Question

What is the $+ R$ effect?

Answer 1

Some molecule’s behaviour can not be explained by a single Lewis structure. In such instances, the molecule can be represented by two or more electronic structures, each of which can explain the majority of the molecule's features but none of which can describe all of them. The molecule's true structure is a close approximation of all of these electrical configurations. Resonance or mesomerism is a term used to describe the behaviour of such molecules. The true structure of these molecules is called resonance hybrid.

Complete answer:
The effect of the interaction between pi bond electrons on the stability of a molecule is known as the resonance effect. If there are any lone pairs on the atoms of a molecule, they can also contribute to the resonance of the molecule.
The delocalization of electrons between atoms is caused by the resonance effect. Resonance is caused by molecules with double bonds. We can use resonance structures to figure out what a molecule's true structure is. A molecule's actual structure is an intermediate structure achieved through resonance stabilisation. The original molecule's resonance structures are not isomers.
If a molecule has only one structure and no alternative resonance structures, it is the most stable structure in which the molecule can exist.
The electrons are transferred away from an atom or substituent group associated to the conjugated system in positive resonance.
In other words, $+ R$ effect refers to atoms that lose electrons towards a carbon atom. Example: $C{l^ - },B{r^ - },{I^ - },NH_2^ - ,O{H^ - }$ .

Note:
The resonance effect is influenced by the number of double bonds and their order. The electrons are transferred towards the atom or substituent group connected to the conjugated system in negative resonance. Resonance effect describes the transmission of electron pairs between atoms in a molecule.