Isomerism: MDCAT Chemistry notes

What isomers are

Isomers have the same molecular formula but different arrangements of atoms. Covalent bonds are rigid and directional, so the same set of atoms can be joined in more than one way. This is why isomerism is so common in covalent (organic) compounds.

Types of isomerism

Structural isomerism	Stereoisomerism
Chain, position, functional group, metamerism, tautomerism	Geometric (cis–trans), optical, conformational

Chain isomerism

Chain isomers differ in the carbon skeleton (straight or branched).

• Butane has 2: n-butane and isobutane (2-methylpropane).
• Pentane has 3: n-pentane, 2-methylbutane and 2,2-dimethylpropane.

Position isomerism

Position isomers have the same chain and the same functional group, but the group or multiple bond is in a different position.

• 1-butene and 2-butene
• 1-propanol and 2-propanol
• 2-pentanone and 3-pentanone, where the carbonyl position differs. MDCAT keys treat these as position isomers.

Functional group isomerism

Functional group isomers have the same formula but different functional groups.

• $\mathrm{C_2H_6O}$: ethanol ($\mathrm{C_2H_5OH}$) and dimethyl ether ($\mathrm{CH_3OCH_3}$)
• $\mathrm{C_3H_6O}$: propanal and acetone

Metamerism

Metamers carry the same functional group but have different alkyl groups on either side of it. Example: diethyl ether and methyl n-propyl ether. Metamerism is a structural isomerism, not a stereoisomerism.

Tautomerism

Tautomers are two structural forms in dynamic equilibrium that differ by the shift of a proton and a double bond. Keto–enol tautomerism is the standard example:

$$\mathrm{CH_2{=}CH{-}OH \;\rightleftharpoons\; CH_3{-}CHO}$$

This shows vinyl alcohol (enol) and acetaldehyde (keto).

Geometric (cis–trans) isomerism

Rotation about a C=C double bond is restricted because the π bond would have to break. If groups are attached differently on the two sides of the double bond, two forms result:

• Cis: similar groups on the same side
• Trans: similar groups on opposite sides

Condition: each carbon of the double bond must carry two different groups. If either carbon has two identical groups, such as $=\mathrm{CH_2}$, there is no geometric isomerism.

Shows cis–trans	Does not
But-2-ene, pent-2-ene, 3-methylpent-2-ene	Pent-1-ene, 3-methylbut-1-ene
1,2-dibromoethene ($\mathrm{CHBr{=}CHBr}$)	2,3-dimethylbut-2-ene (both ends identical)
1-bromo-2-chloropropene	Ethene, $\mathrm{C_2HCl_3}$, alkynes, alcohols, ethers
1,2-dimethylcyclopropane, 1,3-dimethylcyclopentane (rings also restrict rotation)	$\mathrm{C_2H_2Cl_4}$ (saturated, so rotation is free)

Benzene isomers

All six positions of benzene are equivalent, so a monosubstituted benzene has only one isomer. A disubstituted benzene has three: ortho, meta and para.

Common MDCAT traps

• A terminal alkene ($=\mathrm{CH_2}$ end) never shows cis–trans isomerism.
• Alkynes are linear at the triple bond, so they show no geometric isomerism.
• Vinyl alcohol and acetaldehyde are tautomers, not position isomers.
• Metamerism is not a type of stereoisomerism.
• Check both carbons of the double bond, not just one.

Quick revision

• Pentane has 3 chain isomers and butane has 2.
• $\mathrm{C_2H_6O}$ shows functional group isomerism.
• Tautomerism involves a proton shift within the molecule.
• Geometric isomerism needs restricted rotation and two different groups on each carbon.