Crash Course on AS Level Organic Chemistry (9701 Syllabus)

Carboxylic acids can be reduced to primary alcohols, and aldehydes can be reduced to primary alcohols using strong reducing agents like LiAlH4 in dry ether. Ketones can be reduced to secondary alcohols using LiAlH4 or NaBH4 in ethanol.

Long-chain hydrocarbons from crude oil are broken into smaller, more flammable, less viscous, and lower melting/boiling point molecules. Cracking is a random process, producing alkanes, alkenes, and sometimes hydrogen gas. Catalytic cracking uses a zeolite catalyst (SiO2 and Al2O3) at 500°C, while thermal cracking requires high temperatures (450-700°C) and high pressures (around 70 atm).

This reaction occurs in any carbon-hydrogen chain within a molecule. It involves substituting a hydrogen atom with a halogen (typically chlorine or bromine) under UV light. Fluorine is too explosive, and iodine is too unreactive. The detailed mechanism (initiation, propagation, termination) is required knowledge.

Alkenes, or unsaturated hydrocarbons with carbon-carbon double bonds, undergo electrophilic addition. In bromination, the double bond breaks, and two bromine atoms add across it. Conditions include absence of light (to prevent free-radical substitution) and room temperature/pressure. Bromine water (aqueous) can lead to the addition of -OH as well as -Br. This reaction is used to test for unsaturation, decolorizing red-brown bromine.

Alkenes react with hydrogen to form alkanes. The double bond becomes a single bond, and hydrogen atoms are added. This requires a nickel or platinum catalyst and 150°C. Saturated products have higher melting/boiling points due to increased molecular size and van der Waals forces. An application is the production of margarine from vegetable oils.

Alkenes react with water to form alcohols. The double bond breaks, and -H and -OH from water add across it. Conditions are 300°C, 60 atm, and an acid catalyst (phosphoric or sulfuric acid).

Alkenes react with hydrogen halides (e.g., HBr, HCl, HI) where the double bond breaks, and H and the halogen add across it. Concentrated hydrogen halides are used. Markovnikov's Rule states that in unsymmetrical alkenes, the hydrogen atom prefers to bond to the carbon atom already bonded to more hydrogens, leading to a major and minor product.

Alkenes can be mildly oxidized using cold, dilute, or alkaline KMnO4 (potassium manganate(VII)). The double bond breaks, and two hydroxyl (-OH) groups are added to the carbons that were part of the double bond, forming diols.

Strong oxidation of alkenes uses hot, concentrated, and acidified KMnO4. The double bond completely breaks, splitting the molecule into two parts. The products depend on the structure around the double bond: if a carbon is bonded to two hydrogens, it forms CO2 and H2O; if bonded to one hydrogen and one carbon chain, it forms a carboxylic acid; if bonded to two carbon chains, it forms a ketone.

Thousands of alkene monomers (e.g., ethene) join together under high pressure and temperature. The double bonds convert to single bonds, forming a very large macromolecule (polymer). Examples include polyethene, PVC (from chloroethene), and Teflon (from tetrafluoroethene).

Halogenoalkanes react with various nucleophiles. With aqueous NaOH and reflux, the halogen is substituted by -OH (forming an alcohol). With ethanolic NaCN and reflux, the halogen is substituted by -CN, increasing the carbon chain length. With ethanolic NH3 and reflux (or NH3 gas in a sealed container), the halogen is substituted by -NH2, forming an amine.

Primary halogenoalkanes undergo SN2 mechanism (one-step, transition state) where the nucleophile directly attacks. Tertiary halogenoalkanes undergo SN1 mechanism (two-step, carbocation intermediate) where the halogen leaves first, followed by nucleophile attack. The rate of reaction depends on the C-X bond strength: C-I is fastest (weakest bond), C-F is slowest (strongest bond).

A halogen atom and a hydrogen atom from an adjacent carbon are removed, forming a double bond (an alkene) and a hydrogen halide. Conditions are ethanolic NaOH and reflux.

The -OH group in alcohols can be substituted by halogens. For chlorine, dry HCl gas, SOCl2 (thionyl chloride) + heat, or PCl5 at room temperature (or PCl3 + heat) are used. SOCl2 is preferred as byproducts (HCl and SO2) are gases. For bromine or iodine, PBr3 or PI3 + heat are used (often generated in situ from red phosphorus and the halogen).

Alcohols are very weakly acidic but can react very slowly with highly reactive metals like sodium. The hydrogen atom from the -OH group is replaced by sodium, forming an alkoxide salt and hydrogen gas. This slow reaction with visible bubbling can be a test for alcohols.

Alcohols react with carboxylic acids (condensation reaction) to form an ester and water. This is catalyzed by concentrated sulfuric acid and requires reflux. Esters are fruity-smelling. The reverse reaction, hydrolysis, breaks the ester back into an alcohol and carboxylic acid using dilute acid or dilute alkali and heat (reflux). If using alkali, a salt of the carboxylic acid is formed instead of the acid itself.

The -OH group and a hydrogen from an adjacent carbon atom are removed from an alcohol, forming an alkene and water. Conditions include Al2O3 (alumina/pumice) + heat, or concentrated sulfuric acid. Concentrated sulfuric acid can also cause oxidation, producing acidic byproducts, which necessitates collecting the alkene over NaOH to remove them.

Primary alcohols are oxidized to aldehydes and then further to carboxylic acids. The reagent is acidified potassium dichromate(VI) (K2Cr2O7) and reflux. The color changes from orange to green. To stop at the aldehyde stage, distillation is used instead of reflux, as aldehydes have lower boiling points and evaporate quickly.

Aldehydes and ketones (carbonyl compounds) react with HCN to form cyanohydrins. The -CN group adds to the carbonyl carbon, and the oxygen becomes an -OH group. HCN is generated in situ from NaCN and H2SO4; CN- also acts as a catalyst. This reaction mechanism involves nucleophilic attack by CN-.

2,4-dinitrophenylhydrazine (2,4-DNPH) is used to test for aldehydes and ketones. It undergoes a condensation reaction, forming an orange precipitate (a 2,4-dinitrophenylhydrazone). Formal explanation on reaction mechanism provided.

The iodoform test identifies specific structures: a methyl group attached to a carbonyl carbon (CH3-C(=O)-R) or a methyl group attached to a CH-OH group (CH3-CH(OH)-R). When treated with alkaline aqueous iodine, these structures produce a yellow precipitate of triiodomethane (CHI3) and a salt of a carboxylic acid (due to alkaline conditions).

Nitriles (R-C≡N) can be hydrolyzed by heating with a dilute acid or dilute alkali. The -CN group is converted into a carboxylic acid group (-COOH). If conditions are alkaline, a salt of the carboxylic acid is formed instead. Ammonia or ammonium ions are also produced.

Carboxylic acids are weak acids, partially ionizing to release H+. They undergo typical acid reactions: with reactive metals (e.g., Na, Mg) to form a salt and H2 gas; with bases (e.g., NaOH, metal oxides/hydroxides) to form a salt and water; with carbonates (e.g., MgCO3) to form a salt, water, and CO2; and with ammonia to form an ammonium salt.

Tertiary alcohols are resistant to oxidation due to the absence of hydrogen atoms on the carbon bonded to the -OH group. Therefore, they do not react with oxidizing agents like potassium dichromate(VI).

Secondary alcohols are oxidized to ketones. The reagent is acidified potassium dichromate(VI) and reflux (color change orange to green). Ketones cannot be further oxidized.