10.1 - Homologous series
10.1.1: Describe the features of a homologous series. Features include a general formula and neighbouring members differing by CH2, with similar chemical properties and with a gradation in physical properties.
Homologous series are 'families' of organic compounds. They share common characteristics:
They all contain the same functional group. This gives the homologous series name.
They differ in formula from one member to the next by one -CH2- unit
They show a steady change in physical properties from one member to the next
They display similar chemical properties
10.1.2: Predict and explain the trends in boiling points of members of a homologous series. In a homologous series there is a gradual increase in boiling point as the number of carbon atoms increases. Cross reference with 4.3.
Within a homolgous series the intermolecular forces are the same. The fact that each successive member of a homologous series differs by only one -CH2- unit means that the van der Waals component of the intermolecular forces also increases. This leads to a gradual increase in boiling point.
The above may be demonstrated by the boiling points of the alkanes, which only have van der Waals intermolecular forces.
10.1.3: Distinguish between empirical, molecular formula and structural formulae.
The empirical formula is the simplest ratio of atoms within a molecule or formula unit.
The molecular formula shows all of the atoms in a molecule or formula unit
The structural formula shows the actual arrangement of atoms in a molecule. This may be condensed or displayed.
The displayed structural formula shows all of the atoms and all of the bonds.
Example: Ethanoic acid
Empirical formula: CH2O
Molecular formula: C2H4O2
Condensed structural formula: CH3COOH
Displayed structural formula:
10.1.4: Describe structural isomers as compounds with the same molecular formula, but with different arrangements of atoms.
As in the statement, the actual arrangement of the atoms within the molecule is different in structural isomers.
(standard level students do not need stereoisomerism)
10.1.5: Deduce structural formula for the isomers of the non-cyclic alkanes up to C6
The alkane with six carbon atoms has the molecular formula C6H14
10.1.6: Apply IUPAC rules for naming the isomers of the non-cyclic alkanes up to C6
10.1.7: Deduce structural formula for the isomers of the straight chain alkenes up to C6
If the alkene is to remain a straight chain then it cannot have any branching. This means that the only isomerism available is changing the position of the double bond.
Ethene and propene cannot have any functional group positional isomers, while butene can be but-1-ene and but-2-ene.
Pentene has also got two functional group positional isomers, pent-1-ene and pent-2-ene.
Hexene has three isomers, hex-1-ene, hex-2-ene and hex-3-ene.
10.1.8: Apply IUPAC rules for naming the isomers of the straight chain alkenes up to C6
The position on the carbon chain at which the double bond begins is included in the name if there is any ambiguity.
10.1.9: Deduce structural formula for compounds containing up to six carbon atoms with one of the following functional groups, alcohol, aldehyde, ketone, carboxylic acid and halide.
10.1.10: Apply IUPAC rules for naming compounds containing up to six carbon atoms with one of the following functional groups, alcohol, aldehyde, ketone, carboxylic acid and halide.
Nomenclature - alcohols
The -OH group is indicated by the suffix -anol. The locant (locating number) is placed between the 'an' and the 'ol' to locate the position of the OH group if there is any ambiguity.
Hence a carbon chain with four carbons that has the OH group on carbon number 2 in the chain becomes butan-2-ol.
Nomenclature - aldehydes
Aldehydes have a terminating -CHO group (carbonyl with a hydrogen attached), which gives the suffix -anal. This does not need a locant, as it must be at the end of a chain. It must not be forgotten that the carbon atom of the CHO has to be included in the longest chain for the root of the name.
Hence, CH3CH2CHO becomes propanal
Nomenclature - ketones
Ketones have a carbonyl group in the middle of the chain. This is indicated by the suffix -anone. A locant must also be included between the '-an' and the 'one' in the case of any possible ambiguity.
Hence, CH3COCH2CH2CH3 becomes pentan-2-one.
Nomenclature - carboxylic acids
Carboxylic acids have a terminating -COOH group (carbonyl with an OH attached), which gives the suffix -anoic acid. This does not need a locant, as it must be at the end of a chain. It must not be forgotten that the carbon atom of the COOH has to be included in the longest chain for the root of the name.
Hence, CH3CH2COOH becomes propanoic acid
Nomenclature - halides
The halogen group is indicated by prefixes, fluoro-, chloro-, bromo- and iodo-. Locants (locating numbers) are used to locate the position of the halogen group if there is any ambiguity.
Hence, a carbon chain with four carbons that has a Cl atom on carbon number 2 in the chain becomes 2-chlorobutane.
Multiple groups are indicated using multipliers immediately before the group (or atom) that is being multiplied: di- = two of, tri = three of, tetra = four of, penta = five of, hexa = six of.
10.1.11: Identify the following functional groups when present in structural formulas: amino (NH2), benzene ring (hexagon) and esters (RCOOR')
The primary amine group is a nitrogen with two hydrogen atoms which is attached to a carbon chain. R-NH2. This takes the name amino- followed by the carbon chain root, or the carbon chain root followed by -ylamine.
For example: CH3CH2NH2 is either aminoethane or ethylamine - both names are accepted.
Amines are basic compounds. They are found in many natural organic sources, such as plant extracts and fish oils.
Aromatic compounds (benzene containing compounds)
The benzene ring has a ring of six carbon atoms each of which is attached to a hydrogen atom. If the benzene ring is attached to something else a hydrogen must be lost.
Benzene rings are often represented by a hexagon with a ring inside:
Esters are pleasant smelling compounds formed by the reaction between a carboxylic acid and an alcohol. They are often used in the food industry and perfumes industry as flavourings and scents.
They have two alkyl chains linked by a -COO- group of atoms (the ester linkage)
10.1.12: Identify primary secondary and tertiary carbon atoms in alcohols and halogenoalkanes
The terms primary, secondary and tertiary may be applied to any group that can appear in the middle of a carbon chain, such as halogens, alcohols, amines, etc.
The term 'primary' means that there is only one carbon atom directly attached to the carbon atom that holds the functional group.
Example: Butan-1-ol is an example of a primary alcohol as the carbon that is attached to the alcohol group is also attached to only one other carbon atom.
Secondary structures have the functional group attached to a carbon atom that has two other carbon atoms attached to it.
Example: Butan-2-ol is a secondary alcohol.
Tertiary structures have the functional group attached to a carbon atom that has three other carbon atoms attached to it.
Example: Methylpropan-2-ol is a tertiary alcohol.
10.1.13: Discuss the volatility and solubility in water of compounds containing the functional groups listed in 10.1.9
Volatile means that a compound turns from a liquid into a vapour easily. This is a consequence of the vapour pressure being high, because there is limited intermolecular bonding, i.e. the intermolecular forces are weak. A volatile compound has a low boiling point.
Small molecules have weak van der Waals forces and will be very volatile. Polar molecules have extra intermolecular bonding, which lowers the volatility. Molecules with hydrogen bonding have stronger intermolecular forces and even lower volatility.
Alkanes and alkenes are non-polar and have only weak van der Waals forces. The lower members of the homologous series are gases at room temperature.
Aldehydes, ketones and halogenoalkanes are polar molecules with both van der Waals forces and permanent dipole-dipole attractions. This makes them less volatile than alkanes and alkenes.
Alcohols can form hydrogen bonds as well as dipole-dipole interactions and van der Waals forces. This increases the degree of intermolecular force and decreases the volatility.
Carboxylic acids can form two hydrogen bonds per molecule, decreasing volatility even further.
To make a comparison between the different types of homolgous series, examples should be selected with the same, or similar, relative mass in order to cancel out the effects of van der Waals (induced dipole) forces.
|homologous series||compound||formula||relative mass||boiling point / ºC|
|carboxylic acid||methanoic acid||HCOOH||46||101|
Solubility in water
In general, organic compounds are covalent and insoluble in water.
This is true unless the molecules are very small and polar, or can form hydrogen bonds with the water molecules.
The trend within any homologous series is of decreasing solubility as the hydrophobic chain length increases.
Hence, the alkanes are insoluble, while the smaller members of the aldehydes and ketones are soluble or miscible in all proportions. The smaller members of the alcohols are miscible in all proportions up to C4, as are carboxylic acids.