IB Chemistry HL Topic 10

Include the same general formula, neighbouring members differing by CH2, similar chemical properties and gradation in physical properties.
Homologous series: A series of compounds (usually hydrocarbons) that display similar chemical properties and functional groups but different physical properties (generally boiling point), often differing by one repeating unit, usually a CH2/CH3 molecule. They can be given a general formula, eg CnH2n+2
Organic chemistry is the chemistry of carbon compounds (excluding compounds such as metal carbonates and oxides).
The ability of carbon atoms to form chains leads to the existence of a series of compounds that have the same functional group (and hence similar chemical properties) and only differ from each other by the presence of an additional carbon atom and its two associated hydrogen atoms in the molecule (which causes the physical properties to change in a regular manner).

0. The changes in physical properties are a result of the changes that occur in the strength of van der Waals’ forces with increasing molar mass and in some cases a change in molecular polarity.
   The physical properties, especially the melting and boiling point of a compound, depend on the intermolecular forces present. All other factors being unchanged, the greater the molar mass of a molecule the stronger the intermolecular forces.

Empirical formula: The simplest ratio of the atoms in a chemical compound.
A structural formula is one that shows unambiguously how the atoms are arranged together. A full structural formula (sometimes called a graphic formula or displayed formula) shows every atom and bond.
A condensed structural formula can omit bonds between atoms and can show identical groups bracketed together, for example, for hexane: CH3CH2CH2CH2CH2CH3 or CH3(CH2)4CH3.
The use of R to represent an alkyl group and (RING) to represent the benzene ring can be used in condensed structural formulas.
Although skeletal formulas are used for more complex structures in the Chemistry data booklet, such formulas will not be accepted in examination answers.

No distinction need be made between different types of structural isomerism, such as chain
and position isomerism and functional group isomerism. Knowledge of stereoisomerism is not required in the core.
Isomer: Two molecules with the same general formula but different structural formula (ie containing the same amounts of each atom but bonded differently) are said to be isomers of each other.
Two Types:

• structural isomers, in which the atoms are joined in a different order so that they have different structural formulae.
  
• stereoisomers, in which the order that the atoms are joined together is the same, but the molecules have a different arrangement of atoms in space and hence different three dimensional shapes.

The terms primary, secondary and tertiary can also be applied to the molecules containing these carbon atoms.

Most organic compounds are non–polar and hence tend to be insoluble in water owing to the strong hydrogen bonds between the water molecules. If however the functional groups can hydrogen bond to the water (that is those that hydrogen bond to themselves, plus those containing the > C= O group which can form hydrogen bonds to water), then the substance will be water soluble, as long as the hydrocarbon chain is relatively short. As the carbon chain increases there is a gradual decrease in the solubility as is the case with the alcohols.

See notes.

The most familiar reaction of the alkanes is combustion. Like almost all organic compounds, the alkanes are flammable and oxidise when burnt in the air to form carbon dioxide and water if sufficient oxygen is present.
Incomplete = CO and H2O vapor

Alkanes can also react with chlorine or bromine in a substitution reaction, to give an initial product in which one hydrogen atom is replaced by the halogen. Usually these reactions are brought about by exposure to ultraviolet light or sunlight, though they will also occur without light at high temp.

he reaction of an alkane, such as methane, with a halogen is a free radical chain reaction.
Reference should be made to homolytic fission and the reaction steps of initiation, propagation and termination. The use of the half-arrow to represent the movement of a single electron is not required. The formulas of free radicals should include the radical symbol, for example, Cl• .

The general formula for the alkene homologous series is CnH2n.
A reaction in which the double bond of an alkene is converted to a single bond and two new bonds are formed to the species it reacts with is known as an addition reaction and they are typical of alkenes and alkynes.

See notes.

The usual test for the presence of a carbon–carbon double or triple bond is to add bromine water to the compound. If a double or triple bond is present, the bromine water changes colour from yellow–brown to colourless.

Include the formation of poly(ethene), poly(chloroethene) and poly(propene) as examples of addition polymers. Include the identification of the repeating unit, for example, –(–CH2 –CH2 –)n – for poly(ethene).
Polymers are long chain molecules that are formed by the joining together of a large number of repeating units, called monomers, by a process of polymerisation. Polymers, can be made artificially and these are usually referred to as plastics, but there are also a great number of naturally occurring polymers.

Include the hydrogenation of vegetable oils in the manufacture of margarine, the hydration of ethene in the manufacture of ethanol, and polymerization in the manufacture of plastics.