2.4 - Isomers and Naming Based on IUPAC Nomenclature

Structural Isomerism

• Structural isomerism is the phenomenon in which a compound has one molecular formula but two or more different structural formulae.
• Isomers are molecules with the same molecular formula but different structural formulae.
• Chain isomers have different carbon-chain arrangements: straight or branched.
• Position isomers have the same carbon skeleton but different positions of a functional group, double bond or triple bond.
• Isomers with the same functional group have similar chemical properties but different physical properties such as melting and boiling points. More branching generally lowers melting and boiling points.
• The number of possible isomers generally increases with the number of carbon atoms.
• Alkanes form chain isomers only at this level. Alkenes, alkynes and alcohols form both chain and position isomers.

Systematic construction

1. For an alkane, first connect every carbon in a straight chain, then construct distinct branched chains.
2. For an alkene or alkyne, first move the multiple bond to each non-equivalent position on the straight chain.
3. Next, construct distinct branched carbon chains and place the multiple bond at every valid, non-equivalent position.
4. Check that every carbon has four bonds and that rotated or reversed drawings have not been counted twice.

Naming Branched Isomers by IUPAC Nomenclature

• An IUPAC name has three parts:
  • prefix: the alkyl branch and its position; alkyl groups follow ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n+1}$, such as methyl, ethyl and propyl;
  • root: number of carbon atoms in the longest chain;
  • suffix: homologous series and, where needed, the functional-group position.
• Writing conventions: join prefix and root; place a hyphen between a number and a word; place a comma between numbers.

Naming steps

1. Select the longest carbon chain. For alkene, alkyne or alcohol, it must contain the functional group.
2. Identify every branch and the functional group.
3. Number from the end that gives the lowest locant to:
   • the branch in an alkane;
   • the double bond, triple bond or hydroxyl group in an alkene, alkyne or alcohol.
4. State branch positions and names. Use di- for two identical branches.
5. Add the root and suffix with the functional-group locant.

• Examples: 2,2-dimethylpropane, 3-methylbut-1-ene, 3-methylbut-1-yne and 2-methylpropan-2-ol.

Isomers of Alkanes, Alkenes, Alkynes and Alcohols

• Methane, ethane and propane have no isomers because only one distinct carbon skeleton can be constructed. Alkane isomerism begins with butane, ${\mathrm{C}}_4{\mathrm{H}}_{10}$.
• Alcohol isomerism begins at three carbons:
  • ${\mathrm{C}}_3{\mathrm{H}}_7\mathrm{OH}$: propan-1-ol and propan-2-ol;
  • ${\mathrm{C}}_4{\mathrm{H}}_9\mathrm{OH}$: butan-1-ol, butan-2-ol, 2-methylpropan-1-ol and 2-methylpropan-2-ol.
• Alcohol isomers arise from different carbon chains and different positions of –OH.

Isomer	Condensed structural formula
propan-1-ol	$\mathrm{C}{\mathrm{H}}_3-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-\mathrm{OH}$
propan-2-ol	$\mathrm{C}{\mathrm{H}}_3-\mathrm{CH}(\mathrm{OH})-\mathrm{C}{\mathrm{H}}_3$
butan-1-ol	$\mathrm{C}{\mathrm{H}}_3-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-\mathrm{OH}$
butan-2-ol	$\mathrm{C}{\mathrm{H}}_3-\mathrm{CH}(\mathrm{OH})-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_3$
2-methylpropan-1-ol	$(\mathrm{C}{\mathrm{H}}_3{)}_2\mathrm{CH}-\mathrm{C}{\mathrm{H}}_2-\mathrm{OH}$
2-methylpropan-2-ol	$(\mathrm{C}{\mathrm{H}}_3{)}_3\mathrm{C}-\mathrm{OH}$

Uses of Alkanes and Alkenes

• Alkanes have high heats of combustion, so they are used mainly as fuels and petrochemical feedstocks.
• Ethane in liquefied natural gas fuels power stations and is converted to ethene for producing detergents and plastics.
• Butane is used in lighter and portable-stove fuels; mixed with propane, it forms LPG cooking gas.
• Alkenes are petrochemical feedstocks. Ethene produces ethanol, polyethene, polyvinyl chloride (PVC) and polystyrene.
• Buta-1,3-diene produces synthetic rubber for tyres and hot-water bags.

Uses and Misuse of Alcohol

• Fuel: Ethanol is highly flammable and releases much heat without soot; it is used in clean fuel, biofuel and gasohol.
• Solvent: Its colourlessness, volatility, water miscibility and ability to dissolve organic substances suit paints, lacquers, dyes, printing inks, perfumes, nail varnish, creams and lotions.
• Manufacturing: Alcohol is a reactive raw material for vinegar, explosives, Perspex polymer and fibres.
• Pharmaceuticals: Alcohol acts as an antiseptic for injections, surgery and general hygiene, and as a solvent in medicines such as cough mixtures. Hand sanitiser is an example of an antiseptic product containing more than 70% alcohol.
• In Brazil, ethanol from sugar-cane fermentation is blended with petrol as gasohol.

Effects of alcohol misuse

• Long-term consumption of alcoholic drinks adversely affects the central nervous system.
• Effects include addiction; depression and psychosis; intoxication, impaired thinking and slower reflexes; road accidents; birth defects when consumed during pregnancy; cirrhosis or failure of the liver; heart failure; gastritis; ulcers; pancreatitis; and oral cancer.

Uses of Carboxylic Acids and Esters

Carboxylic acids

• Ethanoic acid is used as a food preservative in chilli sauce, ketchup and other flavourings, and as a raw material for dyes, paints, pesticides and plastics.
• Methanoic acid coagulates latex in the rubber industry.
• Long-chain fatty acids are used to make soap.
• Carboxylic acids produce condensation polymers such as polyester (for example, terylene) and polyamide (for example, nylon).

Esters

• Small ester molecules are volatile and fragrant, so they are used in cosmetics, perfumes and food flavourings.
• Methyl butanoate gives apple flavour, pentyl ethanoate banana flavour and ethyl butanoate pineapple flavour.
• Ethyl ethanoate is an organic solvent in lotions, nail varnishes, lacquers and glue.
• Oils and fats are esters of fatty acids and glycerol and are used to make soap.
• Polyesters are used to produce synthetic fabrics.