organic chemistry. homologous series a grouping of organic compounds based on their composition and...
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Organic Chemistry
Homologous Series
A grouping of organic compounds based on their composition and properties
A series has:A general formulaThe same functional groupSimilar chemical propertiesChanging 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 arealkenesalkynesaromatics
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 R – NHR’ R – NR’R’’
(RNH2) (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 moleculeUse 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 (London/Van der Waals)
Dipole – dipole
Hydrogen bonding
* ion – dipole
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