Why are alkenes susceptible to attack by electrophiles?

Alkenes contain a carbon-carbon double bond with a $\pi$ bond. This $\pi$ bond is an area of high electron density that attracts electron-deficient species (electrophiles).

What is the difference between a permanent dipole and an induced dipole in electrophilic addition?

A permanent dipole exists in polar molecules like $HBr$ due to electronegativity differences. An induced dipole is temporary and created in non-polar molecules like $Br_2$ when they are repelled by the high electron density of the alkene's double bond.

What is the definition of heterolytic fission?

Heterolytic fission is the breaking of a covalent bond where one of the bonded atoms takes both electrons from the shared pair, resulting in the formation of a positive ion and a negative ion.

How do alkyl groups stabilize a carbocation?

Alkyl groups have a positive inductive effect, meaning they push electron density toward the positively charged carbon atom. This helps to spread and neutralize the charge, making the intermediate more stable.

What is a common error when drawing curly arrows in this mechanism?

A common error is starting the arrow from a carbon atom instead of the double bond itself, or failing to show the arrow starting exactly from a lone pair on the nucleophile in the second step.

In the reaction of propene with $HCl$, why is 2-chloropropane the major product?

The reaction proceeds via a secondary carbocation intermediate ($CH_3C^+HCH_3$), which is more stable than the primary carbocation ($CH_3CH_2C^+H_2$) due to the inductive effect of two alkyl groups.

What happens if you forget to include the partial charges ($\delta+/\delta-$) on the electrophile in a mechanism diagram?

Failing to show partial charges makes the reason for the electrophilic attack unclear. Examiners often require these labels to demonstrate understanding of why the electrons move toward the electrophile.

How does the addition of steam to an alkene differ from the addition of a halogen?

Addition of steam requires a strong acid catalyst (like $H_3PO_4$) to provide the $H^+$ electrophile, whereas halogens can react directly by forming an induced dipole.

What is the 'major product' in an organic reaction?

The major product is the compound formed in the highest yield, resulting from the most stable reaction pathway or intermediate (usually the most stable carbocation).

What is the trend in carbocation stability?

Tertiary ($3^\circ$) > Secondary ($2^\circ$) > Primary ($1^\circ$). Tertiary carbocations are the most stable because they have three alkyl groups providing an inductive effect to stabilize the positive charge.

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Electrophilic Addition Mechanism

Summary

Electrophilic addition is the fundamental reaction mechanism for alkenes, where the high electron density of the carbon-carbon double bond attracts electron-deficient species called electrophiles. This two-step process involves the heterolytic fission of a reagent and the formation of a carbocation intermediate, ultimately resulting in the saturation of the molecule as two new single bonds are formed.

1. Definition & Core Concepts

Electrophilic addition is a reaction where a double bond is broken and two new atoms or groups are added to the carbon atoms, resulting in a saturated product. This occurs because the $\pi$ bond in an alkene represents a region of high electron density, containing four electrons concentrated above and below the plane of the carbon nuclei.

An electrophile is an 'electron-loving' species that is attracted to these electron-rich areas. Electrophiles are typically positively charged ions (like $H^+$ ) or molecules with a partial positive charge ( $\delta+$ ) created by differences in electronegativity.

The reaction is characterized by the conversion of one strong $\sigma$ bond and one weaker $\pi$ bond into two strong $\sigma$ bonds, which is energetically favorable.

Diagram showing the π electron cloud of an alkene attracting an electrophile with a curly arrow indicating electron movement.

2. Underlying Principles: Polarity and Induction

3. Step-by-Step Mechanism

4. Carbocation Stability and Regioselectivity

5. Key Distinctions

6. Exam Strategy & Tips

Electrophilic Addition Mechanism

Summary

1. Definition & Core Concepts

The reaction is characterized by the conversion of one strong $\sigma$ bond and one weaker $\pi$ bond into two strong $\sigma$ bonds, which is energetically favorable.

Diagram showing the π electron cloud of an alkene attracting an electrophile with a curly arrow indicating electron movement.

2. Underlying Principles: Polarity and Induction

Reagents like hydrogen halides ( $H-X$ ) are naturally polar because the halogen is more electronegative than hydrogen, creating a permanent dipole where the hydrogen atom carries a $\delta+$ charge.

Non-polar molecules like bromine ( $Br_2$ ) can also act as electrophiles through an induced dipole. As the $Br-Br$ molecule approaches the electron-rich double bond, the $\pi$ electrons repel the electrons in the $Br-Br$ bond, pushing them toward the further bromine atom.

This repulsion creates a temporary $\delta+$ charge on the bromine atom closest to the alkene, allowing it to act as an electrophile and accept a pair of electrons from the double bond.

3. Step-by-Step Mechanism

Step 1: Formation of the Carbocation: A curly arrow is drawn from the double bond to the $\delta+$ atom of the electrophile. Simultaneously, the bond within the electrophilic reagent breaks heterolytically, meaning the more electronegative atom takes both electrons from the bond.

The result of Step 1 is the formation of a carbocation intermediate, where one carbon atom from the original double bond is now positively charged because it has lost its share of the $\pi$ electrons.

Step 2: Nucleophilic Attack: The remaining negatively charged ion (the nucleophile, such as $Br^-$ ) donates a lone pair of electrons to the positive carbon atom. A curly arrow is drawn from the lone pair of the nucleophile to the carbocation, forming the final saturated product.

4. Carbocation Stability and Regioselectivity

In reactions with unsymmetrical alkenes, multiple carbocation intermediates are possible. The stability of these intermediates follows the trend: tertiary > secondary > primary.

This stability is due to the inductive effect of alkyl groups (like $-CH_3$ ). Alkyl groups are electron-releasing and help spread the positive charge over a larger volume, stabilizing the ion.

The major product of the reaction is formed from the most stable carbocation intermediate. This is the basis of Markovnikov's Rule, which states that the hydrogen atom of a hydrogen halide will usually add to the carbon that already has the most hydrogen atoms.

5. Key Distinctions

Feature	Addition of $H-X$	Addition of $X_2$
Electrophile Source	Permanent dipole ( $H^{\delta+}$ )	Induced dipole ( $X^{\delta+}$ )
Intermediate	Carbocation	Carbocation (or cyclic halonium)
Regioselectivity	Follows stability rules	Not applicable (symmetrical)

Heterolytic vs. Homolytic Fission: Electrophilic addition always involves heterolytic fission, where one atom takes both electrons, unlike free radical substitution which involves homolytic fission.

6. Exam Strategy & Tips

Curly Arrow Precision: Always start the arrow exactly from the center of the double bond or a lone pair, and point it directly at the atom receiving the electrons. Incorrect placement is a frequent cause of lost marks.
Charge Balance: Ensure that the intermediate step shows both the positive carbocation and the negative ion. The overall charge of the intermediate stage must equal the overall charge of the reactants.
Stability Justification: When asked to explain a major product, always compare the stability of the possible carbocations (e.g., 'The secondary carbocation is more stable than the primary carbocation due to the inductive effect of two alkyl groups').

This repulsion creates a temporary $\delta+$ charge on the bromine atom closest to the alkene, allowing it to act as an electrophile and accept a pair of electrons from the double bond.

In reactions with unsymmetrical alkenes, multiple carbocation intermediates are possible. The stability of these intermediates follows the trend: tertiary > secondary > primary.

This stability is due to the inductive effect of alkyl groups (like $-CH_3$ ). Alkyl groups are electron-releasing and help spread the positive charge over a larger volume, stabilizing the ion.

Feature	Addition of $H-X$	Addition of $X_2$
Electrophile Source	Permanent dipole ( $H^{\delta+}$ )	Induced dipole ( $X^{\delta+}$ )
Intermediate	Carbocation	Carbocation (or cyclic halonium)
Regioselectivity	Follows stability rules	Not applicable (symmetrical)

Heterolytic vs. Homolytic Fission: Electrophilic addition always involves heterolytic fission, where one atom takes both electrons, unlike free radical substitution which involves homolytic fission.