I need help distinguishing between nucleophiles and electrophiles. Like one of the examples I am given shows methanal, chloroethane, and methanamine. Now it says methanal is electrophilic because the carbonyl group is polar since the more electronegative oxygen atom has a greater share of electrons. It says chloroethane is electrophilic because of the polar character of the C-Cl bond. And finally, the methanamine is said to be nucleophilic because it has a pair of electrons to share.
But methanamine is polar so how come it isn't electrophilic? And the oxygen in methanal and the Cl in chloroethane both have lone pair electrons to share, so how come they are not nucleophilic? Basically how would I distinguish between nucleophiles and electrophiles?
To distinguish between nucleophiles and electrophiles, it is important to understand their fundamental characteristics.
Nucleophiles are molecules or ions that have an excess of electrons and are therefore electron-rich. They possess one or more lone pairs of electrons or pi bonds that they can donate to form a new bond. Nucleophiles are attracted to regions of positive charge, such as positively polarized atoms.
On the other hand, electrophiles are molecules or ions that have an electron deficiency and are electron-poor. They typically have a positive charge, a partial positive charge, or an electron-deficient atom. Electrophiles seek to acquire electrons from nucleophiles to complete their octet or stabilize themselves.
Now, let's examine the examples you provided and understand the reasoning behind their classification as electrophiles or nucleophiles.
1. Methanal (formaldehyde): Methanal is classified as electrophilic because of the presence of a polar carbonyl group (C=O). The oxygen (O) atom, being more electronegative than carbon (C), draws electron density towards itself, creating a partial positive charge on carbon. This electron-deficient carbon atom can accept a pair of electrons from a nucleophile.
2. Chloroethane: Chloroethane is also considered electrophilic due to the polar character of the carbon-chlorine (C-Cl) bond. The chlorine atom, being more electronegative than carbon, pulls electron density towards itself, resulting in a partially positive carbon atom. This makes the carbon atom electron-deficient and capable of accepting electrons.
3. Methanamine: Methanamine is nucleophilic since the nitrogen (N) atom possesses a lone pair of electrons. These electrons can be donated to form a new bond with a positive or electron-deficient center.
To answer your question about why methanamine is considered nucleophilic despite being polar: polarity does not determine the nucleophilicity or electrophilicity of a molecule on its own. In the case of methanamine, although the molecule is polar due to the difference in electronegativity between nitrogen and hydrogen, it is the presence of a lone pair of electrons on nitrogen that characterizes it as a nucleophile.
Similarly, while both oxygen in methanal and chlorine in chloroethane have lone pairs of electrons, their classification as electrophiles is due to the partial positive charge on the carbon atom, which makes them electron-deficient and capable of accepting electrons.
In summary, to distinguish between nucleophiles and electrophiles, look for the presence of electron-rich species (lone pairs or pi bonds) indicating nucleophilicity, and electron-deficient species (positive charges or partially positive atoms) indicating electrophilicity. The polarity of a molecule alone does not determine whether it is a nucleophile or an electrophile; rather, it is the availability or deficiency of electrons that dictates its reactivity.