The following notes were written for the previous IB syllabus (2009). The new IB syllabus for first examinations 2016 can be accessed by clicking the link below.

IB syllabus for first examinations 2016

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 CnH2n+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


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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

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

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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.

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10.4 - Alcohols

homologous series name (old name) functional group naming system description
Alkanal (aldehyde) -CHO ends with -anal Has a carbonyl C=O at the end of a carbon chain, with the carbon also attached to a hydrogen
Alkanone (ketone) -CO- ends with -anone Has a carbonyl group C=O but it is in the middle of a carbon chain and the carbon has no hydrogens attached
Alkanol (alcohol) -OH ends with -anol (may also go at the start of the name as hydroxy- if there is another more important group in the molecule) These have an -OH group (or more than one in the case of diols, triols etc) at any position in the chain.
Alkanoic acid (carboxylic acid) -COOH ends with -anoic acid The group must go at the end of a carbon chain as it has a carbon attached to an oxygen by a double bond and also an -OH group (leaving the carbon with only one other bond which it uses to attach to the rest of the carbon chain)
Amine -NH2 ends with -ylamine or starts the name with Amino- (depending on whether there is a more important functional group int he molecule) The NH2 group can go anywhere on the carbon chain. It infers basic characteristics to the molecule. (ability to accept a proton from an acid)
Amide -CONH2 ends with -anamide The amine group is associated with a carbonyl group inferring different characteristics to the molecule. It can also be a linking group -CONH- between two alkyl chains (proteins and nylon)
Halogenoalkanes -X becomes a prefix: chloro- bromo- iodo- folowed by the rest of the name The halogen atom may be added into any position in the chain. If there is more than one then the prefixes di- tri- tetra- etc are used.
Esters -COO- name is derived from the acid and alcohol that were used in making the ester. The alkyl group ending with the -COO was the acid part and becomes -(name)anoate and the part after the -COO group starts the name. For example: ethyl ethanoate Esters are linkage compoounds formed by condensation (esterification) reactions between carboxylic acids (or compounds deriving from them) and alcohols. They also occur naturally and may be polymers (polyester) see amide linkage above
Ethers -O- derives from the shortest alkyl chain ending in -oxy followed by the longest alkyl chain eg. methoxyethane CH3-O-C2H5 These are linkage compounds where the oxygen bridges two carbon chains. They are of little importance except as solvents (ethoxyethane)
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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.
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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.

homologous series Important reaction types reagent and conditions product (s)
alkanes
combustion air/oxygen, heat carbon dioxide and water
free radical substitution of halogens halogen and UV light halogenoalkanes (mixture)
alkenes
combustion air/oxygen heat carbon dioxide and water
addition Br2, Br2(aq), H2 dibromoalkane, bromoalcohol, alkane
polymerisation catalyst polyalkene
alcohols
combustion air/oxygen, heat carbon dioxide and water
dehydration (elimination) phosphoric or sulphuric acid alkene
oxidation sodium dichromate/sulphuric acid alkanal, alkanone or carboxylic acid
substitution of the -OH group phosphorus pentachloride, sodium halogenoalkane, sodium alkoxide
esterification carboxylic acid ester
aldehydes
(alkanals)
addition ammonia compound containing hydroxy and amine
oxidation sodium dichromate/dil. sulphuric acid carboxylic acid
reduction lithium aluminium hydride 1º alcohol
addition - elimination (condensation) amines, hydroxylamine, depends on reagent
     
carboxylic acids
acid - base reactions base (eg sodium hydroxide) salt and water
reduction lithium aluminium hydride alkanal (or 1º alcohol)
esterification alcohol/conc. sulphuric acid ester
haloalkanes
(halogenoalkanes)
substitution of the halogen sodium hydroxide, ammonia alcohol, amine
dehydrohalogenation (elimination) ethanolic NaOH/ reflux alkene
amines
reaction as a base with an acid mineral (strong) acid amine salt
condensation (addition - elimination) carbonyl compounds depends on reagent
amides
hydrolysis (breaking apart in solution) dilute acid/heat amine and carboxylic acid
hydrolysis (breaking apart in solution) dilute base/heat amine and sodium carboxylate
esters
hydrolysis (breaking apart in solution) dilute acid/heat alcohol and carboxylic acid
hydrolysis (breaking apart in solution) dilute base/heat alcohol and sodium carboxylate

Summary of required reactions

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