An introduction to AS Level organic chemistry

A hydrocarbon 碳氢化合物 is a compound of only carbon and hydrogen. Alkanes 烷烃 are the simplest hydrocarbons and have no functional group.

A functional group 官能团 is the reactive part of a molecule. It decides the physical and chemical properties of the compound, so molecules with the same functional group behave alike (for example the $\text{–OH}$ group in alcohols).

Types of formula

You can read off the empirical formula 实验式 (simplest ratio) from any of these.

Naming

Use systematic nomenclature 命名法: a stem for the number of carbons (meth-, eth-, prop-, but-, pent-, hex- for 1 to 6), an ending for the functional group, and numbers to show where groups are.

Some key terms

• a homologous series 同系列 is a family of compounds with the same functional group and general formula, where each member differs by $\text{CH}_2$.
• a saturated 饱和 compound has only single C–C bonds; an unsaturated 不饱和 compound has a C=C double bond (or a triple bond).

Breaking bonds

A covalent bond can break in two ways:

• homolytic fission 均裂: the bond splits evenly, one electron to each atom. This makes two free radicals 自由基 (species with an unpaired electron).
• heterolytic fission 异裂: the bond splits unevenly, both electrons going to one atom. This makes two ions.

Attacking species

• a nucleophile 亲核试剂 is a species with a lone pair that is attracted to a positive (electron-poor) centre.
• an electrophile 亲电试剂 is a species attracted to a negative (electron-rich) centre, such as a C=C double bond.

Types of reaction

For organic redox, the symbol $[\text{O}]$ stands for one oxygen atom from an oxidising agent, and $[\text{H}]$ for one hydrogen atom from a reducing agent.

Types of mechanism

A free-radical reaction happens in three steps: initiation 引发 (radicals are made), propagation 增长 (radicals react and make new radicals), and termination 终止 (two radicals join and stop the chain).

The main mechanisms you meet are free-radical substitution 自由基取代 (alkanes), electrophilic addition 亲电加成 (alkenes), nucleophilic substitution 亲核取代 (halogenoalkanes) and nucleophilic addition 亲核加成 (carbonyls). In mechanisms, a curly arrow 弯箭头 shows a pair of electrons moving; it starts at a bond or a lone pair 孤对电子.

Organic molecules can be straight-chained, branched or cyclic 环状 (in a ring).

The shape around a carbon depends on its hybridisation 杂化:

Every single bond is a sigma bond σ键, made by direct overlap. A double bond is one sigma bond plus one pi bond π键, made by sideways overlap of p orbitals. Ethene is planar because of its $\text{sp}^2$ carbons.

Isomers are compounds with the same molecular formula but a different arrangement of atoms. This is called isomerism 异构.

Structural isomerism

In structural isomerism 结构异构 the atoms are joined in a different order. There are three kinds:

• chain isomerism 链异构: the carbon chain is branched in different ways.
• positional isomerism 位置异构: the functional group is on a different carbon.
• functional group isomerism 官能团异构: the atoms form a different functional group (for example an alcohol and an ether).

Stereoisomerism

In stereoisomerism 立体异构 the atoms are joined in the same order but point in different directions in space.

• geometrical isomerism 几何异构 (cis/trans) happens at a C=C double bond. The pi bond stops the two carbons rotating (restricted rotation 受限旋转), so groups are fixed on the same side (cis 顺式) or opposite sides (trans 反式).

• optical isomerism 旋光异构 happens at a chiral 手性 carbon — a chiral centre 手性中心 is a carbon with four different groups attached. Such a carbon gives two mirror-image forms called enantiomers 对映体. A molecule may have more than one chiral centre.

From a molecular formula you can deduce the possible isomers by trying different chains, positions and functional groups.

Worked example. Explain why but-2-ene shows cis-trans (E/Z) isomerism but but-1-ene does not. Stereoisomerism at a C=C needs two things: restricted rotation about the double bond (both alkenes have that), and two different groups on each of the two double-bond carbons. In but-2-ene, $\text{CH}_3\text{CH=CHCH}_3$, each double-bond carbon carries an $\text{H}$ and a $\text{CH}_3$ - different, so the methyl groups can sit on the same side (cis / Z) or on opposite sides (trans / E). In but-1-ene, $\text{CH}_2\text{=CHCH}_2\text{CH}_3$, the first carbon carries two hydrogens - identical, so swapping them changes nothing and only one form exists. Test each double-bond carbon separately: two identical groups on either carbon kills the isomerism, however different the other carbon may be.

• Know the difference between general, molecular, structural, displayed and skeletal formulas — questions ask for a specific one.
• Name systematically: longest chain, lowest locants, substituents alphabetical; a wrong locant loses the mark.
• Classify each reaction by type and reagent (addition, substitution, elimination, oxidation).
• E/Z isomerism needs restricted rotation about a $\text{C}=\text{C}$ and two different groups on each carbon.