Chemical Formulas and Structures

Think of chemical formulas as different ways to describe the same molecule - like having someone's nickname, full name, and detailed description all referring to the same person.

The general formula gives you the basic pattern for a whole family of compounds. For example, CnH2n works for any alkene - just plug in your number of carbons. Meanwhile, structural formulas like CH3CH2CH3 show you exactly which atoms connect to which, making it dead easy to visualise what's happening.

Displayed formulas take this further by showing every single bond and atom position - think of them as the detailed blueprint. But when things get complex, chemists use skeletal formulas that strip away the hydrogen clutter and just show the carbon backbone, making larger molecules much clearer to read.

Quick Tip: Master reading skeletal formulas early - they're everywhere in A-level papers and make complex molecules look surprisingly simple!

Functional Groups and Compound Types

Functional groups are like the "business end" of organic molecules - they're where all the interesting chemistry happens, and recognising them instantly will save you loads of time in exams.

Alcohols have that distinctive -OH group and end in "-ol" (think ethanol in hand sanitiser). Aldehydes stick their C=O group right at the end of the carbon chain, whilst ketones hide theirs somewhere in the middle - both are reactive because of that double bond.

Carboxylic acids are the acidic ones with HO-C=O at the chain's end, giving compounds like acetic acid (vinegar's active ingredient). Each functional group has its own naming pattern and chemical behaviour, so learning these basics unlocks understanding thousands of different molecules.

Memory Trick: "Ketones are in the middle, aldehydes at the end" - this simple rule will sort out loads of identification questions!

Molecular Architecture and Isomers

Not all molecules with the same formula are identical - structural isomers prove that atom arrangement matters just as much as atom count, like rearranging furniture in identical rooms to create completely different layouts.

Aliphatic compounds keep their carbons in straight or branched chains, whilst aromatic compounds feature benzene's distinctive ring structure (called the phenyl group when it's part of a larger molecule). Alicyclic compounds split the difference with non-aromatic rings that include "cyclo-" in their names.

Positional isomers move functional groups to different spots on the same carbon skeleton. This might seem like a small change, but it can dramatically alter a compound's properties - imagine moving the handle on a mug from one side to the other.

Exam Focus: Drawing different isomers from the same molecular formula is a guaranteed exam question - practise this skill regularly!

Bonding and Stereoisomerism

The pi-bond in double bonds is chemistry's troublemaker - it's weaker than sigma bonds and prevents rotation, creating some fascinating consequences that examiners absolutely love testing.

Alkanes stay relatively unreactive because their sigma bonds have high enthalpy and low polarity, making them stable but boring. Alkenes, however, pack that reactive pi-bond alongside their sigma bond, making them much more chemically interesting but also more fragile.

Cis/trans isomerism happens when double bonds lock groups into fixed positions - "cis" means similar groups on the same side, "trans" means they're opposite. E/Z isomerism extends this concept using priority rules: Z means "zame zide" (same side) for highest priority groups, E means opposite sides.

Reality Check: Think of cis/trans like train carriages locked together - they can't rotate around the coupling, so their arrangement becomes permanent!

Reactions and Alcohol Chemistry

Electrophiles hunt for electrons like moths to a flame, leading to electrophilic addition reactions where they crash into electron-rich areas and form new bonds. The opposite team, nucleophiles, donate electron pairs instead.

Alcohols get classified as primary, secondary, or tertiary based on how many carbons attach to their -OH carbon. This isn't just labelling - it completely determines their chemical behaviour, especially during oxidation reactions.

Primary alcohols follow a clear progression: alcohol → aldehyde → carboxylic acid using acidified potassium dichromate. Secondary alcohols stop at ketones, whilst tertiary alcohols stubbornly refuse to oxidise at all because they lack the necessary hydrogen atom.

Alcohols also undergo elimination reactions with acids to form alkenes, essentially the reverse of adding steam to alkenes. They're cleaner burning than other hydrocarbons because they bring their own oxygen to the combustion party.

Lab Note: The orange dichromate turning green is your visual cue that oxidation is happening - this colour change is exam gold!

Nucleophilic Reactions and Analysis

Nucleophilic substitution flips the electrophile story - now electron-loving nucleophiles attack positive centres, particularly in haloalkanes where the electronegative halogen creates a perfect target.

Haloalkanes undergo hydrolysis with aqueous alkalis, swapping their halogen for an OH group to become alcohols. The reaction rate depends on the carbon-halogen bond strength, with weaker bonds breaking faster.

Nucleophilic addition targets carbonyl compounds where the C=O bond's polarity creates a delta-positive carbon. Adding NaBH4 produces alcohols, whilst HCN creates hydroxynitriles - both reactions exploit the carbonyl group's electron-hungry carbon.

Pattern Recognition: Nucleophiles always attack the most positive part of molecules - spot the δ+ and you'll predict the reaction site every time!

Spectroscopy and Chemical Tests

IR spectroscopy acts like molecular fingerprinting - different bonds vibrate at characteristic frequencies when hit with infrared light, revealing which functional groups lurk in unknown compounds.

Key IR peaks you must memorise: O-H bonds appear around 3200-3600 cm⁻¹ (broader in carboxylic acids), whilst C=O bonds show up near 1700 cm⁻¹. These signatures help distinguish between similar compounds that might fool other tests.

Mass spectrometry identifies compounds through molecular ion peaks (giving molecular weight) and fragment patterns (showing how molecules break apart). Tollen's reagent specifically detects aldehydes by reducing silver ions to metallic silver, whilst 2,4-DNP identifies any carbonyl compound but can't distinguish aldehydes from ketones.

Exam Strategy: Learn the key IR frequencies by heart - they're often worth easy marks and help confirm structures in synthesis questions!

Environmental Chemistry and Purification

Chlorofluorocarbons (CFCs) demonstrate chemistry's environmental impact - UV radiation breaks them down in the upper atmosphere, releasing chlorine radicals that catalytically destroy ozone in a devastating chain reaction.

The ozone destruction cycle shows how one chlorine radical can destroy thousands of ozone molecules before being neutralised, explaining why CFC bans were so crucial for protecting our atmosphere.

Purifying organic liquids requires three key techniques: separating funnel work to separate organic and aqueous layers, drying with anhydrous salts until they stop clumping, and redistillation within narrow boiling point ranges to separate similar compounds.

Real-World Connection: The CFC-ozone story perfectly illustrates how understanding reaction mechanisms helps solve global environmental problems!