Hydrocarbons Answer Key (16/02/2026)
Answer Key: Hydrocarbons
Date: 16/02/2026 | Total Marks: 100
SECTION A: ONE LINE QUESTIONS (25 Marks)
Scoring: 2 points per answer (1 mark each)
1. Define Hydrocarbons.
• Hydrocarbons are defined as organic compounds that contain only carbon and hydrogen as their constituent elements.
• They are broadly classified into open-chain (aliphatic) and cyclic structures.
2. What is the general formula for Alkanes?
• The general molecular formula for the alkane series is CnH2n+2.
• In this formula, 'n' represents the number of carbon atoms present in the molecule.
3. Why are alkanes called 'paraffins'?
• Alkanes are referred to as paraffins because they exhibit very low chemical reactivity.
• The term "paraffin" is derived from their low affinity for reacting with other chemical substances under normal conditions.
4. State the hybridization of carbon atoms in Alkenes.
• The doubly-bonded carbon atoms in alkenes undergo sp2-hybridization.
• This hybridization results in a planar geometry around the double bond.
5. What is the C-C bond length in Alkanes?
• The carbon-carbon single bond length in alkanes is measured at 154 pm.
• This is the longest bond length compared to the carbon bonds found in alkenes and alkynes.
6. Define 'Isomerism'.
• Isomerism is the phenomenon where compounds possess the same molecular formula but have different structural formulas.
• The individual compounds that share the same molecular formula are known as isomers.
7. What is the general formula for Alkynes?
• The general molecular formula representing the alkyne series is CnH2n-2.
• This series is characterized by the presence of at least one carbon-carbon triple bond.
8. Define 'Conformations'.
• Conformations are different spatial arrangements of atoms that can be converted into one another by rotation around a C-C single bond.
• These arrangements are also referred to as conformational isomers or conformers.
9. What is a 'Torsion angle'?
• A torsion angle (or dihedral angle) is the angle between the H-C-C plane and the C-C-H plane in a molecule like ethane.
• It represents the angle of rotation between C-H bonds of adjacent carbon atoms as seen in a Newman projection.
10. Name the parent compound of the entire family of aromatic compounds.
• Benzene (C6H6) is the parent compound for the entire family of aromatic substances.
• All other aromatic compounds are considered derivatives of this basic benzene structure.
11. What is the common name for Alkenes?
• Alkenes are commonly referred to as 'Olefins'.
• This name is used because the lower members of this series form oily products upon reaction with halogens.
12. Give the IUPAC name of the first member of the Alkyne series.
• The first member of the alkyne series, with the formula C2H2, is IUPAC-named Ethyne.
• It is also commonly known as acetylene.
13. State Huckel’s Rule for aromaticity.
• Huckel’s Rule states that for a compound to be aromatic, it must be cyclic and planar and contain (4n+2) π-electrons.
• In this rule, 'n' must be an integer (0, 1, 2, 3...).
14. Which alkane is the major product in a Wurtz reaction using a mixture of alkyl halides?
• When a mixture of different alkyl halides is used, the alkane with an even number of carbon atoms and higher molecular mass is the major product.
• Specifically, this refers to the symmetric products (R-R or R'-R') formed in the reaction.
15. What are 'Arenes'?
• Arenes is the systematic name given to all aromatic hydrocarbons.
• They are characterized by cyclic structures with delocalized π-electron clouds.
16. Name the catalyst used for the hydrogenation of ethene at room temperature.
• Finely divided powders of Platinum (Pt) or Palladium (Pd) are used as catalysts for hydrogenation at room temperature.
• These catalysts allow the addition of hydrogen to occur without requiring the high temperatures needed for nickel.
17. Define 'Vicinal Dihalides'.
• Vicinal dihalides are compounds where two halogen atoms are attached to two adjacent carbon atoms.
• They are commonly used as starting materials in the preparation of alkenes or alkynes.
18. What is the geometry of a benzene molecule?
• The benzene molecule has a planar and cyclic hexagonal structure.
• All six carbon atoms in the ring are sp2 hybridized, contributing to this flat, planar geometry.
19. Name one ortho-para directing group.
• The hydroxyl group (-OH) is a prominent example of an ortho-para directing group.
• Other examples include the amino group (-NH2) and alkyl groups like -CH3.
20. Mention one meta-directing group.
• The nitro group (-NO2) is a common meta-directing group in aromatic substitution.
• Other examples include the carboxylic acid group (-COOH) and the aldehyde group (-CHO).
21. What is the torsion angle in an 'eclipsed' conformation of ethane?
• In the eclipsed conformation of ethane, the torsion angle between the C-H bonds is 0°.
• This arrangement places the hydrogen atoms of the two carbons as close together as possible.
22. Which gas is produced during the complete combustion of alkanes?
• Complete combustion of alkanes in the presence of air or oxygen produces carbon dioxide (CO2) gas.
• Water vapor and a significant amount of heat energy are also released during this reaction.
23. Give the chemical formula for 'Sodium Ethynide'.
• The chemical formula for Sodium Ethynide is Na-C≡C-Na.
• It is formed when ethyne reacts with sodium in liquid ammonia.
24. Define 'Torsional Energy'.
• Torsional energy is the energy required to rotate a molecule around a carbon-carbon single bond.
• It arises due to the repulsion between the electron clouds of the bonds on adjacent carbon atoms.
25. Name a known carcinogenic aromatic hydrocarbon.
• Benzene itself is classified as a known carcinogen.
• Other examples include polycyclic aromatic hydrocarbons like 1,2-Benzpyrene or 3-Methylcholanthrene.
SECTION B: ANSWER IN SHORT (20 Marks)
Scoring: 4 points per answer (2 marks each)
1. Differentiate between 'Cis' and 'Trans' isomers in alkenes.
• Geometrical isomerism in alkenes arises due to different spatial arrangements around the C=C double bond.
• In a Cis-isomer, identical or similar atoms/groups are located on the same side of the double bond.
• In a Trans-isomer, identical or similar atoms/groups are located on opposite sides of the double bond.
• These isomers differ in physical properties such as boiling point and stability.
2. Explain the 'Saytzeff's Rule' with an example.
• Saytzeff's Rule is applied during the dehydrohalogenation or dehydration reactions of hydrocarbons.
• It states that the hydrogen atom is preferentially eliminated from the adjacent carbon atom that has fewer hydrogen atoms.
• This rule ensures the formation of the most highly substituted alkene as the major product.
• For example, in the dehydrohalogenation of 2-bromobutane, 2-butene is the major product over 1-butene.
3. How is benzene prepared from Ethyne? Give the reaction conditions.
• Benzene is prepared from ethyne (acetylene) through a process of cyclic polymerization.
• The reaction requires passing ethyne gas through a red-hot iron tube.
• The specific temperature required for this conversion is approximately 873 K.
• Three molecules of ethyne polymerize to form one molecule of benzene (C6H6).
4. State the physical properties of Alkanes regarding their solubility.
• Alkanes are generally insoluble in water because C-H bonds are almost non-polar.
• They are freely soluble in organic solvents such as ether, acetone, and carbon tetrachloride (CCl4).
• This follows the "like dissolves like" principle for non-polar substances.
• Their solubility in organic solvents remains consistent across the series from gases to solids.
5. What is the 'Markovnikov’s Rule'?
• Markovnikov’s Rule applies to the addition of polar reagents (like HBr) to unsymmetrical alkenes.
• It states that the negative part of the addendum (reagent) attaches to the carbon atom of the double bond that carries fewer hydrogen atoms.
• The hydrogen atom (positive part) attaches to the carbon atom with more hydrogen atoms.
• This rule results in the formation of a specific major product, such as 2-bromopropane from propene.
6. Draw the structural isomers of Butane (C4H10).
• Butane exhibits chain isomerism with two distinct structural formulas.
• The first isomer is n-butane, which consists of a straight four-carbon chain.
• The second isomer is isobutane (2-methylpropane), which consists of a branched three-carbon chain with a methyl group.
• These isomers share the same molecular formula but have different physical properties like boiling points.
7. Why is the staggered conformation of ethane more stable than the eclipsed form?
• In the staggered conformation, the hydrogen atoms on adjacent carbons are as far apart as possible.
• This maximum distance minimizes the repulsive forces between the electron clouds of the C-H bonds.
• In contrast, the eclipsed conformation places hydrogen atoms closest together, leading to maximum repulsion.
• Consequently, the staggered form is a lower energy state, making it more stable than the eclipsed form.
8. Mention any two characteristics of aromatic compounds.
• Aromatic compounds are cyclic, closed-chain structures that possess a planar geometry.
• They contain a delocalized π-electron cloud situated above and below the plane of the ring.
• They are characterized by high resonance energy, which makes them exceptionally stable.
• When burned, aromatic compounds typically produce a sooty flame due to their high carbon content.
9. Show the reaction for the preparation of ethane from Ethyl Bromide using Zinc and dil. HCl.
• Ethyl bromide (C2H5Br) can be converted to ethane through a reduction process.
• The reagent used for this reduction is Zinc metal in the presence of dilute Hydrochloric acid (HCl).
• The reaction involves nascent hydrogen generated by the Zn + HCl mixture reacting with the alkyl halide.
• The chemical equation is: C2H5Br + 2[H] → C2H6 + HBr.
10. Explain the 'Peroxide effect' (Anti-Markovnikov addition) in alkenes.
• The peroxide effect occurs during the addition of HBr to unsymmetrical alkenes specifically in the presence of organic peroxides.
• It causes the addition to take place in the opposite manner to Markovnikov’s Rule.
• The bromine atom attaches to the carbon atom with more hydrogen atoms, rather than fewer.
• This is also known as "abnormal addition" and follows a free-radical mechanism.
11. Give the structural formula for Cyclobutane and Propane.
• Propane is a straight-chain alkane with three carbon atoms: CH3-CH2-CH3.
• Cyclobutane is an alicyclic hydrocarbon with four carbon atoms arranged in a ring.
• The structure of cyclobutane is represented by a square, where each corner is a CH2 group.
• The structural formula for cyclobutane shows single bonds between all carbon atoms in the closed loop.
12. Why do branched-chain alkanes have lower boiling points than straight-chain alkanes?
• Boiling points in alkanes depend on the strength of intermolecular Van der Waals forces.
• Straight-chain alkanes have a larger surface area, allowing for stronger Van der Waals attractions.
• Branching makes the molecule more spherical, which decreases its surface area.
• The reduced surface area leads to weaker intermolecular forces, thus lowering the boiling point.
SECTION C: ANSWER IN BRIEF (30 Marks)
Scoring: 6 points per answer (3 marks each)
1. Describe the 'Wurtz Reaction' for the preparation of Alkanes.
• The Wurtz reaction involves the coupling of two molecules of an alkyl halide (RX).
• The reaction is carried out using sodium metal in the presence of dry ether.
• A new carbon-carbon (C-C) bond is formed during this process, resulting in a higher alkane.
• If a single type of alkyl halide is used, it produces a symmetrical alkane with an even number of carbons.
• If a mixture of different alkyl halides is used, three different alkanes are possible (R-R, R-R', R'-R').
• In a mixture, the alkane with even carbon atoms and higher molecular mass is obtained as the major product.
2. Explain the 'Friedel-Crafts Alkylation' of benzene with a chemical equation.
• Friedel-Crafts Alkylation is an electrophilic substitution reaction used to introduce an alkyl group into the benzene ring.
• Benzene reacts with an alkyl halide (such as CH3Cl) in the presence of an anhydrous AlCl3 catalyst.
• The AlCl3 acts as a Lewis acid to generate the alkyl carbocation (R+) electrophile.
• The alkyl group replaces a hydrogen atom on the benzene ring.
• For example, reacting benzene with methyl chloride produces Methylbenzene (Toluene).
• Equation: C6H6 + CH3Cl → C6H5-CH3 + HCl (in presence of AlCl3).
3. Discuss the acidic nature of Alkynes compared to alkanes and alkenes.
• Alkynes are uniquely acidic compared to alkanes and alkenes due to the nature of their C-H bonds.
• The carbon atoms in alkynes are sp-hybridized, meaning they have 50% s-character.
• This high s-character makes the sp-hybridized carbon more electronegative.
• The electronegative carbon strongly attracts the shared pair of electrons in the C-H bond, allowing the hydrogen to be released as a proton.
• Alkynes react with strong bases like sodium metal in liquid ammonia to form metal salts called alkynides.
• Alkanes and alkenes do not show this acidic behavior because their carbon atoms have less s-character (sp3 and sp2).
4. Explain the 'Nitration' of benzene, including the reagents used.
• Nitration is an electrophilic substitution reaction that introduces a nitro group (-NO2) into benzene.
• The reagents used are a mixture of concentrated Nitric acid (HNO3) and concentrated Sulfuric acid (H2SO4).
• This combination is known as the "nitrating mixture".
• The Sulfuric acid acts as a catalyst and helps in the generation of the nitronium ion (NO2+) electrophile.
• The reaction is typically carried out under heating (reflux) conditions.
• The final product of the reaction is Nitrobenzene (C6H5NO2).
5. Describe the preparation of Alkenes by the dehydration of alcohols.
• Alkenes can be prepared by removing a molecule of water from an alcohol, a process called dehydration.
• The reaction requires a dehydrating agent, most commonly concentrated Sulfuric acid (H2SO4).
• The alcohol is heated with the acid to a temperature of approximately 443 K to 473 K.
• Alternatively, alcohols can be passed over heated Alumina (Al2O3) at 623 K.
• The reaction follows Saytzeff’s rule, where hydrogen is removed from the adjacent carbon with fewer hydrogen atoms.
• The general reaction is: R-CH2-CH2-OH → R-CH=CH2 + H2O.
6. Illustrate the 'Newman Projections' for the eclipsed and staggered conformations of ethane.
• Newman projections view the molecule head-on along the carbon-carbon (C-C) bond axis.
• The front carbon atom is represented by a point at the center of the projection.
• The rear carbon atom is represented by a large circle.
• In the staggered conformation, the H-atoms on the front and rear carbons are as far apart as possible (60° angle).
• In the eclipsed conformation, the H-atoms of the rear carbon are directly behind those of the front carbon (0° angle).
• These projections clearly show the spatial arrangement of H-atoms relative to one another during rotation.
7. Write a note on the 'Pyrolysis' or 'Cracking' of alkanes.
• Pyrolysis, also known as cracking, is the thermal decomposition of higher alkanes into smaller fragments.
• The process involves heating alkanes to high temperatures in the absence of air.
• This causes the C-C and C-H bonds to break, leading to a mixture of lower alkanes and alkenes.
• The specific products obtained depend on the temperature, pressure, and presence of any catalysts.
• Cracking is an essential industrial process used to produce gasoline and other valuable light hydrocarbons.
• For example, hexane can be cracked into a mixture of propane and propene or butane and ethene.
8. Explain the 'Directive Influence' of the hydroxyl group (-OH) in benzene.
• The hydroxyl group (-OH) is an ortho-para directing group in aromatic substitution.
• The oxygen atom in the -OH group possesses lone pairs of electrons that it can donate to the benzene ring via resonance.
• This electron donation increases the electron density specifically at the ortho and para positions.
• Because the ortho and para sites are electron-rich, they become the preferred locations for electrophilic attack.
• The -OH group also "activates" the ring, making it more reactive than benzene itself.
• Substitution reactions with phenols (hydroxyl derivatives) occur much faster and under milder conditions.
9. Describe the preparation of ethyne from Calcium Carbide.
• Ethyne (acetylene) is industrially prepared by the reaction of water with calcium carbide.
• Calcium carbide (CaC2) is a solid substance produced from lime and coke.
• When water (H2O) is added to calcium carbide, a vigorous reaction occurs at room temperature.
• The products of the reaction are ethyne gas and calcium hydroxide.
• The chemical equation is: CaC2 + 2H2O → C2H2 + Ca(OH)2.
• Ethyne produced this way is often used for welding or as a starting material for organic synthesis.
10. What is 'Geometrical Isomerism'? Why is it exhibited by alkenes?
• Geometrical isomerism is a type of stereoisomerism involving different spatial arrangements of atoms about a rigid bond.
• It is exhibited by alkenes because of the restricted rotation around the carbon-carbon double bond (C=C).
• In a single bond, atoms can rotate freely, but the π-bond in a double bond prevents this rotation.
• If each carbon of the double bond is attached to two different groups, two distinct spatial arrangements are possible.
• These arrangements are classified as 'Cis' (groups on the same side) and 'Trans' (groups on opposite sides).
• Geometrical isomers have the same connectivity but different physical and chemical properties.
11. Give the step-by-step mechanism for the generation of a Nitronium ion (NO2+) in benzene nitration.
• The generation of the electrophile is the first step in the nitration mechanism.
• Concentrated Nitric acid (HNO3) and concentrated Sulfuric acid (H2SO4) react together.
• Sulfuric acid, being a stronger acid, donates a proton (H+) to the nitric acid molecule.
• This protonation occurs on the oxygen atom of the -OH group in HNO3, forming a protonated nitric acid intermediate.
• The intermediate then loses a water molecule (H2O) to stabilize itself.
• The resulting species is the Nitronium ion (NO2+), which acts as the electrophile for the benzene ring.
12. Explain the 'Halogenation' of alkanes under UV light.
• Halogenation of alkanes is a substitution reaction where one or more hydrogen atoms are replaced by halogen atoms.
• The reaction (like chlorination) typically requires ultraviolet (UV) light or high temperatures to proceed.
• UV light provides the energy to break the halogen-halogen bond homolytically, creating free radicals.
• This initiates a chain reaction involving initiation, propagation, and termination steps.
• For example, methane reacts with chlorine to form chloromethane, dichloromethane, trichloromethane, and finally carbon tetrachloride.
• The extent of substitution can be controlled by varying the ratio of alkane to halogen.
13. Discuss the 'Resonance' structure of Benzene.
• Benzene is described as a resonance hybrid of two main Kekulé structures.
• In these structures, C-C single bonds and C=C double bonds are shown in alternating positions.
• However, experimental data shows that all six carbon-carbon bonds in benzene are of equal length.
• This equality is due to the uniform delocalization of π-electrons across the entire ring.
• The resonance hybrid is more stable than any single contributing structure, which is reflected in benzene's high resonance energy.
• A circle inside a hexagon is often used to represent this delocalized electron cloud.
14. Explain 'Ozonolysis' of Alkenes with a suitable example.
• Ozonolysis is a reaction where an alkene reacts with ozone (O3) to break the double bond completely.
• The first step forms an unstable intermediate called an ozonide.
• The ozonide is then cleaved using Zinc dust and water (Zn/H2O).
• This cleavage produces carbonyl compounds such as aldehydes or ketones.
• For example, ozonolysis of ethene yields two molecules of Formaldehyde (HCHO).
• This reaction is widely used in organic chemistry to locate the position of double bonds in unknown alkenes.
SECTION D: LONG ANSWERS (25 Marks)
Scoring: 10 points per answer (5 marks each)
1. Classify Hydrocarbons in detail with a flow chart and examples for each category.
• Hydrocarbons are broadly divided into two main categories: Open-chain (aliphatic) and Cyclic (closed-chain).
• Open-chain hydrocarbons consist of straight or branched chains of carbon atoms.
• They are further classified into Saturated (Alkanes) and Unsaturated (Alkenes and Alkynes).
• Alkanes contain only single bonds; an example is Propane (C3H8).
• Alkenes contain at least one double bond; an example is Ethene (C2H4).
• Alkynes contain at least one triple bond; an example is Ethyne (C2H2).
• Cyclic hydrocarbons are categorized into Alicyclic and Aromatic compounds.
• Alicyclic compounds are non-aromatic rings, such as Cyclobutane.
• Aromatic compounds (Arenes) are stable ring structures with delocalized π-electrons, such as Benzene.
• Aromatic compounds can be further divided into Benzenoid and Non-benzenoid types.
2. Explain in detail using both Sawhorse and Newman projections. Discuss their relative stability.
• Ethane (C2H6) has an infinite number of spatial arrangements called conformations due to rotation about the C-C bond.
• The two extreme forms are the Eclipsed and Staggered conformations.
• In the Sawhorse projection, the C-C bond is viewed at an angle as a long diagonal line.
• In the Newman projection, the molecule is viewed head-on along the C-C bond.
• In the Eclipsed form, hydrogen atoms on the two carbons are as close together as possible (0° torsion angle).
• In the Staggered form, hydrogen atoms are as far apart as possible (60° torsion angle).
• The staggered conformation is more stable than the eclipsed conformation.
• The eclipsed form is less stable due to torsional strain, which is the repulsion between electron clouds of C-H bonds.
• The energy difference between these two forms is known as the torsional energy barrier.
• At room temperature, ethane molecules have enough energy to overcome this barrier and rotate freely.
3. Write chemical equations for Hydrogenation, Halogenation, Hydrohalogenation, Hydration, and Hydroboration-Oxidation.
• Hydrogenation: Alkenes react with H2 in the presence of Ni, Pt, or Pd catalysts to form alkanes.
• Equation: CH2=CH2 + H2 → CH3-CH3 (Catalyst: Ni).
• Halogenation: Alkenes react with halogens in CCl4 to form vicinal dihalides.
• Equation: CH2=CH2 + Cl2 → Cl-CH2-CH2-Cl.
• Hydrohalogenation: Addition of haloacids (like HBr) follows Markovnikov's rule.
• Equation: CH3-CH=CH2 + HBr → CH3-CH(Br)-CH3.
• Hydration: Addition of water in the presence of H2SO4 catalyst yields alcohols.
• Equation: CH2=CH2 + H2O → CH3-CH2-OH.
• Hydroboration-Oxidation: Alkenes react with diborane followed by alkaline H2O2 to give primary alcohols.
• This results in an effective Anti-Markovnikov hydration of the alkene.
• These reactions demonstrate the high reactivity of the π-bond in alkenes.
4. Explain the general mechanism of Electrophilic Substitution in benzene, including the formation of the sigma-complex.
• The general mechanism for electrophilic substitution (SE) in benzene consists of three main steps.
• Step 1: Generation of the Electrophile: A reagent and a catalyst react to form a strong electrophile (E+).
• Step 2: Formation of the Carbocation Intermediate (σ-complex): The electrophile attacks the benzene ring, using π-electrons.
• This results in a cyclic carbocation called a σ-complex or arenium ion.
• In this complex, the carbon being attacked becomes sp3 hybridized, breaking aromaticity temporarily.
• The σ-complex is stabilized by resonance, with positive charge delocalized over ortho and para positions.
• Step 3: Removal of a Proton: A base in the reaction mixture attacks the sp3 carbon and removes a proton.
• This restores the aromaticity of the ring and completes the substitution.
• The final product is a substituted benzene ring, and the catalyst is regenerated.
5. Write the structures and IUPAC names of all five chain isomers of Hexane (C6H14).
• n-Hexane: A straight six-carbon chain. CH3-CH2-CH2-CH2-CH2-CH3.
• 2-Methylpentane: A five-carbon chain with a methyl group on the second carbon. CH3-CH(CH3)-CH2-CH2-CH3.
• 3-Methylpentane: A five-carbon chain with a methyl group on the third carbon. CH3-CH2-CH(CH3)-CH2-CH3.
• 2,2-Dimethylbutane: A four-carbon chain with two methyl groups on the second carbon. CH3-C(CH3)2-CH2-CH3.
• 2,3-Dimethylbutane: A four-carbon chain with methyl groups on the second and third carbons. CH3-CH(CH3)-CH(CH3)-CH3.
• These isomers share the same molecular formula but have different boiling points.
• Branching reduces the surface area, leading to weaker Van der Waals forces.
• Consequently, n-hexane has the highest boiling point among these isomers.
6. Explain Hydration, Polymerization to form Benzene, and formation of metal alkynides.
• Hydration: Alkynes add water in the presence of dilute H2SO4 and HgSO4 catalysts.
• Ethyne forms Acetaldehyde (CH3CHO) through an unstable enol intermediate.
• Polymerization: Three molecules of ethyne undergo cyclic polymerization in a red-hot iron tube.
• This reaction occurs at 873 K and produces Benzene (C6H6).
• Formation of Metal Alkynides: Terminal alkynes react with strong bases like Sodium in liquid ammonia.
• The acidic hydrogen atom at the triple bond is replaced by a sodium atom.
• Ethyne reacts with Na to form Sodium Ethynide (Na-C≡C-Na).
• This reaction proves the acidic nature of hydrogen atoms attached to sp-hybridized carbons.
7. Compare the directive influence and activating/deactivating effects of Ortho-Para directing groups versus Meta directing groups.
• Ortho-Para Directing Groups (e.g., -OH, -CH3): These groups direct incoming electrophiles to the ortho and para positions.
• Most ortho-para directors are "activating" because they donate electron density to the ring.
• This donation increases electron density specifically at ortho and para sites via resonance.
• Meta Directing Groups (e.g., -NO2, -COOH): These groups direct electrophiles to the meta position.
• They are "deactivating" because they withdraw electron density from the benzene ring.
• Withdrawal is strongest at ortho and para positions, making meta the relatively most electron-rich site.
• Halogens are unique: they are ortho-para directing but deactivating due to high electronegativity.
• Activating groups make the ring more reactive than benzene, while deactivating groups make it less reactive.
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