The compound whose formula is written below is a major component of a soft fatty substance: (a) State the change that would be observed in the physical stat...

Answer Details

The compound shown is a glyceryl ester (triglyceride): a glycerol backbone CH2-CH-CH2 esterified with three fatty-acid chains.

• The top and bottom chains, -COO(CH2)12CH3, are saturated (no C=C).
• The middle chain, -COOCH=CHCH=CHCH=CHCH=CHCH3, is unsaturated and contains four carbon-carbon double bonds.

(a) Change in physical state on hydrogenation (H₂, finely divided Ni, ~180 °C, long time)

The four C=C double bonds in the unsaturated chain add hydrogen and become saturated (this is hardening, the industrial process used to make margarine). The soft (semi-liquid) fatty substance therefore changes into a hard solid fat. In short: soft/oily → hard solid.

(b) Amount of hydrogen consumed by one mole of the component

Each carbon-carbon double bond adds exactly one mole of hydrogen; the ester (C=O) bonds are not reduced under these conditions.

\[\text{Number of C=C bonds}=4 \;\Rightarrow\; \text{moles of }H_2 = 4\]

\[\text{One mole of the triglyceride} + 4\,\text{mol }H_2 \longrightarrow \text{saturated triglyceride}\]

Amount of hydrogen consumed = 4 moles.

(c) Reaction with hot concentrated sodium hydroxide solution

Hot concentrated NaOH hydrolyses the ester linkages (saponification), giving:

• Soap - the sodium salts of the fatty acids (sodium tetradecanoate from the saturated chains and the sodium salt of the unsaturated acid), and
• Glycerol (propane-1,2,3-triol).