Organic Chemistry
Homologous Series
 A grouping of organic compounds based on their
composition and properties
 A series has:
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A general formula
The same functional group
Similar chemical properties
Changing physical properties as molecular
size changes
Functional Group
 The reactive site of a molecule
 The group of atoms responsible for its chemical
properties
Saturated Hydrocarbons
 Compounds of only hydrogen and carbon
 Contain only single bonds
 Relatively unreactive
 Contain no functional group
Unsaturated Hydrocarbons
 Compounds of hydrogen and carbon only
 Contain double and triple carbon-carbon bonds
 Homologous series are
 alkenes
 alkynes
 aromatics
Molecular Formula
 Show the actual number of atoms present
 eg C6H12O2
Structural Formula
 Shows the arrangement of the atoms in the
molecule, ie the way that the atoms are bonded
Isomers
 Compounds with the same molecular formula, but
different structural formula
Functional Groups
 R represents an alkyl group CnH2n+1
 The alkyl group is generally unreactive in reactions
 It is common to represent different alkyl groups
using R’ and R”
Alcohol
 General Formula: R-OH
 Functional Group: hydroxyl
 Example: ethanol
Aldehyde
 General Formula: R = O
 Functional Group: aldehyde
 Example: pentanal
Ketone
 General Formula:
 Functional Group: Carbonyl
 Example:
propanone
Carboxylic Acid
 General Formula:
 Functional Group: carboxyl
 Example: ethanoic acid
Ester
 General Formula:
 Functional Group: ester
 Example: methyl ethanoate
Amine
 General Formula: R – NH2
(RNH2)
R – NHR’
R – NR’R’’
(R2NH)
(R3N)
 Functional Group: amine
(primary, secondary, tertiary)
 Example: methyl amine
Amide
 General Formula:
 Functional Group: Carboxamide
 Example: acetamide
Naming
 Naming compounds with more than one of the same
functional group per molecule
 Use di and tri to signify 2 and 3 functional groups
 propan–1,2–diol
 Butan–1,2,2–triol
 Ethan-1,2-dioic acid
 Pentan-2,4-dione
Melting and Boiling points
 For molecular substances, melting and boiling points
indicate the strength of the secondary interactions
 Stronger secondary bonds require more heat energy to
break
 Therefore molecules with stronger secondary bonds
have higher melting and boiling points
 The type of secondary interactions is related to the
molecular structure
Secondary Interactions
 In order from lowest to highest
 Dispersion forces
 Dipole – dipole
 Hydrogen bonding
 * ion – dipole
(London/Van der Waals)
Dispersion Forces
 For a reminder of what dispersion forces are turn to pg
30 of you Essentials book
 Dispersion forces increase with increased number of
electrons, therefore in homologous series dispersion
forces increase with chain length
 mp/bp increase with increased chain length or
molecular weight in a homologous series
Dipole - dipole
 Dipole – dipole interactions increase with both the
number of functional groups and the strength of these
bonds
 More polar bonds lead to higher bond strength and
hence higher mp/bp
 Hydrogen bonds are the strongest type of dipole-dipole
interactions and are between N-H and O-H (& F-H)
(note it must be a direct O-H bond to be hydrogen
bonds, the interaction between a carbonyl group and
hydroxyl group is dipole-dipole not hydrogen bond)
Solubility
 “Like dissolves Like” – a guide only, never write this as
an answer
 For a molecule to dissolve in water (or other polar
solvents), it must be able to form secondary bonds with
water
 The stronger the interactions, the more readily it
dissolves
 As a homologous series becomes larger its solubility goes
down