HaloAlkanes OCR A Level Chem

What is a haloalkane?

A compound in which a halogen atom (F, Cl, Br, or I) has replaced at least one of the hydrogen atoms in an alkane chain. The functional group is the carbon-halogen bond (C-X). The halogen atom is more electronegative than carbon making the bond polar.

How are haloalkanes named?

Using the prefix fluoro-, chloro-, bromo-, or iodo- depending on the halogen present. The position of the halogen atom on the alkane chain must be indicated with a number. If more than one halogen is present they are listed alphabetically. Example: 2-chloro-3-iodohexane.

What is a primary haloalkane?

The carbon bearing the halogen is attached to ONE alkyl group. The carbon with the halogen has only one carbon-carbon bond. Example: 1-chloropropane (CH₃CH₂CH₂Cl).

What is a secondary haloalkane?

The carbon bearing the halogen is attached to TWO alkyl groups. The carbon with the halogen has two carbon-carbon bonds. Example: 2-bromopentane (CH₃CHBrCH₂CH₂CH₃).

What is a tertiary haloalkane?

The carbon bearing the halogen is attached to THREE alkyl groups. The carbon with the halogen has three carbon-carbon bonds. Example: 2-bromo-2-methylbutane.

Why are haloalkanes polar?

The carbon-halogen bond is polar because the halogen atom is more electronegative than carbon. This creates a partial positive charge (δ+) on the carbon atom and a partial negative charge (δ-) on the halogen atom. The electron deficient carbon atom attracts nucleophiles.

What is a nucleophile?

An electron pair donor. Must possess a lone pair of electrons which can be donated. Attracted to electron deficient regions (δ+ atoms). Can be negatively charged (like OH⁻) or neutral (like NH₃ and H₂O).

Give examples of nucleophiles.

Hydroxide ion (:OH⁻), ammonia (:NH₃), water (H₂O:), and cyanide ion (:CN⁻). All have lone pairs of electrons.

What is the difference between a nucleophile and an electrophile?

A nucleophile is an electron pair donor (Lewis base). An electrophile is an electron pair acceptor (Lewis acid). Nucleophiles are attracted to positive regions. Electrophiles are attracted to negative regions.

What is nucleophilic substitution?

A reaction where a nucleophile donates an electron pair to replace a halogen atom in a haloalkane. The polar carbon-halogen bond breaks by heterolytic fission. The halogen leaves as a halide ion.

What are the reagents and conditions for reaction of haloalkane with aqueous alkali?

Reagents: NaOH(aq) or KOH(aq). Conditions: Heat under reflux. Nucleophile: OH⁻. Product: Alcohol. Type: Nucleophilic substitution.

Give the equation for 1-chlorobutane reacting with aqueous NaOH.

Full equation: CH₃CH₂CH₂CH₂Cl + NaOH → CH₃CH₂CH₂CH₂OH + NaCl. Ionic equation: CH₃CH₂CH₂CH₂Cl + OH⁻ → CH₃CH₂CH₂CH₂OH + Cl⁻. Product: butan-1-ol.

Describe the mechanism for nucleophilic substitution of a haloalkane with OH⁻.

Step 1: Curly arrow from lone pair on OH⁻ to δ+ carbon of C-Cl bond. Step 2: Dipole shown on C-Cl bond (Cδ+ and Clδ-). Step 3: Curly arrow from C-Cl bond to Cl atom. Step 4: Products are alcohol and Cl⁻ ion.

[DIAGRAM: Nucleophilic substitution mechanism with OH⁻ - show curly arrows, dipole Cδ+ and Clδ-, products]

What type of bond fission occurs in nucleophilic substitution?

Heterolytic fission. Both electrons in the carbon-halogen bond move to the more electronegative halogen atom. This forms two ions: a carbocation and a halide ion.

What are the reagents and conditions for reaction of haloalkane with water?

Reagent: H₂O. Conditions: Heat under reflux. Nucleophile: H₂O. Products: Alcohol and HCl. Type: Hydrolysis. Equation: R-X + H₂O → R-OH + HCl.

Give the equation for 1-chlorobutane reacting with water.

CH₃CH₂CH₂CH₂Cl + H₂O → CH₃CH₂CH₂CH₂OH + HCl. This is a hydrolysis reaction. Product is butan-1-ol.

What is hydrolysis?

A chemical reaction involving water or an aqueous solution that causes the breaking of a bond. The molecule is split into two products. In haloalkane hydrolysis the C-X bond is broken.

What are the reagents and conditions for reaction of haloalkane with cyanide ions?

Reagents: NaCN or KCN. Conditions: Dissolved in ethanol (NOT water). Nucleophile: CN⁻. Product: Nitrile. The carbon chain length increases by one carbon.

Give the equation for 1-chlorobutane reacting with NaCN.

Full equation: CH₃CH₂CH₂CH₂Cl + NaCN → CH₃CH₂CH₂CH₂CN + NaCl. Ionic equation: CH₃CH₂CH₂CH₂Cl + CN⁻ → CH₃CH₂CH₂CH₂CN + Cl⁻. Product: pentanenitrile.

Why is ethanol used as the solvent with cyanide ions?

Water would react with the haloalkane in a hydrolysis reaction producing the alcohol instead of the nitrile. Ethanol is a poor nucleophile so it does not compete with CN⁻.

Why is the reaction with cyanide ions important?

A carbon-carbon bond is formed. The carbon chain length is extended by one carbon atom. The functional group formed is called a nitrile (-C≡N).

What is the reactivity trend of haloalkanes?

Fluoroalkane is least reactive, then chloroalkane, then bromoalkane, then iodoalkane is most reactive. Reactivity increases as the C-X bond gets weaker down Group 7.

Why are iodoalkanes the most reactive haloalkanes?

The carbon-iodine bond is the weakest with the lowest bond enthalpy. Less energy is required to break the C-I bond. Therefore iodoalkanes react fastest.

Why are fluoroalkanes the least reactive haloalkanes?

The carbon-fluorine bond is the strongest with the highest bond enthalpy. More energy is required to break the C-F bond. Therefore fluoroalkanes react slowest.

What are the bond enthalpy values for carbon-halogen bonds?

C-F = +485 kJ mol⁻¹. C-Cl = +338 kJ mol⁻¹. C-Br = +285 kJ mol⁻¹. C-I = +238 kJ mol⁻¹. Weaker bonds break more easily so react faster.

What is the order of C-X bond strength?

C-F > C-Cl > C-Br > C-I. C-F is strongest, C-I is weakest. Bond strength decreases down Group 7.

Describe the experiment to measure the rate of hydrolysis of primary haloalkanes.

Reagents: Aqueous silver nitrate (AgNO₃). Conditions: Ethanol solvent and heat in a water bath at 50°C. Nucleophile: Water (H₂O). The Ag⁺ ions precipitate any halide ions released.

What is the first reaction in the hydrolysis experiment?

CH₃CH₂CH₂CH₂Cl + H₂O → CH₃CH₂CH₂CH₂OH + HCl(aq). HCl dissociates into H⁺(aq) and Cl⁻(aq). Rate depends on C-X bond strength.

What is the second reaction in the hydrolysis experiment?

Ag⁺(aq) + Cl⁻(aq) → AgCl(s). Silver chloride forms as a white precipitate. Appearance of precipitate indicates hydrolysis has occurred.

Describe the method for the hydrolysis experiment.

Step 1: Incubate haloalkane in water bath at 50°C. Step 2: Incubate ethanol and silver nitrate in water bath at 50°C. Step 3: Mix liquids together and replace in water bath. Step 4: Measure time taken for precipitate to form.

How is the rate of hydrolysis calculated?

Rate of hydrolysis = 1 ÷ time taken for precipitate to form. A shorter time means a faster rate of hydrolysis.

What are the observations in the hydrolysis experiment?

1-chlorobutane: white precipitate (slowest). 1-bromobutane: cream precipitate. 1-iodobutane: yellow precipitate (fastest).

[DIAGRAM: Table of hydrolysis experiment results - haloalkane, precipitate colour, time taken, rate]

Why does 1-iodobutane hydrolyse faster than 1-chlorobutane?

The C-I bond is weaker than the C-Cl bond. Less energy is needed to break the C-I bond. Therefore the rate of hydrolysis is faster for 1-iodobutane.

What does the rate of hydrolysis depend upon?

The rate depends upon the strength of the carbon-halogen bond. Weaker bonds break more easily and lead to faster rates of hydrolysis.

What is a leaving group?

The group that departs with the electron pair in heterolytic fission. In haloalkanes this is the halide ion (X⁻). Iodide is a good leaving group. Fluoride is a poor leaving group.

Why is iodide a good leaving group?

Iodide ion is large and polarisable so it can stabilise the negative charge. The C-I bond is weak so it breaks readily. Iodide is a weak base.

Why is fluoride a poor leaving group?

Fluoride ion is small and has a high charge density so it is unstable. The C-F bond is very strong. Fluoride is a strong base.

What are the uses of haloalkanes?

To make polymers. PVC (poly(chloroethene)) used for children's toys, drain-pipes, window frames and packaging. PTFE (Teflon) used for non-stick pans.

[DIAGRAM: PVC repeat unit structure - poly(chloroethene)]

[DIAGRAM: PTFE repeat unit structure - poly(tetrafluoroethene)]

What are CFCs?

Chlorofluorocarbons. Compounds containing chlorine, fluorine and carbon only. They are inert, unreactive, non-toxic and non-flammable. Examples: CFCl₃ and CF₂Cl₂.

What were CFCs used for?

Used as refrigerants, aerosol propellants and solvents for dry cleaning. No longer used because they deplete the ozone layer.

What properties of CFCs made them useful?

Inert, non-toxic, non-flammable, volatile with low boiling points, stable in the lower atmosphere.

Why are CFCs volatile?

They have weak induced dipole-dipole intermolecular forces. This gives them low boiling points so they evaporate easily.

Why are CFCs unreactive in the troposphere?

No polar bonds to attack. C-Cl and C-F bonds are strong. They do not react with common atmospheric species.

What have CFCs been replaced by?

HFCs (hydrofluorocarbons) and HCFCs (hydrochlorofluorocarbons). HFCs contain no chlorine so do not deplete ozone.

What is the environmental impact of HFCs?

HFCs contain no chlorine so they do not deplete the ozone layer. However they have high global warming potential (GWP) as greenhouse gases.

What is the environmental impact of HCFCs?

HCFCs contain chlorine but are broken down in the lower atmosphere. They have lower ozone depletion potential than CFCs but still some.

What is the ideal replacement for CFCs?

Compounds with no chlorine (no ozone depletion). Compounds with low or zero global warming potential. Natural refrigerants like CO₂ and ammonia.

What is the ozone layer?

The ozone layer is in the stratosphere (upper atmosphere 15-30 km above Earth). It contains high concentrations of ozone (O₃). Ozone absorbs most of the Sun's harmful UV radiation.

Why is the ozone layer important?

Ozone absorbs the Sun's harmful UV radiation. UV radiation at Earth's surface is reduced. Without it there would be a rise in skin cancer and eye cataracts.

What is the Chapman cycle?

The natural ozone formation and depletion cycle. O₂ + UV → 2O•. O₂ + O• ⇌ O₃. Rate of formation equals rate of depletion in natural steady state.

[DIAGRAM: Ozone formation/depletion equilibrium cycle showing O₂, O•, O₃ and UV]

How is ozone naturally formed and depleted?

Formation: O₂ + UV → 2O• then O₂ + O• → O₃. Depletion: O₃ + UV → O₂ + O•. Overall: 3O₂ ⇌ 2O₃.

What is the equation for natural ozone formation?

O₂ → 2O• (initiation by UV). Then O₂ + O• → O₃.

What is the equation for natural ozone depletion?

O₃ + UV → O₂ + O•.

What is the steady state concentration of ozone?

When the rate of formation of ozone equals the rate of depletion. The Chapman cycle maintains this equilibrium naturally.

How do CFCs cause the breakdown of ozone?

Initiation: CF₂Cl₂ → Cl• + CF₂Cl•. Propagation 1: Cl• + O₃ → ClO• + O₂. Propagation 2: ClO• + O• → Cl• + O₂. Overall: O₃ + O• → 2O₂.

[DIAGRAM: Chlorine radical ozone depletion mechanism showing initiation and propagation]

What is the role of the chlorine radical in ozone depletion?

The chlorine radical acts as a catalyst. It is regenerated in propagation step 2. One chlorine radical can destroy up to 100,000 ozone molecules.

What is a catalyst?

A substance that increases the rate of a reaction without being consumed in the overall reaction. It provides an alternative pathway with lower activation energy.

Why does the C-Cl bond break in CFCs rather than the C-F bond?

The C-Cl bond is weaker than the C-F bond. Less energy is needed to break the C-Cl bond. UV radiation provides enough energy to break C-Cl but not C-F.

What is the equation for the initiation step in CFC ozone depletion?

CF₂Cl₂ → Cl• + CF₂Cl•. This is homolytic fission of the C-Cl bond by UV radiation.

[DIAGRAM: Initiation step - homolytic fission of CFC showing UV, Cl• and CF₂Cl•]

What is the equation for propagation step 1 in CFC ozone depletion?

Cl• + O₃ → ClO• + O₂.

What is the equation for propagation step 2 in CFC ozone depletion?

ClO• + O• → Cl• + O₂.

[DIAGRAM: Propagation steps for chlorine radical catalysing ozone breakdown]

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

O₃ + O• → 2O₂. The chlorine radical cancels out as it is a catalyst.

What other radicals catalyse the breakdown of ozone?

NOx radicals (such as NO) produced by lightning strikes and jet engines. N₂ + O₂ → 2NO. NO + O₃ → NO₂ + O₂. NO₂ + O• → NO + O₂. Overall: O₃ + O• → 2O₂.

[DIAGRAM: NO radical ozone depletion mechanism showing propagation steps]

What is the equation for formation of NO radicals?

N₂ + O₂ → 2NO. Occurs at high temperatures in lightning strikes and jet engines.

What is propagation step 1 for NO radical ozone depletion?

NO + O₃ → NO₂ + O₂.

What is propagation step 2 for NO radical ozone depletion?

NO₂ + O• → NO + O₂.

What is the overall equation for ozone depletion by NO radicals?

O₃ + O• → 2O₂. The NO radical cancels out as it is a catalyst.

How do OH radicals catalyse ozone breakdown?

OH + O₃ → HO₂ + O₂. HO₂ + O• → OH + O₂. Overall: O₃ + O• → 2O₂.

[DIAGRAM: OH radical ozone depletion mechanism showing propagation steps]

What are the two propagation steps for OH radical ozone depletion?

Step 1: OH + O₃ → HO₂ + O₂. Step 2: HO₂ + O• → OH + O₂.

What is the overall equation for ozone depletion by OH radicals?

O₃ + O• → 2O₂.

What is homolytic fission?

The breaking of a covalent bond where each bonding atom receives one electron from the bonding pair, forming two radicals.

What is heterolytic fission?

The breaking of a covalent bond where one bonding atom receives both electrons from the bonding pair, forming two oppositely charged ions.

What is a radical?

A species with an unpaired electron. Very reactive. Represented with a dot (•). Formed by homolytic fission.

What are the reagents and conditions for reaction of haloalkane with ammonia?

Reagent: NH₃. Conditions: Excess ammonia. Nucleophile: :NH₃. Product: Amine. NH₃ has a lone pair on nitrogen.

What is an elimination reaction of haloalkanes?

Reagents: Alcoholic KOH or NaOH. Conditions: Heat under reflux. Product: Alkene. Equation: CH₃CH₂Br + KOH(alc) → CH₂=CH₂ + KBr + H₂O.

What is the difference between substitution and elimination of haloalkanes?

Substitution: aqueous NaOH, produces alcohol. Elimination: alcoholic KOH, produces alkene. Solvent determines which reaction occurs.

Why is elimination favoured by alcoholic KOH?

Hydroxide ions are less solvated in ethanol. They behave as stronger bases rather than nucleophiles.

Why is substitution favoured by aqueous KOH?

Hydroxide ions are solvated by water. They behave as nucleophiles rather than bases.

What is Saytzeff's rule?

When an elimination reaction can form more than one alkene, the major product is the more substituted alkene (more stable).

Equation for Saytzeff product from 2-bromobutane?

CH₃CH₂CH(Br)CH₃ + KOH(alc) → CH₃CH=CHCH₃ (but-2-ene, major) + KBr + H₂O.

Why are CFCs harmful to the environment?

CFCs release chlorine radicals in the stratosphere. Chlorine radicals catalyse the breakdown of ozone. More UV radiation reaches Earth's surface causing skin cancer and eye cataracts.

What is the overall equation for ozone depletion by CFCs?

O₃ + O• → 2O₂.

What is meant by the term hydrolysis?

A chemical reaction involving water or an aqueous solution that causes the breaking of a bond. The molecule is split into two products.

What is meant by heat under reflux?

The continuous boiling and condensing of a liquid in a condenser. Prevents loss of volatile liquids from the heated reaction vessel.

[DIAGRAM: Reflux apparatus - round bottom flask, condenser, water in/out, heat source]

What is the general formula for a haloalkane?

R-X where R is an alkyl group and X is a halogen atom (F, Cl, Br, or I).

What is a chlorofluorocarbon?

A compound containing chlorine, fluorine and carbon only. Inert, non-toxic and non-flammable. Volatile with low boiling points.

Why do CFCs reach the stratosphere?

CFCs are volatile and have low boiling points. They are unreactive in the lower atmosphere so they diffuse up to the stratosphere.

What is the effect of ozone depletion on humans?

More UV radiation reaches Earth's surface. Causes increase in skin cancer and eye cataracts. UV suppresses the immune system.

What is the effect of ozone depletion on the environment?

Damages plant life and marine ecosystems. UV reduces phytoplankton populations affecting the food chain. Reduces crop yields.