Introduction to Organic Chemistry
Senior 5 · Chemistry
Introduction to Organic Chemistry
Lesson 4 of 4
25 min
Chewing gum flavoured with spearmint and bread baked with caraway seeds smell completely different, yet the main smell molecule in each is carvone, . The two carvones have the same atoms joined in the same order. They differ only in the way their atoms are arranged in space: one is the mirror image of the other. The smell receptors in your nose are themselves built from chiral molecules, so they can tell the two apart. This is isomerism at its subtlest; this lesson starts with the more obvious kinds.
Definition
Isomers: compounds with the same molecular formula but different arrangements of atoms.
Structural isomers: isomers whose atoms are joined in a different order: a different chain (chain isomers), the same chain with the group in a different place (position isomers), or a different functional group (functional-group isomers).
Stereoisomers: isomers with the same structural formula but a different arrangement of atoms in space. There are two kinds: geometric and optical.
Figure 4.1: The types of isomerism met in Senior 5.
The formula fits the general formula , shared by alcohols and ethers. It has seven structural isomers.
| Isomer | Structural formula | Related to butan-1-ol as |
|---|---|---|
| butan-1-ol | (reference) | |
| butan-2-ol | position isomer | |
| 2-methylpropan-1-ol | chain isomer | |
| 2-methylpropan-2-ol | chain and position isomer | |
| 1-methoxypropane | functional-group isomer | |
| 2-methoxypropane | functional-group isomer | |
| ethoxyethane | functional-group isomer |
Other common functional-group pairs: aldehydes and ketones (propanal and propanone, ); carboxylic acids and esters (propanoic acid and methyl ethanoate, ); alkenes and cycloalkanes (propene and cyclopropane, ). A special case is tautomerism, in which two isomers interconvert rapidly by moving a hydrogen atom: ethenol, , changes at once into ethanal, . You will meet this in Unit 3.
Definition
Geometric (cis–trans) isomers: stereoisomers that differ in the positions of groups on either side of a bond that cannot rotate, usually C=C. They exist only when each carbon of the double bond carries two different groups.
cis: the two similar groups on the same side; trans: on opposite sides.
E/Z notation: on each carbon of the C=C, rank the two groups by the atomic number of the atom directly attached (higher number = higher priority; if they tie, compare the next atoms out). Z (German zusammen, together): the two higher-priority groups on the same side. E (entgegen, opposite): on opposite sides.
Figure 4.2: The geometric isomers of but-2-ene. The groups cannot swap sides without breaking the π bond.
Restricted rotation about C=C (Lesson 1) locks the groups in place. But-1-ene, , has no geometric isomers, because C1 carries two identical H atoms: swapping them gives the same molecule. The cis–trans names work when the two carbons carry a similar group; E/Z works for any alkene.
Table 4.1: Geometric isomers have different physical properties
| Compound | cis (Z) | trans (E) | Why they differ |
|---|---|---|---|
| but-2-ene, boiling point | 4 °C | 1 °C | the cis isomer is slightly polar; the trans isomer's dipoles cancel |
| butenedioic acid, melting point | 135 °C (maleic acid) | 287 °C (fumaric acid) | the trans isomer packs better and forms hydrogen bonds between molecules; in the cis isomer some hydrogen bonding is within the molecule |
The cis acid also loses water on heating to form a cyclic anhydride, because its two –COOH groups are close together. The trans acid cannot.
Definition
Chiral: not superimposable on its mirror image, like your left and right hands.
Chiral centre (asymmetric carbon): a carbon atom bonded to four different atoms or groups; it is often marked with an asterisk, C*.
Enantiomers (optical isomers): a pair of stereoisomers that are non-superimposable mirror images of each other.
Optically active: able to rotate the plane of plane-polarised light. One enantiomer rotates it clockwise (dextrorotatory, (+)), the other by the same angle anticlockwise (laevorotatory, (−)).
Racemic mixture (racemate): a 50 : 50 mixture of two enantiomers. It is optically inactive, because the two rotations cancel.
Figure 4.3: The enantiomers of lactic acid. The chiral carbon (*) carries four different groups.
Enantiomers have identical melting points, boiling points, solubilities and reactions with non-chiral reagents. They differ in only two ways: the direction in which they rotate plane-polarised light, and the way they interact with other chiral molecules, such as enzymes, receptors and many drugs. This matters in medicine. The sedative thalidomide, sold in the late 1950s as a mixture of its two enantiomers, caused severe birth defects; one enantiomer is linked to the harm, and the body converts each enantiomer into the other, so even the "safe" one alone would not have been safe.
Figure 4.4: A polarimeter measures the angle α through which a chiral substance rotates plane-polarised light.
Worked example
Problem. Name all the alkenes with formula , and say which show geometric isomerism.
Solution. Unbranched chain, C=C at C1: but-1-ene. Unbranched, C=C at C2: but-2-ene, which exists as (Z)- and (E)-but-2-ene, because C2 carries H and , and so does C3. Branched chain: 2-methylpropene, . That is four alkenes in all. But-1-ene (two H on C1) and 2-methylpropene (two H on C1, and two on C2) have no geometric isomers. (Cyclobutane and methylcyclopropane are also : functional-group isomers of the alkenes.)
Worked example
Problem. (a) In one isomer of 2-bromobut-2-ene, , the Br atom and the group on C3 are on the same side. Is it E or Z? (b) Which of these is chiral: 2-chlorobutane, 1-chlorobutane, 3-methylhexane?
Solution. (a) On C2: Br (atomic number 35) outranks C of (6). On C3: C of (6) outranks H (1). The two higher-priority groups are on the same side: (Z)-2-bromobut-2-ene.
(b) In 2-chlorobutane, C2 carries H, Cl, and : four different groups, so it is chiral. In 1-chlorobutane every carbon has at least two H atoms: not chiral. In 3-methylhexane, C3 carries H, , and : chiral.
Warning
E is not always trans. E/Z follows atomic-number priority, not "similar groups". In the isomer whose two groups are on the same side (looking "cis") is E, because Br outranks on C2. And a carbon with two identical groups, such as a inside a chain, is never a chiral centre.
Tip
To spot a chiral centre, look only at carbons with exactly one H (or none). Then check that all four groups differ all the way out: and are different groups.
Allery Chemistry: Stereoisomerism (14 min). An A-level overview of the two kinds of stereoisomerism: E/Z isomerism about a C=C bond and optical isomerism at a chiral carbon.
Practice
Further reading
Sources and credits
