Romania Clasa a X-a Chemistry
Chapters: 4
1. Introduction to organic chemistry
Organic compounds and formulas
- Organic Compounds: Structure, Formulae and Classification – Organic chemistry is the chemistry of carbon compounds. A carbon atom has 4 outer electrons, so it always makes 4 bonds. This is called tetravalence. With 4 single bonds the carbon is sp³ and its bonds point to the corners of a tetrahedron (109.5°). With one double bond it is sp² and flat (120°). With a triple bond it is sp and straight (180°). We can draw the same molecule in several ways: complete, condensed, bond-line and 3D wedge-dash. Finally we sort organic compounds into open-chain and ring compounds, and into families by their functional group. Members of one family form a homologous series, where each member differs by one CH₂.
2. Hydrocarbons
Fuels and petroleum · Alkanes · Alkenes and dienes · Alkynes · Arenes
- Alkanes – Alkanes are hydrocarbons with only single C–C bonds. Their general formula is CnH2n+2. We make them by adding hydrogen to alkenes, by the Wurtz reaction, by reducing alkyl halides and by decarboxylation. They are non-polar, burn with a lot of heat and swap H for Cl in light. Around the C–C bond the molecule can twist into staggered and eclipsed shapes called conformations.
- Alkenes – Alkenes are hydrocarbons with one C=C double bond and the formula CnH2n. The double bond is one σ bond plus one π bond, so the molecule is flat around it and cannot twist. That gives cis and trans isomers. We make alkenes by removing small molecules (H₂O, HX, X₂) or by partly adding H₂ to alkynes. Alkenes mainly do addition reactions: H₂, X₂, HX (Markovnikov rule, or anti-Markovnikov with peroxide), water, ozone, KMnO₄ and polymerisation.
- Alkynes – Alkynes have a C≡C triple bond and the formula CnH2n−2. The two triple-bond carbons are sp hybridised, so the molecule is a straight line. A hydrogen on a triple-bond carbon is weakly acidic because an sp carbon holds electrons tightly: strong bases like NaNH₂ remove it. The triple bond can open twice, so alkynes add two molecules of H₂, X₂ or HX. With water (Hg²⁺ catalyst) they give aldehydes or ketones, and three ethyne molecules can join into benzene.
- Benzene and Aromaticity – Benzene (C₆H₆) is a flat ring of six sp² carbons. Its six π electrons are spread over the whole ring, so all C–C bonds are equal and the ring is extra stable. A ring like this is called aromatic when it is cyclic, planar, fully conjugated and has 4n + 2 π electrons (Hückel rule). Benzene prefers electrophilic substitution (nitration, halogenation, sulphonation, Friedel–Crafts) over addition, because substitution keeps the stable ring. A group already on the ring decides where the next group goes: ortho/para or meta. Benzene and fused-ring hydrocarbons are toxic and can cause cancer.
3. Compounds with functional groups
Alcohols · Acetic acid · Soaps and detergents
- Alcohols – An alcohol is a carbon compound with an –OH group on an sp³ carbon (R–OH). Count the carbons joined to the C–OH carbon to classify it as primary (1°), secondary (2°) or tertiary (3°). Name it as alkan-n-ol, giving –OH the lowest number. Make alcohols from alkenes (acid-catalysed hydration or hydroboration–oxidation), by reducing aldehydes, ketones and acids, or from Grignard reagents. H-bonding gives them high boiling points and water solubility. Reactions break either the O–H bond (acidic: Na, esterification) or the C–O bond (HX, dehydration), and 1°/2° alcohols can be oxidised. Methanol and ethanol are the key commercial alcohols.
- Carboxylic Acids: Carboxyl Group, Acidity and Reactions – A carboxylic acid has the carboxyl group, −COOH: a C=O and an O−H on the same carbon. It can give away H⁺, and the ion left behind (carboxylate, −COO⁻) is stable because its negative charge is shared equally by two oxygen atoms. That is why these acids are stronger than phenols and alcohols (but weaker than mineral acids like HCl). Groups that pull electrons (Cl, F, NO₂) make the acid stronger; groups that push electrons (alkyl) make it weaker. We make carboxylic acids by oxidising alcohols, aldehydes or alkylbenzenes, by hydrolysing nitriles, esters and amides, or from Grignard reagents and CO₂. Their key reactions: salt formation with NaHCO₃ (CO₂ fizz), esterification, acid chloride and anhydride formation, reduction to alcohols, decarboxylation and the HVZ reaction.
- Soaps and Detergents: How a Micelle Cleans Oily Dirt – A soap molecule is the sodium or potassium salt of a long-chain fatty acid. Its ionic head loves water and its long hydrocarbon tail loves oil. In water, soap molecules bury their tails in oily dirt and point their heads outward, forming a micelle that is rinsed away. In hard water, Ca²⁺ and Mg²⁺ ions turn soap into insoluble scum; detergents (sulphonate or ammonium salts) do not form scum, so they work in hard water too.
4. Organic compounds in everyday life
Rubber, plastics and fibres · Dyes, paints, flavours and perfumes · Biologically active compounds
- Polymers – A polymer is a giant molecule made by joining many small molecules (monomers) into a long chain. In addition polymerisation, monomers with a C=C double bond open up and link with nothing lost (ethene → poly(ethene)). In condensation polymerisation, two kinds of monomer with reactive groups at both ends join and give off a small molecule such as water at every link (nylon, polyester). Nature makes polymers too: starch, cellulose, proteins, DNA, rubber. Separate chains give thermoplastics that melt and can be recycled; cross-linked chains give thermosets that never melt. Most plastics do not rot, so we must reduce, reuse and recycle them.