1. If a carbon has four different groups attached to it, it is a chirality or stereogenic center with R or S configurations. In 2.3-dichlorohexane, the second carbon has four groups attached: 1) CH3, 2) Cl, 3) H, and 4) the rest of the molecule. Thus carbon 2 is a stereogenic carbon. Same with carbon 3. In 2,4-dichlorohexane, you have the same situation. In 2,2-dichlorohexane, no carbon has four different groups attached, so it is not optically active.
If you have an even number of stereogenic centers in a molecule, it is possible for it to still be optically inactive, if it possesses a plane of symmetry. Such a molecules is classified as being meso. But that is not the case with 2,3- and 2,4-dichlorohexane.
2. NO2+, the nitronium ion, is the electrophile in aromatic nitrations using nitric acid and sulfuric acid.
3. CH3-CH-OH=CH-CH-Br-CH2-NH2 There seems to be an extra H, but if the oxygen atom has two bonds attached to it, it will have two lone pairs on it. Just as in the H2O molecule.
Steve
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