When 100 mL of water is mixed with 400 mL of ethanol, the total volume is not simply additive — because hydrogen bonding causes volume contraction.

---

Reason:

Water and ethanol molecules form strong hydrogen bonds, allowing the molecules to pack more closely.
So the mixture’s final volume is less than 500 mL.

---

Experimental result:

When equal amounts of water and ethanol are mixed, the volume contraction is about 4–5%.

The maximum number of carbon atoms found in naturally occurring monosaccharides is nine (9).

---

Explanation:

Monosaccharides are classified by the number of carbon atoms:

No. of Carbons	Type	Example
3	Triose	Glyceraldehyde, Dihydroxyacetone
4	Tetrose	Erythrose
5	Pentose	Ribose, Xylose
6	Hexose	Glucose, Fructose
7	Heptose	Sedoheptulose
8	Octose	2-Keto-3-deoxy-octonate
9	Nonose	Neuraminic acid (and its derivative sialic acid)

A native protein is a protein in its natural, functional, and properly folded form (the form found in living cells).

---

Structure of native protein

A native protein usually has its secondary, tertiary, and sometimes quaternary structure intact.

That’s what gives it its specific shape and biological activity.

---

Summary

Term	Meaning
Primary structure	Sequence of amino acids
Secondary structure	α-helix or β-pleated sheet (H-bonds)
Tertiary structure	3D folding → active shape
Native protein	Protein in its functional 3D (tertiary or quaternary) form

---

Secondary Structure Proteins

• Meaning: Local folding of polypeptide chains into α-helix or β-pleated sheets (held by hydrogen bonds).
• Examples:
  • Keratin → mostly α-helix (found in hair, nails, wool).
  • Fibroin → mostly β-pleated sheet (found in silk).

---

Tertiary Structure Proteins

• Meaning: 3D folding of a single polypeptide chain → forms a globular protein.
• Forces involved: H-bonds, disulfide bridges, ionic bonds, hydrophobic interactions.
• Examples:
  • Myoglobin → stores oxygen in muscles.
  • Lysozyme → enzyme that breaks bacterial cell walls.
  • Ribonuclease → enzyme for RNA breakdown.

---

Quaternary Structure Proteins

• Meaning: Association of two or more polypeptide chains (subunits) in a functional complex.
• Examples:
  • Hemoglobin → 4 subunits (2 α + 2 β chains).
  • Insulin → made of multiple polypeptide chains (A and B chains linked by disulfide bonds).
  • DNA polymerase → multi-subunit enzyme.

Fructose is a reducing sugar, even though it’s a ketose (contains a keto group), not an aldehyde.
Let’s see how?

---

1. Reducing sugar meaning

A reducing sugar is one that can reduce mild oxidizing agents like Tollens’ reagent (Ag⁺ → Ag) or Fehling’s solution (Cu²⁺ → Cu⁺) — because it has a free aldehyde or ketone group (or can form one in solution).

---

2. Fructose has a ketone group (not aldehyde)

At first glance, this looks non-reducing because it lacks a free aldehyde group.

---

3. In alkaline medium → tautomerization

In basic solution, fructose undergoes keto–enol tautomerism, forming an enediol intermediate.
This intermediate can rearrange to give glucose and mannose, both of which are aldehydes (aldoses). Fructose⇌enediol formalkaline mediumGlucose+Mannose\text{Fructose} \xrightleftharpoons[\text{enediol form}]{\text{alkaline medium}} \text{Glucose} + \text{Mannose}Fructosealkaline mediumenediol form​Glucose+Mannose

---

4. Hence → Reducing property

Because glucose and mannose can reduce Tollens’ and Fehling’s reagents, fructose also gives a positive test indirectly.
So, fructose is a reducing sugar — not directly, but via tautomeric conversion to an aldehyde sugar.

Urotropin is another name for hexamethylenetetramine (chemical formula: C₆H₁₂N₄).

🔹 Chemical Information

• IUPAC Name: Hexamethylenetetramine
• Molecular Formula: C₆H₁₂N₄
• Molar Mass: 140.19 g/mol
• Structure: It is a cage-like compound made up of four nitrogen atoms linked by methylene (-CH₂-) groups.

🔹 Preparation

Urotropin is formed by the reaction of formaldehyde with ammonia.

6HCHO + 4NH₃→ C₆H₁₂N₄ + 6H₂O

🔹 Uses

1. Medicine: Used in the treatment of urinary tract infections (UTIs) because in acidic urine, it releases formaldehyde, which acts as an antiseptic.
2. Explosives: Used in the preparation of RDX (Cyclonite), a powerful explosive.
3. Fuel tablets: Used in solid fuel tablets for camping and military use (burns smokelessly).
4. Plastics & resins: Used in making phenol-formaldehyde resins.

The correct order of basic strength is:

piperidine > pyridine > pyrrole

Reasoning (in brief):

• In piperidine, the nitrogen is sp³-hybridized and the lone pair is “free” (not delocalized into any aromatic system), so it can readily accept a proton → very basic.
• In pyridine, the nitrogen is sp² and its lone pair resides in an orbital perpendicular to the aromatic π system (i.e. not part of the aromatic sextet), so it is available for protonation, but less “free” than in piperidine → moderate base.
• In pyrrole, the nitrogen’s lone pair is part of the aromatic π system (delocalized), and protonating it would disrupt aromaticity; hence the lone pair is not readily available → very weak base

Major product: bromine replaces the –OH to give CH₃–CH(Br)–CH=CH₂ (IUPAC: 3-bromobut-1-ene) — with a minor product CH₃–CH=CH–CH₂Br (1-bromo-2-butene) possible.

Why (mechanism, stepwise):

1. Protonation of the alcohol.
   CH₃–CH(OH)–CH=CH₂ + H⁺ → CH₃–CH(OH₂⁺)–CH=CH₂
2. Loss of water → allylic carbocation (RDS).
   CH₃–CH(OH₂⁺)–CH=CH₂ → CH₃–C⁺H–CH=CH₂ (carbocation at C-2).
   This carbocation is allylic and is resonance stabilized: the + charge is delocalized between C-2 and the terminal C-4 (move the π electrons C3–C4 → C2–C3). Resonance forms:
   • A: CH₃–C⁺H–CH=CH₂ (positive at C2)
   • B: CH₃–CH=CH–CH₂⁺ (positive at terminal C4)
3. Nucleophilic attack by Br⁻.
   Bromide attacks the resonance-stabilized cation. Attack at C-2 (the more substituted resonance form) is favored, giving CH₃–CH(Br)–CH=CH₂ (major). Attack at the terminal C (resonance form B) gives CH₃–CH=CH–CH₂Br (minor).

Minor product is more stable alkene?

minor product can be the thermodynamically more stable alkene, but the reaction gives the major product for kinetic/reaction-path reasons. Here’s why, step-by-step, in plain language.

1) What the two products are (reminder)

• Major product (fast): bromide ends up on the more substituted carbon (attack at C-2) → gives CH₃–CH(Br)–CH=CH₂ (formed quickly by attack on the resonance form with the positive charge at C-2).
• Minor product (more stable alkene): bromide ends up at the terminal site (attack at C-4 resonance form) → gives CH₃–CH=CH–CH₂Br, which may correspond to the more substituted/thermodynamically more stable alkene after any isomerization.

2) Why the less stable alkene is the major product

• After loss of water you form an allylic carbocation that is resonance-stabilized. The two resonance contributors are not equally important: the contributor with the positive charge on C-2 (the more substituted position) is a major contributor (it has greater carbocation stabilization by adjacent alkyl groups). That means Br⁻ encounters more positive character at C-2, so nucleophilic attack there is faster. The product from that fast attack is the observed major product — this is kinetic control.
• The route that would give the thermodynamically more stable alkene requires attack on the resonance contributor with the charge at the terminal carbon (less favored contributor), so that pathway is slower → hence minor product.

3) Stability vs. kinetics

• Thermodynamic stability (more substituted internal alkene) and kinetic accessibility (where Br⁻ attacks fastest) are two different things. The reaction pathway here is under kinetic control (fast capture of the carbocation by Br⁻), so the kinetically favored product predominates even if it’s not the most stable alkene.

What is Ring Expansion?

Ring expansion is a rearrangement reaction where a cyclic compound increases its ring size (e.g., 5-membered → 6-membered ring).
It usually happens during carbocation rearrangements or nitrene/carbene insertions.

---

🔹 Why Does It Happen?

Rings expand mainly for stability reasons:

1. Carbocation stability – a less stable carbocation shifts to form a more stable one.
2. Ring strain relief – small rings (3, 4, 5-membered) are strained; expansion to 6-membered reduces strain.
3. Formation of more stable rings – 6-membered rings are especially stable due to minimal angle and torsional strain.

---

🔹 General Mechanism

Typical situation: a carbocation is adjacent to a ring.

• A bond adjacent to the carbocation migrates (1,2-shift) into the positive center.
• This migration enlarges the ring size by one carbon.

Stepwise:

1. Generate a carbocation (often via protonation, halonium ion, or leaving group loss).
2. Adjacent C–C bond migrates toward the carbocation center.
3. New carbocation forms inside the enlarged ring.

Do ring expansion always to cyclohexane?

Ring expansion does not always stop at cyclohexane, but cyclohexane is the most common and stable product when smaller rings expand, because:

• Cyclohexane is strain-free (ideal bond angles ~109.5°, no angle strain).
• Rings smaller than 6 (3-, 4-, 5-membered) often expand to reach 6.
• Rings larger than 6 (like 7 or 8) may rearrange in different ways but they don’t necessarily “shrink” back to 6.

---

General trends:

1. Cyclopropane (3-membered) → expands to cyclobutane, cyclopentane, or cyclohexane (depending on the mechanism).
2. Cyclobutane (4-membered) → prefers expansion to cyclopentane or cyclohexane (to reduce strain).
3. Cyclopentane (5-membered) → expands most often to cyclohexane (since 6 is especially stable).
4. Cyclohexane (6-membered) → usually does not expand, because it’s already the most stable.
5. Cycloheptane or larger → can undergo rearrangements, but not necessarily to cyclohexane; instead, they may give different products depending on the reaction condition

In SN1 (Substitution Nucleophilic Unimolecular) reactions involving alcohols (R-OH), the mechanism requires the departure of a leaving group to form a carbocation intermediate in the rate-determining step. The hydroxyl group (OH) in alcohols is a poor leaving group because it is a strong base (OH⁻) and holds onto the substrate too tightly, making ionization difficult.

To facilitate the reaction, the alcohol is first protonated under acidic conditions (e.g., using H₂SO₄ or HCl) to form an alkyloxonium ion (R-OH₂⁺). This protonation weakens the C-O bond and converts the leaving group to neutral water (H₂O), which is a much better leaving group due to its weaker basicity and greater stability. The water then departs, generating the carbocation (R⁺), which can be attacked by the nucleophile in the subsequent fast step.

Without this protonation, the SN1 pathway would be inefficient or impossible for alcohols, as the energy barrier for OH⁻ departure is too high. This is why SN1 reactions on alcohols are typically conducted in acidic media.