It was noted previously (here) that there were great similarities in SN and E in terms of the starting materials and reagents – both involve a molecule containing a leaving group, and a reagent that is either a nucleophile or a base (it was also noted in the Nucleophilic Substitution chapter that these two share similar properties). So why do some reactions go as … Read more
The issue of which proton is removed when there is a choice is not as simple in E2 as it is in the other mechanisms. Two empirical rules have been formulated which apply to different systems, and essentially say the opposite things. These rules were developed by Hofmann and Saytzev (working separately) in the 19th century. Hofmann’s work applied … Read more
The stereochemistry of E1 and E1CB is relatively easily to dissect. In terms of the first step the regiochemistry is reasonably clear (and there should be no stereochemistry to consider) – in E1CB the most acidic proton (adjacent to the leaving group) is taken and in E1 the leaving group leaves! From the charged intermediate there are two possible olefinic outcomes and the more … Read more
E1 1. The similarity in rate determining steps between E1 and SN1 helps to explain why the same factors govern whether a mechanism is E1 or E2 (E1CB disregarded for the time being) – those being; possibility of a stabilised cation and a polar solvent which can solvate said cation. Therefore, the E1 order of reactivity for alkyl halides runs tertiary > secondary > primary. 2. Where there … Read more
An elimination reaction involves the removal of two species from a molecule: The most common type of elimination is a so-called 1, 2- elimination where the two species are removed from adjacent carbon atoms, usually causing the formation of a multiple bond. For example; This 1, 2- style elimination can happen in one of three distinct mechanisms, … Read more
There are a number of fairly commonly encountered electrophilic additions that have their own very particular mechanisms, some of which have been collected here. Some sections of the mechanisms may be unfamiliar; if they are, simply focus upon the electrophilic addition step that is important for this chapter – the other aspects of the mechanism should become more clear as later … Read more
In its most basic form, an alkene is an electron rich species – the electrons of the π system are quite diffuse and easily polarised, forming a cloud of negative charge around the carbon atoms. This makes most alkenes attractive to electrophiles – those species that are electron deficient. The reactions they will undergo together are addition reactions – similar in overall … Read more
A mesomeric effect is an electron redistribution that occurs via a pi orbital, quite often via conjugated systems. A good example of this effect is seen in the carbonyl group: The properties of the carbonyl are not properly explained by the classical image (far left), nor by the dipole (centre), which represents the total relocation of the pi electrons; the “real” … Read more
The factors affecting the electron availability of a compound might reasonably be thought to have far-reaching consequences upon its reactivity with various compounds. For example, an area of high electron density is unlikely to be attacked by OH–, but an area of low electron density is likely to be far more susceptible to attack by … Read more
This final section of the chapter deals with a few named reactions in which enolate anions react with other carbonyl compounds as nucleophiles – so these could realistically have been placed under the nucleophilic addition chapter. These are just meant to be representative of the huge number of possibilities present for carbonyls reacting with other carbonyls – by noting the patterns and … Read more