Edexcel International A Level Chemistry
Topic 10: Organic Chemistry: Halogenoalkanes, Alcohols and Spectra
Ethane ignores hydroxide, bromoethane does not. This topic explains why, uses the answer on halogenoalkanes and alcohols, and ends with the question every preparation asks: did we make what we meant to make?
The one idea
Warm ethane with aqueous potassium hydroxide and nothing happens. Warm bromoethane with the same solution and ethanol forms. The difference is bond polarity. Bromine is more electronegative than carbon, so the carbon in C–Br is δ+. A species with a lone pair, a nucleophile, is attracted to it and can attack. C–H and C–C bonds carry no charge, so the hydroxide ion has nothing to attack.
That one idea runs through the topic:
- Find the δ+ carbon. Ask whether a bond is polar and which end is δ+.
- Follow the electrons. A curly arrow starts at a lone pair or a bond and ends where a pair of electrons arrives.
- Pick the reagent and the conditions. The same hydroxide ion substitutes in water and eliminates in ethanol. Heating under reflux or distilling decides how far an alcohol is oxidised.
- Check the product. The mass spectrum gives the relative molecular mass and the fragments. The infrared spectrum gives the functional groups.
Almost every question in this topic is one of four moves:
- Name the reaction type and the reagent, with the condition that matters (aqueous or ethanolic, distil or reflux).
- Draw a mechanism with curly arrows that start in the right place.
- Explain a rate with bond enthalpy, not bond polarity, and describe how Core Practical 5 shows it.
- Identify a compound from a mass spectrum, an infrared spectrum or a test result.
Where to find things
| Question | Pages |
|---|---|
| What types of reaction are there, and what is a mechanism? | 1, 2 |
| What is a nucleophile, and why does polarity decide the mechanism? | 3 |
| How do I name and draw a halogenoalkane, and classify it? | 4, 5 |
| What do halogenoalkanes react with? | 6, 7, 8 |
| How do I draw the substitution mechanisms? | 9, 10 |
| Why do halogenoalkanes hydrolyse at different rates? | 11, 12 |
| Core Practical 5 | 13, 14 |
| How do I name and classify alcohols? | 15 |
| What do alcohols react with? | 16, 17 |
| How do I heat, distil, separate and dry a liquid? | 18, 19 |
| Core Practical 6 | 20 |
| How are alcohols oxidised, and how do I tell the products apart? | 21, 22, 23 |
| Core Practical 7 | 24 |
| How do I use a mass spectrum or an infrared spectrum? | 25, 26, 27 |
How to use these notes
- Read the pages in order the first time. Pages 1 to 3 give the vocabulary. Pages 4 to 14 are halogenoalkanes. Pages 15 to 24 are alcohols, the techniques of preparation and purification, and two more core practicals. Pages 25 to 27 are spectra.
- Each page opens with its one idea in a sentence. If you can say it to someone else, you know the page.
- Coloured boxes mean the same thing on every page: definitions you may be asked to quote, key ideas, common mistakes, exam technique, practical steps to describe, and beyond the spec (interesting, but not examined).
- Each page ends with questions and fully worked answers. Cover the answers, try the questions, then mark yourself.
- Use the topic on a page the night before an exam. It ends with a mixed test.
Core Practicals 5, 6 and 7 are on pages 13 and 14, 20, and 24. Core Practical 6 starts from an alcohol, so it sits in the alcohol pages even though the specification lists it with the halogenoalkanes.
Times, yields and spectra in the worked examples are made up to show the method and are marked illustrative or schematic. Reference values (boiling temperatures, bond enthalpies, infrared ranges) are standard rounded values; check them against your data booklet.
Constants used in this topic: relative atomic masses are the Edexcel IAL values to one decimal place: H 1.0, C 12.0, N 14.0, O 16.0, Cl 35.5, Br 79.9. Mean bond enthalpies: C–Cl 346, C–Br 290, C–I 228 kJ mol⁻¹. Pauling electronegativities: H 2.1, C 2.5, N 3.0, O 3.5, Cl 3.0, Br 2.8, I 2.5.
The skills that earn most marks here are starting a curly arrow on a lone pair or a bond, saying "ethanolic" and "aqueous" when you name the reagent, explaining halogenoalkane rates with bond enthalpy rather than polarity, saying "distil as it forms" for an aldehyde and "heat under reflux" for an acid, and naming the bond that gives each peak in a spectrum.
Pages
- Seven types of reaction
Every organic reaction in this topic can be labelled as addition, elimination, substitution, oxidation, reduction, hydrolysis or polymerisation. One reaction can carry two labels, and the question tells you which to give.
- Mechanisms and curly arrows
A mechanism is a step-by-step account of which bonds break and which form. Curly arrows show a pair of electrons moving, and breaking a bond unevenly, heterolytic fission, makes ions.
- Nucleophiles, electrophiles and bond polarity
A nucleophile donates a pair of electrons, an electrophile accepts one. Bond polarity tells you which is needed: a δ+ carbon is attacked by a nucleophile, an electron-rich bond by an electrophile, and a non-polar bond by neither.
- Naming and drawing halogenoalkanes
A halogenoalkane is named from its longest carbon chain, with the halogen as a numbered prefix. You need to draw it three ways, displayed, skeletal and structural, and to go from a name to a drawing and back.
- Primary, secondary and tertiary halogenoalkanes
A halogenoalkane is primary, secondary or tertiary according to how many other carbon atoms are bonded to the carbon that carries the halogen. The class decides how fast it reacts.
- Hydroxide: substitution or elimination
The hydroxide ion does two different jobs with a halogenoalkane. In aqueous potassium hydroxide it is a nucleophile and gives an alcohol. In ethanolic potassium hydroxide it is a base and gives an alkene.
- Ammonia and cyanide
Ammonia turns a halogenoalkane into an amine and cyanide turns it into a nitrile. Both are nucleophiles attacking the δ+ carbon, and the cyanide reaction adds a carbon atom to the chain.
- Water as the nucleophile
Water is a weak nucleophile and hydrolyses a halogenoalkane slowly. Silver ions in aqueous silver nitrate catch the halide ion as a coloured precipitate, so the time it takes to appear measures how fast the C–X bond breaks.
- Mechanism: hydroxide and a primary halogenoalkane
The hydroxide ion donates a lone pair to the δ+ carbon while the C–Br bond breaks and bromine leaves with the bonding pair. Two curly arrows, one from a lone pair and one from the bond, describe the whole mechanism.
- Mechanism: ammonia and a primary halogenoalkane
Ammonia is a neutral nucleophile, so the first product carries a positive charge on nitrogen. A second ammonia molecule removes an H⁺ in a second step, which is why two moles of ammonia appear in the equation.
- Rates of hydrolysis: primary, secondary, tertiary
With the same halogen, a tertiary halogenoalkane is hydrolysed faster than a secondary one, and a secondary one faster than a primary one. The evidence is the time for a silver halide precipitate to appear.
- Rates of hydrolysis: chloro, bromo, iodo
For primary halogenoalkanes the order of reactivity is iodo > bromo > chloro. Bond polarity predicts the opposite order, so the explanation is bond enthalpy: the weaker the C–X bond, the faster it breaks.
- Core Practical 5: method
Time how long the silver halide precipitate takes to hide a cross, for halogenoalkanes of different class or different halogen. Everything else is kept the same, so the time measures only the rate of hydrolysis.
- Core Practical 5: results and conclusion
Turn the times into rates, draw the two conclusions, class and halogen, and explain each with bond enthalpy or structure. Then evaluate: the end point is a judgement by eye, and fast reactions carry the biggest percentage error.
- Naming and classifying alcohols
An alcohol is named from its longest chain with the ending -ol and a number for the OH group. It is primary, secondary or tertiary according to the carbon bonded to OH, and that class decides what oxidation does to it.
- Burning alcohols and losing water
Alcohols burn to carbon dioxide and water in plenty of air, and to carbon monoxide or soot when air is short. With concentrated phosphoric acid and heat an alcohol loses water and becomes an alkene, which is an elimination.
- Replacing the OH group with a halogen
Three reagent systems turn an alcohol into a chloro-, bromo- or iodoalkane. PCl₅ also gives a test for –OH, and the bromine and iodine reagents make the active compound in the flask.
- Heating under reflux and distillation
Reflux keeps a reaction mixture hot without losing anything, because the vapour condenses and runs back. Distillation does the opposite: it boils a liquid off and collects it elsewhere. The apparatus and its set-up decide which you get.
- Separating, drying and checking a liquid
A crude liquid product is separated from water with a separating funnel, dried with an anhydrous salt, and then distilled. The boiling temperature shows whether it is pure.
- Core Practical 6: chlorination of 2-methylpropan-2-ol
Shake a tertiary alcohol with concentrated hydrochloric acid, separate the upper layer, wash it, dry it and distil it. Every technique in 10.19 is used once, and the boiling temperature shows whether the chloroalkane is pure.
- Oxidation of alcohols
Acidified potassium dichromate(VI) oxidises a primary alcohol to an aldehyde and then to a carboxylic acid, a secondary alcohol to a ketone, and a tertiary alcohol not at all. The class of the alcohol decides the outcome.
- Distil to stop, reflux to go on
A primary alcohol gives an aldehyde if the product is distilled out as it forms, and a carboxylic acid if the mixture is heated under reflux with excess oxidising agent. The boiling temperatures explain why.
- Telling the products apart
Dichromate turns green when anything is oxidised. Benedict's or Fehling's solution gives a brick-red precipitate with an aldehyde but not a ketone, and sodium carbonate fizzes with a carboxylic acid. Together the tests name the product.
- Core Practical 7: propan-1-ol to propanal and propanoic acid
Part A distils propanal out of the oxidation mixture as it forms. Part B heats the alcohol under reflux with excess oxidising agent to reach propanoic acid. The tests and the boiling temperatures show what was made.
- Mass spectra
The peak at the highest m/z in a mass spectrum is the molecular ion, and its m/z is the relative molecular mass. The smaller peaks are fragments, and the way a molecule breaks up helps to show its structure.
- Infrared spectra
Each bond absorbs infrared radiation at a characteristic wavenumber. Finding the absorptions in a spectrum, and matching them to the data table, shows which functional groups are present.
- Identifying an unknown
Use the mass spectrum for the relative molecular mass, the infrared spectrum for the functional groups, and the fragments to choose between isomers. Every preparation ends with this question: did we make what we meant to make?
Also in this topic: Topic 10 on a page
Spec checklist
Every point you can be examined on in this topic, and where it is explained. Tick them off as you go.
| Spec | You should be able to | Where |
|---|---|---|
| 10.1 | be able to classify reactions (including those in Unit 1) as addition, elimination, substitution, oxidation, reduction, hydrolysis or polymerisation | Seven labels How to decide |
| 10.2 | understand the concept of a reaction mechanism | What a mechanism shows Curly arrows |
| 10.3 | understand that heterolytic bond breaking results in species that are electrophiles or nucleophiles | Heterolytic fission makes ions Nucleophiles and electrophiles |
| 10.4 | know the definition of the term 'nucleophile' | Nucleophiles and electrophiles |
| 10.5 | understand the link between bond polarity and the type of reaction mechanism a compound will undergo | Bond polarity decides the mechanism |
| 10.6 | understand the nomenclature of halogenoalkanes and be able to draw their structural, displayed and skeletal formulae | Naming halogenoalkanes Three ways to draw one molecule |
| 10.7 | understand the distinction between primary, secondary and tertiary halogenoalkanes | Counting carbons on the C–X carbon Isomers have different classes |
| 10.8 | understand the reactions of halogenoalkanes with: (i) aqueous alkali, including KOH(aq) to produce alcohols (where the hydroxide ion acts as a nucleophile) (ii) ethanolic potassium hydroxide to produce alkenes by an elimination reaction (where the hydroxide ion acts as a base) (iii) aqueous silver nitrate in ethanol (where water acts as a nucleophile) (iv) alcoholic ammonia under pressure to produce amines (where the ammonia acts as a nucleophile) (v) alcoholic potassium cyanide to produce nitriles (where the cyanide ion acts as a nucleophile). Note: students should know this is an example of increasing the length of the carbon chain. | Aqueous alkali: substitution Ethanolic alkali: elimination Ammonia: an amine Cyanide: a nitrile Water attacks the δ+ carbon Which precipitate? Five reagents on one page |
| 10.8(i) | (i) aqueous alkali, including KOH(aq) to produce alcohols (where the hydroxide ion acts as a nucleophile) | Aqueous alkali: substitution |
| 10.8(ii) | (ii) ethanolic potassium hydroxide to produce alkenes by an elimination reaction (where the hydroxide ion acts as a base) | Ethanolic alkali: elimination |
| 10.8(iii) | (iii) aqueous silver nitrate in ethanol (where water acts as a nucleophile) | Water attacks the δ+ carbon Which precipitate? |
| 10.8(iv) | (iv) alcoholic ammonia under pressure to produce amines (where the ammonia acts as a nucleophile) | Ammonia: an amine |
| 10.8(v) | (v) alcoholic potassium cyanide to produce nitriles (where the cyanide ion acts as a nucleophile). Note: students should know this is an example of increasing the length of the carbon chain. | Cyanide: a nitrile |
| 10.9 | understand the mechanisms of the nucleophilic substitution reactions between primary halogenoalkanes and: (i) aqueous potassium hydroxide (ii) ammonia. Note: SN1 and SN2 substitution mechanisms will be tested in Unit 4. | The mechanism Other primary halogenoalkanes Step 1: ammonia attacks the carbon Step 2: a second ammonia removes H⁺ |
| 10.9(i) | (i) aqueous potassium hydroxide | The mechanism Other primary halogenoalkanes |
| 10.9(ii) | (ii) ammonia | Step 1: ammonia attacks the carbon Step 2: a second ammonia removes H⁺ |
| 10.10 | understand that experimental observations and data can be used to compare the relative rates of hydrolysis of: (i) primary, secondary and tertiary structural isomers of a halogenoalkane (ii) primary chloro-, bromo- and iodoalkanes using aqueous silver nitrate in ethanol | Comparing three isomers Same skeleton, different halogen Conclusions |
| 10.10(i) | (i) primary, secondary and tertiary structural isomers of a halogenoalkane | Comparing three isomers |
| 10.10(ii) | (ii) primary chloro-, bromo- and iodoalkanes using aqueous silver nitrate in ethanol | Same skeleton, different halogen |
| 10.11 | CORE PRACTICAL 5: Investigation of the rates of hydrolysis of some halogenoalkanes. | Aim and idea Method Safety and good practice Processing the results Evaluating the method |
| 10.12 | know the trend in reactivity of primary, secondary and tertiary halogenoalkanes | Comparing three isomers Conclusions |
| 10.13 | understand, in terms of bond enthalpy, the trend in reactivity of chloro-, bromo- and iodoalkanes | Why: bond enthalpy, not polarity Conclusions |
| 10.14 | CORE PRACTICAL 6: Chlorination of 2-methylpropan-2-ol with concentrated hydrochloric acid. | The reaction Method Yield Safety |
| 10.15 | understand the nomenclature of alcohols and be able to draw their structural, displayed and skeletal formulae | Naming alcohols Three ways to draw an alcohol |
| 10.16 | understand the distinction between primary, secondary and tertiary alcohols | Primary, secondary and tertiary alcohols |
| 10.17 | understand the reactions of alcohols with: (i) oxygen in air (combustion) (ii) halogenating agents: (a) PCl₅ to produce chloroalkanes (including its use as a qualitative test for the presence of the –OH group), (b) 50% concentrated sulfuric acid and potassium bromide to produce bromoalkanes, (c) red phosphorus and iodine to produce iodoalkanes (iii) concentrated phosphoric acid to form alkenes by elimination. Note: descriptions of the mechanisms of these reactions are not required. | Combustion Dehydration: losing water Three ways to swap OH for a halogen Chloroalkanes with PCl₅ Bromoalkanes with KBr and sulfuric acid Iodoalkanes with red phosphorus and iodine |
| 10.17(i) | (i) oxygen in air (combustion) | Combustion |
| 10.17(ii) | (ii) halogenating agents: (a) PCl₅ to produce chloroalkanes (including its use as a qualitative test for the presence of the –OH group), (b) 50% concentrated sulfuric acid and potassium bromide to produce bromoalkanes, (c) red phosphorus and iodine to produce iodoalkanes | Three ways to swap OH for a halogen Chloroalkanes with PCl₅ Bromoalkanes with KBr and sulfuric acid Iodoalkanes with red phosphorus and iodine |
| 10.17(iii) | (iii) concentrated phosphoric acid to form alkenes by elimination | Dehydration: losing water |
| 10.18 | understand that potassium dichromate(VI) in dilute sulfuric acid can oxidise: (i) primary alcohols to produce aldehydes (which give a positive result with Benedict's or Fehling's solution) if the product is distilled as it forms (ii) primary alcohols to produce carboxylic acids (which give a positive result with sodium carbonate or sodium hydrogencarbonate) if the reagents are heated under reflux (iii) secondary alcohols to produce ketones. Note: in equations, the oxidising agent can be represented by [O]. | The oxidising agent Primary alcohols: aldehyde, then carboxylic acid Secondary alcohols: ketones Tertiary alcohols: no oxidation Why the conditions matter The boiling temperatures explain it Secondary alcohols Three tests Part A: stop at propanal Part B: on to propanoic acid |
| 10.18(i) | (i) primary alcohols to produce aldehydes (which give a positive result with Benedict's or Fehling's solution) if the product is distilled as it forms | Primary alcohols: aldehyde, then carboxylic acid Why the conditions matter Three tests |
| 10.18(ii) | (ii) primary alcohols to produce carboxylic acids (which give a positive result with sodium carbonate or sodium hydrogencarbonate) if the reagents are heated under reflux | Primary alcohols: aldehyde, then carboxylic acid Why the conditions matter Three tests |
| 10.18(iii) | (iii) secondary alcohols to produce ketones. Note: in equations, the oxidising agent can be represented by [O]. | Secondary alcohols: ketones Secondary alcohols |
| 10.19 | understand the following techniques in the preparation and purification of a liquid organic compound: (i) heating under reflux (ii) extraction with a solvent using a separating funnel (iii) distillation (iv) drying with an anhydrous salt (v) boiling temperature determination | Heating under reflux Distillation Extraction with a separating funnel Drying with an anhydrous salt Checking purity by boiling temperature Method The boiling temperatures explain it Part A: stop at propanal |
| 10.19(i) | (i) heating under reflux | Heating under reflux Why the conditions matter |
| 10.19(ii) | (ii) extraction with a solvent using a separating funnel | Extraction with a separating funnel |
| 10.19(iii) | (iii) distillation | Distillation Why the conditions matter |
| 10.19(iv) | (iv) drying with an anhydrous salt | Drying with an anhydrous salt |
| 10.19(v) | (v) boiling temperature determination | Checking purity by boiling temperature |
| 10.20 | CORE PRACTICAL 7: The oxidation of propan-1-ol to produce propanal and propanoic acid. | Aim and reactions Part A: stop at propanal Part B: on to propanoic acid Results and evaluation Safety |
| 10.21 | be able to interpret data from mass spectra to suggest possible structures of simple organic compounds using the m/z of the molecular ion and fragmentation patterns | The molecular ion Fragmentation Same molecular ion, different fragments A routine for unknowns Worked problems |
| 10.22 | be able to use infrared spectra, or data from infrared spectra, to deduce functional groups present in organic compounds, and predict infrared absorptions, given wavenumber data, due to familiar functional groups including: (i) C–H stretching absorptions in alkanes, alkenes and aldehydes (ii) C=C stretching absorption in alkenes (iii) O–H stretching absorptions in alcohols and carboxylic acids (iv) C=O stretching absorptions in aldehydes, ketones and carboxylic acids (v) C–X stretching absorption in halogenoalkanes (vi) N–H stretching absorption in amines | What an infrared spectrum shows Reading three spectra Predicting absorptions A routine for unknowns Worked problems |
| 10.22(i) | (i) C–H stretching absorptions in alkanes, alkenes and aldehydes | What an infrared spectrum shows Predicting absorptions |
| 10.22(ii) | (ii) C=C stretching absorption in alkenes | What an infrared spectrum shows |
| 10.22(iii) | (iii) O–H stretching absorptions in alcohols and carboxylic acids | What an infrared spectrum shows Reading three spectra |
| 10.22(iv) | (iv) C=O stretching absorptions in aldehydes, ketones and carboxylic acids | What an infrared spectrum shows Reading three spectra |
| 10.22(v) | (v) C–X stretching absorption in halogenoalkanes | What an infrared spectrum shows |
| 10.22(vi) | (vi) N–H stretching absorption in amines | What an infrared spectrum shows |