Organic Chemistry & Hydrocarbons

The study of carbon-containing compounds. Carbon is present in every organic compound, and most also contain hydrogen.

The old, wrong claim that organic compounds form only inside living bodies.

Made urea in the lab from ammonium cyanate, a non-living salt — celebrated as the birth of modern organic chemistry.

Carbon also combines with oxygen, chlorine and sulphur to build medicines, plastics and living tissue.

Group 14 (with silicon, germanium, tin, lead). It has 4 valence electrons (2s² 2p²) and forms four covalent bonds.

CO₂ makes up about 0.03% of air; carbon is the 17th most abundant element by mass.

A covalent bond is formed by sharing a pair of electrons between two atoms.

Carbon is a non-metal that always shares electrons; it never gives or grabs them.

The carbon family mostly shows +4 and +2 oxidation states; +4 compounds are usually covalent.

Carbon – The Backbone Element

Carbon's ability to link with other carbon atoms into chains and rings. Order: Carbon (very high) > Silicon > Germanium > Tin > Lead (lowest).

¹⁴C, half-life about 5770 years — used for radiocarbon dating of fossils. ¹²C and ¹³C are the stable isotopes.

Diamond, graphite and fullerenes (like C₆₀).

Catenation uses single, double or triple bonds and creates millions of compounds.

Allotropy means the same element exists in different structural forms.

Catenation tendency

Very high

Catenation tendency

Moderate

Catenation tendency

Low

Catenation tendency

Very low

Catenation tendency

Lowest

Catenation, Isotopes and Allotropes

A triple bond (C≡O) with bond order 3 — and CO is a neutral oxide.

It binds haemoglobin far more strongly than oxygen, causing suffocation. It forms by the incomplete burning of fuels like petrol and diesel.

CO: neutral, triple bond, highly toxic, from incomplete burning. CO₂: acidic, double bonds, not toxic, from complete burning.

CO's lone pair lets it bond to metals, forming metal carbonyls.

Carbon dioxide (CO₂) is an acidic oxide that drives the greenhouse effect and global warming.

Lime-water test: CO₂ turns lime-water milky as calcium carbonate (CaCO₃) forms.

CO

Neutral

CO₂

Acidic

CO

Triple (C≡O)

CO₂

Double bonds

CO

Highly toxic

CO₂

Not toxic

CO

Incomplete burning

CO₂

Complete burning

A compound of only carbon and hydrogen — petrol, LPG and natural gas are everyday examples.

Saturated has only C–C single bonds and the maximum hydrogen. Unsaturated has a C=C double or C≡C triple bond and is more reactive.

Alkane (paraffin): single, CnH2n+2, methane CH₄. Alkene (olefin): one double, CnH2n, ethene C₂H₄. Alkyne: one triple, CnH2n−2, ethyne C₂H₂.

Alkanes (old name paraffin) are saturated and unreactive.

Alkenes (olefins) have one double bond; alkynes have one triple bond.

Bond

Single

Formula

CnH2n+2

Example

Methane CH₄

Bond

One double

Formula

CnH2n

Example

Ethene C₂H₄

Bond

One triple

Formula

CnH2n-2

Example

Ethyne C₂H₂

Hydrocarbons and Their Types

No. It is one strong sigma (σ) bond from head-on overlap (~397 kJ/mol) plus one weaker pi (π) bond from sideways p-orbital overlap (~284 kJ/mol).

134 pm versus 154 pm — the double bond is shorter.

Their loosely held pi electrons are exposed, making alkenes more reactive.

The sigma bond is always the first bond between any two atoms.

Sigma (σ)

Head-on

Pi (π)

Sideways

Sigma (σ)

Stronger (~397)

Pi (π)

Weaker (~284)

Sigma (σ)

First bond

Pi (π)

Second bond

An electron-seeking, electron-deficient species with an empty orbital: H⁺, NO₂⁺, BF₃, Cl⁺.

A nucleophile is electron-rich and donates electrons: OH⁻, NH₃, CN⁻. Negative ions and lone-pair species are NOT electrophiles — and stable cations like Na⁺ are not either, since they have no vacant orbital.

C₆H₆ — a flat six-carbon ring with alternating single and double bonds; the simplest aromatic hydrocarbon.

12 sigma (6 C–C plus 6 C–H) and 3 pi — 15 covalent bonds in all.

Delocalisation — the spreading of its pi electrons around the ring.

It is the simplest aromatic hydrocarbon, a class of unusually stable rings.

Spreading of pi electrons (delocalisation) gives benzene its remarkable stability.

A family of compounds sharing one functional group and a general formula. Each member (a homologue) differs from the next by one –CH₂– unit, so boiling point changes gradually while chemical properties stay similar.

Individual members are called homologues.

Properties like boiling point change gradually as you move up the series.

–OH hydroxyl (alcohol) · –COOH carboxyl (acid) · –CHO (aldehyde) · C=O carbonyl (ketone).

HOOC–COOH (C₂H₂O₄) — the simplest dicarboxylic acid, two –COOH groups joined together.

Acetic acid (CH₃COOH) — sharp and pungent, and it is the acid in vinegar.

A functional group is the reactive atom/group that decides a compound's chemistry.

Alcohols contain –OH; aldehydes contain –CHO; ketones contain C=O.

Carboxylic acids contain the –COOH group and are weak acids.

Formula

–OH

Class

Alcohol

Formula

–COOH

Class

Acid

Formula

–CHO

Class

Aldehyde

Formula

C=O

Class

Ketone

Common Functional Groups and Acids

Methane (CH₄) for both.

Butane and propane — with butane as the major constituent.

Ethyl mercaptan (C₂H₅SH) is deliberately added to LPG and natural gas so leaks can be detected by smell.

Mainly methane (the fuel gas, ~50–70%) with carbon dioxide; produced by anaerobic digestion of animal and plant waste.

Main constituent

Methane

Main constituent

Methane

Main constituent

Butane / Propane

Main constituent

Methane

A large molecule made by joining many small repeating units (monomers).

Isoprene (2-methyl-1,3-butadiene), giving cis-polyisoprene.

Thermoplastics soften on heating and can be remoulded (polythene, PVC, Teflon, polystyrene). Thermosetting plastics set permanently and do NOT soften again (bakelite, melamine).

Natural: cotton, silk, wool, rubber. Synthetic: nylon, polyester, plastics. Neoprene is a synthetic rubber (an elastomer), not a thermoplastic.

Property

Re-mouldable

Example

Polythene, PVC

Property

Permanent set

Example

Bakelite