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\(\text{E}^+\)

Electrophilic Addition
to Alkenes

Master how electrophiles attack carbon-carbon double bonds, learn Markovnikov's Rule, and predict addition products with confidence.

Organic Chemistry 1

What is an Alkene?

An alkene is a hydrocarbon containing at least one carbon-carbon double bond \( \text{C}{=}\text{C} \). The double bond consists of one sigma bond \( \sigma \) and one pi bond \( \pi \).

\(\text{C}\)
\(\text{C}\)
\(\sigma\) bond
\(\pi\) bond
\(\text{H}\)
\(\text{H}\)
\(\text{H}\)
\(\text{H}\)
Ethene (simplest alkene)

What is an Electrophile?

An electrophile ("electron-lover") is a species that is electron-poor and seeks to accept an electron pair. In electrophilic addition, common electrophiles include:

\( \text{H}^+ \)
From HCl, HBr, HI: the proton is the electrophile
\( \text{Br}^+ \)
From \( \text{Br}_2 \): bromine polarized by the π electrons
\( \text{H}_3\text{O}^+ \)
Acid-catalysed hydration adds water across the double bond

The General Mechanism

Electrophilic addition proceeds in two key steps. Let's use the addition of HBr to an alkene as our model reaction:

Step 1: Electrophilic Attack
\(\text{C}\)
\(\text{C}\)
\(\text{H-Br}\)
→ Carbocation + \(\text{Br}^-\)
Step 2: Nucleophilic Attack
\(\text{C}^+\)
\(\text{C}\)
\(\text{Br}^-\)
→ Addition product
Overall:
\(\text{C}=\text{C} + \text{H-Br} \rightarrow \text{C(H)-C(Br)}\)
The \(\pi\) bond breaks, two new \(\sigma\) bonds form

Carbocation Stability Order

The stability of the carbocation intermediate determines which carbon gets the electrophile. More substituted carbocations are more stable due to hyperconjugation and inductive effects.

Increasing stability
Methyl
\(\text{CH}_3^+\)
\(0^\circ\) (least)
Primary
\(\text{RCH}_2^+\)
\(1^\circ\)
Secondary
\(\text{R}_2\text{CH}^+\)
\(2^\circ\)
Tertiary
\(\text{R}_3\text{C}^+\)
\(3^\circ\) (most)

Markovnikov's Rule

Markovnikov's Rule (1870): When an unsymmetrical reagent (HX) adds to an unsymmetrical alkene, the hydrogen attaches to the carbon with more hydrogens, and the halide attaches to the carbon with fewer hydrogens.

Propene + HBr
\(\text{CH}_2\)
\(\text{CH}\)
\(\text{CH}_3\)
more H's
fewer H's
+ HBr →
Markovnikov Product (major)
\(\text{CH}_3\)
\(\text{CHBr}\)
\(\text{CH}_3\)
Br on more substituted C

Anti-Markovnikov Addition

When a peroxide (ROOR) is present, the addition of HBr reverses regioselectivity. This is called anti-Markovnikov addition, and proceeds through a radical mechanism instead of an ionic one.

Propene + HBr + ROOR
\(\text{CH}_2\)
\(\text{CH}\)
\(\text{CH}_3\)
+ ROOR (peroxide)
radical mechanism →
Anti-Markovnikov Product (major)
\(\text{CH}_2\text{Br}\)
\(\text{CH}_2\)
\(\text{CH}_3\)
Br on less substituted C

Markovnikov vs Anti-Markovnikov

Markovnikov
Conditions: HX alone (no peroxides)
Mechanism: Ionic (carbocation)
H goes to: Less substituted C
X goes to: More substituted C
Key factor: Most stable carbocation
Anti-Markovnikov
Conditions: HBr + ROOR (peroxide)
Mechanism: Radical chain
H goes to: More substituted C
X goes to: Less substituted C
Key factor: Most stable radical

Common Addition Reactions

Reagent Product Selectivity
HCl, HBr, HI Alkyl halide Markovnikov
HBr + ROOR Alkyl bromide Anti-Markovnikov
\( \text{H}_2\text{O} \) / \( \text{H}^+ \) Alcohol Markovnikov
\( \text{Br}_2 \) Vicinal dihalide anti addition (both sides)
\( \text{BH}_3 \) then \( \text{H}_2\text{O}_2 \) Alcohol Anti-Markovnikov, syn

How to Predict the Product

1
Identify the alkene. Locate the \( \text{C}{=}\text{C} \) double bond and note if it is symmetrical or unsymmetrical.
2
Identify the reagent. Is it HX, \( \text{Br}_2 \), \( \text{H}_2\text{O}/\text{H}^+ \), or HBr + peroxide?
3
Check for peroxides. If ROOR is present with HBr, use anti-Markovnikov. Otherwise, use Markovnikov.
4
Form the carbocation (or radical). Place \( \text{H}^+ \) on the less substituted carbon to form the more stable intermediate.
5
Add the nucleophile. The halide (or \( \text{OH} \)) attacks the carbocation to give the final product.

Example 1: HCl + 2-Methylpropene

2-Methylpropene + HCl
\(\text{CH}_3\)
\(\text{C}\)
\(\text{CH}_3\)
\(\text{CH}_2\)
+ HCl
Carbocation Intermediate
\(\text{CH}_3\)
\(\text{C}^+\)
\(\text{CH}_3\)
\(\text{CH}_3\)
\(3^\circ\) carbocation ✓
Product: 2-Chloro-2-methylpropane
\(\text{C}\)
\(\text{CH}_3\)
\(\text{CH}_3\)
\(\text{CH}_3\)
\(\text{Cl}\)

Example 2: HBr + ROOR + 1-Butene

Predict the major product when 1-butene reacts with HBr in the presence of peroxides.

1
Alkene: \(\text{CH}_2=\text{CHCH}_2\text{CH}_3\) (unsymmetrical, double bond at C1-C2)
2
Reagent: HBr + peroxide → anti-Markovnikov (radical mechanism)
3
Br radical adds to the less substituted carbon (C1), forming a \(2^\circ\) radical at C2 (more stable than \(1^\circ\))
4
Product: 1-bromobutane (\(\text{CH}_2\text{BrCH}_2\text{CH}_2\text{CH}_3\)) : Br ends up on the less substituted carbon

Example 3: Acid-Catalysed Hydration

Predict the product when 1-methylcyclohexene undergoes acid-catalysed hydration \( \text{H}_2\text{O}/\text{H}^+ \).

\(\text{CH}_3\)
C1
C2
\(\text{H}_2\text{O} / \text{H}^+\)
\(\text{CH}_3\)
\(\text{OH}\)
(+H)
1-Methylcyclohexanol

Hydroboration-Oxidation: Anti-Markovnikov Alcohol

To get the anti-Markovnikov alcohol, use a two-step sequence:

1
Hydroboration: \( \text{BH}_3 \cdot \text{THF} \) adds across the double bond in a syn fashion. Boron adds to the less substituted carbon.
2
Oxidation: \( \text{H}_2\text{O}_2 \), \( \text{NaOH} \) replaces boron with \( \text{OH} \), giving the anti-Markovnikov alcohol with syn stereochemistry.

Common Mistakes

✕ Applying Peroxide Rule to HCl/HI
Anti-Markovnikov (radical) addition only works with HBr + peroxides. HCl and HI always follow Markovnikov.
✕ Ignoring Carbocation Rearrangements
A \(2^\circ\) carbocation can rearrange to a \(3^\circ\) via a hydride or methyl shift. Always check for possible rearrangements.
✕ Confusing Regio- and Stereoselectivity
Regioselectivity (Markovnikov) tells you which carbon. Stereoselectivity (syn/anti) tells you which face.

Summary Comparison

Feature Markovnikov Anti-Markovnikov
Conditions HX, \( \text{H}_2\text{O}/\text{H}^+ \) HBr + ROOR, \( \text{BH}_3 \)
Mechanism Ionic (carbocation) Radical or concerted
Intermediate More substituted carbocation More substituted radical
H adds to Less substituted C More substituted C
X adds to More substituted C Less substituted C
Stereochemistry Not controlled (racemic) syn (\( \text{BH}_3 \))
Rearrangements? Yes (possible) No

Question 1 of 5

What is the major product when propene reacts with HBr without peroxides?

Question 2 of 5

Which reagent combination produces an anti-Markovnikov addition product?

Question 3 of 5

In the Markovnikov addition of HCl to 2-methylpropene, what is the intermediate?

Question 4 of 5

Hydroboration-oxidation (\( \text{BH}_3 \) then \( \text{H}_2\text{O}_2/\text{NaOH} \)) of 1-methylcyclohexene gives an alcohol at which position?

Question 5 of 5

Why does Markovnikov's Rule predict the major product in ionic HX additions?