Its yellow color is consistent with the absorption of high energy blue light necessary to excite a t 2g to the e g levels. If we use a weak field ligand like Br - , the e g levels will be closer to the t 2g in energy, requiring a lower energy photon to be absorbed, resulting in the transmission of shorter wavelengths of light. The compound will appear less reddish and in a colder color. Now that the gap, D , between the e g and t 2g has been lowered, the six electrons can occupy that upper level even in the ground state, in obedience to Hund's Rule. The bulky Br atom would tend to shield its carbon's double bond a bit, and so the addition would give the 3,4,5-tribromo in somewhat greater abundance.
Esters require an organic acid and an alcohol to condense by the elimination of water.
Poly esters require a di oic acid and a di ol so that the condensation can continue ad infinitum . The optically active carbons are in red . Each bonds to four different substituents. Each has its mirror form with which it is not superimposable. Two carbons with two mirror forms each gives four optical isomers of Threonine.