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Acetylation and Osmotic Pressure

Acetylation reaction is the introduction of an acetyl functional group (CH3COO) into an organic compound. Phenols, amines and quinones can be conveniently acetylated. The acetylating reagent used depends on the nature of the compound to be acetylated. The acetylation reaction involves the generalization of acetic acid as by product.

Osmosis is the process of spontaneous movement of water across a membrane driven by a gradient of water concentration. The water moves from an area of high concentration of water to an area of low concentration. The osmotic pressure of a solution is defined as the pressure necessary to stop the net movement of water across a selectively permeable membrane that separates the two solutions of different osmotic pressure.

Acetylation Organic Chemistry

Compounds of the types ROH (alcohols and phenols) and also compounds of the type RNH2 and R2NH (primary and secondary amines) can be directly acetylated, the reactive H atom being replaced by the acetyl radical. In acetylation reaction part of acetic anhydride, molecule is used up in acetylation and part of the molecule is converted to acetic acid.

C2H5OH (ethyl alcohol) + (CH3CO)2O (acetic anhydride) → CH3COOC2H5 (ethyl acetate) + CH3COOH

C6H5NH2 (aniline) + (CH3CO)2O (acetic anhydride) → C6H5NHCOCH3 (acetanilide) + CH3COOH (acetic acid)

Acetylation will proceed particularly smoothly with acetyl chloride if pyridine is present to absorb the hydrogen chloride, as fast as it is formed.


Acetic anhydride is used industrially as an acetylating agent in the manufacture of cellulose acetate, acetanilide, aspirin and many other acetyl compounds. In large scale industrial work acetic anhydride is preferred to acetyl chloride as an acetylating agent because it is less corrosive and easier to handle. Acetylation reaction is used in the detection and estimation of the hydroxyl and amino groups in organic compounds.

Factors affecting Osmotic Pressure

The following are the factors which affect osmotic pressure.

  1. Temperature
    → Osmotic pressure is directly proportional to the absolute temperature.
  2. Concentration
    → Osmotic pressure is directly proportional to the molar concentration of the solute in it.
    → Equimolar solutions of non-ionized solutes exert the same osmotic pressure.
  3. Ionization of solute
    → The ionized solute exerts more osmotic pressure than an equimolar solution of non-ionizable solution.
    → This is because each ion exerts osmotic pressure as a separate solute particle.
  4. Molecular weight of the solute
    → Osmotic pressure is inversely proportional to the molecular weight of the solute.
    → Low molecular weight of a solute exerts more osmotic pressure than high molecular weight of solute.

The excess hydrostatic pressure which must be applied on the solution to prevent the inflow of solvent (water) to the solution when they are separated by semipermeable membrane and establish an equilibrium is called osmotic pressure of the solution.

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