Question

The physical properties of soap depend on the very different water solubility of two ends of tghe same molecule. Normally, soap is the sodium salt of a long-chain carboxylic acid.

A. Explain, using structures and words, the observation that soap does not work properly in very low pH water, or hard water which has calcium and magnesium ions.

B. would a soap make from N,N,N-trimethyl octadecyl ammonium chloride (shown below) have the same problems as a long-chain carboxylic acid? Explain.

CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-N+H3 CL-

Answer 1

A)Soaps are sodium salt of long chain carboxylic acid(fatty acids).It has a large non-ionic hydrocarbon group (hydrophiobic end ) and an ionic-group,-COO-Na+(hydrophillic end), responsible for cleansing action of soap.

example-sodium stearate C17H35 COO-Na+ of the form RCOO-Na+ (R= long chain alkyl group)

At low pH, under high H+ concentrations(acidic conditions),

RCOO-Na+ +HRCOOH +Na+ (soap reduces to its corresponding fatty acid and so no ionic end remains.Consequently , soap loses its cleansing action.

In the presence of Ca2+/Mg2+,(hard water) , soap precipitates in the form of insoluble calcium and magnesium salts

RCOO-Na+ +Ca2+(RCOO)2Ca +Na+

RCOO-Na+ +Mg2+(RCOO)2Mg +Na+

B) N,N,N-trimethyl octadecyl ammonium chloride has the structure, CH3(CH2)17N(CH3)3 Cl

The ionic end +N(CH3)3Cl- does not react with H+ or Ca2+ or Mg2+

R +N(CH3)3Cl- +H+ =No reaction

R +N(CH3)3Cl- +Ca2+/Mg2+=no reaction

So acts even in hard water and low pH