有机化学

There are several ways to write an organic molecule:

• The molecular formula 分子式 shows how many of each atom there are, for example $\text{C}_2\text{H}_6$.
• The displayed formula 结构式 shows every atom and every bond drawn out in full.
• The structural formula 结构简式 shows how the atoms are arranged without drawing every bond, for example $\text{CH}_3\text{CH}_2\text{OH}$.
• The general formula 通式 works for a whole family, for example $\text{C}_n\text{H}_{2n+2}$ for alkanes.

A homologous series 同系物 is a family of compounds with the same functional group 官能团 — the atom or group of atoms that gives the family its chemical properties. Members of a series have the same general formula, differ by a $\text{CH}_2$ unit each step, and have similar chemical properties with a gradual change in physical properties.

These are the simple open-chain families used here. For alkanes, alkenes and alcohols, $n$ is the total carbon count. In $\text{C}_n\text{H}_{2n+1}\text{COOH}$, $n$ counts the alkyl-group carbons; the COOH group adds one more carbon.

A saturated 饱和 compound has only single carbon–carbon bonds. An unsaturated 不饱和 compound has one or more carbon–carbon bonds that are not single (such as a C=C double bond).

Structural isomers 同分异构体 are compounds with the same molecular formula but different structural formulae. For example, $\text{C}_4\text{H}_{10}$ can be a straight chain or a branched chain.

The end of the name tells you the family:

The start of the name tells you the number of carbon atoms: meth- = 1, eth- = 2, prop- = 3, but- = 4. For longer alkenes and alcohols, a number shows where the functional group is, for example but-1-ene and but-2-ene, or propan-1-ol and propan-2-ol.

The three fossil fuels 化石燃料 are coal 煤, natural gas 天然气 and petroleum 石油 (crude oil). Methane 甲烷 is the main part of natural gas.

A hydrocarbon 碳氢化合物 is a compound made of hydrogen and carbon only. Petroleum is a mixture of many different hydrocarbons.

Fractional distillation

Petroleum is separated into useful fractions 馏分 by fractional distillation 分馏. The mixture is heated, and the different hydrocarbons turn to gas and rise up a tall fractionating column 分馏塔. The column is hot at the bottom and cool at the top, so each fraction turns back to liquid at a different height.

Going from the bottom to the top of the column, the fractions have:

• shorter chain length 链长 (smaller molecules);
• higher volatility 挥发性 (they turn to gas more easily);
• lower boiling points;
• lower viscosity 黏度 (they flow more easily).

Alkanes 烷烃 have only single covalent bonds, so they are saturated hydrocarbons. They are generally unreactive. Their two important reactions are:

• Combustion 燃烧: they burn in plenty of oxygen to give carbon dioxide and water.
• Substitution with chlorine.

In a substitution reaction 取代反应, one atom (or group of atoms) is replaced by another. Alkanes react with chlorine only in ultraviolet light 紫外线 — this is a photochemical reaction 光化学反应, where the light provides the activation energy 活化能. For example:

Alkenes 烯烃 have a carbon–carbon double bond (C=C), so they are unsaturated hydrocarbons.

Cracking

Large alkane molecules are not very useful. Cracking 裂化 breaks them into smaller, more useful molecules — smaller alkanes and alkenes — using a high temperature and a catalyst 催化剂. Cracking also makes hydrogen 氢气 and provides alkenes for making plastics.

Worked example. Cracking one molecule of decane, $\text{C}_{10}\text{H}_{22}$, gives octane, $\text{C}_8\text{H}_{18}$, and one other product. Identify it, and say how you would tell the two products apart. Atoms are conserved, so subtract: carbon $10 - 8 = 2$, hydrogen $22 - 18 = 4$. The other product is $\text{C}_2\text{H}_4$, ethene - an alkene, because it fits $\text{C}_n\text{H}_{2n}$. To tell them apart, shake each with orange bromine water: ethene has a C=C, so it decolourises the bromine water, while octane is saturated and leaves it orange. Balance a cracking equation by counting atoms on each side; the alkene is whatever is left over.

Reactions of alkenes

The C=C double bond makes alkenes reactive. They take part in addition reactions 加成反应, where two molecules join to form a single product.

• Test for unsaturation: shake the compound with bromine 溴 water (which is orange). An alkene turns the bromine water colourless; an alkane does not change it.

• With hydrogen and a nickel 镍 catalyst, an alkene becomes an alkane.
• With steam 蒸汽 and an acid catalyst, an alkene becomes an alcohol.

(Supplement) Drawing the product. Only one product forms in an addition reaction. To draw it, change the C=C into a single bond, then add one part of the added molecule to each of the two carbons. Br₂ adds Br and Br; H₂ adds H and H; H₂O adds H and OH. Every carbon still has four bonds. With propene, $\text{CH}_2\text{=CHCH}_3$, the same rules give $\text{CH}_2\text{BrCHBrCH}_3$ with bromine, and propane with hydrogen.

Alcohols 醇 contain the –OH functional group. The most important one is ethanol 乙醇. There are two ways to make ethanol.

In fermentation, yeast 酵母 turns glucose 葡萄糖 into ethanol and carbon dioxide.

Ethanol burns well (combustion), so it is used as a fuel. It also dissolves many substances, so it is used as a solvent 溶剂.

Carboxylic acids 羧酸 contain the –COOH functional group. Ethanoic acid 乙酸 is the one to know. Like other acids, it reacts with:

• metals 金属 → a salt + hydrogen;
• bases 碱 → a salt 盐 + water;
• carbonates 碳酸盐 → a salt + water + carbon dioxide.

The salts formed are called ethanoates.

Ethanoic acid can be made by the oxidation 氧化 of ethanol, either using acidified potassium manganate(VII) 高锰酸钾, or by bacteria during the making of vinegar 醋.

When a carboxylic acid reacts with an alcohol (using an acid catalyst), it forms an ester 酯. For example, ethanol + ethanoic acid → ethyl ethanoate + water. The ester's functional group is $-\text{COO}-$, and you name it from its two parents: the alcohol gives the first part with an -yl ending (ethyl), the acid gives the second part with an -oate ending (ethanoate). Its structural formula is $\text{CH}_3\text{COOCH}_2\text{CH}_3$; a displayed formula draws every bond, including the C=O.

A polymer 聚合物 is a very large molecule built from many small molecules called monomers 单体.

Addition polymerisation

In addition polymerisation 加聚, many unsaturated monomers join together with no other product. For example, many ethene monomers join to make poly(ethene). The small part that repeats along the chain is the repeat unit 重复单元.

(Supplement) From monomer to repeat unit, and back. Draw the two carbons of the C=C side by side with a single bond between them. Keep every other group exactly where it was on those carbons. Then add a bond out from each carbon through the brackets, and write $n$ after them.

Worked example. A repeat unit has two carbons in the main chain, and each carbon carries one H and one CH₃. Find the monomer. Go backwards: take the two carbons out of the brackets, remove the bonds through the brackets, and put a C=C between them, keeping the groups. The monomer is $\text{CH}_3\text{CH=CHCH}_3$, but-2-ene. It has four carbons, but only the two of the C=C go into the main chain; the other two stay as side groups. For these simple alkene examples, the two atoms of the C=C become the two main-chain carbons in the repeat unit.

Condensation polymerisation

In condensation polymerisation 缩聚, monomers join and a small molecule (usually water) is lost each time a bond forms. This makes two important types:

• Polyamides 聚酰胺 are made from a dicarboxylic acid and a diamine. Nylon 尼龙 is a polyamide.
• Polyesters 聚酯 are made from a dicarboxylic acid and a diol. PET is a polyester, and it can be broken back into its monomers and re-made.

The new bond joining the monomers is the linkage: an amide linkage 酰胺键 (written $-\text{CO-NH}-$) in a polyamide such as nylon, and an ester linkage 酯键 (written $-\text{CO-O}-$) in a polyester such as PET. A repeat unit is drawn in brackets, e.g. nylon as $-[\text{CO-}(\text{CH}_2)_4\text{-CO-NH-}(\text{CH}_2)_6\text{-NH}]-$; in an exam you may be asked to circle the linkage in a drawn repeat unit.

(Supplement) Addition or condensation?

Common mistakes.

• Leaving a C=C in the repeat unit of an addition polymer. The double bond is what opens up, so the chain has only single bonds.
• In these alkene examples, keep the side group (CH₃ or Cl) off the main chain. The two carbons of the C=C form the repeated main-chain section.
• Drawing a condensation polymer without showing where water came from: the OH from the acid group and an H from the amine or alcohol group.
• Calling nylon a polyester or PET a polyamide. Amide linkage: N–H next to the C=O. Ester linkage: O next to the C=O.
• Forgetting the bonds through the brackets, or the $n$.

Plastics and the environment

Plastics 塑料 are made from polymers. Because many plastics do not break down, getting rid of them is a problem:

• they build up in landfill 填埋 sites;
• they collect in the oceans and harm sea life;
• burning them can make toxic gases.

Proteins

Proteins 蛋白质 are natural polyamides. They are made from amino acid 氨基酸 monomers joined in long chains.

• Learn each homologous series by its functional group and general formula: alkanes $\text{C}_n\text{H}_{2n+2}$, alkenes $\text{C}_n\text{H}_{2n}$, alcohols $-\text{OH}$, carboxylic acids $-\text{COOH}$.
• The bromine water test for unsaturation: an alkene (with its C=C) turns orange bromine water colourless; an alkane leaves it orange.
• Cracking breaks large, less useful alkanes into smaller alkanes and alkenes, using heat and a catalyst.
• Two routes to ethanol: fermentation of glucose (renewable but slow and impure) and adding steam to ethene (fast and pure, but from petroleum).
• Addition polymerisation joins unsaturated monomers with no other product; condensation polymerisation joins monomers and loses a small molecule (usually water) each time.