After mechanisms had been developed which brought order and reason to many organic reactions, a few were still elusive. They did not appear to proceed with a polar or radical pathway. A famous example of one such reaction is the Diels-Alder reaction: This involves the addition of an alkene to a conjugated diene. Reactions such as the Diels-Alder have one very … Read more
A substitution reaction is one with the general form; For example; For a nucleophilic substitution, Z is the nucleophile. This nucleophile will attack an area of a molecule which has partial positive charge – or in other words is electrophilic. In the example above, the carbon-iodine bond is polar (because iodine is more electronegative than carbon), so there is partial positive charge … Read more
Carbonyls The other unsaturated system that will commonly undergo nucleophilic addition. However, a carbonyl system is most likely to undergo nucleophilic substitution with a different mechanism to SN1, SN2 etc. and this is dealt with in a separate section. The reason for the attractiveness of carbonyl systems to nucleophiles is that the Carbon-Oxygen double bond is quite polar, as shown by the resonance structures which leave a δ+ on the carbon. Compared … Read more
Nucleophilic addition is essentially the opposite of elimination – shown in general form below; The two main types of unsaturated systems that will undergo this are alkenes and carbonyls. These two systems are very different, so their examples will be kept separate, but a general pattern should become evident. The other type of addition that occurs (also onto unsaturated systems) is electrophilicaddition, … Read more
The main points about sulphur 1. A range of oxidation states are available – predominantly 2, 4 and 6. 2. Quite nucleophilic in its divalent state, even more so as S–. 3. Hydrogens alpha to sulphur atoms are noticeably more acidic than they would be if the sulphur were absent. The carbocation generated by deprotonation is stabilised inductively by the sulphur, … Read more
A widely used method of protecting hydroxyl groups in organic molecules is to turn them into silyl ethers. A protecting group is one which reduces the reactivity of the functional group it protects so that chemistry may be carried out elsewhere on the molecule without fear of interference from the protected group. A silyl ether is quite easy to make – all that … Read more
Recall from the first page of this chapter that silicon stabilises carbocations β to it and carbanions α to it. Thus vinylsilanes can undergo both electrophilic substitution and nucleophilic addition. Electrophilic Substitution For maximum stabilisation of the carbocation, the C-Si bond must be in the same plane as the empty p orbital (of the carbocation) as this gives maximum overlap of the C-Si σ bond into … Read more
Whereas enols are seen only as a small proportion of the keto-enol tautomerism, silyl enol ethers are stable and very useful sources of regiochemically pure enolate ions. They are easily prepared – following the route to the enolate ion then treating with a silyl reagent like Tri-Methyl-Silyl-Chloride (‘TMSCl’). For example; If the ketone is asymmetrical then there are two possibilities for deprotonation, and the proton that … Read more
1. Silicon forms very strong bonds to oxygen and fluorine (particularly to oxygen) – this provides a driving force for much of silicon’s chemistry. 2. Silicon is more electropositive than carbon, so when bonded to carbon it often represents an attractive site for nucleophilic attack (i.e. it has δ+) – particularly if a halogen is attached. 3. The mechanism for nucleophilic substitution is … Read more
Some assorted reactions to show the versatility of phosphorus and to exemplify the points made on page 1. 1. i. Shows the potency of P=O as a driving force – in the penultimate step a carbanion is created (fairly unstable) while the P=O is formed. ii. Ph-CH2– is the only possible carbanion is the system with any stabilisation (see below), so it … Read more