ORGANIC CHEMISTRY

• Organic chemistry is the chemistry of carbon compounds

• Carbon is in the group 4 or 14 of the periodic table.

• Carbon always forms covalent bonds.

• As carbon has 4 electrons in its valence level (2,4) it forms 4 covalent bonds.

• When all of them are single bonds they have a tetrahedral arrangement.

• In organic molecules:

• Carbon always forms 4 bonds.

• Oxygen forms 2 bonds

• Halogen and Hydrogen form 1 bond

• Functional group

• A homologous series is a series of compounds with the same functional group, in which each member differs from the next by -CH2- .

• Features of homologous series:!

• Successive compounds differ from each other by a -CH2- group(methylene group)

• The compounds can all be represented by a general formula. eg. Alkanes =

• CnH2n+2!

• The compounds have similar chemical properties.

• Successive compounds have physical properties that vary in a regular manner as the number of carbon atoms present increases.

• Cambridge Chemistry 422

• As the number of carbon atoms in a molecule in a particular homologous series increases the boiling point increases. !

• The boiling point of straight chain alkanes increases when a metalyne group is added.

• Reason:

• As the relative molecular mass of the alkane increases the strength of the Van der Waals forces between molecule increases.

• The boiling point of the alcohols(-OH) tend to be higher due to the presence of hydrogen bonding.

• Other functional groups such as carboxylic acid (-COOH), amine(-NH2) and amide(-CONH2) also gives rise to hydrogen bonding.

• Therefore, they have higher melting and boiling point than non-polar or polar organic compounds of similar molar masses.

• The physical properties, especially the melting and boiling point, depend on intermolecular forces.

• Some function groups such as Aldehyde and Ketones give rise to polarity within molecules and this results in Dipole-Dipole force hence slightly higher melting and boiling point.

• Hydrocarbons: Compound containing only C-H bonds.

• Empirical formula: The simplest whole number ratio of the elements present in a compound.

• Molecular formula: The total number of atoms of each element present in the molecule of the compound.

• The full structural formula shows all the atoms and bonds. eg. Butanoic acid.

• A structural formula indicates in a fixed manner how the atoms are arranged in the molecule.

• An alkyl group CnH2n+1 because of the removal of 1 electron.

• Alcohols can be expressed by the addition of the hydroxyl group -OH.

• Benzene ring:

• Phenol-Benzene ring with -OH attached.

• pg. 425 Cambridge

• pg. 426 Cambridge

• Isomers are different compounds which have the same molecular formula.

• Different isomers have different chemical and physical properties

• Structural isomers: Two or more compounds in which the atoms are joint in a different order so that they have the same molecular formula but different structural formula.

• pg. 426 Cambridge

• Position isomers: Have the same hydrocarbon skeleton and the same functional group:

• The functional group is joined to a different part of the skeleton.

• eg. Propan-1-ol and propan-2-ol

• Hydrocarbon chain isomers: Have different hydrocarbon skeletons that the functional group is attached to.

• Example: Butane and methylpropane

• Functional group isomers: Some functional groups will usually have isomers containing another functional group.

• Example: Alcohols usually have an alcoxyalkane that is isomeric to them hence ethanol has an isomer methoxymethane.

• Stereoisomers: The order that the atoms are joined together is the same, but the molecules a different arrangement of atoms in space and hence 3-d shapes.

• In general, branched chain isomers have lower boiling point then straight chain isomers. !

• Reason: Branches prevent the main chains from getting as close together and so Van der Waals forces are weaker between the molecules.

• Rules for IUPAC:

• Find the longest continuous carbon chain.!

• Use the prefix from the table to the number of carbon atoms in the longest continuous carbon chain.

• Look for the substituent groups(alkyle groups)!

• Number the position of these substituent groups using the combination that includes the lowest individual numbers.

• Use the prefix to indicate the number of each substituent group!

• Arrange the number of substituent groups in alphabetical order. eg. di, tri, tetra

• Saturated-Carbon has a single bond can be eight In an open or closed system

• Unsaturated: Carbon has a double bond/triple bond. In an open or closed system.

• Benzene with ring = aromatic

• Benzine without ring=aliphatic.

• The alkanes are non-polar molecules with only Van der Waals forces between molecules.

• This means that they are volatile(evaporate easily) with these first four members being gases at room temperature.

• Due to their non-polar nature, they are insoluble in water.!

• The most familiar reaction of alkanes is combustion!

• Complete combustion of hydrocarbons: Requires the presence of excess oxygen and produces carbon dioxide and water.

• Complete combustion of ethane:!

• pg. 432

INCOMPLETE COMBUSTION

• When there is a limited supply of oxygen, incomplete combustion occurs, which produces carbon monoxide, soot ( C ) and Water.

• Example:

• Alkanes react with chlorine or bromine in a substitution reaction, to give an initial product in which 1 hydrogen atom is replaced by the halogen.

• Usually, these reactions take place by exposure to UV light or sunlight, though they will also occur without light at very high temperature.

• eg. Ethane Chloroethane

• The reaction of an alkane (methane), with a halogen is a free radical chain reaction.!

• The process can be split into 3 distinct changes:!

Initiation: Produces the radicals (reactive species with unpaired electrons.

Propagation which forms most of the product in which the radicals are reformed (that is one radical is used up but another formed).

Termination: Which consumes radicals.

• As a result, the initiation stage occurring once can cause the propagation steps to occur many times before the radicals are consumed in a termination step.

• The alkanes are said to be saturated hydrocarbons because they contain only single C-C bonds.

• The compounds with multiple bonds are called unsaturated hydrocarbons.!

• The simplest unsaturated hydrocarbons are alkenes, which contain a carbon-carbon double bond.

• The general formula for alkene homologous series is CnH2n

• Though double bond are stronger than single bond, they are not twice as strong:

!

!

• This means that it is energetically favorable for a double bond to be converted into two single bonds.

• The activation energy for these reactions is also relatively low, owing to the high electron density in the double bond.

• This means that alkenes are considerably more reactive than alkanes.

Alkenes and hydrogen!

• Alkenes react with hydrogen when heated in the presence of a catalyst such as finely divided nickel to form alkanes.

• This reaction is called hydrogenation.

• Reaction of alkenes and halogen!

• Alkenes react with halogen at room temperature.

• The reaction with iodine is very slow at room temperature.

• Alkenes and hydrogen halides!

• Alkenes react with hydrogen halides (HBr) by bubbling the alkene through a concentrated solution of hydrogen halide at 100 degrees celcius.

• The reaction with HI is rapid but that with HCl is very slow.

• So a catalyst such as AlCl-3 is used.

• Alkenes and water!

• Alkenes react with water to form alcohol.

• Alkenes may be distinguished from alkanes by shaking them with bromine water.

• The bromine water, which is orange, is decolorized to colorless when shaken with alkene.

• There is no change in color when shaken with alkane.

• Alkene: bromine water ——->orange to colorless

• Alkane: bromine water ———>no color change

• 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 polymerization.

• Alkenes undergo addition polymerization in which a large number of monomers are joined together into a polymer chain.

• It is important to realize that it is only the C C group that reacts when the polymerization reaction occurs-all the other groups attached to the C C are unaffected

• Reaction page 443 Addition polymerization. Cambridge

Poly(ethene)• In the production of low density poly(ethene) the

reaction takes place under high temperature and high pressure in the presence of small amount of oxygen or an organic peroxide.

• In the production of high density poly(ethene) the reaction takes place under much lower temperature and lower pressure in the presence of a catalyst.

POLY(VINYL) CHLORIDE (PVC)

• It is made by polymerization of chloroethane monomers at moderate temperature in the presence of an organic peroxide.

POLY(PROPENE)• Poly(propene) is formed by the polymerization of

propene.

• The polymers more commonly known as plastics are extensively used in every day life.

• Among other things polyethene is used for plastic bags washing up bowls and bottles.

• Polychloroethane is used for electrical insulation and window frames.

• Polypropene is used for car bumpers and carpet fibers.

ALCOHOLS• The functional group of alcohol is -OH.

• The general molecular formula is CnH2n+1OH.

• Alcohols have a much higher boiling point than alkanes as a result of hydrogen bonding.

• The lower members are soluble in water because of hydrogen bonding.

• Alcohols burn readily in air to form carbon dioxide and water, hence they are used for fuels in the presence of excess oxygen.

Primary alcohols !

• Primary alcohols are oxidized first of all to an aldehyde (partial oxidation)

• Then the aldehyde is oxidized further to a carboxylic acid (complete oxidation)