NDA Chemistry · Carbon and Its Compounds

Tetra-valency, Catenation and Isomerism

Carbon forms four covalent bonds and links to itself in chains and rings, so a handful of atoms can be arranged many different ways — which is why carbon has more compounds than any other element.

All Carbon and Its Compounds notes NDA Chemistry strategy

Why this matters

The foundation of the whole chapter. The bank tests two ideas directly — the properties that make carbon special (tetra-valency + catenation), and counting the structural isomers of a small alkane. Get these two right and every later allotrope and functional-group fact has a frame to hang on.

Concept 1 of 2: Why carbon forms so many compounds

Carbon has 4 electrons in its outer shell, so it shares 4 electrons to complete its octet — it forms exactly four covalent bonds (tetra-valency). It is also small, so those bonds are strong, and it bonds happily to other carbons — chains, branches and rings of any length (catenation). Four strong bonds plus self-linking is the whole reason organic chemistry exists.

Definition

The two defining properties and what follows from them:

Tetra-valency — carbon (atomic number 6, configuration 2,4) shares 4 electrons to form four covalent bonds.
Catenation — carbon atoms bond to one another in long chains, branches and rings; no other element does this to the same extent.
Carbon forms single, double and triple bonds with itself — but never a quadruple (four) bond.
Bonds are covalent, so most carbon compounds are poor conductors of electricity, have low melting points and are often volatile.

Worked example

Carbon has the electron configuration 2,4. Explain how many bonds it forms and why it cannot form a positive or negative ion easily.

Practice this concept4 quick reps

The same idea in a real exam question:

NDA · Sep 2023 · Q117Moderate

Example 1 · Carbon and Its Compounds · Catenation, Tetra-valency and Isomerism

Which among the following statements with respect to carbon is/are correct? 1. Carbon forms the basis for all living organisms and many things we use 2. Carbon shows tetra-valency and the property of catenation 3. Carbon forms covalent bonds with itself and other elements 4. Carbon forms compounds containing triple and tetra bonds between carbon atoms Select the correct answer using the code given below:

Carbon forms a triple bond, not a 'four' bond

A statement that 'carbon forms compounds with quadruple bonds between carbon atoms' is false. The maximum is a triple bond (as in ethyne, HC≡CH).

Covalent means poor conductor

'Most carbon compounds are good conductors of electricity' is NOT correct — covalent compounds have no free charges. (Graphite is the famous exception, because of its delocalised electrons.)

Drill 2 more on why carbon forms so many compounds

Concept 2 of 2: Structural isomerism and counting isomers

The same molecular formula can be built in more than one way — straight chain, branched chain, or a different branch position. These are structural isomers: same atoms, different skeletons, different compounds. For the small alkanes the bank simply asks 'how many isomers?', so learn the running count.

Definition

Structural isomers share a molecular formula but differ in the arrangement of atoms. The number of chain isomers of the straight-chain alkanes:

C4H10 (butane) → 2 isomers (n-butane, isobutane).
C5H12 (pentane) → 3 isomers (n-pentane, isopentane, neopentane).
C6H14 (hexane) → 5 isomers.

Methane, ethane and propane have no chain isomers (only one possible skeleton).

Structural isomer counts (alkanes)

\text{C}_4\text{H}_{10}\!: 2 \qquad \text{C}_5\text{H}_{12}\!: 3 \qquad \text{C}_6\text{H}_{14}\!: 5

Worked example

How many structural isomers does butane (C4H10) have? Name them.

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

NDA · Sep 2022 · Q107Moderate

Example 2 · Carbon and Its Compounds · Catenation, Tetra-valency and Isomerism

The number of structural isomers of pentane is

Pentane has 3 isomers, not 5

Students confuse the number of carbons with the number of isomers. C5H12 has 3 isomers (the carbon count is 5, the isomer count is 3). Hexane (6 carbons) is the one with 5 isomers.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Structural isomerism and counting isomers
Structural isomer counts (alkanes)
$\text{C}_4\text{H}_{10}\!: 2 \qquad \text{C}_5\text{H}_{12}\!: 3 \qquad \text{C}_6\text{H}_{14}\!: 5$

Watch out for (3)

Carbon forms a triple bond, not a 'four' bond
→ Why carbon forms so many compounds
Covalent means poor conductor
→ Why carbon forms so many compounds
Pentane has 3 isomers, not 5
→ Structural isomerism and counting isomers

Test yourself on Carbon and Its Compounds

15 past NDA questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Take the Carbon and Its Compounds chapter test Drill all 4 questions