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