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 occurs in non-polar molecules like $Br_2$ when the high electron density of the alkene's double bond repels the electrons in the $Br-Br$ bond, creating a temporary $\delta+$ end.

How does the product of hydrogenation differ from the product of hydration?

Hydrogenation adds $H_2$ across the double bond to produce an alkane, typically using a Nickel catalyst. Hydration adds $H_2O$ (as steam) across the double bond to produce an alcohol, requiring an acid catalyst like phosphoric acid.

Why is a tertiary carbocation more stable than a primary carbocation?

Tertiary carbocations are bonded to three alkyl groups, which exert an inductive effect, pushing electron density toward the positive carbon. This spreads the charge over a larger volume, lowering the potential energy and increasing stability compared to a primary carbocation with only one alkyl group.

What is a common mistake when drawing the first curly arrow in an electrophilic addition mechanism?

Students often start the arrow from a carbon atom or the 'space' near the bond. The arrow must start specifically from the double bond line itself, as it represents the movement of the $\pi$ pair of electrons.

What happens if the positive charge is omitted from the carbocation intermediate in an exam diagram?

The mechanism is considered chemically incorrect because the charge balance is lost. The carbon must be shown with a $+$ charge to justify the subsequent attack by the electron-rich nucleophile.

Why is it incorrect to show a full arrow from the nucleophile to the double bond in the first step?

The double bond is electron-rich and the nucleophile is also electron-rich; they repel each other. The reaction must be initiated by an electrophile (electron-seeker) attacking the double bond.

Define 'Heterolytic Fission' in the context of this reaction.

Heterolytic fission is the breaking of a covalent bond where both electrons from the shared pair move to one of the atoms, resulting in the formation of a cation and an anion.

What is an 'Electrophile'?

An electrophile is an electron-deficient species (an atom or molecule) that can accept a pair of electrons to form a new covalent bond. They are attracted to regions of high electron density.

What are the standard conditions for the hydrogenation of alkenes?

The reaction typically requires a Nickel ($Ni$) or Platinum ($Pt$) catalyst and a temperature of approximately $150^{\circ}C$ to facilitate the addition of hydrogen gas.

How does the 'Saturation Test' using bromine water work?

Bromine water (orange/brown) undergoes electrophilic addition with an alkene. As the $Br_2$ adds across the double bond to form a colorless dibromoalkane, the solution decolourises, confirming the presence of unsaturation.

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Electrophilic Addition

Summary

Electrophilic addition is a fundamental organic reaction where an electrophile attacks the electron-rich carbon-carbon double bond of an alkene, resulting in the saturation of the bond. This two-step mechanism involves the formation of a carbocation intermediate and is governed by the stability of that intermediate, which determines the regioselectivity of the final product.

1. Definition & Core Concepts

Diagram showing the electron-rich pi bond of an alkene being attacked by an electrophile (E+).

2. The Reaction Mechanism

The mechanism begins with heterolytic fission of the reagent bond. If the reagent is polar (like $H-Br$ ), the $\delta+$ atom acts as the electrophile; if non-polar (like $Br-Br$ ), a dipole is induced as the molecule approaches the electron-rich double bond.

In the first step, the $\pi$ electrons from the $C=C$ bond move to form a new bond with the electrophile. This leaves one of the carbon atoms with only three bonds and a positive charge, creating a highly reactive carbocation intermediate.

In the second step, the remaining part of the reagent (now a negatively charged nucleophile) donates a lone pair of electrons to the positively charged carbon atom. This completes the addition, resulting in a stable, saturated product with two new single bonds.

3. Carbocation Stability & Regioselectivity

4. Key Distinctions

5. Exam Strategy & Tips

Electrophilic Addition

Summary

1. Definition & Core Concepts

Electrophilic addition is a reaction specific to unsaturated hydrocarbons, such as alkenes, where the $\pi$ bond of a double bond is broken to form two new $\sigma$ bonds. The reaction is initiated by an electrophile, which is an electron-deficient species (often positively charged or polar) that is attracted to areas of high electron density.

The carbon-carbon double bond ( $C=C$ ) serves as the reactive center because it contains four electrons concentrated in a region of high electron density. This density makes the double bond susceptible to attack by 'electron-loving' species that seek to accept an electron pair to form a new covalent bond.

Common reagents that undergo this reaction include hydrogen halides ( $HX$ ), halogens ( $X_2$ ), steam ( $H_2O$ ), and hydrogen ( $H_2$ ). Each reagent adds across the double bond, converting the alkene into a saturated compound such as a halogenoalkane, dihalogenoalkane, or alcohol.

Diagram showing the electron-rich pi bond of an alkene being attacked by an electrophile (E+).

2. The Reaction Mechanism

3. Carbocation Stability & Regioselectivity

When an unsymmetrical alkene reacts with an unsymmetrical reagent (like $HBr$ adding to propene), two different products are possible. The outcome is determined by the stability of the carbocation intermediate formed during the first step.

Carbocations are stabilized by alkyl groups, which push electron density toward the positive carbon (the inductive effect). Stability follows the trend: tertiary ( $3^\circ$ ) > secondary ( $2^\circ$ ) > primary ( $1^\circ$ ).

The major product is formed via the most stable carbocation intermediate. This principle, often referred to as Markovnikov's Rule, states that the hydrogen atom (or the electrophilic part) will preferentially bond to the carbon atom that already has the greater number of hydrogen atoms.

4. Key Distinctions

Feature	Addition of $HX$	Addition of $X_2$
Electrophile	Permanent dipole ( $H^{\delta+}$ )	Induced dipole ( $X^{\delta+}$ )
Intermediate	Carbocation	Carbocation (or cyclic halonium)
Product	Halogenoalkane	Dihalogenoalkane
Solvent	Often gaseous or in inert solvent	Often aqueous (Bromine water)

Hydrogenation vs. Hydration: Hydrogenation requires a metallic catalyst (Ni/Pt) and heat to add $H_2$ , whereas hydration (adding steam) requires an acid catalyst like $H_3PO_4$ to produce an alcohol.
Induced vs. Permanent Dipoles: Students must distinguish between reagents that are naturally polar (like $HCl$ ) and those that become polar only when near the alkene's $\pi$ electrons (like $Cl_2$ ).

5. Exam Strategy & Tips

Curly Arrow Precision: Always start the curly arrow from the center of the double bond (representing the $\pi$ electrons) and point it exactly at the electrophilic atom. A second arrow must show the bond breaking within the reagent ( $E-Nu$ ) toward the nucleophilic atom.
Intermediate Charges: Never forget to draw the full positive charge ( $+$ ) on the carbon atom in the carbocation intermediate. Missing this charge is a frequent cause of lost marks.
Major vs. Minor: If asked to predict the major product, always draw the intermediate carbocations for both possibilities and explicitly state which one is more stable (e.g., 'The secondary carbocation is more stable than the primary carbocation due to the inductive effect of two alkyl groups').

Feature	Addition of $HX$	Addition of $X_2$
Electrophile	Permanent dipole ( $H^{\delta+}$ )	Induced dipole ( $X^{\delta+}$ )
Intermediate	Carbocation	Carbocation (or cyclic halonium)
Product	Halogenoalkane	Dihalogenoalkane
Solvent	Often gaseous or in inert solvent	Often aqueous (Bromine water)

Hydrogenation vs. Hydration: Hydrogenation requires a metallic catalyst (Ni/Pt) and heat to add $H_2$ , whereas hydration (adding steam) requires an acid catalyst like $H_3PO_4$ to produce an alcohol.
Induced vs. Permanent Dipoles: Students must distinguish between reagents that are naturally polar (like $HCl$ ) and those that become polar only when near the alkene's $\pi$ electrons (like $Cl_2$ ).

Curly Arrow Precision: Always start the curly arrow from the center of the double bond (representing the $\pi$ electrons) and point it exactly at the electrophilic atom. A second arrow must show the bond breaking within the reagent ( $E-Nu$ ) toward the nucleophilic atom.
Intermediate Charges: Never forget to draw the full positive charge ( $+$ ) on the carbon atom in the carbocation intermediate. Missing this charge is a frequent cause of lost marks.
Major vs. Minor: If asked to predict the major product, always draw the intermediate carbocations for both possibilities and explicitly state which one is more stable (e.g., 'The secondary carbocation is more stable than the primary carbocation due to the inductive effect of two alkyl groups').