Question

Laboratory 11B: Methyl Orange

1) What changes to the synthetic sequence used for methyl orange would need to be made to generate FD&C Orange #1 (see lab manual for the structure). (Some structure drawings will be helpful when answering this question).

2) Some fabrics stained strongly with methyl orange while others stained poorly. Using one of the strongly stained fabrics as an example, explain what features of that material result in it being strongly stained.

3) Explain why methyl orange changes color at different pH.

Answer 1

Q.1: FD&C Orange #1 can be prepared by treating diazotized sulfanilic acid with 1-naphthol. The complete reaction is shown in the figure below;

Q.3: Methyl orange shows orange color in acidic medium and yellow color in basic medium.

The unionized methyl orange is red in color. When the red color methyl orange loses the H+ ion attached to the N - atom of (-N=N-) it becomes ionized methyl orange whose color is yellow. The ionized and unionized form maintian an equilibrium as shown below

Methyl orange(orange color) < ---------- > H⁺(aq) + ionized methyl orange(yellow color)

When we add acid, the concentration of  H⁺(aq) increases and according to Lechatelier's principle the equilibrium is shifted towards left and more methyl orange is formed making the color red.

When we add base, the concentration of  H⁺(aq) decreases and according to Lechatelier's principle the equilibrium is shifted towards right and more ionized methyl orange is formed making the color yellow.