- Which of the following structures corresponds to the IUPAC name
trans-1-tert-butyl-3-methylcyclohexane? Answer: A cyclohexane ring with a tert-butyl group at C1 (wedge) and a methyl group at C3 (dash).
The name specifies a cyclohexane ring with two substituents: tert-butyl and methyl. The numbering is determined by alphabetical order ('butyl' before 'methyl'), so the tert-butyl group must be at C1. The methyl group is at C3. The stereochemical descriptor 'trans' means the two groups are on opposite sides of the ring. If the C1 tert-butyl group is represented by a wedge (up), the C3 methyl group must be represented by a dash (down). Option A is the only choice that satisfies all these requirements.
- An alkane has the IUPAC name 4-ethyl-2,2,5-trimethylheptane. How many quaternary carbons does this molecule contain?
Answer: 1
To determine the number of quaternary carbons, we must first visualize or draw the structure from its IUPAC name. The parent chain is heptane (7 carbons). There are two methyl groups at C2, one ethyl group at C4, and one methyl group at C5. A quaternary carbon is a carbon atom bonded to four other carbon atoms. Let's examine the carbons in the chain: C1 is primary. C2 is bonded to C1, C3, and two methyl carbons, making it a quaternary carbon. C3 is secondary. C4 is tertiary. C5 is tertiary. C6 is secondary. C7 is primary. None of the carbons within the substituent groups are quaternary. Therefore, the molecule contains exactly one quaternary carbon (C2).
- Consider the structure of 1-bromo-3-chloro-5-fluorocyclohexane, where all three halogen substituents are on the same side of the ring (all wedges or all dashes). Which statement accurately describes this molecule?
Answer: The correct name is 1-bromo-3-chloro-5-fluorocyclohexane, and the bromo and chloro groups are cis to each other.
First, determine the correct numbering. The locants will be {1, 3, 5} regardless of which substituent is C1. In this case of a tie, numbering priority is determined alphabetically: bromo > chloro > fluoro. Thus, the bromine is at C1, chlorine at C3, and fluorine at C5. The base name is 1-bromo-3-chloro-5-fluorocyclohexane. Since all three substituents are on the same side of the ring, any pair is cis to each other. The bromo group (at C1) and the chloro group (at C3) are therefore cis. Statement A correctly identifies both the name and the stereochemical relationship.
