Organic Chemistry

Module 7 is worth about 24 marks a year. Alcohols is the biggest single inquiry question (about 8 marks a year, often including an enthalpy of combustion calculation or a reaction pathway), followed by reactions of organic acids and bases (about 5 marks). Nomenclature, hydrocarbons, hydrocarbon reactions and polymers are each worth 2–3 marks a year but appear almost every year, often in multiple choice.

• Nomenclature: IUPAC names and structures for chains up to C8 with the functional groups listed below; structural isomers.
• Hydrocarbons: alkanes, alkenes and alkynes; trends in physical properties; environmental, health and safety issues.
• Products of reactions involving hydrocarbons: addition reactions of alkenes and alkynes; substitution reactions of alkanes.
• Alcohols: properties, combustion, dehydration, substitution with hydrogen halides, oxidation, fermentation, and biofuels versus fossil fuels.
• Reactions of organic acids and bases: properties of carboxylic acids, amines and amides; esterification; soaps and detergents.
• Polymers: addition polymers (polyethylene, PVC, polystyrene, PTFE) and condensation polymers (polyesters, polyamides).

Name a compound in four steps:

1. Find the longest carbon chain containing the main functional group; this gives the stem (meth, eth, prop, but, pent, hex, hept, oct).
2. Number the chain so the main functional group gets the lowest number.
3. Name and number the branches and halogen substituents (methyl, ethyl, chloro, bromo), in alphabetical order, with di/tri for repeats.
4. Add the suffix for the main functional group.

Commas separate numbers (2,3-dimethyl) and hyphens separate numbers from words. Aldehydes and carboxylic acids are always carbon 1, so they need no number.

Example. ${\text{CH}}_3{\text{CH(CH}}_3){\text{CH}}_2{\text{CH(OH)CH}}_3$ has a five-carbon chain with the –OH on carbon 2 (numbering from the right) and a methyl on carbon 4: 4-methylpentan-2-ol.

Structural isomers

Isomers have the same molecular formula but different structures:

• Chain isomers: different carbon skeletons (pentane and 2-methylbutane).
• Position isomers: same group in a different position (butan-1-ol and butan-2-ol).
• Functional group isomers: different functional groups (propanal and propanone, or propanoic acid and methyl ethanoate).

Alkanes (${\text{C}}_n{\text{H}}_{2n+2}$) are saturated; alkenes (${\text{C}}_n{\text{H}}_{2n}$, one C=C) and alkynes (${\text{C}}_n{\text{H}}_{2n-2}$, one C≡C) are unsaturated.

All hydrocarbons are non-polar, so the only intermolecular forces are dispersion forces. As chain length increases, molecules have more electrons and larger surface area, dispersion forces get stronger, so boiling point and viscosity increase. Branching makes molecules more compact, reducing contact area, so branched isomers have lower boiling points. Hydrocarbons are insoluble in water but dissolve in non-polar solvents.

Safety and environment: short-chain hydrocarbons are volatile and highly flammable; vapours are often heavier than air. Incomplete combustion produces carbon monoxide (toxic) and soot. Spills of crude oil and fuels harm aquatic life; burning fossil fuels releases ${\text{CO}}_2$. Some aromatic hydrocarbons (such as benzene) are carcinogenic.

Addition reactions (alkenes and alkynes)

The C=C double bond breaks and two atoms or groups add across it. One product forms, with no by-product.

For example, propene with bromine gives 1,2-dibromopropane. With an unsymmetrical alkene such as propene, HBr or water can add two ways, giving a mixture (here 1-bromopropane and 2-bromopropane). An alkyne can add two molecules of reagent.

Test for unsaturation: bromine water decolourises rapidly (orange/brown to colourless) with an alkene or alkyne at room temperature. With an alkane there is no fast change in the dark.

Substitution reactions (alkanes)

Alkanes are unreactive, but in UV light a halogen substitutes for a hydrogen atom, producing a haloalkane and a hydrogen halide:

Further substitution can occur, so a mixture of products forms. Note the difference: addition gives one product, substitution gives two.

Alcohols are classified by how many carbons are bonded to the C–OH carbon:

• Primary: one (propan-1-ol).
• Secondary: two (propan-2-ol).
• Tertiary: three (2-methylpropan-2-ol).

The –OH group can hydrogen-bond, so alcohols have much higher boiling points than alkanes of similar molar mass. Short-chain alcohols are soluble in water (they hydrogen-bond with it); solubility falls as the non-polar chain gets longer.

Reactions of alcohols

• Combustion: complete combustion gives ${\text{CO}}_2$ and ${\text{H}}_2\text{O}$.
• Dehydration: heating with concentrated ${\text{H}}_2{\text{SO}}_4$ or ${\text{H}}_3{\text{PO}}_4$ removes water to form an alkene (the reverse of hydration).
• Substitution: reacting with a hydrogen halide replaces –OH with a halogen to give a haloalkane.
• Production: alcohols are made by hydration of alkenes, by fermentation, and by substitution of a haloalkane: heating with aqueous NaOH replaces the halogen with –OH (for example 1-chloropropane gives propan-1-ol).
• Oxidation with acidified potassium dichromate (${\text{Cr}}_2{\text{O}}_7^{2-}$, orange to green ${\text{Cr}}^{3+}$) or acidified permanganate (${\text{MnO}}_4^{-}$, purple to colourless):

To stop at the aldehyde, distil it off as it forms; to make the carboxylic acid, heat under reflux with excess oxidant.

Enthalpy of combustion

The molar enthalpy of combustion is the heat released when one mole burns completely. In the school experiment a spirit burner heats a known mass of water in a metal can, and the burner is weighed before and after.

On the data sheet $c=4.18\times 10^3{\text{J kg}}^{-1}{\text{K}}^{-1}$ for water, so use $m$ in kg (or use $4.18{\text{J g}}^{-1}{\text{K}}^{-1}$ with grams).

Worked example. Burning 0.85 g of propan-1-ol (${\text{C}}_3{\text{H}}_7\text{OH}$, $M=60.09{\text{g mol}}^{-1}$) heats 200 g of water by 22.0 °C. Calculate the molar enthalpy of combustion.

The accepted value is about $-2021{\text{kJ mol}}^{-1}$, so the experiment captured only about 64% of the heat. Reasons: heat lost to the air and the can, incomplete combustion (soot on the can), and evaporation of alcohol from the wick. Remember the negative sign: combustion is exothermic.

Down the homologous series, each extra ${\text{CH}}_2$ adds roughly the same amount (about 650 kJ per mole) to the enthalpy of combustion.

Fermentation and biofuels

Yeast converts glucose to ethanol under anaerobic conditions at about 30–37 °C:

Fermentation stops at about 15% ethanol, when the yeast is killed; ethanol is then concentrated by distillation. Ethanol can also be made by hydrating ethene from crude oil.

Comparing fuels: ethanol is renewable and burns more cleanly, and the ${\text{CO}}_2$ it releases was recently absorbed by the crop. But it releases less energy per gram than octane, its production uses farmland, water and fertiliser, and fossil energy is used in farming and distillation, so it is not truly carbon neutral. Evaluate answers weigh these against each other.

Carboxylic acids are weak acids: they ionise partially in water. They react like other acids with bases, carbonates and reactive metals, forming salts such as sodium ethanoate. Their boiling points are higher than alcohols of similar mass because they form two hydrogen bonds per pair of molecules (dimers).

Amines are weak bases: the lone pair on nitrogen accepts a proton from water. Amides are formed from carboxylic acids and amines and are essentially neutral. Both amines and amides with N–H bonds can hydrogen-bond.

Boiling point order for compounds of similar molar mass: alkane < aldehyde or ketone ≈ primary amine < alcohol < carboxylic acid < primary amide (amides are highest because of extensive hydrogen bonding).

Esterification

A carboxylic acid and an alcohol form an ester and water, catalysed by concentrated sulfuric acid. It is a condensation reaction and an equilibrium:

That ester is propyl ethanoate: the alcohol part is named first (propyl), then the acid part (ethanoate).

In the laboratory:

1. Mix the acid and alcohol with a few drops of concentrated ${\text{H}}_2{\text{SO}}_4$, plus boiling chips.
2. Heat under reflux, so volatile reactants and product condense and return to the flask instead of escaping.
3. Wash in a separating funnel with sodium carbonate (or hydrogen carbonate) solution to remove leftover acid, then with water.
4. Dry and distil, collecting the fraction that boils at the ester's boiling point.

To increase yield, use an excess of one reactant (usually the cheaper one) or remove water, both shifting the equilibrium forward.

Soaps and detergents

Saponification is the base hydrolysis of a fat or oil (a triglyceride, an ester of glycerol) with NaOH, producing glycerol and soap (the sodium salt of a long-chain carboxylic acid).

A soap or detergent particle has a long non-polar hydrocarbon tail and a polar or ionic head. In water, tails dissolve in grease and heads face the water, forming micelles that lift grease off surfaces and keep it suspended.

• Soaps form scum (insoluble precipitates) with the ${\text{Ca}}^{2+}$ and ${\text{Mg}}^{2+}$ in hard water.
• Anionic detergents (sulfonate heads) work in hard water; used in laundry products.
• Cationic detergents (ammonium heads) are also antibacterial; used in fabric softeners and hair conditioners.
• Non-ionic detergents have no charge, produce little foam; used in dishwasher (machine) detergents and other low-suds products.

Addition polymers

Made from alkene monomers: the C=C opens and monomers join with no by-product.

Linear chains pack closely, so HDPE has stronger dispersion forces, higher density and a higher melting point than branched LDPE. Bulky or polar side groups (Cl in PVC, the benzene ring in polystyrene) change rigidity and strength.

Condensation polymers

Monomers with two functional groups each join, and a small molecule (usually water) is released at every link.

• Polyesters: a diol and a dicarboxylic acid form ester links, for example PET from ethane-1,2-diol and benzene-1,4-dicarboxylic acid (terephthalic acid). Used in bottles and clothing fibres.
• Polyamides: a diamine and a dicarboxylic acid form amide links, for example nylon 6,6. Hydrogen bonding between chains makes it strong; used in ropes and fabrics.
• Natural examples: cellulose and starch (from glucose) and proteins (from amino acids, joined by amide links).

To draw a polymer, draw the repeating unit in brackets with bonds extending through the brackets and a subscript $n$.

• Numbering a chain from the wrong end, or naming a side chain as part of the main chain (there is no "2-ethylbutane"; it is 3-methylpentane).
• Writing a branched chain's name with spaces or forgetting hyphens and commas.
• Saying tertiary alcohols oxidise, or that secondary alcohols give carboxylic acids.
• Saying ethanol is carbon neutral without qualification.
• Forgetting the negative sign on $\Delta H$, or using the mass of fuel in $q=mc\Delta T$ instead of the mass of water.
• Calling esterification an addition reaction, or drawing PVC with a by-product.
• Naming esters back to front (the alcohol part comes first).
• Explaining boiling points with "stronger bonds": boiling breaks intermolecular forces, not covalent bonds.

• Outline a reaction pathway (for example propene to propanoic acid): give each step's reagent, conditions and intermediate name. Each step is usually one mark.
• Explain trends in boiling point or solubility: name the intermediate forces, say which is strongest and why, and link it to the property.
• For practical method questions (esterification, combustion), name the equipment and give a purpose for each step, such as why reflux and why sodium carbonate.
• Compare means similarities and differences; for polymers include monomer type, by-product and an example of each.
• Evaluate fuels or polymers: give advantages and disadvantages, use data if supplied, and finish with a clear judgement.
• Draw structural formulas showing every bond for functional groups; skeletal shortcuts are risky unless the question uses them.

• Module 5: esterification is an equilibrium; Le Chatelier's principle explains how to increase ester yield.
• Module 6: carboxylic acids are weak acids and amines are weak bases; $K_a$ and conjugate pairs apply.
• Module 8: IR, NMR and mass spectrometry identify the functional groups named here; chemical tests for unsaturation, alcohols and acids; synthesis design.
• Year 11 Module 1 and 4: intermolecular forces, and enthalpy and calorimetry.