Reaction Intermediates
Table of Contents
Reaction intermediates are short-lived species that form during multi-step chemical reactions. They are produced in one step and consumed in the next, so they exist only briefly and usually in very small amounts. Although they are often difficult to detect directly, they are important because they help explain how reactants are converted into products. [1–3]
Types
1. Carbocation
A carbocation is an intermediate in which a carbon atom carries a positive charge. Because this carbon has only six electrons in its outer shell, it is electron-deficient and highly reactive. Carbocations are stabilized by alkyl groups, hyperconjugation, and resonance, and they often form in reactions where a leaving group departs before nucleophilic attack. [1,2]
A common example is the hydrolysis of tert-butyl chloride. First, the chloride ion leaves, forming a tert-butyl carbocation:
(CH3)3CCl → (CH3)3C+ + Cl−
The carbocation then reacts with water:
(CH3)3C+ + H2O → (CH3)3COH2+
After losing a proton, tert-butyl alcohol is formed:
(CH3)3COH2+ + H2O → (CH3)3COH + H3O+
2. Carbanion
A carbanion is an intermediate in which a carbon atom carries a negative charge and a lone pair of electrons. Because it is electron-rich, it usually reacts with positively charged or electron-poor species. Carbanions are often stabilized by electron-withdrawing groups and resonance.
In carbonyl compounds, such as aldehydes, ketones, or esters, this intermediate is often better described as an enolate ion, a resonance-stabilized anion formed when a base removes a proton from the α-carbon. The α-carbon is the carbon atom next to the carbonyl group.
For example, when a strong base removes an α-hydrogen from the α-carbon in acetaldehyde, an enolate forms:
CH3CHO + B– ⇌ [CH2CHO]− + HB
This intermediate is resonance-stabilized, in which the negative charge is shared between carbon and oxygen.
3. Free Radical
A free radical is an intermediate that contains an unpaired electron. This makes it highly reactive in chain reactions.
A well-known example is the chlorination of methane. Under ultraviolet light, a chlorine molecule splits into two chlorine radicals:
Cl2 → 2 Cl⋅
One chlorine radical then removes a hydrogen atom from methane:
Cl⋅ + CH4 → HCl + CH3⋅
This produces a methyl radical, which continues the chain reaction as another intermediate.
4. Carbene
A carbene is a neutral intermediate in which a carbon atom forms only two bonds and has six valence electrons. Because it does not have a complete octet, it is highly reactive. Carbenes can exist in two electronic states: singlet and triplet, depending on the arrangement of their electrons.
An important example is dichlorocarbene, which can be generated from chloroform in the presence of a strong base:
CHCl3 + OH− → CCl3− + H2O
CCl3− → :CCl2 + Cl−
The dichlorocarbene formed can add across a carbon–carbon double bond, forming cyclopropane derivatives.
To better understand reaction intermediates, it is helpful to briefly compare them with the transition state, another important part of a reaction pathway that is often confused with an intermediate.
Transition State vs. Reaction Intermediate
| Basis | Transition State | Reaction Intermediate |
|---|---|---|
| Meaning | A high-energy arrangement of atoms at the top of the energy barrier for a single reaction step | A species formed during the reaction that exists between two steps |
| Position on energy diagram | At the top of an energy peak | At an energy valley between two peaks |
| Stability | Extremely unstable; exists for an instant. Cannot be isolated. | More stable than a transition state, but usually less stable than reactants/products. May exist for a short time; sometimes can be detected or isolated. |
| Representation | Shown with double dagger symbol, like ‡ | Written as a normal chemical species |
| Bonding | Bonds are partly broken and partly formed | Has definite, though often unstable, bonds |
| Role in mechanism | Represents the moment of conversion from one structure to another | Represents an actual stage in a multistep reaction |
| Example idea | The instant when an old bond is breaking while a new bond is forming | A carbocation formed during an SN1 reaction |
A simple way to remember the difference is that an intermediate is like a valley, while a transition state is like a peak. [4]
