- understand that the compounds in the table on page 47 contain a functional group which dictates their physical and chemical properties
- interpret and use the general, structural, displayed and skeletal formulas of the classes of compound stated in the table on page 47
- understand and use systematic nomenclature of simple aliphatic organic molecules (including cyclic compounds containing a single ring of up to six carbon atoms) with functional groups detailed in the table on page 47, up to six carbon atoms (six plus six for esters and amides, straight chains only for esters and nitriles)
- understand and use systematic nomenclature of simple aromatic molecules with one benzene ring and one or more simple substituents, for example 3-nitrobenzoic acid or 2,4,6-tribromophenol
An introduction to A Level organic chemistry
A-Level Chemistry · Topic 29
8:28
An introduction to A Level organic chemistry
Two molecules. Same atoms, same bonds. The only difference: one is the mirror image of the other, like your left hand and your right hand. And yet one of them…
English narration · English + 中文 subtitles burned in
29.1
New functional groups and naming
Syllabus
Source: Cambridge International syllabus
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.
Explore
Aromatic naming lab
Classify aromatic substituents by how they are named.
Vocabulary
Train
| English |
|---|
| functional group/ˈfʌŋkʃənl ɡruːp/ |
| amide/əˈmaɪd/ |
| aromatic/ərəʊˈmætɪk/ |
| benzene/ˈbenziːn/ |
| benzene ring/ˈbenziːn rɪŋ/ |
29.2
Two new types of mechanism
Syllabus
- understand and use the following terminology associated with types of organic mechanisms: (a) electrophilic substitution (b) addition–elimination
Source: Cambridge International syllabus
- 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).
Explore
Electrophilic substitution on benzene
Step through how benzene reacts. An electrophile swaps for a hydrogen — keeping the stable ring — instead of adding across it.
Vocabulary
Train
| English |
|---|
| electrophilic substitution/ɪˌlektrəʊˈfɪlɪk ˌsʌbstɪˈtjuːʃn/ |
| addition–elimination/əˈdɪʃn ɪˌlɪmɪˈneɪʃn/ |
29.3
The shape of benzene
Syllabus
- describe and explain the shape of benzene and other aromatic molecules, including $\text{sp}^2$ hybridisation, in terms of $\sigma$ bonds and a delocalised $\pi$ system
Source: Cambridge International syllabus
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.
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.
Explore
Benzene bonding lab
Follow the evidence for a planar delocalised benzene ring.
Vocabulary
Train
| English |
|---|
| hybridisation/ˌhaɪbrɪdaɪˈzeɪʃn/ |
| sigma bond/ˈsɪɡmə bɒnd/ |
| delocalised/dɪˈlɒkəlaɪzd/ |
| pi bond/paɪ bɒnd/ |
| enthalpy change of hydrogenation/enˈθælpi tʃeɪndʒ ɒv haɪˈdrɒdʒəneɪʃn/ |
29.4
Optical isomerism
Syllabus
- understand that enantiomers have identical physical and chemical properties apart from their ability to rotate plane polarised light and their potential biological activity
- understand and use the terms optically active and racemic mixture
- describe the effect on plane polarised light of the two optical isomers of a single substance
- explain the relevance of chirality to the synthetic preparation of drug molecules including: (a) the potential different biological activity of the two enantiomers (b) the need to separate a racemic mixture into two pure enantiomers (c) the use of chiral catalysts to produce a single pure optical isomer (Candidates should appreciate that compounds can contain more than one chiral centre, but knowledge of meso compounds and nomenclature such as diastereoisomers is not required.)
Source: Cambridge International syllabus
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. 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.
Explore
Optical isomerism lab
Identify when a molecule can have non-superimposable mirror images.
Vocabulary
Train
| English |
|---|
| enantiomers/eˈnæntɪəməz/ |
| plane polarised light/pleɪn ˈpəʊləraɪzd laɪt/ |
| optically active/ˈɒptɪkli ˈæktɪv/ |
| racemic mixture/rəˈsiːmɪk ˈmɪkstʃə/ |
| chirality/kaɪˈrælɪti/ |
| chiral centre/ˈkaɪrəl ˈsentə/ |
| chiral catalyst/ˈkaɪrəl ˈkætəlɪst/ |
29.4
Exam tips
- 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.
Interactive lessons on this topic
Work through it step by step, with instant-check exercises.