Introduction to AS Level Organic Chemistry

0. What Examiners Want

Where this topic appears. Recent AS papers have asked for: pairs of structural isomers and counts of isomers of a given formula (including primary halogenoalkanes of C₅H₁₁Br), the number of chiral centres in a drug molecule, whether a molecule shows cis/trans isomerism, which of several molecules are chiral, the type of stereoisomerism in a table, the hybridisation of an atom, the nature of an attacking group (nucleophile) and a leaving group, and the repeat unit or monomer of an addition polymer. Almost every organic question in Papers 1, 2, 4 and 5 uses at least one skill from this topic.

1. Representing Organic Molecules

An organic compound is a compound of carbon (with H, O, N, halogens and so on), apart from the oxides of carbon, carbonates and hydrogencarbonates, which are treated as inorganic. A hydrocarbon contains only carbon and hydrogen. Carbon atoms bond to each other in chains and rings, which is why there are so many organic compounds.

From empirical to molecular formula. Divide each mass (or percentage) by A_r to get moles, divide by the smallest, then scale to whole numbers. Divide the relative molecular mass by the mass of the empirical formula to find the multiple.

Worked example. A compound is 54.5% C, 9.1% H and 36.4% O by mass, with M_r = 88.

• C: 54.5 ÷ 12.0 = 4.54; H: 9.1 ÷ 1.0 = 9.1; O: 36.4 ÷ 16.0 = 2.275
• Divide by 2.275: C : H : O = 2.00 : 4.00 : 1.00, so the empirical formula is C₂H₄O (mass 44).
• 88 ÷ 44 = 2, so the molecular formula is C₄H₈O₂.

2. Homologous Series and Naming

A homologous series is a family of compounds with the same functional group and general formula, and each member differs from the next by CH₂. The functional group is the atom or group of atoms that gives the compound its characteristic reactions.

These general formulae assume one functional group and an open chain. Cycloalkanes, C_nH_2n, do not follow the alkane formula.

Stems for the carbon chain: 1 meth-, 2 eth-, 3 prop-, 4 but-, 5 pent-, 6 hex- (then hept-, oct-, non-, dec-).

Rules for naming

1. Find the longest carbon chain that contains the functional group. This gives the stem.
2. Number the carbon atoms from the end that gives the lowest numbers, with the functional group taking the lowest number.
3. Name each side chain as an alkyl group (methyl, ethyl, propyl). Use di-, tri-, tetra- for repeats, and put the groups in alphabetical order (ignore di- and tri- when ordering).
4. Put commas between numbers and hyphens between numbers and words, for example 3-ethyl-2-methylpentane and 2,2,3-trimethylpentane.
5. A number is not needed when the group can only be at the end of the chain (aldehydes and carboxylic acids, such as pentanal and butanoic acid) or when there is only one possible position (such as methanol or chloroethane).

Aryl compounds contain a benzene ring (for example propylbenzene). With one substituent on the ring no number is needed, because all six ring carbon atoms are equivalent.

3. Bonding and Shapes (σ, π, Hybridisation)

A σ bond is formed by head-on overlap of orbitals and lies on the line between the two nuclei. Every single bond is a σ bond. A π bond is formed by sideways overlap of two p orbitals, with the electron density above and below the plane of the σ bonds. A double bond is one σ plus one π, and a triple bond is one σ plus two π.

• sp³ carbon (alkanes, and the CH₃ or CH₂ parts of any molecule): four σ bonds, tetrahedral, about 109.5°.
• sp² carbon (C=C, and C=O): three σ bonds and one π. The carbon and the atoms directly attached to it lie in one plane, with bond angles of 120°.
• sp carbon (C≡C and C≡N): two σ bonds and two π. The arrangement is linear, 180°.

Worked example: propene, CH₃CH=CH₂. There are 8 σ bonds (2 C–C and 6 C–H) and 1 π bond. The CH₃ carbon is sp³; the two carbon atoms of the double bond are sp².

Rotation. There is free rotation about a σ bond, so a chain can twist into many shapes (all the same molecule). The π bond prevents rotation about C=C, which is the origin of cis/trans isomerism. The electron-rich π bond is also why alkenes are attacked by electrophiles.

4. Structural Isomerism

Isomers are compounds with the same molecular formula but different structures. Structural isomers have different structural formulae.

Isomers you must be able to list

A reliable method. Start with the longest straight chain. Then shorten it by one carbon and place that carbon as a branch on every different position in turn. Repeat with a shorter chain. Move the functional group along each skeleton. Stop placing a group when the position gives a structure you have already drawn (check by finding the longest chain and the lowest numbers).

5. Stereoisomerism

Stereoisomers have the same structural formula (the same atoms bonded in the same order) but a different arrangement of the atoms in space.

Cis/trans (geometrical) isomerism needs:

• restricted rotation, which comes from a C=C double bond (the π bond stops rotation) or from a ring, and
• two different groups on each of the two carbon atoms.

In the cis isomer the two matching groups are on the same side; in the trans isomer they are on opposite sides. The isomers are different compounds, with different physical properties (boiling point, melting point) and sometimes different reactivity. They cannot interconvert at room temperature because the π bond would have to break. Use cis and trans to describe where the matching (identical) groups on each carbon sit. Examples: but-2-ene, pent-2-ene, 1,2-dibromoethene. Not possible: propene, but-1-ene, 2-methylpropene (one carbon of the C=C carries two identical groups).

Optical isomerism arises when a molecule has a chiral centre: a carbon atom bonded to four different atoms or groups. The molecule and its mirror image cannot be superimposed (like left and right hands). The two forms are enantiomers. They have identical physical properties, except that each rotates the plane of plane-polarised light by the same angle in opposite directions.

• Examples with one chiral centre: butan-2-ol, CHBrClF, 2-chlorobutane, 2-hydroxypropanoic acid.
• Molecules with n chiral centres have up to 2ⁿ stereoisomers (two centres give up to four).
• Ring carbon atoms count: test a ring carbon by following the ring both ways. If the two paths are different, the groups are different. In a symmetrical substituted ring the two paths are identical, so there is no chiral centre.
• Carbon atoms with two identical groups (any CH₂ or CH₃) and C=C or C=O carbons are never chiral.

6. Bond Fission, Carbocations and Reaction Types

Organic reactions involve breaking and making covalent bonds. A bond can break in two ways.

• Homolytic fission: each atom keeps one electron of the bonding pair, forming two free radicals (species with an unpaired electron, written with a dot). Example: Cl–Cl → 2Cl• in UV light. Radicals are very reactive. A radical reaction has three stages: initiation (radicals formed from a molecule), propagation (a radical and a molecule give a different radical and molecule, so the chain continues) and termination (two radicals join, so no radical is left).
• Heterolytic fission: the more electronegative atom takes both electrons, giving a positive and a negative ion. Breaking C–Br gives a carbocation, CH₃⁺, and Br⁻.

Nucleophile: an electron-rich species that donates a pair of electrons to form a covalent bond (for example OH⁻, CN⁻, NH₃, H₂O). It is attracted to a δ+ or positive centre. Electrophile: an electron-deficient species that accepts a pair of electrons (for example H⁺, a carbocation, or the δ+ hydrogen of HBr).

Alkyl groups donate electron density along the bond (the positive inductive effect), which reduces the charge on C⁺. The more alkyl groups, the more stable the carbocation: tertiary > secondary > primary. This explains the products of addition to alkenes (Topic 14) and the substitution mechanisms in Topic 15.

Types of reaction

[O] and [H] stand for an oxygen or hydrogen atom supplied by the reagent. Equations written with them must still balance: complete oxidation of ethanol to ethanoic acid is C₂H₅OH + 2[O] → CH₃COOH + H₂O.

7. Quick Sheet and Checklist

Before you leave the question, check:

• The molecular formula counts hidden hydrogen atoms in skeletal formulae.
• The name uses the longest chain, the lowest numbers and alphabetical order.
• Each structural isomer is genuinely different (name each one) and none has been drawn twice.
• Cis/trans: two different groups on each carbon; chirality: four different groups, marked with an asterisk.
• Curly arrows start at a bond or lone pair; full heads for pairs, half heads for single electrons.
• Reaction type is named exactly (addition, substitution, elimination, hydrolysis, condensation, oxidation or reduction).