Organic Chemistry (sl)

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

 

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)

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

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.