Q.Would you expect the hydrides of N, O and F to have lower boiling points than the hydrides of their subsequent group members ? Give reasons.

The question asks which protein structure represents its constitution. In chemistry and biology, "constitution" means the fundamental composition — the specific identity and order of the building blocks. For a protein, the building blocks are amino acids, and their exact sequence is the most basic, defining feature. All higher levels of structure (secondary, tertiary, quaternary) depend on this sequence, but they are about shape and assembly, not the raw "ingredient list."

1. Understand what "constitution" means in context.

   The word "constitution" here is not about health or strength; it’s a technical term meaning the makeup or composition of a molecule — which atoms are present and how they are connected. For a protein, this translates to the order of amino acids along the polypeptide chain.

2. Recall the four levels of protein structure.

   • Primary structure: The linear sequence of amino acids, held together by peptide bonds. This is the covalent backbone — the actual chemical identity of the protein.
   • Secondary structure: Local folding patterns (α-helices, β-sheets) stabilized by hydrogen bonds. This is about shape, not composition.
   • Tertiary structure: The overall 3D folding of a single polypeptide chain, driven by side-chain interactions.
   • Quaternary structure: The arrangement of multiple polypeptide subunits into a functional complex.

3. Match "constitution" to the correct level. …

Hydrogen bonding is a special type of dipole-dipole attraction. It happens when a hydrogen atom is covalently bonded to a very electronegative atom (like oxygen, nitrogen, or fluorine) and is also attracted to a lone pair of electrons on another electronegative atom nearby. The key is that the hydrogen must be directly attached to N, O, or F to be sufficiently polarized.

Let's examine each molecule.

1. Salicylaldehyde has an –OH group (phenolic) and an –CHO group (aldehyde). The hydrogen of the –OH is attached to oxygen, so it can form a hydrogen bond. In fact, it famously forms an intramolecular hydrogen bond between the –OH hydrogen and the oxygen of the –CHO group. So hydrogen bonding exists here.

2. Catechol has two –OH groups on adjacent carbons of a benzene ring. Each –OH has a hydrogen attached to oxygen. These can form hydrogen bonds with each other (intramolecular) or with other molecules (intermolecular). So hydrogen bonding exists.

3. Salicylic acid has an –OH group and a –COOH group. The –OH hydrogen and the –COOH hydrogen are both attached to oxygen. It forms a strong intramolecular hydrogen bond between the –OH hydrogen and the carbonyl oxygen of the –COOH group. Hydrogen bonding definitely exists. …

The concept first

Melting point measures how strongly molecules cling to one another in the solid. Hydrogen bonding is the strongest of the ordinary intermolecular forces — but only if it is spent between molecules.

When a hydrogen-bond donor ($\ce{-OH}$) and an acceptor ($\ce{C=O}$) sit close enough on the same molecule to form a comfortable five- or six-membered ring, the molecule bonds to itself. This is intramolecular hydrogen bonding (chelation), and it has a striking consequence: the $\ce{-OH}$ is now unavailable to its neighbours, so the substance behaves almost like a non-polar molecule — lower melting point, lower boiling point, greater solubility in non-polar solvents.

Step 1 — The ortho isomer (A)

In 2-hydroxybenzaldehyde (salicylaldehyde) the $\ce{-OH}$ at C-2 is right beside the $\ce{-CHO}$ at C-1. The $\ce{O-H\bond{...}O=C}$ contact closes a six-membered chelate ring:

Each molecule is self-satisfied; only weak dispersion forces act between molecules.

Step 2 — The para isomer (B) …

Intramolecular hydrogen bonding occurs when a hydrogen atom covalently bonded to an electronegative atom (O, N, F) forms a hydrogen bond with another electronegative atom within the same molecule. The key constraint is geometry: the donor and acceptor must be positioned such that they can fold into a stable ring structure, typically forming 5- or 6-membered rings.

This is fundamentally different from intermolecular hydrogen bonding, where molecules interact with each other. For intramolecular bonding, we need to examine whether the molecular architecture permits the necessary spatial proximity.

Without seeing the structures labeled a, b, c, d, and e in your question, I can guide you through the analysis:

1. Identify potential donors and acceptors

   Look for –OH, –NH₂, –COOH groups (donors) and lone-pair-bearing atoms like O, N (acceptors) within each molecule.

2. Check the spatial arrangement

   The donor and acceptor must be on the same molecule and positioned such that rotation around single bonds allows them to approach each other. Common examples:

   • ortho-substituted phenols (like o-nitrophenol): the –OH can hydrogen-bond to the adjacent –NO₂ oxygen, forming a 6-membered ring
   • Salicylic acid: the carboxylic –OH bonds to the phenolic oxygen
   • 1,2-diols or β-keto-enols in their enol form

3. Evaluate ring size

   5- and 6-membered rings are most stable. A 4-membered ring is too strained; rings larger than 7 are entropically disfavored and rarely form intramolecularly.

4. Rule out unfavorable geometries

   • meta- or para-substituted aromatics place groups too far apart
   • Linear chains without functional groups in 1,2- or 1,3-positions typically cannot fold appropriately …

Concept and Intuition

Hydrated salts often contain water molecules that play different roles within the crystal structure. It's crucial to distinguish between two main types of water molecules:

1. Coordinated Water: These water molecules act as ligands and are directly bonded to the central metal ion, forming a complex ion.
2. Lattice Water (or Water of Crystallization): These water molecules are not directly bonded to the metal ion. Instead, they are held within the crystal lattice by hydrogen bonds, often bridging between the complex ion and counter-ions (like sulfate ions) or other water molecules.

When a question asks about water molecules "participating in hydrogen bonding" in such a context, it typically refers to the lattice water molecules whose primary role is to form hydrogen bonds and are not coordinated to the metal ion. While coordinated water molecules can also form hydrogen bonds (e.g., their hydrogen atoms can bond to counter-ions), the distinction is usually made for the non-coordinated water.

Step-by-Step Solution

1. Understand the Structure of $\text{CuSO}_4 \cdot \text{5H}_2\text{O}$:

   Copper(II) sulfate pentahydrate, commonly known as blue vitriol, is not simply $\text{CuSO}_4$ mixed with five separate water molecules. Its crystal structure is well-defined and involves specific interactions between the copper ion, sulfate ion, and water molecules. The chemical formula is more accurately represented as $[\text{Cu}(\text{H}_2\text{O})_4]\text{SO}_4 \cdot \text{H}_2\text{O}$.

2. Identify Coordinated Water Molecules:

   The central $\text{Cu}^{2+}$ ion is a transition metal ion and typically forms coordination complexes. In $\text{CuSO}_4 \cdot \text{5H}_2\text{O}$, four of the five water molecules are directly coordinated to the $\text{Cu}^{2+}$ ion. These four water molecules act as ligands, forming a square planar arrangement around the copper ion. The oxygen atoms of two sulfate ions complete a distorted octahedral geometry around the $\text{Cu}^{2+}$ ion. This forms the complex ion $[\text{Cu}(\text{H}_2\text{O})_4]^{2+}$.

3. Identify the Fifth Water Molecule:

   The fifth water molecule is distinct from the other four. It is not directly bonded to the $\text{Cu}^{2+}$ ion. Instead, it resides within the crystal lattice, outside the coordination sphere of the copper ion.

4. Describe the Role of the Fifth Water Molecule: …