Organic Chemistry · An Overview of Organic Reactions

Describing a Reaction: Intermediates

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

An is a species formed in one elementary step of a mechanism and consumed in the next. On an energy diagram it sits in a valley — a local energy minimum — between two transition states, giving it a real, if sometimes microscopic, lifetime. That is the crucial distinction from a transition state, an energy maximum lasting only a single bond vibration.

Most organic reactions pass through intermediates far more reactive than the reactants themselves: carbocations, carbanions, radicals, and carbenes. Understanding what these species look like, how stable they are, and why they form explains a great deal of reactivity — why HBr adds to an alkene through a , why radicals drive polymerization, and why some intermediates rearrange before reacting. This topic defines intermediates, surveys the common classes, and shows how chemists detect them.

Why this matters

Intermediates are where the "action" of a mechanism happens, so most mechanistic evidence is really evidence about intermediates. A rate law that is second-order in one reagent tells you which species meet in the slow step; a rearranged product tells you a carbocation shifted a hydrogen or alkyl group before being trapped; a racemic product from a chiral reactant tells you a planar intermediate erased the stereochemistry. In industry the same logic matters: carbocations drive acid-catalyzed rearrangements that upgrade hydrocarbons to gasoline, and radicals are the chain carriers in polymerization and in biological damage by reactive oxygen species.

The college version

Core Concepts

What makes a species an intermediate

Three tests identify an intermediate:

It is formed in one elementary step and consumed in the next — never appearing in the overall balanced equation.
It sits at a local energy minimum: both the step that forms it and the step that consumes it are downhill.
It has a finite lifetime, however short — long enough to be a real molecule or ion with a defined structure.

A transition state, by contrast, is a maximum: the atoms are "halfway" between two structures, and the geometry is not a stable molecule. Intermediates can sometimes be observed spectroscopically or trapped with a reactive reagent; transition states never can. As a rule of thumb, intermediates live at least long enough for bond rotation or collision (picoseconds to seconds); a transition state lasts about 10^-13 s — one vibration.

Carbocations: electron-poor, planar, and rearranging

A carbocation is a carbon bearing only six valence electrons and a formal positive charge, such as the tert-butyl cation (CH₃)₃C⁺. Its carbon is sp²-hybridized and trigonal planar, with an empty p orbital perpendicular to the plane. Stability follows the order:

$3 ° > 2 ° > 1 ° > methyl$

Tertiary carbocations are most stable because alkyl groups release electron density by (overlap of a neighboring C–H or C–C σ bond with the empty p orbital) and by induction. Carbocations react immediately with any electron-rich species — a π bond, a lone pair, a solvent molecule — and frequently rearrange by shifting a hydrogen or alkyl group to become more stable. Their planar shape allows attack from either face.

Carbanions: electron-rich, pyramidal

A is a carbon bearing a lone pair and a formal negative charge, as in methyllithium (CH₃Li) or the enolate of a ketone. The carbon is sp³-hybridized and roughly tetrahedral, with the lone pair occupying one vertex. Stability is the reverse of carbocations:

$methyl > 1 ° > 2 ° > 3 °$

Alkyl groups destabilize the negative charge by pushing electron density onto it (induction), so less-substituted carbanions are more stable. Carbanions are strong bases and nucleophiles — key players in carbon–carbon bond-forming reactions.

Radicals: neutral, unpaired, and chain-carrying

A (free radical) is a neutral species with one unpaired electron, such as the methyl radical CH₃^•. The carbon is sp²-hybridized and planar, with the unpaired electron in a p orbital. Radical stability follows the carbocation order — 3^°> 2^°> 1^°> methyl — but radicals are uncharged, so they are neither acids nor bases. They are the chain carriers in halogenation, combustion, and polymerization.

Carbenes: divalent carbon

A is a neutral, divalent carbon with only two bonds and two nonbonding electrons, as in methylene :CH₂. Carbenes exist in two electronic states — singlet (electrons paired) and triplet (electrons unpaired) — and are highly reactive, inserting into C–H bonds and adding to π bonds. They appear in reactions such as the Simmons–Smith cyclopropanation.

Detecting and proving intermediates

Because intermediates are short-lived, chemists rely on indirect evidence:

Kinetics: the rate law identifies which species meet in the slow step, implying which intermediate forms next.
Trapping: a reagent that reacts selectively with the proposed intermediate intercepts it and changes the product.
products: a hydride or alkyl shift before capture proves a carbocation existed long enough to rearrange.
Low-temperature spectroscopy: stable intermediates (e.g., the tert-butyl cation in superacid media at -60 ^°C) have been observed by NMR.
Stereochemistry: racemization at a stereocenter points to a planar intermediate.

Common Confusions

Do not confuse	With	Difference
Intermediate	Transition state	Intermediate = energy minimum, finite lifetime; transition state = energy maximum, 10^-13 s
Carbocation stability order	Carbanion stability order	Carbocations: 3^°> 2^°> 1^°> methyl; carbanions: exactly reversed
Carbocation geometry	Tetrahedral carbon	Carbocations are planar sp² with an empty p orbital; carbanions are pyramidal sp³
Radical	Ion	Radicals are neutral with one unpaired electron; ions carry full charges
"Intermediates appear in the equation"	Intermediates cancel out	They never appear in the overall balanced equation — they form and are consumed within it
Every species on the diagram	Intermediates only	Peaks are transition states; valleys are intermediates

Eli explains

The same idea, in plain words

Explain it like I’m 10

A reaction is like a relay race. The intermediate is the runner who receives the baton from the first runner and passes it to the third — she exists for a moment and then is gone. The transition state is not a runner at all but the exact instant the baton changes hands.

Worked example

Example 1: Finding the intermediate in HBr addition to propene

Propene (CH₃CH=CH₂) reacts with HBr in two elementary steps:

Step 1 — the π electrons of the C=C bond attack the hydrogen of HBr, forming a new C–H bond and breaking the H–Br bond; the bond's electrons move to bromine, giving Br^-. The product is the secondary carbocation CH₃CH⁺CH₃.

Step 2 — bromide ion donates its lone pair to the carbocation's empty p orbital, forming the C–Br bond and giving 2-bromopropane.

The carbocation is the intermediate: formed in step 1, consumed in step 2, absent from the overall equation. As a local minimum it sits in the valley between the two transition states. (This is the Markovnikov outcome — the more stable carbocation forms — detailed in Chapter 7.)

Example 2: Predicting a rearrangement

In the addition of HBr to 3-methyl-1-butene, protonation puts the positive charge on C-2, giving a secondary carbocation:

$(CH 3) 2 CH - CH + CH 3$

This cation sits next to a tertiary carbon bearing a hydrogen. A 1,2-hydride shift moves that hydrogen (with its electron pair) to the cationic carbon, giving the more stable tertiary carbocation:

$(CH 3) 2 C + - CH 2 CH 3$

Bromide captures the tertiary cation to give 2-bromo-2-methylbutane as the major product — not the product expected from simple Markovnikov addition. The rearranged product is experimental evidence that a carbocation shifted a hydride before being trapped.

Example 3: Why a planar intermediate erases stereochemistry

If a reaction at a tetrahedral carbon goes through a carbocation, the planar intermediate's empty p orbital can be attacked from either face. A chiral starting material therefore gives a racemic mixture when the cationic center is the stereocenter. Observing racemization is classic evidence for a carbocation intermediate; a direct backside attack (no intermediate) would instead invert stereochemistry cleanly.

Key takeaways

An intermediate is formed in one step and consumed in the next; it sits in an energy valley with a finite lifetime.
A transition state is an energy maximum with essentially zero lifetime — never confuse the two.
Carbocation stability: 3^°> 2^°> 1^°> methyl (hyperconjugation, induction); sp², planar, empty p orbital.
Carbanion stability is reversed: methyl > 1^°> 2^°> 3^°; sp³, lone pair, basic, nucleophilic.
Radicals are neutral, unpaired-electron species; stability order matches carbocations.
Carbocations rearrange (hydride/alkyl shift) to more stable isomers; planar geometry erases stereochemistry.
Evidence: rate laws, trapping, rearranged products, low-temperature NMR, stereochemical outcomes.

Check yourself

5 review questions from the chapter. Try each one, then open the answer.

State the three tests that identify a reaction intermediate.
Show answer
(a) Formed in one elementary step, consumed in the next; (b) sits at a local energy minimum between two transition states; (c) has a finite lifetime as a real species.
Why is a transition state not an intermediate?
Show answer
A transition state is an energy maximum — a fleeting geometry with bonds partially broken and formed — lasting about one bond vibration (10^-13 s). It cannot be trapped, so it is not a molecule or intermediate.
Rank by stability: (CH₃)₃C⁺, CH₃CH₂⁺, (CH₃)₂CH⁺, CH₃⁺. Explain.
Show answer
Tertiary (CH₃)₃C⁺ > secondary (CH₃)₂CH⁺ > primary CH₃CH₂⁺ > methyl CH₃⁺. More alkyl groups mean more hyperconjugation and inductive donation.
In the addition of HBr to propene, which species is the intermediate, and how do you know?
Show answer
The secondary carbocation CH₃CH⁺CH₃: it forms when the alkene π bond protonates and is consumed when bromide attacks — absent from the overall equation and sitting between the two transition states.
What observations suggest a reaction passes through a carbocation intermediate?
Show answer
A rate law consistent with a stepwise mechanism, trapping of the cation by a nucleophile, rearranged products (hydride or alkyl shifts), racemization at a stereocenter, and — for stable cations — direct NMR observation at low temperature in superacid media.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

intermediate: Species formed in one step, consumed in the next; sits in an energy valley
carbocation: Carbon with 6 valence electrons and a positive charge; sp², planar
carbanion: Carbon with a lone pair and a negative charge; sp³, pyramidal
radical: Neutral species with one unpaired electron
carbene: Neutral divalent carbon with two nonbonding electrons
hyperconjugation: Stabilization by σ-bond overlap with an adjacent empty p orbital
rearrangement: Migration of H or alkyl group to an adjacent electron-poor atom

Sources & references

openstax.org — Organic Chemistry

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