Introduction to Organic Chemistry
Senior 5 · Chemistry
Introduction to Organic Chemistry
Lesson 1 of 4
25 min
Caffeine, the stimulant in a cup of Rwandan coffee or tea, has the molecular formula . Its carbon atoms sit in two fused rings, and those rings also contain nitrogen atoms. The butane in a camping-gas cylinder has its four carbon atoms in an open chain. Methylbenzene, a common paint thinner, has six carbon atoms in a flat ring whose electrons are shared all the way round. There are many millions of organic compounds, and the first step in making sense of them is to sort them: first by the shape of the carbon skeleton, then by the functional groups attached to it.
In Senior 3 you saw why carbon forms so many compounds: it makes four bonds and joins to itself in chains and rings. This lesson goes one level deeper and asks how carbon makes those four bonds, because that decides the shape of every molecule in this unit and the next seven.
Definition
Organic compound: a compound of carbon, usually with hydrogen, in which carbon atoms are bonded to each other or to hydrogen. Carbon monoxide, carbon dioxide, carbonates, hydrogencarbonates and cyanides are traditionally treated as inorganic.
Acyclic (open-chain or aliphatic) compound: one whose carbon atoms form chains, straight or branched, with no ring. Example: butane, .
Carbocyclic compound: one with a ring made only of carbon atoms. It is alicyclic if the ring behaves like an open chain (cyclohexane, ) and aromatic if it contains a benzene-type ring of delocalised π electrons (benzene, ).
Heterocyclic compound: one whose ring contains at least one atom other than carbon, usually N, O or S. Examples: pyridine, ; caffeine.
Saturated: having only single bonds between carbon atoms. Unsaturated: having at least one C=C or C≡C bond.
Figure 1.1: Classifying organic compounds by their carbon skeleton.
A second, independent way to sort compounds is by functional group, the atom or group of atoms that gives a family its typical reactions: –OH in alcohols, C=O in aldehydes and ketones, –COOH in carboxylic acids. Lesson 3 lists them all. A compound can belong to several boxes at once: cyclohexanol is alicyclic, saturated and an alcohol.
Definition
Primary (1°), secondary (2°), tertiary (3°) and quaternary (4°) carbon: a carbon atom bonded to one, two, three or four other carbon atoms. The same labels are used later for halogenoalkanes and alcohols, where they decide how a compound reacts.
Figure 1.2: The carbon atoms of 2,2,4-trimethylpentane ("isooctane", the reference fuel for octane numbers), classified by how many other carbon atoms each one is bonded to.
A carbon atom in its ground state has the configuration : only two unpaired electrons. Yet in methane all four C–H bonds are identical. The hybridisation model explains this. One 2s electron is promoted to the empty 2p orbital, giving four unpaired electrons, and the 2s orbital then mixes with some or all of the 2p orbitals to make a set of identical hybrid orbitals. How many p orbitals join the mixture depends on how many atoms the carbon is bonded to.
Figure 1.3: The hybridisation of carbon sets the shape around it. A wedge bond points towards you; a hashed bond points away from you.
Definition
Sigma (σ) bond: a covalent bond formed by head-on overlap of two orbitals along the line joining the nuclei. Every single bond is a σ bond.
Pi (π) bond: a covalent bond formed by sideways overlap of two parallel p orbitals, with electron density above and below the bond axis. A double bond is one σ + one π bond; a triple bond is one σ + two π bonds.
Figure 1.4: A σ bond forms by head-on overlap along the axis between the nuclei; a π bond forms by sideways overlap of p orbitals, above and below that axis.
Because a π bond needs the two p orbitals to stay parallel, the atoms cannot rotate about a C=C bond without breaking it. This restricted rotation is the origin of cis–trans isomerism (Lesson 4). Sideways overlap is also less effective than head-on overlap, so a π bond is weaker than a σ bond and is the first to break when an alkene reacts (Unit 3).
Table 1.1: The three kinds of carbon
| Hybridisation | Atoms bonded to the carbon | Bonds formed | Shape around carbon | Bond angle | Example |
|---|---|---|---|---|---|
| 4 | 4 σ | tetrahedral | 109.5° | methane, every carbon in ethane | |
| 3 | 3 σ + 1 π | trigonal planar | 120° | ethene, the carbon of C=O in methanal | |
| 2 | 2 σ + 2 π | linear | 180° | ethyne, the carbon in |
Table 1.2: Carbon–carbon bonds compared (typical data-book values)
| Bond | Made of | Bond length / pm | Mean bond enthalpy / kJ mol⁻¹ |
|---|---|---|---|
| C–C | 1 σ | 154 | 347 |
| C=C | 1 σ + 1 π | 134 | 612 |
| C≡C | 1 σ + 2 π | 120 | 838 |
More shared electron pairs pull the nuclei closer, so the bond gets shorter and stronger. But C=C (612) is less than twice C–C (): the extra π bond adds only about 265 kJ mol⁻¹.
Worked example
Problem. Classify each compound by its skeleton and say whether it is saturated: (a) ; (b) cyclohexane; (c) methylbenzene; (d) oxolane, a five-membered ring of four groups and one oxygen atom.
Solution. (a) An open chain with a C=C bond: acyclic, unsaturated. (b) A ring of six carbons, all single bonds: carbocyclic, alicyclic, saturated. (c) A benzene ring with a methyl group: carbocyclic, aromatic; the ring is unsaturated. (d) The ring contains an oxygen atom: heterocyclic, and saturated because every bond in the ring is single.
Worked example
Problem. Propenenitrile, , is used to make acrylic fibres. State the hybridisation of each carbon atom and the bond angle around it, and count the σ and π bonds in the molecule.
Solution. Count the atoms bonded to each carbon.
σ bonds: 3 C–H + C1–C2 + C2–C3 + C3–N = 6 σ. π bonds: 1 in C=C + 2 in C≡N = 3 π.
Warning
"Aromatic" is about structure, not smell. Cyclohexane has a ring of six carbons but no delocalised π electrons, so it is alicyclic, not aromatic. And do not count a double bond as "two single bonds": it is one σ and one π bond, and the π bond is the weaker one.
Tip
For any carbon, count the atoms bonded to it (lone pairs count too, for N and O): 4 → , 109.5°; 3 → , 120°; 2 → , 180°. Then σ bonds = number of bonded atoms, and every extra bond in a multiple bond is a π bond.
Chemistry Student: sp2 and sp3 Hybridisation of Carbon (11 min). Watch how one 2s electron is promoted and how the orbitals mix, and how sideways overlap of the leftover p orbital gives the π bond in ethene. The same reasoning with one p orbital gives the sp carbon of ethyne.
Practice
Sources and credits
