(Received: SeptemAccepted: October 9, 2003) Universidad Andrés Bello, Santiago, Chile andĢ Departamento de Química, Facultad de Ciencias, Universidad de Chile, Santiago, Chile. Contreras 2ġ Departamento de Ciencias Químicas, Facultad de Ecología y Recursos Naturales, This proton-transfer step can occur both in S N2 and S N1 mechanisms.QUANTITATIVE REPRESENTATION OF REACTIVITY, SELECTIVITYĪND SITE ACTIVATION CONCEPTS IN ORGANIC Pérez 1, Julia Parra-Mouchet 2 and Renato R. Remember, curved arrows show movement of electrons and since the nucleophile is providing the electrons, the arrows should start from its lone pair or the negative charge.Īfter the nucleophilic attack, there is also a deprotonation step to form the alcohol. In any case, you need to show the curved arrow starting from the nucleophile and point it exactly to the carbon with (or without the LG if it is S N1 mechanism). When the processes happen in turn, this is called a stepwise mechanism – the S N1 mechanism. When everything happens simultaneously, it is called a concerted mechanism. There are two main types of substitution reactions: One, in which the nucleophilic attack and the loss of the leaving group happen at the same time, and the second, in which the loss of the leaving group happens before the nucleophile can attack. You can read more about the acidity and p Ka here. The more stable it is, the stronger the acid. So, the conjugate base is, in a way, a “leaving group”. Well, think about it this way a strong acid is the one with a large dissociation of the proton from the counterion (the conjugate base). In order for it to be expelled by the nucleophile, it needs to be able to handle the negative charge well – it needs to be a weak base.Ī good leaving group is usually the conjugate base of a strong acid. The trick here is that you always need to look at the leaving group (usually abbreviated as LG) as well. For example, the following reaction doesn’t work even though the carbon is connected to oxygen which is a very electronegative atom: Having a nucleophile and a carbon atom connected to an electronegative atom is not enough for a substitution to happen. Below are some common nucleophiles that you will encounter in your Organic Chemistry 1 class:
However, very often it is going to be negatively charged since the negative charge makes it more reactive. The nucleophile has to have at least one lone pair of electrons. Just like we call the carbon electrophilic, the species with a high electron density is called a nucleophile – “loving nucleolus”, i.e. The most common electrophile are the alkyl halides since the Cl –, Br –, and I – are good leaving groups. The molecule with the leaving group and the electrophilic carbon is called an electrophile. And, species with a high electron density are going to attack this carbon. The carbon is, therefore, electrophilic – it is looking for some electron density to compensate the partial charge. The more electronegative halogen pulls the electron density thus making the carbon partially positivity charged: These reactions occur because of the imbalance of the electron density between the carbon and halogen (leaving group) since it is a polar covalent bond. The replacing group is called a “ nucleophile” and the group being kicked out is called a “ leaving group”:
In a substitution reaction, one atom (or a functional group) replaces another one.
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