Organic Chemistry
Uploaded by mahnoormukhtar on Jan 24, 2015
TOPIC 10 ORGANIC CHEMISTRY
10. 1 Introduction
Homologous series
A homologous series is a set of compounds which has the following features:
share a general formula (i.e. same elements in the same ratio);
members share the same functional group; a functional group is a group of atoms which determine the chemical properties of the homologous series;
whose nearest neighbours differ by one repeating unit, most often a methyl group -(CH3 ) or a methylene group -(CH2)- ;
have similar chemical properties (same functional group);
show a gradual change (gradation) in physical properties as shown by the table below which shows the melting points and boiling points of some alkanes:
examples of homologous series: alkanes, alkenes, alkynes, alcohols, esters, alkanals and amines.
name molecular
formula melting
point
(oC) boiling
point
(oC) state at
25oC
methane CH4 -183 -164 gas
ethane C2H6 -183 -89
propane C3H8 -190 -42
butane C4H10 -138 -0.5
pentane C5H12 -130 36
liquid
hexane C6H14 -95 69
heptane C7H16 -91 98
octane C8H18 -57 125
nonane C9H20 -51 151
decane C10H22 -30 174
undecane C11H24 -25 196
dodecane C12H26 -10 216
eicosane C20H42 37 343
solid
triacontane C30H62 66 450
The table on the left shows a gradual increase in boiling point with increasing number of carbon atoms and therefore increasing molar mass.
A trend caused by the fact that as the number of carbon atoms in the molecules increases so does the number of electrons within the compound which creates greater polarity during instantaneous polarisation (which causes the Van der Waals’ forces) and therefore produces greater Van der Waals’ forces. There is also a greater surface area over which instantaneous polarization can occur.
A graph of boiling points of alkanes against chain length gives a steep line at first but then flattens out at higher numbers of carbon atoms suggesting that the size of molecules becomes less influential in affecting boiling point.
Formula of organic compounds
type of formula description example
empirical formula shows most simple ratio CH2
molecular formula shows the different atoms and how many of each; no information on how they are arranged. C6H14
structural formula structural formula show how atoms are arranged together in the molecule; a
full structural formula (sometimes called a graphic formula or displayed formula) shows every atom and bond.
condensed structural formula structural formula which shows order in which atoms are arranged but which omits bonds CH3CH2CH2CH2CH2CH3
or CH3(CH2)4CH3
Naming of organic compounds
When naming an organic compound we want to give a lot of information in its name which is why the name of an organic compound consists of at least two parts: one to indicate the number of carbon atoms in the chain and the other the functional group. Other parts will indicate length and number of branches or if the compound is cyclic.
Example:
eth ane
• this part tells us how many carbons atoms there are in the molecule;
• this part...