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Reaction Conditions
and Reagent Recognition

Master the reagents, catalysts, solvents, and conditions that drive the most important reactions in organic chemistry.

Organic Chemistry 1

What Are Reaction Conditions?

In organic chemistry, simply mixing two molecules together is rarely enough. You need to specify the reagents, catalysts, solvents, and physical conditions (temperature, pressure, light) to make a reaction proceed correctly.

Reagent
A substance consumed in the reaction that transforms the substrate
Catalyst
Speeds up the reaction without being consumed: lowers \(E_a\)
Solvent
Dissolves reactants and can influence mechanism (polar vs. nonpolar)

Why Conditions Matter

The same substrate can yield completely different products depending on the reagents and conditions. Recognizing the correct reagent set is how you predict the product on an exam.

Alkene (\(C=C\))
\(\text{HBr}\)
 Alkyl Halide
\(\text{H}_2\text{O}, \text{H}^+\)
 Alcohol
\(\text{OsO}_4\)
 Diol
\(\text{Br}_2\)
 Dihalide

The Arrow Convention

In organic chemistry, reactions are written with an arrow. Reagents and catalysts are placed above or below the arrow, not mixed in with the substrate.

Substrate Reagent Solvent, Temperature Product above = consumed or catalytic below = solvent/conditions

Reagent Sets as Reaction Fingerprints

Every named reaction in organic chemistry has a unique combination of reagents and conditions that acts like a fingerprint. Memorize the fingerprint, and you can instantly identify the reaction and predict the product.

1
Read the reagents written above and below the arrow.
2
Match the set to a known reaction type (addition, substitution, elimination, oxidation, reduction).
3
Apply regiochemistry and stereochemistry rules for that reaction to draw the product.

Major Reagent Categories

Electrophilic
\(\text{HBr}\), \(\text{HCl}\), \(\text{Br}_2\), \(\text{H}_3\text{O}^+\): attack electron-rich sites
Nucleophilic
\(\text{NaOH}\), \(\text{NaCN}\), \(\text{NaOR}\): donate electrons to electrophilic carbon
Reducing
\(\text{NaBH}_4\), \(\text{LiAlH}_4\), \(\text{H}_2/\text{Pd}\): add \(\text{H}\) or remove \(\text{O}\)
Oxidizing
\(\text{KMnO}_4\), \(\text{OsO}_4\), \(\text{CrO}_3\), PCC: add \(\text{O}\) or remove \(\text{H}\)

\(\text{S}_\text{N}\) vs. Elimination Conditions

The same substrate (e.g., an alkyl halide) can undergo substitution or elimination depending on the reagent strength, temperature, and substrate structure.

\(\text{S}_\text{N}2\) \(\text{S}_\text{N}1\) \(\text{E}2\) \(\text{E}1\)
Base/Nuc. Strong nuc. Weak nuc. Strong base Weak base
Substrate \(1^\circ > 2^\circ\) \(3^\circ > 2^\circ\) \(3^\circ > 2^\circ > 1^\circ\) \(3^\circ > 2^\circ\)
Solvent Polar aprotic Polar protic Any Polar protic
Temperature Low-moderate Moderate High High

The Oxidation Reagent Ladder

Different oxidizing agents stop at different oxidation levels. Knowing which reagent stops where is critical.

\(1^\circ\) Alcohol
\(\text{R-CH}_2\text{OH}\)
Aldehyde
\(\text{R-CHO}\)
Carboxylic Acid
\(\text{R-COOH}\)
 PCC
\(\text{KMnO}_4\)
Jones (\(\text{CrO}_3/\text{H}^+\)) or \(\text{KMnO}_4\)
 STOPS here

Reduction Reagents Compared

Reagent Reduces Does NOT Reduce
\(\text{NaBH}_4\) Aldehydes, Ketones Esters, Carboxylic acids, Amides
\(\text{LiAlH}_4\) All carbonyls, Esters, Acids, Amides Alkenes, Aromatics
\(\text{H}_2 / \text{Pd}\) \(C=C\) double bonds (catalytic hydrogenation) Carbonyls (usually)

5-Step Reagent Recognition

1
Identify the functional group in the substrate (alkene, alcohol, carbonyl, alkyl halide, etc.).
2
Read all reagents above and below the arrow, including solvents and temperature.
3
Classify the reagent type: electrophilic, nucleophilic, oxidizing, reducing, acidic, or basic.
4
Match to a named reaction and determine the transformation: what bond breaks, what bond forms.
5
Apply regio- and stereoselectivity rules (Markovnikov, anti-Markovnikov, syn/anti addition).

Example 1: \(\text{HBr}\) Addition to an Alkene

propene
\(\text{CH}_3\)
\(C=C\)
\(\text{HBr}\)
 2-bromopropane
\(\text{CH}_3\)
\(\text{CH}_3\)
\(\text{Br}\)
\(\text{H}\)

Example 2: \(\text{NaBH}_4\) Reduction

ketone
O
R
R'
\(\text{NaBH}_4\)
 MeOH
\(2^\circ\) alcohol
OH
H
R
R'

Example 3: \(\text{LiAlH}_4\) on an Ester

ester
O
O
R
R'
\(\text{LiAlH}_4\)
THF, then \(\text{H}_3\text{O}^+\)
\(1^\circ\) alcohol
OH
R
 +
\(\text{R'OH}\)

Alkene Addition Reagent Map

\(\text{HX}\) (no peroxides)
Markovnikov addition: \(\text{X}\) goes to more substituted C
\(\text{HBr} + \text{ROOR}\)
Anti-Markovnikov: \(\text{Br}\) goes to less substituted C (radical mechanism)
\(\text{H}_2\text{O} / \text{H}^+\)
Acid-catalyzed hydration: Markovnikov alcohol
\(\text{BH}_3\) / THF, then \(\text{H}_2\text{O}_2\)
Hydroboration-oxidation: anti-Markovnikov, syn alcohol

Common Mistakes

✕ Confusing \(\text{NaBH}_4\) and \(\text{LiAlH}_4\)
Using \(\text{NaBH}_4\) to reduce an ester or carboxylic acid: it will not react. Only \(\text{LiAlH}_4\) is strong enough.
✕ Ignoring Peroxides
\(\text{HBr}\) alone gives Markovnikov. \(\text{HBr}\) with ROOR (peroxides) gives anti-Markovnikov via a radical pathway.
✕ Wrong Oxidation Level
PCC only oxidizes a \(1^\circ\) alcohol to an aldehyde. Using \(\text{KMnO}_4\) or Jones goes further to the carboxylic acid.

Summary: Key Reagent Sets

Reagent Reaction Type Key Product
\(\text{HBr}\) Electrophilic addition Markovnikov alkyl halide
\(\text{HBr} + \text{ROOR}\) Radical addition Anti-Markovnikov alkyl halide
\(\text{NaBH}_4\) Mild reduction Alcohol (from aldehyde/ketone)
\(\text{LiAlH}_4\) Strong reduction Alcohol (from ester/acid too)
\(\text{PCC}\) Mild oxidation Aldehyde (from \(1^\circ\) alcohol)
\(\text{KMnO}_4 / \text{H}^+\) Strong oxidation Carboxylic acid
\(\text{H}_2 / \text{Pd}\) Catalytic hydrogenation Alkane (from alkene)

Question 1 of 5

Which reagent will reduce a ketone to a secondary alcohol but will not reduce an ester?

Question 2 of 5

When \(\text{HBr}\) is added to propene in the presence of peroxides (\(\text{ROOR}\)), the major product has bromine on which carbon?

Question 3 of 5

A primary alcohol is treated with \(\text{PCC}\). What is the product?

Question 4 of 5

Which set of conditions favors an \(\text{S}_\text{N}2\) reaction on a primary alkyl halide?

Question 5 of 5

Which reagent performs catalytic hydrogenation of an alkene to form an alkane?