Covalent bonds in carbon
Carbon (atomic number 6) has the electronic configuration 2, 4. It has 4 valence electrons. To reach a stable octet it would have to gain 4 electrons (forming C⁴⁻) or lose 4 (forming C⁴⁺). Six protons cannot hold 10 electrons tightly, and removing 4 electrons needs a huge amount of energy. So carbon does neither.
Instead carbon shares electrons. A pair of electrons shared between two atoms is a covalent bond. One shared pair is a single bond (–), two pairs a double bond (=) and three pairs a triple bond (≡).
Electron dot structures
In methane (CH₄) carbon shares one electron with each of four hydrogen atoms: 4 single bonds, carbon gets 8 outer electrons and each hydrogen gets 2. In O₂ each oxygen shares 2 electrons (a double bond); in N₂ each nitrogen shares 3 (a triple bond).
Properties of covalent compounds
- No ions are formed, so they are generally poor conductors of electricity.
- Forces between molecules are weak, so they usually have low melting and boiling points.
- The bonds inside a molecule are strong, but the molecules are only loosely held together.
Versatile nature of carbon: catenation and tetravalency
About 3 million-plus known carbon compounds exist, more than all other elements put together. Two special features make this possible:
- Catenation: carbon atoms can bond to other carbon atoms to form long chains, branched chains and rings. The C–C bond is very strong and stable. Silicon shows some catenation, but its chains are short and reactive.
- Tetravalency: carbon has a valency of 4, so each carbon can bond to four other atoms: carbon, hydrogen, oxygen, nitrogen, sulphur, chlorine and more.
Carbon is also small, so its nucleus holds the shared pairs tightly, which makes its bonds strong. Allotropes of carbon (diamond, graphite, fullerene C-60) show the same bonding in giant structures.
Saturated and unsaturated hydrocarbons
A hydrocarbon contains only carbon and hydrogen.
- Saturated hydrocarbons have only single C–C bonds. These are the alkanes, general formula CₙH₂ₙ₊₂ (methane CH₄, ethane C₂H₆).
- Unsaturated hydrocarbons have at least one double or triple C–C bond. Alkenes (C=C) have the formula CₙH₂ₙ, like ethene C₂H₄. Alkynes (C≡C) have CₙH₂ₙ₋₂, like ethyne C₂H₂.
Unsaturated compounds are more reactive (they take part in addition reactions) and usually burn with a yellow, sooty flame.
Chains, branches and rings
Carbon skeletons come in three shapes: straight chains (butane), branched chains (isobutane) and rings (cyclohexane C₆H₁₂, a saturated ring; benzene C₆H₆, a ring with alternate single and double bonds). Compounds with the same molecular formula but different structures are called structural isomers: butane and isobutane are both C₄H₁₀.
Functional groups and heteroatoms
When an atom other than C or H (a heteroatom such as O, Cl, N) replaces hydrogen in a hydrocarbon, it gives the compound its own typical properties. That atom or group is the functional group.
| Group | Formula | Name ends | Example |
|---|---|---|---|
| Halo (chloro/bromo) | –Cl, –Br | prefix chloro-/bromo- | chloropropane |
| Alcohol | –OH | -ol | propanol |
| Aldehyde | –CHO | -al | propanal |
| Ketone | >C=O | -one | propanone |
| Carboxylic acid | –COOH | -oic acid | propanoic acid |
| Double bond | C=C | -ene | propene |
| Triple bond | C≡C | -yne | propyne |
Homologous series
A homologous series is a family of compounds with the same functional group and the same general formula, in which each member differs from the next by one –CH₂– unit (14 u of mass).
- Same functional group, so similar chemical properties.
- Molecular mass goes up step by step, so physical properties change gradually: melting point, boiling point and density rise; solubility in water falls.
Examples: CH₃OH, C₂H₅OH, C₃H₇OH (alcohols); CH₄, C₂H₆, C₃H₈ (alkanes); C₂H₄, C₃H₆, C₄H₈ (alkenes).
Nomenclature (naming) of carbon compounds
Follow three steps:
- Count the carbons in the longest chain and pick the root: 1 meth, 2 eth, 3 prop, 4 but, 5 pent, 6 hex.
- Find the functional group. If it is a prefix group (chloro, bromo), write it in front: chloroethane.
- If it is a suffix group, drop the final "e" of the alkane name when the suffix starts with a vowel and add the ending: propane → propanol, propanal, propanone, propanoic acid. For double and triple bonds change "ane" to "ene" or "yne".
Board exams usually ask you to name a structure, draw the structure from a name, or spot the functional group (2–3 marks).
Key formulas and definitions
- Alkane: CₙH₂ₙ₊₂ (single bonds, saturated)
- Alkene: CₙH₂ₙ (one C=C, unsaturated)
- Alkyne: CₙH₂ₙ₋₂ (one C≡C, unsaturated)
- Successive members differ by –CH₂– (14 u)
- Roots: meth 1, eth 2, prop 3, but 4, pent 5, hex 6
- Endings: -ol (–OH), -al (–CHO), -one (>C=O), -oic acid (–COOH), -ene (C=C), -yne (C≡C)
Worked examples
1. How many covalent bonds are there in a molecule of ethane, C₂H₆?
One C–C bond + six C–H bonds = 7 single covalent bonds (7 shared pairs).
2. Is C₃H₆ saturated or unsaturated? Name it.
For n = 3, an alkane would be C₃H₈. C₃H₆ fits CₙH₂ₙ, so it is an alkene with one C=C: propene. It is unsaturated.
3. Write the next two members of the series CH₃OH, C₂H₅OH.
Add –CH₂– each time: C₃H₇OH (propanol) and C₄H₉OH (butanol).
4. Name CH₃–CH₂–CHO.
3 carbons → prop. Group –CHO → aldehyde, ending -al. Propane → propanal.
5. Name CH₃–CO–CH₃ and draw its functional group.
3 carbons, a >C=O in the middle → ketone, ending -one: propanone (acetone).
6. Draw the electron dot structure of ethene and say why it is unsaturated.
Each carbon shares one pair with each of two hydrogens and two pairs with the other carbon: H₂C::CH₂. The C=C double bond means it can add more atoms, so it is unsaturated.
Common mistakes
- Writing that carbon forms C⁴⁺ or C⁴⁻ ions. Carbon shares electrons; it does not form these ions in simple compounds.
- Counting a double bond as one shared electron pair. A double bond is two pairs (4 electrons); a triple bond is three pairs (6 electrons).
- Naming CH₃CH₂OH as "ethanoil" or "ethaneol". Drop the "e" before a vowel ending: ethanol.
- Thinking members of a homologous series have the same physical properties. Chemical properties are similar; physical ones change gradually with mass.