A Level 有机化学导论

At A Level you meet more functional group 官能团 families, including the amide 酰胺 group and aromatic 芳香 compounds (those built on a benzene ring). As before, the functional group decides the properties, and you read it from the general, structural, displayed or skeletal formula.

Naming aromatic compounds

A benzene 苯 molecule is a ring of six carbons. When you name an aromatic compound, use the benzene ring 苯环 as the parent and number the positions of the substituents. For example, 3-nitrobenzoic acid has a $\text{–NO}_2$ group on carbon 3, and 2,4,6-tribromophenol has three bromine atoms on a phenol ring. You can also name cyclic compounds with a single ring of up to six carbons.

• electrophilic substitution 亲电取代: an electrophile replaces a hydrogen atom on a benzene ring (this is how benzene reacts).
• addition–elimination 加成消去: a molecule first adds on, then a small molecule is removed (seen with 2,4-DNPH and with acyl chlorides).

Benzene is a flat, regular hexagon. Each carbon is sp² hybridised: hybridisation 杂化 mixes its 2s orbital and two of its 2p orbitals into three equal hybrid orbitals. The carbon uses these three orbitals to make sigma bonds σ键 to two neighbouring carbons and one hydrogen. This gives the ring of σ bonds.

Each carbon also has one electron left in a p orbital, standing up at right angles to the ring. These p orbitals overlap sideways all the way round, making a single delocalised 离域 pi bond π键 system — a ring of electrons above and below the plane. Because the electrons are shared evenly, all six C–C bonds are the same length, and benzene is very stable.

How do we know benzene really is delocalised? One strong piece of evidence is its enthalpy change of hydrogenation 氢化焓变. Adding hydrogen to one C=C double bond (as in cyclohexene) releases about $120\ \text{kJ}\,\text{mol}^{-1}$, so a Kekulé ring of three separate double bonds should release about $3 \times 120 = 360\ \text{kJ}\,\text{mol}^{-1}$. Real benzene releases only $208\ \text{kJ}\,\text{mol}^{-1}$ — it is about $152\ \text{kJ}\,\text{mol}^{-1}$ more stable than the model predicts.

Two enantiomers 对映体 (mirror-image isomers) have identical physical and chemical properties, with two exceptions:

• they rotate plane polarised light 平面偏振光 in opposite directions. A substance that does this is optically active 旋光活性.
• they may have different effects in living things (biological activity).

A racemic mixture 外消旋混合物 is a 50:50 mix of the two enantiomers. It does not rotate plane polarised light, because the two opposite rotations cancel out.

Why chirality matters for drugs

Chirality 手性 is important when making medicines. A molecule with a chiral centre 手性中心 has two enantiomers, and they can behave very differently in the body — one may cure while the other does harm. So drug makers either:

• separate a racemic mixture into the two pure enantiomers, or
• use a chiral catalyst 手性催化剂 to make just the single enantiomer they want.

Worked example. Which of butan-1-ol, butan-2-ol and 2-methylpropan-2-ol is chiral? A molecule is chiral if it has a carbon carrying four different groups. In butan-2-ol, $\text{CH}_3\text{CH(OH)CH}_2\text{CH}_3$, carbon 2 carries $\text{OH}$, $\text{H}$, $\text{CH}_3$ and $\text{C}_2\text{H}_5$ - four different groups, so it is a chiral centre and butan-2-ol exists as two optical isomers. Butan-1-ol's carbon 1 carries two hydrogens, and the central carbon of 2-methylpropan-2-ol carries two identical methyl groups, so neither is chiral. It takes only one repeated group on a carbon to destroy the chirality there - and compare groups properly: $\text{CH}_3$ and $\text{C}_2\text{H}_5$ differ only beyond the first atom.

Common mistakes.

• Drawing benzene as three fixed C=C double bonds and explaining its reactions that way. The six p electrons are delocalised round the ring, so all C–C bonds are the same length and benzene resists addition.
• Getting the stability sign wrong. Benzene releases less energy on hydrogenation ($208$ against the Kekulé $360\ \text{kJ}\,\text{mol}^{-1}$), so it is more stable than the Kekulé model.
• Calling a carbon chiral when two of its groups are the same element. The four groups must be different as whole groups: $\text{CH}_3$ and $\text{C}_2\text{H}_5$ count as different.
• Saying enantiomers have different boiling points or react differently with ordinary reagents. Their physical and chemical properties are identical; only the direction they rotate plane-polarised light and their effect in the body differ.
• Saying a racemic mixture is optically inactive because it has no chiral centre. Each molecule is chiral; the 50:50 mixture shows no rotation because the two opposite rotations cancel.
• Numbering a substituted benzene ring to give large locants. Number from the parent group (for example the $\text{–COOH}$ of benzoic acid, carbon 1) and give the other substituents the lowest numbers.

• Optical isomers need a chiral carbon (four different groups); they are non-superimposable mirror images that rotate plane-polarised light oppositely.
• A racemic mixture forms when a planar intermediate (carbocation or $\text{C}=\text{O}$) is attacked equally from both sides.
• Benzene's delocalised ring (equal bond lengths, planar) explains its stability versus the Kekulé model.
• New mechanisms: electrophilic substitution and nucleophilic addition–elimination.