Halogenoalkanes with ozone depletion

What is the functional group for the halogenoalkanes homologous series?

-C-X (X=F,Cl,Br,I)

One way in which halogenoalkanes can be formed from an alkane

In the presence of UV light (Free radical substitution)

What does primary, secondary and tertiary mean in relation to halogenoalkane structure?

The number of carbon atoms bonded to the carbon atom that the halogen is bonded to.

Physical properties of halogenoalkanes

Short-chained hydrocarbons are only slightly soluble in water

Polar molecules

Can interact with polar water molecules but attractive forces are not as strong as the hydrogen bonding present in water

Two types of bond breaking

Homolytic bond breaking/fission

Heterolytic bond breaking/fission

What is homolytic bond breaking/fission?

The breaking of a covalent bond to form atoms with unpaired electrons (free radicals)

What is heterolytic bond breaking/fission?

The breaking of bond so that the pair of electrons both go to one atom resulting in the formation of two ions. (one ion will be -1 charged, the other +1 charged)

What is heterolytic bond breaking represented by?

A curly arrow showing the movement of the pair of electrons.

What is a substitution reaction?

Reactions in which one group replaces another in a molecule

What are addition reactions?

Reactions in which one or more groups are added onto a molecule to give a single product.

What are elimination reactions?

Reactions in which one or more groups are removed from a molecule.

3 types of reagent

Free radicals

Nucleophiles

Electrophiles

What are free radicals?

Species which have an odd number of electrons, leaving one electron unpaired.

Free radical charge

No charge

How are free radicals shown in an equation?

As a dot next to the formula.

What are nucleophiles?

Species which donate a pair of electrons. They have a lone pair of electrons which are donated to form a new covalent bond.

What are nucleophiles attracted to?

Areas of electron deficiency

Charge on a nucleophile

Often negatively charged.

What do curly arrows show in a reaction mechanism?

The movement of a pair of electrons. They can also show the formation of new covalent bonds and the breaking of existing covalent bonds.

If a curly arrow points from a bond, what is happening?

A bond is being broken.

If a curly arrow points from an atom, what is happening?

A bond is being formed.

What happens during a nucleophilic substitution reaction?

A nucleophile replaces another group in a molecule.

How can you confirm the presence of halide ions in the products of a reactant?

Add silver nitrate and a precipitate (Silver halide) will form

(Chloride = white)

(Bromide = cream)

(Iodide = yellow)

What is the leaving group in a reaction

The atom/group of atoms that are substituted

Why does the rate at which halogenoalkanes react differ due to the halogen?

Halogens with higher bond enthalpy and a shorter bond are least reactive and take longer to react.

How does bond length affect reactivity?

The longer the bond length, the further the shared pair of electrons are from the nucleus, therefore, there is a weaker force of attraction between the nucleus and the shared pair of electrons (weaker covalent bond).

How is a nitrile formed from reaction with a halogenoalkane?

Nucleophilic substitution with cyanide

Equation for the formation of nitrile from a halogenoalkane

R-X + KCN → R-CN +KX

Cyanide formula

:C^-N

Conditions for formation of nitrile

Aqueous ethanol solvent, heat under reflex (constant heat to prevent evaporation)

How can the resulting nitrile from nucleophilic substitution be converted into a carboxylic acid?

Reaction with dilute acid or alkali

How is a primary amine made from a halogenoalkane?

Nucleophilic substitution with ammonia

Why does the reaction to form amines happen heated in a sealed vessel?

Ammonia is a gas so escape is prevented.

First step to form amine through nucleophilic substitution

Halogenoalkane + ammonia → R-NH₃ + halide leaving group

Second step to form amine through nucelophilic subtitution

R-NH₃ + ammonia + halide leaving group → primary amine + ammonium + halide leaving group

What happens if the amine reaction is carried out using excess ammonia?

The primary amine will be made and no further substitution will occur

What happens if the amine reaction is carried out using excess halogenoalkane?

A further reaction may occur to produce a secondary amine and then a tertiary amine etc…

Conditions required for nucleophilic substitution to make an alcohol

Water molecule

First step to form an alcohol through nucleophilic substitution

Halogenoalkane + water → (1 extra hydrogen on alcohol) + Halide ion

Second step to form an alcohol through nucleophilic substitution

Extra hydrogen alcohol + Halide ion → Alcohol + hydrogen halide (hydrogen is lost)

What happens during a reaction with an alcoholic alkali?

Elimination reaction and an alkene is formed

What type of reactions do aqueous solutions promote?

Nucleophilic Substitution reactions

What type of reactions do alcoholic/ethanolic solutions promote?

Elimination reactions

Conditions required for elimination reaction with an alcoholic alkali

OH^- ion (potassium hydroxide)

Elimination reaction equation with an alcoholic alkali

Halogenoalkane + OH ion → Alkene + Halide ion + Water

What happens to the reactants during an elimination reaction

1. OH^- ion bonds to hydrogen ion adjacent to the carbon with the halogen

2. Electron pair becomes double bond between the carbon atom that has lost hydrogen and the carbon atom that has lost the halogen.

3. Halide ion is removed

Why do we need ozone in the atmosphere?

Ozone in the upper atmosphere (stratosphere) protects Earth from harmful UV radiation by absorbing it

What does ozone do with the UV it absorbs? (3 equations)

O₃ → O₂ + O^_O

O₂ → O^_O + O^_O

O₂ + O^_O → O₃

How does the concentration of ozone remain constant?

In unpolluted air, the 3 reactions combine to form an equilibrium.

Compounds implicated in the destruction of the ozone layer

CFCs (Chlorofluorocarbons)

Nitrogen oxides (Produced high in the atmosphere in jet engines)

How did CFCs contribute to the hole in the ozone layer?

The C-Cl bond breaks when UV, therefore, a chlorine free radical (atom) is formed which catalyses the breakdown of ozone.

What were CFCs used for?

Refrigerants

Aerosol propellants

Cleaning solvents

Reaction between chlorine free radical and ozone

Cl_^O + O₃ → O₂ + ClO_^O

Reaction to regenerate Cl free radicals

ClO_^O + O₃ → Cl_^O + 2O₂

Overall equation for the decomposition of ozone

2O₃ → 3O₂