Halogenoalkanes Overview
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Questions and Answers

What is the main process by which halogenoalkanes can be formed from alkanes, and what conditions are necessary for this reaction?

Halogenoalkanes can be formed from alkanes through free-radical substitution in the presence of UV light.

Describe the steps involved in the free-radical substitution reaction when an alkane reacts with a halogen.

The reaction occurs in three steps: initiation, propagation, and termination.

What observation is made during the reaction of an alkene with bromine, and what does it indicate?

A red-brown color of bromine vapor is liberated during the reaction, indicating that bromine is being consumed.

Provide an example of an alkene reacting with fluorine and the product formed.

<p>An example is propene reacting with fluorine to form 1,2-difluoropropane.</p> Signup and view all the answers

What is the general reaction mechanism for the electrophilic addition of alkenes with halogens?

<p>Alkenes react with halogens to form halogenoalkanes in a straightforward addition reaction.</p> Signup and view all the answers

What type of carbocation is formed during the SN1 reaction and why is it considered stable?

<p>A tertiary (3°) carbocation is formed during the SN1 reaction, considered stable due to the inductive effect from three electron-donating alkyl groups.</p> Signup and view all the answers

What is the primary product of the reaction between bromoethane and excess ammonia?

<p>The primary product is ethyl amine, CH3CH2NH2.</p> Signup and view all the answers

Explain the effect of concentration on the rate of SN1 reactions with 2-chloro-2-methylpropane.

<p>Higher concentration of 2-chloro-2-methylpropane increases the reaction rate, while lower concentration decreases it.</p> Signup and view all the answers

Describe the product formation in the reaction of bromoethane with cyanide ion.

<p>The reaction of bromoethane with cyanide ion forms propane nitrile and KBr.</p> Signup and view all the answers

In an elimination reaction, what is the role of the hydroxide ion (OH-) from sodium or potassium hydroxide?

<p>The hydroxide ion acts as a base, accepting a proton (H+) to form water.</p> Signup and view all the answers

In the SN2 mechanism, what determines the rate of the reaction?

<p>The rate of the SN2 reaction depends on the concentrations of both the halogenoalkane and the nucleophile.</p> Signup and view all the answers

Write the overall equation for an elimination reaction of 1-bromo propane using sodium hydroxide in ethanol.

<p>1-bromo propane + NaOH (in ethanol) → propene + H2O + NaBr.</p> Signup and view all the answers

What is the significance of bromine's electronegativity in bromoethane's reaction with hydroxide?

<p>Bromine's higher electronegativity creates a partial positive charge on carbon, facilitating nucleophilic attack by hydroxide.</p> Signup and view all the answers

Why does pent-2-ene become the major product in the elimination reaction of 2-chloro pentane?

<p>Pent-2-ene is the major product because it has two alkyl groups attached to the double bond, stabilizing it.</p> Signup and view all the answers

What type of amine is formed when 2-chloro propane reacts with excess ammonia?

<p>A secondary amine, specifically isopropyl amine, CH3CH(NH2)CH3, is formed.</p> Signup and view all the answers

What type of reaction mechanism does 2-chloropropane undergo when reacting with cyanide?

<p>2-chloropropane undergoes an SN1 reaction mechanism with cyanide due to the formation of a stable carbocation.</p> Signup and view all the answers

Describe the initial step involved in an SN2 reaction mechanism with bromoethane and cyanide ion.

<p>The cyanide ion attacks the carbon atom bonded to the bromine, leading to the breaking of the C-Br bond.</p> Signup and view all the answers

What role does the slow step play in the SN2 reaction mechanism?

<p>The slow step, or rate-determining step, involves the simultaneous breaking and forming of bonds between the halogenoalkane and nucleophile.</p> Signup and view all the answers

What are the expected products when treating 2-bromost 2-methyl propane with excess ammonia?

<p>The expected product is the tertiary amine, 2-methyl-2-aminopropane, along with HBr.</p> Signup and view all the answers

In the context of SN2 reactions, how does the structure of the halogenoalkane affect the reaction pathway?

<p>Primary halogenoalkanes, like bromoethane, favor the SN2 pathway due to lower steric hindrance compared to secondary or tertiary halogenoalkanes.</p> Signup and view all the answers

What distinguishes the electrophilic nature of the carbon atom in haloalkanes during reactions?

<p>The carbon atom becomes partially positive due to the electronegativity of the halogen (like bromine).</p> Signup and view all the answers

What defines a halogenoalkane and how is it formed?

<p>A halogenoalkane is an organic compound formed when a hydrogen atom in an alkane is replaced by a halogen.</p> Signup and view all the answers

What is the general formula for halogenoalkanes?

<p>The general formula for halogenoalkanes is CnH2n+1X, where X represents any halogen.</p> Signup and view all the answers

Explain the physical state change of halogenoalkanes with increasing carbon atoms.

<p>Halogenoalkanes transition from gas to liquid as the number of carbon atoms increases.</p> Signup and view all the answers

How does the polarity of the C-X bond affect the physical properties of halogenoalkanes?

<p>The polarity of the C-X bond leads to dipole-dipole attractions, increasing melting and boiling points.</p> Signup and view all the answers

Which halogenoalkane has the highest melting and boiling points and why?

<p>Iodoethane has the highest melting and boiling point due to its stronger dispersion forces.</p> Signup and view all the answers

Distinguish between primary and secondary halogenoalkanes.

<p>A primary halogenoalkane has one alkyl group attached to the carbon containing the halogen.</p> Signup and view all the answers

What role do the electronegativities of elements play in determining bond polarity in halogenoalkanes?

<p>Electronegativity differences result in bond polarity, with higher differences creating more polar bonds.</p> Signup and view all the answers

Describe the type of intermolecular forces present in halogenoalkanes.

<p>Halogenoalkanes exhibit dispersion forces and permanent dipole-dipole attractions due to polar C-X bonds.</p> Signup and view all the answers

What is the product of hydrolyzing a halogenoalkane with water?

<p>The product is an alcohol and HX.</p> Signup and view all the answers

Which halogenoalkane undergoes hydrolysis the fastest and why?

<p>Iodoalkanes undergo hydrolysis the fastest due to the weaker C-I bond, which is easier to break.</p> Signup and view all the answers

How does the rate of hydrolysis differ among tertiary, secondary, and primary halogenoalkanes?

<p>Tertiary halogenoalkanes hydrolyze faster than secondary, which hydrolyze faster than primary halogenoalkanes.</p> Signup and view all the answers

What role do CFCs play in ozone depletion?

<p>CFCs deplete ozone by releasing chlorine free radicals that react with ozone, breaking down O3 into O2.</p> Signup and view all the answers

What is the main disadvantage of using CFCs as refrigerants?

<p>The main disadvantage is their contribution to ozone layer depletion, which leads to increased UV radiation exposure.</p> Signup and view all the answers

In the context of hydrolysis, why is water a less effective nucleophile than OH-?

<p>Water is a neutral molecule and a less effective nucleophile compared to the negatively charged OH- ion.</p> Signup and view all the answers

What is the overall equation for ozone depletion caused by chlorine free radicals?

<p>The overall equation is $2 O_3 ightarrow 3 O_2$.</p> Signup and view all the answers

What alternative can be used instead of CFCs to mitigate environmental issues?

<p>Hydrofluorocarbons (HFCs) can be used as they do not contain chlorine and are less harmful to the ozone layer.</p> Signup and view all the answers

What is the primary product formed when propene reacts with hydrogen fluoride according to Markovnikov's rule?

<p>The primary product is 1-fluoropropane (CH3CHFCH2F).</p> Signup and view all the answers

Explain why aqueous hydrogen halides do not react effectively with alcohol to produce halogenoalkanes.

<p>Aqueous hydrogen halides provide a wet environment which does not allow for the in-situ preparation of dry hydrogen halides, making them less effective nucleophiles.</p> Signup and view all the answers

What are the products formed when an alcohol reacts with phosphorus trichloride (PCl3)?

<p>The products formed are halogenoalkane, hydrogen chloride (HCl), and phosphorus oxychloride (POCl3).</p> Signup and view all the answers

State the role of sulfur dichloride oxide (SOCl2) in the conversion of alcohol to halogenoalkane.

<p>SOCl2 acts as a reagent that facilitates the substitution of -OH group in alcohols to form halogenoalkanes.</p> Signup and view all the answers

What should be prepared in-situ to improve the nucleophilicity in the reaction between alcohol and hydrogen halide?

<p>Dry hydrogen halide should be prepared in-situ to enhance its nucleophilicity.</p> Signup and view all the answers

What major observation can be made when HCl gas is produced in reactions involving alcohol and phosphorus trichloride?

<p>Mist fumes of HCl gas are observed during the reaction.</p> Signup and view all the answers

What is the effect of heat in the reaction of alcohol with PCl3 in the presence of phosphorus?

<p>Heat is applied to facilitate the reaction and increase the substitution efficiency of -OH group with chlorine.</p> Signup and view all the answers

Which hydrogen halide reacts with propene to yield 2-chloro propane as the major product?

<p>Hydrogen chloride (HCl) reacts with propene to yield 2-chloro propane as the major product.</p> Signup and view all the answers

Flashcards

Halogenoalkane

An organic compound formed by replacing one or more hydrogen atoms in an alkane with a halogen atom.

CnH2n+1X

The general formula for halogenoalkanes, where 'n' represents the number of carbon atoms and 'X' represents a halogen.

Dipole-dipole attraction

The intermolecular force present in halogenoalkanes, caused by the unequal sharing of electrons between the carbon and halogen atoms, creating partial positive and negative charges.

Physical State of Halogenoalkanes

The physical state of halogenoalkanes changes from gas to liquid as the number of carbon atoms increases, leading to increased London Dispersion Forces.

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Solubility of Halogenoalkanes

Halogenoalkanes are slightly soluble in water but dissolve readily in organic solvents. This is due to the non-polar nature of the C-X bond.

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Melting and Boiling Points of Halogenoalkanes

Halogenoalkanes with a larger number of carbon atoms have higher melting and boiling points due to increased strength of dispersion force.

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Primary Halogenoalkane

A halogenoalkane where the carbon atom attached to the halogen has only one alkyl group attached to it.

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Polarity of C-X Bond

The strength of the dipole-dipole attraction in halogenoalkanes depends on electronegativity difference between carbon and halogen atoms. C-F bond has the highest polarity due to greatest electronegativity difference.

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Why are tertiary carbocations more stable?

Tertiary carbocations are extremely stable. This is because the carbon bearing the positive charge is attached to three electron-donating alkyl groups. The inductive effect reduces the positive charge density on the cation.

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Describe the SN1 reaction mechanism.

The SN1 reaction involves a two-step mechanism. The first step is the slow step, where the leaving group departs to form a carbocation. The second step is fast, where the nucleophile attacks the carbocation to form the product.

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What factors influence the rate of SN1 reaction?

The rate of an SN1 reaction depends only on the concentration of the alkyl halide and the rate is independent of the nucleophile.

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Explain the SN2 reaction mechanism.

The SN2 reaction is a one-step process where the nucleophile attacks the alkyl halide from the backside. The nucleophile displaces the leaving group in a single step.

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What affects the rate of SN2 reaction?

The rate of an SN2 reaction depends on the concentration of both the alkyl halide and the nucleophile.

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What is a primary (1°) halogenoalkane?

A primary (1°) halogenoalkane is a compound where the carbon atom bonded to the halogen is also bonded to one other carbon atom.

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What is a tertiary (3°) halogenoalkane?

A tertiary (3°) halogenoalkane is a compound where the carbon atom bonded to the halogen is also bonded to three other carbon atoms.

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What happens when a halogenoalkane reacts with KCN?

The reaction of halogenoalkanes with KCN in ethanol increases the chain length by one carbon atom. This reaction forms an alkane nitrile.

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SN2 Reaction

A reaction where a nucleophile attacks an electrophilic carbon atom, replacing a leaving group. The nucleophile and leaving group approach from opposite sides of the electrophilic carbon, resulting in an inversion of configuration.

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SN1 Reaction

A reaction where a nucleophile attacks an electrophilic carbon atom, replacing a leaving group. The nucleophile and leaving group approach from the same side of the electrophilic carbon, resulting in retention of configuration.

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Nucleophile

A molecule that is capable of donating an electron pair to form a new covalent bond.

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Leaving Group

A molecule that can leave a reaction as a stable ion or molecule.

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Elimination Reaction

A reaction where a halogen atom is removed from a halogenoalkane, forming a double bond (alkene).

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Amine

A compound that contains a nitrogen atom with one or more alkyl groups attached.

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Reaction of Halogenoalkanes with Ammonia

The formation of a new C-N bond by reacting a halogenoalkane with ammonia (NH3) in ethanol.

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Zaitsev's Rule

The regioselectivity of elimination reactions, favoring the formation of the alkene with the more substituted double bond.

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Free radical substitution

A type of chemical reaction involving the replacement of one or more hydrogen atoms in an alkane with halogen atoms. This reaction typically requires the presence of UV light to initiate the process.

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Electrophilic addition of an alkene with a halogen (Halogenation)

A chemical reaction where an alkene reacts with a halogen, resulting in the addition of the halogen across the double bond, forming a halogenoalkane.

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Chloroalkane

A halogenoalkane formed by the reaction of an alkene with chlorine.

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Bromoalkane

A halogenoalkane formed by the reaction of an alkene with bromine.

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Iodoalkane

A halogenoalkane formed by the reaction of an alkene with iodine.

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Hydrolysis of Halogenoalkanes

A reaction where a halogenoalkane reacts with water to form an alcohol and a hydrogen halide.

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Effect of Halogen Atom Size

The rate of hydrolysis increases as the size of the halogen atom increases. This is because the C-X bond becomes weaker as the halogen atom gets larger.

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Effect of Carbon Chain Structure

Tertiary halogenoalkanes hydrolyze faster than secondary halogenoalkanes, which hydrolyze faster than primary halogenoalkanes. This is due to the stability of the carbocation intermediate.

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Chlorofluorocarbons (CFCs)

Chlorofluorocarbons (CFCs) are compounds that contain chlorine and fluorine atoms. They were widely used as refrigerants, propellants, and solvents.

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CFCs and Ozone Depletion

CFCs contribute to the depletion of the ozone layer because they release chlorine atoms, which catalyze the breakdown of ozone molecules.

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Hydrofluorocarbons (HFCs)

Hydrofluorocarbons (HFCs) are a type of refrigerant that does not contain chlorine. They are used as a replacement for CFCs.

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Reduction

A chemical reaction where a molecule gains an electron, resulting in a negative charge.

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Oxidation

A chemical reaction where a molecule loses an electron, resulting in a positive charge.

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Markovnikov's Rule

An alkene reacts with hydrogen halide to form a halogenoalkane. The hydrogen atom from the hydrogen halide adds to the carbon with more hydrogen atoms, while the halide attaches to the carbon with fewer hydrogen atoms.

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Alcohol to Halogenoalkane: Reaction 1

A reaction where an alcohol is converted to a halogenoalkane using hydrogen halide.

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Alcohol to Halogenoalkane: Reaction 2

A reaction where an alcohol is converted to a halogenoalkane using thionyl chloride (SOCl2).

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Alcohol to Halogenoalkane: Reaction 3

A reaction where an alcohol is converted to a halogenoalkane using phosphorus(V) chloride (PCl5).

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Alcohol to Halogenoalkane: Reaction 4

A reaction where an alcohol reacts with phosphorus trichloride (PCl3) in the presence of heat to form a halogenoalkane.

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Testing for -OH Group

A reaction that can be used to test the presence of the -OH functional group in an alcohol. This reaction involves reacting the alcohol with phosphorus(V) chloride (PCl5), which produces misty fumes of hydrogen chloride (HCl).

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Reaction with Hydrogen Halide

The reaction between an alkene and a hydrogen halide.

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Substitution Reaction

A reaction that involves replacing an atom or group of atoms in a molecule with another atom or group.

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Study Notes

Halogenoalkanes

  • Halogenoalkanes are organic compounds where a hydrogen atom in an alkane is replaced by a halogen atom (X).
  • General formula for halogenoalkanes is CnH2n+1X (where X = any halogen).

Physical Properties

  • The physical state changes from gases to liquids as the number of carbon atoms increases.
  • They are slightly soluble in water but soluble in organic solvents.
  • Melting and boiling points increase with the increasing number of carbon atoms due to increased strength of dispersion forces.
  • Electronegativity difference between carbon and the halogen atom results in permanent dipole-dipole attraction as an intermolecular force.

Classification

  • Primary, Secondary, Tertiary halogenoalkanes are based on the number of alkyl groups attached to the carbon atom containing the halogen.
    • Primary halogenoalkane: one alkyl group attached to the carbon with the halogen.
    • Secondary halogenoalkane: two alkyl groups attached to the carbon with the halogen.
    • Tertiary halogenoalkane: three alkyl groups attached to the carbon with the halogen.

Chemical Properties

  • Substitution Reaction: Halogenoalkanes undergo nucleophilic substitution (SN1 or SN2) reactions when reacting with nucleophiles like OH⁻, CN⁻, and NH₃.
    • SN1 mechanism: favoured by tertiary halogenoalkanes
    • SN2 mechanism: favoured by primary halogenoalkanes
  • Hydrolysis: Halogenoalkanes react with water (hydrolysis) to form alcohols.
    • Primary and secondary halogenoalkanes react with water via SN2 reactions.
    • Tertiary halogenoalkanes react with water via SN1 reactions.
  • Elimination Reaction: Halogenoalkanes react with ethanolic KOH or ethanolic NaOH to produce alkenes.
  • Reaction with KCN (in ethanol): This reaction increases the chain length of the compound by one carbon atom.
  • SN1 Reaction Mechanism (with CN⁻): The reaction with CN⁻ involves a carbocation intermediate.
  • SN2 Reaction Mechanism (with OH⁻): The reaction with OH⁻ involves a single step where the hydroxyl group attacks the carbon directly.
  • Reaction with NH₃ (in ethanol): This reaction yields amines, depending on the excess of ammonia.
  • Free Radical Substitution: This reaction involves alkanes and halogens in the presence of ultraviolet light. Reaction occurs in initiation, propagation and termination steps.
  • Electrophilic Addition of Alkene with Halogen: Alkenes react with halogens (Cl₂, Br₂, I₂) to form halogenoalkanes.
  • Electrophilic Addition of Alkene with Hydrogen Halide: Alkenes react with hydrogen halide (HCl, HBr, HI) to form halogenoalkanes, following Markovnikov's rule (the halogen adds to the carbon with more hydrogen atoms).

Uses

  • Halogenoalkanes are used as anaesthetics, refrigerants, and in the production of polymers (e.g., PTFE).

Problems with CFCs

  • CFCs deplete the ozone layer, which allows harmful UV rays from the sun to reach the Earth's surface, increasing skin cancer risks.
  • CFCs contain carbon-chlorine bonds that are broken down by UV radiation, creating chlorine radicals that catalyze the breakdown of ozone.
  • HFCs are alternatives to CFCs to prevent ozone layer depletion.

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Description

This quiz explores the properties, classifications, and characteristics of halogenoalkanes. Participants will learn about the physical states, solubility, and intermolecular forces of these organic compounds. Additionally, the classification into primary, secondary, and tertiary halogenoalkanes will be covered.

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