Introduction to organic compounds

Every plastic bag, every drop of cooking oil, the sugar in your tea, the vinegar in your salad, the DNA inside every one of your cells — all of it is built from one element behaving in one very unusual way. Carbon can bond to four other atoms at once, and, more remarkably, it can bond to itself again and again to build chains and rings of almost any length. Out of this single trick comes organic chemistry: the study of carbon compounds, a family so vast that it dwarfs the combined compounds of every other element in the periodic table put together. This article answers two questions. First, why carbon and not some other element — what two properties of carbon cause this explosion of compounds? Second, how do chemists sort millions of carbon compounds into a few manageable buckets — chain versus ring, saturated versus unsaturated? By the end you can also predict how many hydrogen atoms a straight-chain saturated hydrocarbon carries, just from counting its carbon atoms.

The core insight

Because carbon is tetravalent (forms 4 covalent bonds) and shows exceptional catenation (carbon-to-carbon self-linking), it builds chains and rings of almost unlimited length and variety, which chemists classify by shape (chain or cyclic) and by bond type (saturated or unsaturated).

The core idea

Organic chemistry is the branch of chemistry that studies compounds of carbon. A handful of simple carbon compounds — carbon dioxide and other oxides of carbon, carbonates, bicarbonates, and metal carbides — are, by long-standing convention, still classed as inorganic; everything else built around a carbon skeleton is organic.

Two properties of carbon, working together, explain why: Carbon has 4 valence electrons, and in essentially every stable compound it completes its octet by forming exactly 4 covalent bonds. This property is called tetravalency. A single carbon atom can therefore attach to up to four other atoms or groups at once, which already gives enormous room for structural variety. The second property is catenation — the exceptional tendency of carbon atoms to bond to other carbon atoms, forming long straight chains, branched chains, or closed rings, all held together by strong, stable covalent bonds. No other element in the periodic table forms self-linked chains anywhere near as long or as stable as carbon does; even silicon, carbon's neighbour in Group 14, manages only short and comparatively weak Si–Si chains.

Classifying carbon compounds. Once you accept that carbon can link to itself in almost any pattern, chemists sort the resulting compounds along two independent axes. By shape: a chain (open-chain, or acyclic) compound has its carbon atoms strung in a chain with two free ends, whether straight or branched — propane, CH3−CH2−CH3CH_3-CH_2-CH_3, is a simple example. A cyclic (ring) compound has its carbon atoms closed into a loop with no free ends (at Class 10 level this is recognised only descriptively; benzene-type aromatic rings are outside this syllabus). By bond type: a saturated hydrocarbon contains only single covalent bonds between its carbon atoms — these are the alkanes. An unsaturated hydrocarbon contains at least one carbon–carbon double bond (an alkene) or triple bond (an alkyne). The word "saturated" is chosen deliberately: a saturated chain is completely "full" of hydrogen, carrying the maximum number of hydrogen atoms its carbon skeleton can hold, while an unsaturated chain has "room" for more hydrogen if the double or triple bond is broken open. A carbon compound's chemical personality is set not just by its skeleton but by its functional group — a specific atom or small group of atoms, such as –OH (alcohols) or –COOH (carboxylic acids), attached to the carbon chain. Two molecules with very similar skeletons can behave completely differently once different functional groups are attached; each functional group is studied as its own topic later in this unit. Keep the two ideas separate: tetravalency fixes how many bonds each carbon atom can make (always 4); catenation is the fact that carbon prefers to spend some of those bonds linking to other carbon atoms, which is what makes chains possible at all.

Figure
A chain of four carbon atoms in butane showing each carbon using two bonds to its neighbours and the remaining bonds filled by hydrogen atoms
Butane C4H10 — 4 carbons, each carbon uses all 4 bonds C C C C end carbon: 1 C-C bond + 3 C-H bonds internal carbon: 2 C-C bonds + 2 C-H bonds Every carbon still totals exactly 4 bonds — tetravalency never changes
Derivation APredicting the hydrogen count of a straight-chain saturated hydrocarbon

The goal of this derivation is to turn the two ideas above, tetravalency and catenation, into a single counting rule that predicts exactly how many hydrogen atoms are attached to a straight-chain saturated hydrocarbon (an alkane) once you know how many carbon atoms, nn, it contains, without having to draw out every bond by hand each time.

Step 1. Count the total number of bond-connections carbon has available. Each of the nn carbon atoms is tetravalent, so each one contributes 4 bond-ends. Across the whole molecule this gives a total of 4n4n bond-ends waiting to be used, whether for linking to another carbon or to a hydrogen atom.

Step 2. Count how many of those bond-ends are used up joining carbon to carbon. An open (straight) chain of nn carbon atoms has exactly (n−1)(n-1) carbon–carbon single bonds — one fewer bond than the number of atoms, since a chain of nn beads needs (n−1)(n-1) links to join them in a row. Each of those C–C bonds uses one bond-end from each of the two carbons it joins, so it consumes 22 bond-ends per bond, for a total of 2(n−1)2(n-1) bond-ends spent on the carbon skeleton itself.

Step 3. Subtract to find what is left for hydrogen. Since the chain is saturated, every remaining bond-end must be filled by a hydrogen atom (a single C–H bond, no double or triple bonds anywhere). The number of hydrogen atoms is therefore 4n−2(n−1)=4n−2n+2=2n+24n - 2(n-1) = 4n - 2n + 2 = 2n+2.

H=2n+2so the general formula is CnH2n+2.(1)H = 2n + 2 \qquad \text{so the general formula is } C_nH_{2n+2}. \qquad (1)

where: nn is the number of carbon atoms in the straight (unbranched) saturated chain, and H=2n+2H = 2n+2 is the number of hydrogen atoms the formula CnH2n+2C_nH_{2n+2} predicts.

Step 4. Check the rule against compounds you already know. For propane, n=3n=3, so H=2(3)+2=8H = 2(3)+2 = 8, giving C3H8C_3H_8 — correct. For butane, n=4n=4, H=2(4)+2=10H = 2(4)+2 = 10, giving C4H10C_4H_{10} — exactly the formula used in the worked reasoning above. The rule CnH2n+2C_nH_{2n+2} holds for every straight-chain alkane, because the bond-counting argument never depended on which specific value of nn was chosen.

Worked example

Question: A straight, unbranched, fully saturated hydrocarbon chain has 6 carbon atoms and no ring. Predict its molecular formula, and verify the hydrogen count by bond counting.

Step 1. Apply the general formula derived above, CnH2n+2C_nH_{2n+2}, with n=6n = 6: H=2(6)+2=14H = 2(6) + 2 = 14. So the predicted molecular formula is C6H14C_6H_{14}.

Step 2. Verify by direct bond counting, the way the derivation was built. Total bond-ends available: 4n=4×6=244n = 4 \times 6 = 24. Bond-ends used for the carbon skeleton: an open chain of 6 carbons needs (n−1)=5(n-1) = 5 C–C bonds, using 2×5=102 \times 5 = 10 bond-ends. Bond-ends left for hydrogen: 24−10=1424 - 10 = 14, matching Step 1 exactly.

Step 3. Name the shape and bond type. Because the chain has two free ends and no ring, it is a chain (open-chain) compound; because every bond is a single bond, it is saturated — this is the alkane called hexane.

Final answer: the molecular formula is C6H14C_6H_{14}, with 14 hydrogen atoms, and the compound is the saturated open-chain hydrocarbon hexane.

Competitive stretch

If the same 6-carbon chain instead had one carbon–carbon double bond (an alkene), that double bond uses 2 extra bond-ends within the skeleton, leaving 2 fewer for hydrogen: H=14−2=12H = 14 - 2 = 12, giving C6H12C_6H_{12}. This matches the general pattern for a one-double-bond alkene, CnH2nC_nH_{2n} — always 2 hydrogens short of the saturated count for the same nn, exactly what "unsaturated" means.

Common traps

  • Thinking "organic" means "made by living things" — modern organic chemistry means the chemistry of carbon compounds, the overwhelming majority of which are made in laboratories and factories, not extracted from organisms.
  • Mixing up tetravalency and catenation — tetravalency fixes the fixed number of bonds (always 4) each carbon forms; catenation is carbon's specific willingness to spend some of those bonds linking to other carbons, which is what actually builds the chain.
  • Forgetting that carbonates, bicarbonates, oxides of carbon (CO, CO2) and metal carbides are conventionally still classed as inorganic even though every one of them contains carbon.
  • Applying CnH2n+2C_nH_{2n+2} to a branched chain, a ring, or an unsaturated compound — the formula was derived strictly for a straight-chain, fully saturated, acyclic hydrocarbon; a ring or a double bond changes the bond count and the formula.

Ask the tutor: Want to practise predicting molecular formulas for longer chains, or work through how the general formula changes for alkenes and alkynes? Ask the AI tutor for a few more examples.

Why doesn't an element like oxygen or nitrogen form long self-linked chains the way carbon does

Oxygen has only 2 valence electrons available for bonding and typically forms just 2 bonds, so any O–O–O chain has very few spare bonds left for anything else, and O–O bonds themselves are comparatively weak and reactive. Carbon's 4 available bonds mean that even in a long chain, each carbon still has spare bonds left over for hydrogen or other atoms, and its C–C bonds are strong and stable. That combination, not just having 4 bonds, is what makes carbon's chains so much longer and more stable.

Does the formula CnH2n+2 work for cyclic compounds too

No. That formula was derived specifically by counting the bonds used in an open chain, which needs only (n−1)(n-1) carbon–carbon bonds to link nn carbons in a row. A ring needs one extra carbon–carbon bond to close the loop, using 2 more bond-ends, so a cyclic saturated hydrocarbon with nn carbons instead carries 2n2n hydrogen atoms, two fewer than the open-chain version with the same number of carbons.

Is silicon a second element that shows real catenation like carbon

Silicon sits directly below carbon in Group 14 and is also tetravalent, so it can form Si–Si chains in principle, and a few silicon-chain compounds do exist. But Si–Si bonds are noticeably weaker than C–C bonds and react much more readily with oxygen and moisture, so silicon chains stay short and unstable compared to carbon's. This is why silicon-based life or silicon-based "organic-style" chemistry never rivals carbon's, even though silicon shares carbon's tetravalency.

Before this

Where this leads