An introduction to A Level organic chemistry

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.

Number the ring from the principal group (carbon 1); the substituent positions then give the name

• 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).

Two aromatic mechanisms: electrophilic substitution and addition-elimination

Benzene is a flat, regular hexagon. Each carbon is sp² hybridised, using its three hybridisation 杂化 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.

Benzene's bonding: an sp$^2$ $\sigma$ framework makes the flat hexagon, while the p orbitals overlap into one delocalised $\pi$ system above and below the ring

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.

Evidence for delocalisation: real benzene releases far less on hydrogenation ($-208\ \text{kJ}\,\text{mol}^{-1}$) than the Kekulé model predicts ($-360 = 3\times$ cyclohexene), so it is about $152\ \text{kJ}\,\text{mol}^{-1}$ more stable than expected

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.

The two enantiomers rotate plane-polarised light in opposite directions; a 50:50 racemic mixture gives no net rotation

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. Take the carbons one at a time. 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, but only once you look past the first atom.

• 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.
• Learn the two new mechanisms — electrophilic substitution of arenes, and nucleophilic addition-elimination.