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Organic chemistry is the study of compounds which contain carbon. The name 'organic' refers to the historical link between these compounds and living organisms. 'Inorganic' compounds were those which could be derived from non-living sources. Confusingly, some inorganic compounds contain carbon.
This material in this chapter is restricted to that needed by SL Chemistry students For further material see Organic Chemistry
[edit] 10.1
10.1.1 Describe the features of a homologous series. This is how organic compounds are classified into 'families'. Successive members of a homologous series differ by a -CH2. Molecular mass increases by a fixed amount as you go up the series. For example, alkanes.
Methane (CH4), ethane (C2H6, propane (C3H8), butane (C4H10). General formula: CnH2n+2 Other series have an additional 'functional group' which is also shown in the general formula. The functional group is usually a small group of atoms attached to a carbon atom in the molecule. For example, alcohols have the functional group -OH. General formula: CnH2n+1OH.
10.1.2 Predict and explain the trends in boiling points of members of a homologous series. Since successive members differ by -CH2, their carbon chains will continue to increase in length.
Boiling point increases with increasing carbon number. Note that if you know the chemical characteristics of a functional group, you can predict the chemical properties of the series.
10.1.3 Distinguish between empirical, molecular and structural formulas. The empirical formula is the lowest whole number ratio of the atoms. The molecular formula is the actual number of atoms. The structural formula is a representation of how the molecules are bonded together. Full structural shows all bonds, whereas condensed structural omits bonds where they can be
assumed. Ethane... Empirical: CH3, Molecular: C2H6, Condensed structural: CH3CH3 Full structural:
H H | |
H — C — C — H
| |
87 H H
10.1.4 Describe structural isomers. Compounds with the same molecular formula but different arrangements of atoms.
10.1.5 Deduce structural formulas for the isomers of the non-cyclic alkanes up to C6. C4H10: Butane and 2-methyl propane.
C5H12: Pentane, 2-methylbutane, 2,2-dimethylpropane.
H H H H H H H H H H
Organic chemistry is concerned with the compounds of carbon. Since there are more compounds of carbon known than all the other elements together, it is helpful to have a systemic way of naming them.
Identify longest carbon chain 1 carbon = meth- 2 carbons = eth- 3 carbons = prop- 4 carbons = but- 5 carbons = pent- 6 carbons = hex- 7 carbons = hept- 8 carbons = oct-
88 Identify type of bonding in the chain All single bonds in the carbon chain = -an- One double bond in the carbon chain = -en- One triple bond in the carbon chain = -yn-
Identify functioning group
Formula Name Example R-H Alkane Methane R-OH Alcohol ethanol R-NH3 Amine Ethylamine R-X Halogenoalkane Bromoethane (Where X = F, Cl, Br, I) R-C=O Aldehyde Ethanal
| H
R-C-R Ketone Propanone
||
O
R-C-OH Carboxylix Acid Ethanoic Acid
||
O
R-C-OR Ester Ethyl Ethanoate
||
O
Numbers are used to give the positions of groups or bonds along the chain.
Homologous Series The alkanes form a series of compounds all with the same general formula CnH2n+2 e.g. Methane CH4 Ethane C2H6 Propane C3H8
If one of the hydrogen atoms is removed what is left is known as an alkyl radical R- (e.g. methyl CH3- ) When other atoms or groups are attached to an alkyl radical they can form different series of compounds. These atoms or groups are known as functioning goups and the series formed are all homologous eries. Homologous series have the same general formula with the neighbouring members of the series differing by by -CH2: for example the general formula of alcohols is CnH2n+1OH. The chemical properties of the individual members of an homologous series are similar and they show a gradual change in physical properties.
Boiling Points As the carbon chain gets longer the mass of the moleculaes increases and the van der Waals' forces of attraction increase. A plot of boiling points against the number of carbon atoms show a sharp increse at first, as the percentage increase in mass is high, but as successive -CH2- groups are added the rate of increase in boiling point decreases. When branching occurs the molecules become more spherical in shape, which reduces the contact surface area between them and lowers the boiling point. Other homologous series show similar trends but the actual
temeratures at which the compounds boilwill depend on the type of attractive forces between the
89 molecules. The volatility of the compounds also follows the same pattern. The lower members of the alkanes are all gases as the attractive forces are weak and the next few members are volatile liquids. Methanol, the first member of the alcohols is a liquid at room temperature, due to the presence of hydrogen bonding. Methanol is calssed as volatile as its boiling point is 337.5K but when there are four or more carbon atoms in the chain the boiling points exceed 373K and the hugher alcohols have low volatility.
Functional Group Strongest type of intermolecular attraction Alkane van der Waals' Alkene van der Waals' Alkyne van der Waals' Ester Dipole:Dipole Aldehyde Dipole:Dipole Ketone
Dipole:Dipole Amine Hydrogen bonding Alcohol Hydrogen bonding Carboxylix Acid Hydrogen bonding
Low Reactivity of Alkanes Because of the strong C-H and C-C in alkanes they have low reactivity and only usually undergo combustion in the presence of Oxygen and substitution reactions with halogens in ultra-violet light.
MECHANISM OF SUBSTITUTION REACTION WITH HALOGENS Chemical bonds can break homolytically or heterolytically. Heterolytically is when both of the shared electrons go to one of the atoms resulting in a positive and one negative ion. When halogen are exposed to ultraviolet light the halogen molecule bond can break homololytically meaning each of the atoms retains one of the pair of previously shared electrons. The bond between two halogen atoms is weaker than C-H or C-C bond in alkanes and so is more likely to become a free radical due to homolytic fission of the covalent bond. When a Chlorine molecule is exposed to ultraviolet rays and becomes two free radical chlorine atoms it is called the intiation stage. Free radicals have an unpaired electron and are therefore very reactive.
Topic 20 is the additional HL material for Topic 11.
Determination of structure
This HL Sub-topic is required for Sub-topic 1 of SL Option A Hydrocarbons
The material on benzene in this HL Sub-topic is required for Sub-topic 1 of SL Option A Nucleophilic substitution reactions
This HL Sub-topic is Sub-topic 4 of SL Option A Alcohols
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