Chemistry

4.31. The electron pair involved in sharing between two atoms during covalent bonding is called shared pair or bond pair. At the same time, the electron pair which is not involved in sharing is called lone pair of electrons.

For example: CH₄ has 4 bond pairs but H₂O has 2 bond pairs and 2 lone pairs.

(a) Here, HO^– acts as a nucleophile as it is an electron-rich species, i.e., it is a nucleus-seeking species.

(b) Here, –CN acts as a nucleophile as it is an electron-rich species, i.e., it is a nucleus-seeking species.

(c) CH₃C⁺O acts as an electrophile as it is an electron deficient species.

Electrophiles: The name electrophiles mean electron loving. Electrophiles are electron deficient. They may be positive ions or neutral molecules.
Ex: H⁺, Cl⁺, Br⁺, NO₂⁺, R₃C⁺, RN₂⁺, AlCl₃, BF₃
Nucleophiles: The name nucleophiles means 'nucleus loving' and indicates that it attacks the region of low electron density (positive centres) in a substrate molecule. They are electron rich they may be negative ions or neutral molecules.
Ex: Cl^– Br^–, CN^–, OH^–, RCR₂^–, NH₃, RNH₂, H₂O, ROH etc.

Due to hyperconjugation, alkyl groups act as electron donors when attached to a π - system as shown below:

Nitroethoxide ion (O₂NCH₂CH₂O^–) is more stable than ethoxide ion (CH₃CH₂O^–) due to -I effect of nitro group which decreases the negative charge of oxide ion of nitroethoxide ion leading to stability. In contrast, CH₃CH₂ has +I-effect. It, therefore, tends to intensify the -ve charge and hence destabilizes it.

4.29 (c) It will not form a sigma bond because taking x-axis as the inter-nuclear axis, there will be lateral overlap between the two 2p_y orbitals forming a -bond.

4.28.  (a) Acetylene C_2? H₂? contains 3? bonds and 2? bonds.

(b) Ethylene C_2? H₄? contains 5? bonds and 1? bond.