Organic chemistry HL
20.1 - Introduction
The following homologous series must be familiar: Amines, amides, esters and nitriles
Amines are the simplest organic molecules containing a nitrogen atom. They consist of the amine group, NH2, attached to an alkyl chain, such as methyl ethyl, propyl etc.
They are named after the alkyl chain followed by the suffix '-amine'
Amides are derived from carboxylic acids and contain the carbonyl linkage followed by an amine group. Together these atoms make up the amide group., CONH2.
Their names come from the name of the carboxylic acid from which they aqre derived plus the suffix '-amide'.
Esters are formed from the reaction between an alcohol and a carbopxylic acid. They consist of two hydrocarbon chains connected by a carbonyl and an oxygen, the ester group, -COO-. Their names come from the hydrocarbon chain that orginated from the alcohol followed by the acid derivative name, eg ethanoate.
Nitriles are described in many texts as derivatives of carboxylic acids. However, I don't find this a very useful description, as they have no carbon - oxygen double bond.
They consist of an alkyl chain attached to a nitrogen atom via a triple bond. This means that the last carbon in the hydrocarbon chain has no hydrogens attached. Another way to think of nitriles is as organic cyanide molecules.
It is important to remember that the carbon of the -CN at the end counts as part of the longest carbon chain. The name comes from the longest alkyl chain as if it were an acid derivative, plus the suffix - 'nitrile'.
When the bond breaks, one atom gets a lone pair of electron, the other gets none.
The most common reaction of the halogenoalkanes is nucleophilic substitution (SN1 and SN2 mechanisms) The type of mechanism depends on the nature of the halogenoalkane - primary halogenoalkanes react via SN2 and tertiary halogenoalkanes via SN1.
The reason for the different mechanisms lies with the stability of the intermediate tertiary carbocation which forms in the case of the SN1 mechanism, whereas a primary carbocation would not be stable encouraging SN2. Another factor encouraging the SN1 mechanism in tertiary halogenoalkanes is the steric hindrance on approaching nucleophile experiences, preventing easy access to the partially positive carbon atom
First, due to the electron withdrawing effect of the halogen, the carbon-halogen bond breaks heterolytically, resulting in
(CH3)3C-Cl (CH3)3C+ + Cl-
This is the rate determining step (hence the 1st order reaction). The nucleophile then attacks the positive carbon atom and forms (CH3)3C-Nu.
Rather than completely breaking the bond, the polar bond between the halogen and carbon produces a partial +ve charge on the carbon. This is enough to attract a nuleophile to form an intermediate with effectively 5 bonds, one to the nucleophile, one with the halogen and 3 others. This is the rate determining step, hence the second order reaction. The halide ion then breaks off heterolytically forming CH3Nu + Cl-.
Some good nucleophiles are ROH, CN-, OH-, and RNH2.
Rates of nucleophilic substitution
The type of halogen determines the bond strength between the carbon and the halogen. F-C is the strongest and consequently fluoro alkanes are the least reactive (slowest rate)
Primary (1º) halogenoalkanes tend to react via the SN2 mechanism which is slower. Consequently the order of reactivity is 3º > 2º > 1º.
Dehydration to form alkenes or alkoxyalkanes
The products formed depend on the conditions used: Alkenes are formed in the presence of H2SO4 (or H3PO4 better, as it doesn't produce as many by-products) and the correct temperature (hot for primary, warm for secondary and cool for tertiary) alcohols lose a water molecule.
CH2H-CH2OH -- H2SO4 (and heat, 170ºC) CH2=CH2 + H2O
Alkoxyalkanes are produced under moderate temperature conditions (140ºC for primary alcohols) Water and a proton are then split off producing an ether (alkoxyalkane) and water (and regenerating the acid as a catalyst).
CH2H-CH2OH -- H2SO4 (and heat, 170ºC) CH3OCH3 + H2O
20.4 - Condensation reactions
Reactions of amines with halogeno compounds
Condensation of aldehydes and ketones
Polyester and polyamide formation
Carbonyl compounds are reactive because they contain a delta+ve carbon atom (caused by polarisation of the C=O bond), and are unsaturated. Thus, the pi electrons can be relatively easily shifted to form a new bond on both the carbon and oxygen atoms, and since nucleophiles are attracted to the carbon atom, this happens relatively quickly.
Alkanals are oxidised in the same way as the alcohols and form carboxylic acid as follows.
CH3CHO (aldehyde/alkanal) --Cr2O72- CH3COOH (alkanoic acid)
Reduction is the reverse of the process shown in the alcohols above. Alkanals will be reduced to primary alkanols, alkanones will be reduced to secondary alkanols by LiAlH4.
Alkanals ... CH3CHO --LiAlH4 CH3CH2OH (primary alkanol)
Alkanones ... CH3-CO-CH3 --LiAlH4 CH3-CH(OH)CH3 (secondary alkanol)
20.5 - Reaction pathways
20.6 - Isomerism
Sub-divided into structural (positions of the atoms in the molecule) or stereoisomerism (relative poisitions in space)