Question

Is 1-bromo-2,3-dimethylbutane chiral? If chiral, give the number(s) of the chirality center(s) (increasing numberical order, separated by a comma, no space). Write 'none' if there are no chirality centers. . Is 4,5-dimethyl-2,6-octadiyne chiral? If chiral, give the number(s) of the chirality center(s) (increasing numberical order, separated by a comma, no space). Write 'none' if there are no chirality centers. .

Answer 1

Answer:

1-bromo-2,3-dimethylbutane is chiral. There is one chiral center at carbon number 2.

4,5-dimethyl-2,6-octadiyne is chiral. There are two chiral centers: one at carbon number 4 and the other at carbon number 5.

Explanation:

Start by sketching the structure of the two compounds. (A colored sketch of the two compounds is attached.) The first compound is a branched halogen-alkane with four carbon atoms in its backbone. The second is a branched alkyne with two triple bonds.

Each carbon atom contains four valence electrons. It forms four chemical bonds. A carbon atom is chiral if each of its four bonds connects to a different group of atoms.

For example, the second carbon atom (number two) in 1-bromo-2,3-dimethylbutane is connected to:

• one $\rm \text{-CH}_2\text{Br}$,
• one $\text{-CH}_3$,
• one $\text{-H}$, and
• one $\text{-CH}(\text{CH}_3)_2$.

All four of these groups are different. As a result, this carbon atom is chiral.

Another example: the third carbon atom in 1-bromo-2,3-dimethylbutane is connected to

• one $\text{-H}$,
• two $\text{-CH}_3$, and
• one $\text{-CH}(\text{CH}_3)(\text{CH}_2\text{Br})$.

This carbon atom is connected to two instances of the same group $\text{-CH}_3$. As a result, it is not chiral.

One more example: the first carbon atom in in 1-bromo-2,3-dimethylbutane is connected to:

• two $\text{-H}$,
• one $\text{-Br}$, and
• one $\text{-CH}(\text{CH}_3)(\text{CH}(\text{CH}_3)_2)$.

This carbon atom is connected to two of the same group. As a result, it is not chiral. This example shows that it is likely helpful for the sketch to include all the hydrogen atoms in this compound.

Carbon atoms 2, 3, 6, and 7 in 4,5-dimethyl-2,6-octadiyne are all connected to triple bonds. None of these atoms is bonded to four groups. Therefore, they cannot be chiral. In other words, out of the eight carbon atoms in this compound, only four of them need to be checked for chirality.