Organic chemistry HL

20.1- Introduction

The following homologous series must be familiar: Amines, amides, esters and nitriles

Amines

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'

methylamine	ethylamine	propylamine

Amides

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'.

methanamide	ethanamide	propanamide

Esters

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.

methyl ethanoate	ethyl methanoate	ethyl ethanoate

Nitriles

Nitriles are described in many texts as derivatives of carboxylic acids. However, I personally 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'.

methanonitrile	ethanonitrile	propanonitrile

20.2 - Nucleophilic substitution reactions

Heterolytic fission

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 (S_N1 and S_N2 mechanisms) The type of mechanism depends on the nature of the halogenoalkane - primary halogenoalkanes react via S_N2 and tertiary halogenoalkanes via S_N1.

The reason for the different mechanisms lies with the stability of the intermediate tertiary carbocation which forms in the case of the S_N1 mechanism, whereas a primary carbocation would not be stable encouraging S_N2. Another factor encouraging the S_N1 mechanism in tertiary halogenoalkanes is the steric hindrance on approaching nucleophile experiences, preventing easy access to the partially positive carbon atom

S_N1 mechanism

First, due to the electron withdrawing effect of the halogen, the carbon-halogen bond breaks heterolytically, resulting in

Example:

(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.

S_N2 mechanism

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

1. Depend on the identity of the halogen (F, Cl, Br or I)
2. The nature of the halogenoalkane (1º, 2º or 3º)

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 S_N2 mechanism which is slower. Consequently the order of reactivity is 3º > 2º > 1º.

20.3 - Elimination reactions

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.

Example:

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).

Example:

CH₂H-CH₂OH -- H₂SO₄ (and heat, 170ºC) CH3OCH3 + H₂O

20.4 - Condensation reactions

Esterification

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) --Cr₂O₇^2- 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.

Examples:

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)

Structural isomerism

• Positional
• Functional group

Stereoisomerism

• Geometric
• Optical

Resources

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