Topic 4
Topic 4 on a page
Everything from this topic in one place, for the last read before an exam. If anything here is unfamiliar, go back to the full page.
Definitions to learn word for word
- Hazard: a substance, object or activity with the potential to cause harm (Hazard and risk).
- Risk: the chance (likelihood) that harm will actually happen, taking into account how serious it would be.
- Homologous series: a family of compounds with the same general formula and the same functional group, each member differing from the next by CH₂ (Homologous series).
- Functional group: an atom or group of atoms responsible for the characteristic chemical reactions of a molecule.
- Hydrocarbon: a compound of carbon and hydrogen only. Saturated: single bonds only (Alkanes and cycloalkanes).
- Structural isomers: the same molecular formula, different structural formulae (Structural isomerism).
- Free radical: a species with an unpaired electron. Electrophile: an electron-pair acceptor (Bond breaking).
- Cracking: breaking long-chain alkanes into shorter alkanes and alkenes. Reforming: converting straight-chain alkanes into branched-chain alkanes and cyclic hydrocarbons.
- Carbon neutral: no net release of CO₂ to the atmosphere (Alternative fuels).
Formulae and names
| Series | General formula | Functional group |
|---|---|---|
| Alkane | CₙH₂ₙ₊₂ | none |
| Cycloalkane | CₙH₂ₙ | none (a ring) |
| Alkene | CₙH₂ₙ | C=C |
| Halogenoalkane | CₙH₂ₙ₊₁X | C–X |
| Alcohol | CₙH₂ₙ₊₁OH | –OH |
Stems: meth, eth, prop, but, pent, hex, hept, oct, non, dec (1 to 10 carbons). Branches: methyl, ethyl; halogens: fluoro, chloro, bromo, iodo.
Naming: longest chain, then number from the end that gives the lowest numbers, alphabetical order for different groups, di-, tri- for repeats, commas between numbers and hyphens between numbers and letters (Naming).
Four formulae: molecular, structural, displayed (every atom and bond) and skeletal (corners and ends are carbons, hydrogens on carbon not shown, functional groups written out) (Ways of writing a formula).
Reaction types and bond breaking
| Term | Meaning |
|---|---|
| Addition | two molecules join to give one product |
| Substitution | an atom or group is replaced |
| Oxidation | gain of oxygen or loss of hydrogen |
| Reduction | gain of hydrogen or loss of oxygen |
| Polymerisation | many monomers join into a polymer |
| Homolytic fission | one electron to each atom: two free radicals, shown with curly half-arrows |
| Heterolytic fission | both electrons to one atom: a positive and a negative ion, shown with a full curly arrow |
Isomers
| Formula | Alkane isomers | Ring isomers (cycloalkane) |
|---|---|---|
| C₄ | 2 (butane, methylpropane) | 2 for C₄H₈ |
| C₅ | 3 (pentane, 2-methylbutane, 2,2-dimethylpropane) | 5 for C₅H₁₀ |
| C₆ | 5 (hexane, 2- and 3-methylpentane, 2,2- and 2,3-dimethylbutane) | 12 for C₆H₁₂ |
Method: longest chain first, then shorten by one carbon and move the branch. Name every drawing: the same name means the same molecule (Isomers of alkanes and cycloalkanes).
Fuels from crude oil
- Fractional distillation (physical): hot at the bottom, cooler at the top. Longer chains have higher boiling temperatures and condense lower down. Each fraction is a mixture (Crude oil).
- Cracking: high temperature, zeolite catalyst. C₁₀H₂₂ → C₈H₁₈ + C₂H₄. There is always an alkene.
- Reforming: platinum catalyst. C₆H₁₄ → C₆H₁₂ + H₂ (ring), or hexane to 2-methylpentane (branched) (Cracking and reforming).
Combustion and pollutants
- Complete: alkane + O₂ → CO₂ + H₂O. Balance C, then H, then O last. For CₙH₂ₙ₊₂ the oxygen needed is (3n + 1)/2 mol (Combustion).
- Incomplete: CO and/or C, and H₂O.
- Pollutants: CO (incomplete combustion), carbon particulates, unburned hydrocarbons, NOₓ (N₂ + O₂ from the air, in a hot engine), SO₂ (sulfur impurities in the fuel).
- CO is toxic because it binds to haemoglobin more strongly than oxygen does, so less oxygen is carried.
- NO₂ and SO₂ are acidic and make acid rain: 4NO₂ + O₂ + 2H₂O → 4HNO₃; SO₂ + H₂O → H₂SO₃ (Pollutants).
- Alternative fuels: finite crude oil, pollutants, CO₂ as a greenhouse gas. Bioethanol: 6 CO₂ in, 2 + 4 out, so neutral on paper, but not in practice. Hydrogen: no CO₂ on burning, but it depends on how it is made.
Halogens and the mechanism
Alkane + halogen, UV light: substitution. .
| Stage | Example |
|---|---|
| Initiation | Cl₂ → 2Cl• (homolytic fission, two half-arrows) |
| Propagation | CH₄ + Cl• → CH₃• + HCl; CH₃• + Cl₂ → CH₃Cl + Cl• |
| Termination | Cl• + Cl• → Cl₂; CH₃• + Cl• → CH₃Cl; CH₃• + CH₃• → C₂H₆ |
Radicals attack the products too, so there is further substitution (CH₂Cl₂, CHCl₃, CCl₄), a mixture, a low yield and limited use in synthesis (Free radical substitution, Further substitution).
The ten traps
- Saying you can reduce a hazard. You reduce the risk.
- Using "wear gloves" as a precaution with no hazard and no reason.
- Writing "2-ethyl" or "1-methyl" in a name: the longest chain was counted wrongly.
- Putting hydrogens on a skeletal formula, or leaving out the O–H bond in a displayed formula.
- Describing fractional distillation as breaking bonds, or writing an equation for it.
- Writing a cracking equation with no alkene, or one that does not balance.
- Balancing the oxygen first in a combustion equation.
- Saying NOₓ comes from the fuel. It comes from the air.
- A radical drawn as an ion (Cl⁻ instead of Cl•), or a full arrow used for one electron.
- Writing H• in the first propagation step, or forgetting that the termination step makes a molecule.
Mixed test
Try these with no notes, then check the answers.
- Explain the difference between a hazard and a risk, and suggest one precaution for heating a flammable organic liquid.
- Name CH₃CH(CH₃)CH₂CH₂CH₃ and give its molecular formula.
- Name the three structural isomers of C₅H₁₂ and say which has the lowest boiling temperature.
- Complete and classify: C₂H₆ + Cl₂ → ……
- Explain the difference between homolytic and heterolytic fission. Which gives free radicals?
- Complete the cracking equation C₁₄H₃₀ → C₁₀H₂₂ + 2……
- Write a balanced equation for the complete combustion of pentane. Calculate the volume of oxygen needed to burn 20.0 cm³ of pentane, and the volume of carbon dioxide formed, at constant temperature and pressure.
- Calculate the mass of carbon dioxide formed when 5.80 g of butane burns completely.
- Explain why nitrogen oxides are found in the exhaust of a petrol engine, and why carbon monoxide is toxic.
- Bioethanol is claimed to be carbon neutral. Explain why it could be, and why it is not completely.
- Write the propagation steps for the reaction of methane with bromine, and explain why they are called a chain reaction.
- Explain why the reaction of methane with chlorine in UV light gives CH₂Cl₂ and C₂H₆ as well as CH₃Cl.
- How many different monochloro compounds can be made from 2,2-dimethylpropane? Name it.
Answers cover these until you have tried the questions
- A hazard is something that has the potential to cause harm. Risk is the chance that harm will actually happen. Precaution: the liquid is flammable, so heat it in a water bath or with an electric heater instead of a naked flame, which removes the source of ignition.
- Longest chain: 5 carbons, with the methyl on carbon 2 when numbered from the right: 2-methylpentane. Molecular formula C₆H₁₄.
- Pentane, 2-methylbutane and 2,2-dimethylpropane. The most branched, 2,2-dimethylpropane, has the lowest boiling temperature: its molecules are the most compact, so there is less contact between molecules, weaker London forces and less energy needed to separate them.
- C₂H₆ + Cl₂ → C₂H₅Cl + HCl, UV light. It is a substitution.
- Homolytic: each atom takes one electron of the bonding pair, giving two free radicals. Heterolytic: one atom takes both electrons, giving a positive and a negative ion. Homolytic fission gives free radicals.
- Carbon: 14 − 10 = 4. Hydrogen: 30 − 22 = 8. Two molecules of the same alkene share this, so each is C₂H₄: C₁₄H₃₀ → C₁₀H₂₂ + 2C₂H₄ (ethene).
- C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O. (Check O: 8 × 2 = 16; 5 × 2 + 6 = 16.) Ratio pentane : O₂ : CO₂ = 1 : 8 : 5. O₂ = 20.0 × 8 = 160 cm³. CO₂ = 20.0 × 5 = 100 cm³.
- 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O. (C₄H₁₀) = 4 × 12.0 + 10 × 1.0 = 58.0. n(butane) = 5.80 ÷ 58.0 = 0.100 mol. n(CO₂) = 0.100 × 4 = 0.400 mol (from 2 : 8). (CO₂) = 44.0. Mass = 0.400 × 44.0 = 17.6 g.
- In the hot engine, nitrogen and oxygen from the air react: N₂ + O₂ → 2NO, and NO is oxidised to NO₂. CO is toxic because it binds to haemoglobin more strongly than oxygen does, so less oxygen is carried to the cells.
- On paper: photosynthesis takes in 6 CO₂, and fermentation (2) plus burning (4) release 6 CO₂, so there is no net release. In practice, fossil fuels are used in farming, fertiliser production, transport and distillation, so extra CO₂ is released.
- CH₄ + Br• → CH₃• + HBr, then CH₃• + Br₂ → CH₃Br + Br•. The bromine radical used in the first step is made again in the second step, so the sequence repeats and one initiation produces many molecules of product.
- Further substitution: Cl• radicals attack CH₃Cl as well as CH₄, giving CH₂Cl₂. Termination: two CH₃• radicals combine to give C₂H₆. The reaction therefore gives a mixture.
- All twelve hydrogens in 2,2-dimethylpropane are equivalent (four CH₃ groups on a central carbon), so there is only one monochloro compound: 1-chloro-2,2-dimethylpropane.