- $\ce{OH^-}$, $\ce{RO^-}$ (alkoxides),
- $\ce{CN^-}$,
- $\ce{NH3}$ and amines,
- water, alcohols (as weaker nucleophiles).
Nucleophilic reagents are central to:
- Nucleophilic substitution at saturated carbon (e.g. $\ce{S_N1}$, $\ce{S_N2}$),
- Nucleophilic addition to polar multiple bonds (e.g. $\ce{C=O}$).
Radical Reagents
- Species with one unpaired electron.
- Extremely reactive; they react with other radicals or with ordinary molecules to form new radicals.
Radical reagents are involved in:
- Radical substitutions (e.g. halogenation of alkanes),
- Radical additions (e.g. certain polymerizations).
Examples:
- Halogen atoms $\ce{Cl·}$, $\ce{Br·}$ generated from $\ce{Cl2}$, $\ce{Br2}$ by light,
- Organic radicals derived from peroxides.
Other Descriptive Terms for Reagents
Depending on the transformation they promote, reagents are also categorized as:
- Oxidizing reagents (oxidants): cause oxidation of the substrate.
- Example: $\ce{KMnO4}$, $\ce{CrO3}$ in oxidation of alcohols.
- Reducing reagents (reducing agents): cause reduction of the substrate.
- Example: $\ce{LiAlH4}$, $\ce{NaBH4}$ in reduction of carbonyl compounds.
- Acidic reagents: donate protons or create strongly acidic conditions.
- Example: $\ce{H2SO4}$ as catalyst in esterification.
- Basic reagents: accept protons or create basic conditions.
- Example: $\ce{NaOH}$ in elimination or substitution reactions.
- Organometallic reagents: contain metal–carbon bonds, highly reactive toward electrophilic or protic substrates.
- Example: Grignard reagents $\ce{RMgX}$.
Precise classification of these as acids/bases or redox reagents is covered in the respective chapters; here the focus is on their role in organic transformations.
Reaction Conditions vs. Reagents
Not every substance present in the reaction mixture is a reagent in the strict sense. You should distinguish:
- Reagents: chemically transform the substrate (consumed or changed).
- Solvents: provide a medium, may or may not participate.
- Catalysts: accelerate the reaction without being consumed overall.
- Additives (e.g. salts, drying agents): influence conditions (ionic strength, water removal) but are not always directly part of the main transformation.
Example: Nucleophilic substitution:
$$
\ce{CH3Br + OH^- -> CH3OH + Br^-}
$$
- Substrate: $\ce{CH3Br}$ (bromomethane).
- Reagent: $\ce{OH^-}$, often provided by $\ce{NaOH}$ or $\ce{KOH}$.
- Solvent: water or an alcohol.
- Product: $\ce{CH3OH}$ (methanol).
Here, $\ce{Na^+}$ is a spectator ion and not part of the actual bond-making/breaking at carbon.
Reaction Types from the Viewpoint of Substrate and Reagent
Detailed mechanisms are treated separately; here we only relate the roles of substrate and reagent to the name of the reaction type.
Substitution Reactions
In substitution reactions, one atom or group in the substrate is replaced by another.
General form:
$$
\ce{R–X + Y^- -> R–Y + X^-}
$$
- Substrate: $\ce{R–X}$.
- Reagent: nucleophile $\ce{Y^-}$.
- Leaving group: $\ce{X^-}$ (initially part of the substrate; it departs).
From the viewpoint of roles:
- Substrate provides the carbon framework.
- Reagent provides the incoming group.
Addition Reactions
In addition reactions, atoms or groups are added to a multiple bond (e.g. C=C or C≡C) in the substrate.
Example:
$$
\ce{CH2=CH2 + Br2 -> BrCH2-CH2Br}
$$
- Substrate: $\ce{CH2=CH2}$.
- Reagent: $\ce{Br2}$ (can act as an electrophile once polarized).
- Product: a dibromoalkane; the double bond is converted to a single bond.
Here, the organic skeleton of the substrate gains new atoms/groups supplied by the reagent.
Elimination Reactions
In elimination reactions, atoms or groups are removed from a substrate, often forming a multiple bond.
Example:
$$
\ce{CH3-CH2Br + OH^- -> CH2=CH2 + H2O + Br^-}
$$
- Substrate: $\ce{CH3-CH2Br}$.
- Reagent: base $\ce{OH^-}$, which abstracts a proton and assists leaving group departure.
- Product: $\ce{CH2=CH2}$ (ethene) plus by-products.
Base here is both reagent and (in some cases) also the solvent if a strong base is used in excess alcohol.
Rearrangement Reactions
In rearrangement reactions, the connectivity in the substrate changes without necessarily adding or removing external atoms.
Example (schematic):
$$
\ce{R-CH2-CH2^+ -> R-CH(CH3)^+}
$$
- Substrate: the original carbocation.
- Reagent: not always a separate species; the system rearranges internally.
In such cases, the main “reagent” effect is sometimes provided by acid or heat, but the restructuring occurs within the substrate itself.
Stoichiometric vs. Catalytic Reagents
From a quantitative point of view, reagents can be used:
- Stoichiometrically: in amounts comparable to the substrate; they are consumed.
- Example: $\ce{Br2}$ in the bromination of an alkene.
- Catalytically: in small amounts, regenerated in the reaction cycle.
- Example: small amounts of $\ce{H2SO4}$ in esterification.
A catalyst might also be called a “catalytic reagent,” but its key feature is that its role is to facilitate the transformation, not to appear as part of the final product in net consumption.
Writing and Interpreting Organic Reaction Schemes
When you see or write an organic reaction, ask:
- What is the substrate?
- The organic species whose transformation you care about.
- What are the reagents?
- What is added to drive the transformation (acids, bases, oxidants, nucleophiles, etc.)?
- What conditions are specified?
- Solvent, temperature, catalysts, light, pressure.
- What are the products?
- Which new bonds have formed, and which bonds have broken in the substrate?
For example, a typical reaction scheme might look like:
$$
\ce{Ph-CHO} \xrightarrow[\text{EtOH}]{\ce{NaBH4}} \ce{Ph-CH2OH}
$$
- Substrate: $\ce{Ph-CHO}$ (benzaldehyde).
- Reagent: $\ce{NaBH4}$ (reducing reagent).
- Solvent: ethanol (EtOH).
- Product: $\ce{Ph-CH2OH}$ (benzyl alcohol).
Understanding these roles is essential preparation for studying the specific reaction types in organic chemistry and their mechanisms, which are discussed in the corresponding later chapters.