S3.2Structure 3 — Classification of matter
Functional groups: classification of organic compounds — IB Chemistry SL Revision Notes & Topical Questions
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Lesson summary
S3.2 Functional groups: classification of organic compounds — IB CHEMISTRY SL notes: Identify functional groups, name organic compounds with IUPAC rules and deduce structural isomers.Functional group = an atom or group of atoms that gives an organic compound its characteristic ph…
- Unsaturated
- Catenation
- Homologous series
- Chemical properties
- Physical properties
- Alkyl groups
Section 1 of 11Functional groups and classes
Functional groups and classes
Functional group
Functional group an atom or group of atoms that gives an organic compound its characteristic physical and chemical properties.
Class
- Class compounds with the same functional group belong to the same class, which is not the same as belonging to the same homologous series.
Saturated
- Saturated compounds with only C–C single bonds.
Unsaturated
- Unsaturated compounds with C=C or C≡C bonds.
| Class | Functional group | Suffix or prefix | Example |
|---|---|---|---|
| Alkane | Alkyl | –ane | Ethane |
| Alkene | Alkenyl, C=C | –ene | Ethene |
| Alkyne | Alkynyl, C≡C | –yne | Ethyne |
| Halogenoalkane | Halogeno, –X | fluoro–, chloro–, bromo–, iodo– | Chloroethane |
| Alcohol | Hydroxyl, –OH | –ol or hydroxy– | Ethanol |
| Aldehyde | Carbonyl at chain end, –CHO | –al | Ethanal |
| Ketone | Carbonyl within chain, C=O | –one | Propanone |
| Carboxylic acid | Carboxyl, –COOH | –oic acid | Ethanoic acid |
| Ester | Ester, –COO– | alkyl –oate | Methyl ethanoate |
| Ether | Alkoxy, –O– | alkoxy– | Ethoxyethane |
| Amine | Amino, –NH2 | –amine | Ethanamine |
| Amide | Amido, –CONH2 | –amide | Ethanamide |
| Arene | Phenyl, –C6H5 | phenyl– or –benzene | Phenylethene |
Section 2 of 11Catenation and homologous series
Catenation
- Catenation carbon forms strong covalent bonds to itself and always makes four bonds,
- so it builds chains, branches and rings and gives a vast number of compounds.
Homologous series
Homologous series a family of compounds with the same functional group and general formula, in which successive members differ by CH2 (14 g mol−1).
Chemical properties
- Chemical properties are similar because the functional group is the same.
Physical properties
- Physical properties change gradually, with boiling point rising as chain length grows because the larger electron cloud gives stronger London forces.
Branching
- Branching lowers the boiling point by reducing surface contact.
| Series | General formula |
|---|---|
| Alkanes | CnH2n+2 |
| Alkenes | CnH2n |
| Alkynes | CnH2n−2 |
| Alcohols | CnH2n+1OH |
| Carboxylic acids | CnH2n+1COOH |
| Halogenoalkanes | CnH2n+1X |
Section 3 of 11IUPAC naming rules
1
Find the longest continuous carbon chain containing the main functional group to give the stem.
2
Add the suffix of the principal functional group.
3
Number the chain from the end that gives the functional group the lowest locant.
4
Name substituents as prefixes with locants, e.g. methyl and chloro–.
5
List prefixes alphabetically and use di–, tri– and tetra– for repeated groups.
6
Use commas between numbers and hyphens between numbers and letters, e.g. 2-methylbutan-2-ol.
IUPAC naming rules
Stems
- Stems meth, eth, prop, but, pent, hex.
- (names up to six carbons are required)
Alkyl groups
- Alkyl groups side chains take the –yl ending, e.g. methyl.
Esters
- Esters are named alkyl alkanoate, the alkyl group coming from the alcohol, so ethyl propanoate comes from ethanol and propanoic acid.
Section 4 of 11Naming alkenes, alkynes, halogenoalkanes and alcohols
Alkenes
- Alkenes alk + ene, with a locant from four carbons, e.g. but-1-ene and but-2-ene, using the lower numbered carbon of the double bond.
Alkynes
- Alkynes alk + yne, with a locant from four carbons, e.g. but-2-yne.
Halogenoalkanes
- Halogenoalkanes prefix chloro–, bromo– or iodo–, with a locant from three carbons, e.g. 2-bromopropane and 1,1,2-trichloroethane.
Alcohols
- Alcohols alkan + ol, with two –OH groups called a diol, e.g. propan-1-ol.
Section 5 of 11Naming carbonyl compounds and carboxylic acids
Aldehydes
- Aldehydes alkan + al, with no number because the group is always on carbon 1.
Ketones
- Ketones alkan + one, from three carbons, and a locant is needed from pentanone onward, e.g. pentan-2-one and pentan-3-one.
Carboxylic acids
- Carboxylic acids alkan + oic acid, with no number because the group is always on carbon 1.
Polar carbonyl
- O is more electronegative than C,
- so aldehydes and ketones have dipole-dipole forces, higher boiling points than alkanes of similar mass, and the smallest members dissolve in water.
Isomers
- Isomers aldehydes and ketones with the same number of carbons are functional group isomers.
Section 6 of 11Naming benzene compounds and classifying amines
Naming benzene compounds and classifying amines
Ring as parent
- Ring as parent alkyl + benzene, e.g. methylbenzene, and 1-ethyl-2-methylbenzene when two groups are present.
Numbering
- Numbering the longest alkyl chain is carbon 1 and substituents are listed alphabetically.
Ring as substituent
- Ring as substituent called phenyl, e.g. phenylethene and phenylamine.
Amines
- Amines classified by the number of carbon groups on nitrogen: CH3CH2NH2 is primary, (CH3)2NH secondary and (CH3)3N tertiary.
Alcohols and halogenoalkanes
- Alcohols and halogenoalkanes classified by the number of carbon atoms bonded to the carbon bearing the functional group.
Section 7 of 11Structural isomers
Structural isomers
Definition
Definition compounds with the same molecular formula but different structural formulae.
Chain (branched) isomers
- Chain (branched) isomers different carbon skeletons, e.g. butane and 2-methylpropane.
Position isomers
- Position isomers the same group on a different carbon, e.g. propan-1-ol and propan-2-ol, and butan-1-ol and butan-2-ol.
Functional group isomers
- Functional group isomers different groups, e.g. propanal and propanone, ethanol and methoxymethane, and alcohols and ethers.
Series pairs
- Series pairs alkenes and cycloalkanes, alcohols and ethers, and aldehydes and ketones share general formulae.
Amines
- Amines ethanamine and N-methylmethanamine are isomers of C2H7N.
Properties
- Properties isomers have different physical properties and sometimes different chemical properties.
Section 8 of 11Finding all isomers of a formula
1
Draw one possible structural formula.
2
Check for functional group isomers.
3
Change the carbon skeleton to find branched isomers.
4
Move the functional group to find position isomers.
5
Name each structure to remove duplicates, because different names mean different isomers.
- Bent or turned drawings
- rotating or bending a structure does not make a new isomer.
Section 9 of 11More from the specification
1Empirical vs molecular formula
- The empirical formula expresses the simplest whole-number ratio of atoms in a compound, whereas molecular formula shows actual atom counts.
2Displayed and condensed structural formulas
- Displayed formulas show every single bond and atom, while condensed structural formulas group functional units along the chain.
3IUPAC nomenclature rules
- Identifying the principal functional group attached to the longest continuous carbon chain to establish standard systematic names.
4Structural isomerism
- Compounds sharing identical molecular formulas but possessing different connectivity or positions of functional groups attached to the chain.
5Empirical formula
- The simplest whole-number ratio of atoms or ions.
6Displayed formula
- A formula showing all atoms and all bonds.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Functional group | The atom or group responsible for the characteristic reactions of a molecule. |
| Homologous series | A family of compounds with the same functional group and general formula, differing by CH2. |
| Structural isomers | Compounds with the same molecular formula but different structures. |
| Catenation | The ability of carbon atoms to bond to each other to form chains and rings. |
| Saturated | Containing only single bonds between carbon atoms. |
| IUPAC name | The systematic name built from a stem, suffix and numbered prefixes. |
Section 11 of 11Exam technique
1
Omitting the locant when it is needed — alkenes and alkynes from four carbons and ketones from five carbons need a number.
2
Numbering the chain from the wrong end — the functional group must get the lowest locant.
3
Giving locants for aldehydes and carboxylic acids — the group is always on carbon 1, so no number is used.
4
Counting a rotated drawing as a new isomer — name the structures to compare them.
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