AQA Chemistry Unit 2: 14 Haloalkanes

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14.1 Haloalkane Introduction

  • General Formula: CnH2n+1X

Bond Polarity

  • Haloalkanes have C-X bonds, C-X bonds are polar due to halogens being more electronegative than carbon
  • Down the group the bonds get less polar

Physical Properties: Solubility

  • The polar bonds aren't polar enough to make haloalkanes soluble in water
  • Intermolecular forces are dipole-dipole attraction and van der Waal forces

Boiling Point

  • Increases with increased chain length and when you go down the halogen group
  • This is due to increased electron number which increases van der Waals forces
  • Melting point is lowered if the haloalkanes are branched
  • Haloalkanes have higher BP to alkanes due to higher molecular masses and being polar
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Haloalkanes Cont.

How Haloalkanes react: the reactivity of the C-X bond

  • Factors that determnine how readily the C-X bond reacts:
  • the C-X bond polarity and the C-X bond enthalpy

Bond Polarity

  • Carbon on the C-X bond is partial positive in charge due to the halogen being more electronegative so it is electron deficient
  • It can be attacked by reagents called nucleophiles
  • Nucleophile: an electron pair donor
  • The more positive the charge of C the more easily attacked by a nucleophile

Bond Enthalpies

  • Bond is weaker down the group due to the shared electrons in the C-X bond being further away from the Halogen nucleus
  • Experiments show reactivity increases as we go down the group so bond enthalpy is more important than bond polarity
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14.2 Nucleophilic Substitution

Nucleophiles:

  • Reagents that attack and form bonds with positively charged carbon atoms
  • It is a negatively charged ion or atom with a delta minus charge
  • It has a lone pair which it can use to form a covalent bond
  • It is a species that can donate its lone pair to an electron deficient carbon atom
  • Common: Hydroxide Ion, Ammonia, Cyanide Ion
  • When they replace a halogen in a haloalkane this in nucleophilic substitution

The Nucleophilic Substitution

  • A curly rrow is drawn from the lone pair on the nucleophile to the partially positive carbon
  • Another curly arrow is draw from the carbon-halogen bond to the halogen forming a halide ion (This is the leaving group)
  • Going down the halogen group the rate of reaction increases as the C-Halogen bond strength decreases
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Nucleophilic Substitution 2

Haloalkanes with Hydroxide Ions

  • Conditions: Room temperature, Ethanol solvent in which the haloalkane and ions mix
  • This reaction is called a hydrolysis reaction
  • It forms an alcohol

Haloalkanes with Cyanide Ions

  • Product: Nitrile
  • On a nitrile there is a triple bond between the carbon and the nitrogen

Haloalkanes with Ammonia

  • Conditions: Excess concentrated ammonia in ethanol, under pressure
  • Product: Primary amine
  • The primary amine is formed when the NH3 (ammonia) joins the removes the halogen from the haloalkane and joins its self to the carbon. Since ammonia is a neutral nucleophile a H+ is lost to form a neutral product. The H= then reacts with a second ammonia to form NH4+
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The Uses of Nucleophilic Substitution

  • Allow the introduction of new functional groups

Nitriles can form primary amines and carboxylic acid

Alcohols can form aldehyde which then forms carboxylic acid

Primary amines can form secondary and then tertiary amines

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14.3 Elimination Reactions

  • Elimination forms unsaturated alkenes

Hydroxide Ions acting as a Base

  • It can remove a H+ ion from a haloalkane
  • This is known as elimination
  • Conditions: The sodium/potassium hydroxide is dissolved in ethanol and heated
  • Their is no water present

The Mechanism

  • The base uses its lone pair to form a bond with one of the hydrogen adjacent to the          C-Halogen bond. The Hydrogen atoms are slightly delta positive
  • The electron between that hydrogen and the attached carbon forms a C=C
  • The halogen is the leaving group
  • Water and a halide ion are formed
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Elimination Reactions 2

Substitution or Elimination?

  • If the haloalkane structure is primary it will be use substitution
  • Tertiary haloalkanes use elimination
  • Secondary haloalkanes can use both methods but it depends on the conditions

Conditions

  • Substitution: Hydroxide ions are at room temperature and dissolved in water
  • Elimination: Hydroxide ions at a high temperature and dissolved in ethanol
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Chlorofluorocarbons (CFCs)

  • Contain no hydrogen
  • Very unreactive under normal conditions
  • Short chain are gases and used to be used in aerosols and refrigerants
  • Longer chains used as dry cleaning and degreasing solvents
  • CFC end up in the atmosphere and decompose into chlorine atoms
  • Chlorine atoms decompose the ozone: O3
  • CFCs are being reduced and replaced by hydrochlorofluorocarbos (HCFCs)
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14.4 Haloalkane Formation

  • A haloalkane with form when UV light shines onto a halogen and an alkane mixture
  • A substutition reaction takes place (Free Radical Substitution)

Chain Reactions

  • Has three stages: Initiation, Propagation and Termination

Initiation:

  • Break down of the Cl-Cl bond using UV lighting
  • Each Chlorine in the bond receives one electron each
  • This forms two free radicals with unpaired electrons 
  • Free radicals are highly reactive
  • C-H bonds don't break as the UV lighting doesn't have enough energy to break them
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Haloalkane Formation 2

Propagation:

  • Part 1: Chlorine free radical takes a hydrogen atom from the alkane to form hydrogen chloride and a methyl free radical
  • Part 2: The methyl free radical reacts with a chlorine molecule to form a chlorine free radical and a haloalkane
  • This step can take place thousands of times before the radicals are destroyed

Termination:

  • Free radicals are removed as they react together to form stable compounds with no unpaired electrons

Other Products

  • Longer chain alkanes will have many isomers formed as free radicals can replace any hydrogen atom
  • Chain reactions are not very useful as a mixture is produced, they need light and high temperatures
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Why Chain Reactions are Important

  • Too much ozone at ground level causes lung irritation and degrades paints
  • But it is vital in the higher atmosphere
  • It protects the Earth from harmful UV light exposure
  • UV light can cause skin cancer and affect the food chains of the oceans
  • Chlorine free radicals are formed from CFCs in UV lighting
  • These chlorine radicals attack the ozone forming the free radical ClO and Oxygen
  • This free radical can then go on to attack osone forming oxygen and chlorine free radicals
  • Adding the equations together it shows ozone breaks down to oxygen and chlorine acts as a catalyst

Cl + O3 -> ClO + O2

ClO + O3 -> 2O2 + Cl

2O3 -> 3O2

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