Organic Chemistry (sl)

10.1 - Introduction

An homologous series is a set of compounds whose components differ by a single repeating functional group. In the case of (straight chain) alkanes, CH₂. Their general formula is C_nH_2n+2.

Physical properties

The boiling points of alkanes increase as the chains get longer (increased number of electrons causes increased Van de Waal's forces), increasing rapidly initially but flattening off. Click diagram to enlarge

10.2 - Alkanes

Compounds containing only hydrogen and carbon. There are three types alkanes, alkenes and alkynes.

Alkanes have a CH₃ group at each end (except methane has only one CH₄) and fill out the required number with CH₂ groups.

Nomenclature (Naming system)

Methane, ethane, propane, butane, pentane, hexane.

methane	ethane	propane	butane

Isomerism and branching

Any structure that can be drawn can exist providing the fundamental rules have been fulfilled.

1. Each carbon forms four bonds (may be all single, one double and two singles, etc)
2. Each hydrogen forms one bond
3. Each oxygen forms two bonds

This means that one molecular formula can have other possible structures. This is called isomerism. The chains formed by the alkanes and other organic molecules do not have to be straight (actually zigzag) but may be branched ie having "branches" of carbon atoms attached to the main unbroken chain.

Example:

The alkane C₄H₁₀ exists in two isomeric forms - a straight chain form and a branched form

butane	methyl propane

10.3 - Alkenes

These structures will be similar to those of the alkanes except two hydrogens on adjacent carbons are replaced by a double bond between those carbons. The number '1' in the names refers to the position of the carbon starting the double bond. No numbering is needed in the first two members as there can be no ambiguity.

ethene	propene	but-1-ene	pent-1-ene

Combustion

This is effectively a technical word for burning. Most organic compounds burn with the exception of chlorinated (halogenated ) hydrocarbons.

Complete combustion produces CO₂ and H₂O, incomplete combustion produces CO, C and H₂O (usually occurs with unsaturated compounds, where there is a limited supply of oxygen). C produces a 'dirty' flame leaving carbon deposits on everything, CO is toxic and CO₂ is a greenhouse gas. Incomplete combustion is where the carbon is not completely oxidised.

The combustion of hydrocarbons is an exothermic process (otherwise there wouldn't be much point in burning them to produce energy for fuel and heat). This is because the O-H bond is stronger than the C-H bond, and the C=O bond is stronger than the C-C. This means that, the C-C and C-H bonds breaking requires energy, but this is more than made up for by the energy released by the formation of the C=O and O-H bonds.

10.4 - Alcohols

10.5 - Halogenoalkanes

Halogenoalkanes (also called haloalkanes) have a halogen atom attached to a hydrocarbon chain. The halogen atom may be at the end of the chain or on any of the carbons.

1-chloropropane	2-chloropropane

Halogenoalkanes may be classified as primary, secondarty or tertiary, depending on the number of carbon atoms attached to the carbon holding the halogen.

1-chloropropane is a primary halgenoalkane, as there is only one carbon attached to the carbon holding the halogen, whereas 2-chloropropane is a secondary halogenoalkane as there are two carbon atoms attached to the carbon holding the halogen.

Reactions of halogenoalkanes

• Nucleophilic substitution
• Elimination reactions (not SL)

Substitution involves removal of the halogen and replacing it with another ion, or group (just like substitution in football). The nucleophilic refers to the mechanism of the reaction and says that the attacking species must be looking for a positive charge (nucleophile = 'positive seeking'), i.e. it has a lone pair of electrons and may be negatively charged.

The simplest reaction is with dilute sodium hydroxide which containes free hydroxide ions OH^-(aq).

CH₃CH₂-Br + NaOH CH₃CH₂-OH + NaBr

The mechanism of the reaction depends on the nature (1º, 2º or 3º) of the halogenoalkane.

• 1º Halogenoalkanes react via sN2 mechanism
• 3º Halogenoalkanes react via sN1 mechanism
• 2º probabaly use a mixture of both.

10.6 - Reaction pathways

Chemical properties of the different functional groups

The following table summarises the chemical properties of the functional groups studied at this level. Some of the reactions must be known in greater detail than others. If in doubt consult the relevant section of the syllabus.

Summary of required reactions