Organic Chemistry

Homologous Series A grouping of organic compounds based on their composition and properties A series has: 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 C 6 H 12 O 2

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 C n H 2n+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 – NH 2 R – NHR’ R – NR’R’’ (RNH 2 ) (R 2 NH)(R 3 N) 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 (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