reactivity of carbon centered nucleophile with electrophile

However, whereas with Bronsted-Lowry we can quantify acid/base strength using the pK a/pK b, this is not possible with electrophiles & nucleophiles. Just as with Bronsted-Lowry acids/bases, we can have strong Lewis acids (electrophiles) and strong Lewis bases (nucleophiles). To draw a mechanism, we need to think about which atom is the nucleophile – usually giving away a lone pair or a bond – and which atom is the electrophile – gaining a lone pair or bond. The pair of electrons in a mechanistic curved arrow begins at a nucleophilic center and end at an electrophilic center.

However, organic chemists usually refer to a Lewis acid as an electrophile (which is electron poor), and a Lewis base as a nucleophile (electron rich). The majority of organic reactions can in fact be classified as Lewis acid-base reactions. The carbonyl oxygen (the Lewis base) donates a pair of electrons to the magnesium cation (the Lewis acid). The interaction between a magnesium cation (Mg 2+) and a carbonyl oxygen is a common example of a Lewis acid-base reaction in enzyme-catalyzed biological reactions. Other molecules can also act as either an acid or a base. Thus, the medium which a molecule is placed in has an effect on the properties of that molecule.

Water does not act as an acid in an acid medium and does not act as a base in a basic medium. You may have noticed that the degree to which a molecule acts depends on the medium in which the molecule has been placed in. However, water can also act as a base by accepting a proton from an acid to become its conjugate base, H 3O +. Water can act as an acid by donating its proton to the base and thus becoming its conjugate acid, OH. This ability of water to do this makes it an amphoteric molecule. You may have noticed this with water, which can act as both an acid or a base. Again, aldehydes are more reactive than ketones.As of now you should know that acids and bases are distinguished as two separate things however some substances can be both an acid and a base.The partial positive charge on an aldehyde carbonyl carbon is less stable than the partial positive charge on a ketone carbonyl carbon.The carbonyl carbon in the ketone is a little more stable than the carbonyl carbon in the aldehyde. In a more sophisticated explanation, the cation becomes stabilized by a molecular orbital interaction involving the empty p orbital on the carbocation and C-H bonds on the neighbouring carbons.Ī similar situation results in the partially positive carbon in the carbonyl. In that way, it s charge is delocalized and stabilized. It can polarize the neighbouring carbons, drawing some negative charge towards itself and leaving some positive charge on the other carbons. The positively charged carbon is more electronegative than the uncharged carbons, so it draws electrons away from them. This observation is sometimes explained as an inductive effect. There is a general rule about cation stability on carbon atoms: a carbocation with more carbons attached to it is more stable than a carbocation with more hydrogens attached to it.

Reactivity of carbon centered nucleophile with electrophile